Autonomic Nervous System Anatomy

The autonomic nervous system (ANS) is a very complex, multifaceted neural network that maintains internal physiologic homeostasis. This network includes the cardiovascular, thermoregulatory, gastrointestinal (GI), genitourinary (GU), and ophthalmologic (pupillary) systems (see the following image). Given the complex nature of this system, a stepwise approach to autonomic disorders is required for proper understanding.

The goal for this article remains focused at step III on the anatomy of the ANS, as follows.

• Step I - Understand the reason for testing
• Step II - Recognition and etiology (especially small fiber neuropathy [SFN])
• Step III - Understand basic anatomy and neurophysiology
• Step IV - Learn the methods for testing
• Step V - Diagnosis and management

Almost 10% of the population (or >30 million people in the United States) may acquire an autonomic disorder requiring medical attention. Because the ANS maintains internal physiologic homeostasis, disorders of this system can be present with both central as well as peripheral nervous system localization.

Causes of autonomic nervous system dysfunction

ANS dysfunction, often referred to as dysautonomia, can arise from primary or idiopathic and secondary causes. ^{[1, 2, 3, 4]}

The primary causes of ANS dysfunction: Primary autonomic disorders are those where dysfunction is the main disease process. This means that autonomic dysfunction occurs in the absence of other conditions that may contribute to it. These include: ^{[1, 2, 3, 4]}

• Familial dysautonomia - A genetic disorder affecting the development of the ANS, primarily seen in individuals of Ashkenazi Jewish descent
• Multiple system atrophy (MSA) - A progressive neurodegenerative disorder that affects multiple systems in the body, including the ANS
• Pure autonomic failure - A rare condition characterized by a gradual decline in autonomic function without accompanying neurological symptoms
• Postural orthostatic tachycardia syndrome (POTS) - A condition where severe tachycardia occurs upon standing in the presence of orthostatic intolerance
• Idiopathic dysautonomia - A disorder where the cause remains unknown despite thorough investigation

Secondary causes of ANS dysfunction: Secondary autonomic dysfunction occurs because of another underlying condition or external factors. Common secondary causes include: ^{[1, 2, 3, 4]}

• Diabetes mellitus - Particularly when poorly controlled, diabetes can lead to diabetic autonomic neuropathy, affecting the heart rate, blood pressure, and digestion
• Neurodegenerative diseases - Conditions such as Parkinson's disease and multiple sclerosis can disrupt autonomic functions due to neurodegeneration
• Infectious diseases - Viral infections (e.g., human immunodeficiency virus (HIV), Lyme disease, coronavirus disease 2019 (COVID-19)) and bacterial infections (e.g., botulism) can damage autonomic nerves
• Autoimmune disorders - Conditions such as Guillain-Barré syndrome and lupus can lead to an immune-mediated attack on the ANS
• Metabolic disorders - Deficiencies in vitamins such as B12 (cobalamin) and conditions such as amyloidosis can impair autonomic function
• Trauma - Physical injuries, particularly spinal cord injuries, can disrupt autonomic pathways
• Medications - Certain drugs, especially those used in chemotherapy or for treating other conditions, can induce autonomic dysfunction as a side effect
• Toxins - Chronic alcohol use is another major contributor to autonomic dysfunction, primarily through toxic neuropathy

Inherited disorders: In addition to acquired causes, there are inherited conditions associated with ANS dysfunction: ^{[1, 2, 3, 4]}

• Hereditary sensory-autonomic neuropathy (HSAN) - A group of genetic disorders affecting the sensory and autonomic nerves
• Familial amyloid polyneuropathy (FAP) - A hereditary condition that leads to amyloid deposits affecting the nerve function
• Tangier disease and Fabry disease - Both genetic disorders that can affect the nerve function due to lipid metabolism abnormalities

Autonomic failure is seen in MSA, pure or progressive autonomic failure, Parkinson and other neurodegenerative diseases, metabolic diseases such as Wernicke and cobalamin deficiency, diabetes mellitus, hyperlipidemia, trauma, vascular diseases, neoplastic diseases, and multiple sclerosis. In addition, autonomic dysfunction is associated with various medications.

In addition to diabetes, autonomic dysfunction is associated with other neuropathies, including Guillain-Barré syndrome, Lyme disease, HIV infection, leprosy, acute idiopathic dysautonomia, amyloidosis, porphyria, uremia, and alcoholism. Besides nerve localization in the peripheral nervous system, it occurs in diseases of the presynaptic neuromuscular junction such as botulism and myasthenic syndrome.

In addition to the acquired causes, inherited disorders such as HSAN, FAP, Tangier disease, and Fabry disease also exist.

The interplay between genetic predispositions, environmental factors, and underlying health conditions highlights the complexity of managing dysautonomia. Identifying whether the dysfunction is primary or secondary can significantly influence treatment strategies and patient outcomes. ^{[1, 2, 3, 4]}

Clinical presentation

Clinically, postural lightheadedness, dry mouth, dry eyes, impotence, loss of sweating or hyperthermia, nocturnal diarrhea, gastroparesis,impaired accommodation, urinary or bowel incontinence, and SFN such as burning pain in the feet are some of the presenting features. These symptoms can arise due to the involvement of small nerve fibers, which are responsible for autonomic functions and sensory perception. In particular, burning pain in the feet is linked to sensory neuropathy involving small fibers, with studies indicating that this occurs in over 90% of the cases. ^{[5, 6]}

Most peripheral neuropathies affect all fiber sizes. Few peripheral neuropathies are associated with pure or predominantly small fiber involvement. Notably, diabetes mellitus is a significant contributor to SFN; it is estimated that up to 50% of individuals with diabetes or prediabetes may develop this condition. For conditions such as Sjögren's syndrome, SFN often presents with autonomic symptoms such as dry eyes and dry mouth. Other autoimmune conditions, including lupus and psoriatic arthritis, can also involve SFN. ^{[5, 6]}

Patients with pure small fiber involvement display normal large fiber function. Muscle bulk, strength, muscle stretch reflexes, and large fiber sensory function (i.e., vibration, proprioception) are normal.

Myelinated vs unmyelinated small fibers

Small fibers are both myelinated and unmyelinated.

Myelinated small fibers: Small myelinated fibers transmit preganglionic autonomic efferents (B fibers) and somatic afferents (A delta fibers). ^{[7, 8]}

Types: The myelinated small fibers primarily include A delta fibers — responsible for transmitting sharp pain and temperature sensations — and B fibers — involved in autonomic functions. ^{[7, 8]}

Function: A delta fibers conduct impulses rapidly due to their myelination, facilitating quick responses to stimuli. They transmit somatic afferents from the skin and deep tissues, as well as preganglionic autonomic efferents. The myelination allows for a saltatory conduction, where the action potential jumps between the nodes of Ranvier, increasing the conduction velocity. ^{[7, 8]}

Unmyelinated fibers: Unmyelinated (C) fibers transmit postganglionic autonomic efferents as well as somatic and autonomic afferents. ^{[7, 8]}

Types: Unmyelinated fibers are primarily C fibers. These fibers transmit dull pain, temperature sensations, and autonomic signals.

Function: C fibers conduct impulses more slowly than A delta fibers because they lack myelin. They engage in transmitting postganglionic autonomic efferents and somatic and autonomic afferents. Their slower conduction speed is suitable for functions requiring prolonged signaling such as chronic pain sensations or autonomic regulation.

Both A delta and C fibers are widely distributed in the skin and deep tissues, playing critical roles in sensory perception and autonomic regulation.

The neurotransmitter for preganglionic sympathetic and parasympathetic nervous system (PNS) as well as postganglionic PNS is acetylcholine (ACh). The neurotransmitter for the postganglionic sympathetic nervous system (SNS) (innervating sweat glands) is also ACh, whereas that for the remaining postganglionic SNS is norepinephrine (NE).

Studies have highlighted the importance of these small fiber types in various clinical conditions. For instance, research has shown that conditions such as postherpetic neuralgia predominantly affect the unmyelinated C fibers while sparing the autonomic nerve fibers. ^{[7, 8]}

Electromyography

Electromyography (EMG) plays a key role in the evaluation of most peripheral neuropathies and helps in assessing only the large myelinated fibers. Thus, pure small fiber neuropathies may be associated with normal findings on routine electrophysiologic studies. Older patients who lack sural sensory responses can still be diagnosed with SFN. Patients with symptoms other than neuropathic ones certainly need autonomic function testing for appropriate diagnosis.

SFN can present with symptoms such as burning pain, allodynia, and autonomic dysfunction, even when routine EMG results are normal. Therefore, EMG should be combined with other diagnostics to fully assess neuropathy, particularly small fiber involvement. The diagnosis of SFN typically involves clinical evaluation, skin biopsy to measure intraepidermal nerve fiber density, and autonomic function tests such as the quantitative sudomotor axon reflex test (QSART). Autonomic testing is especially important for patients with symptoms such as orthostatic hypotension (OH) and palpitations, aiding in the differentiation of neuropathy types and guiding treatment. ^{[9, 10, 11]}

Autonomic function tests that are essential for diagnosing conditions where autonomic fibers are involved may include: ^{[9, 10, 11]}

• QSART - This test measures the sweat response to ACh stimulation and helps assess postganglionic sympathetic nerve function; it has shown sensitivity rates up to 80% for diagnosing SFN.
• Sudoscan - A device that quantifies electrochemical skin conductance in the palms and soles, providing rapid assessment for small fiber dysfunction; it has been validated against traditional methods and is particularly useful for monitoring SFN in clinical practice.

While EMG remains a valuable tool for assessing peripheral neuropathies, its limitations necessitate the use of additional diagnostic approaches for conditions such as SFN. Autonomic function testing plays a pivotal role in accurately diagnosing SFN, especially in patients presenting with atypical symptoms. ^{[9, 10, 11]}

Central integration

The central autonomic network (CAN) is a complex network in the central nervous system (CNS) that integrates and regulates autonomic function. The network involves the cerebral cortex (the insular and medial prefrontal regions), amygdala, stria terminalis, hypothalamus, and brainstem centers (periaqueductal gray, parabrachial pons, nucleus of the tractus solitarius, and intermediate reticular zone of the medulla). ^[12] Studies show that CAN has a role in linking the brain and body dynamics, including the regulation of cardiovascular function and its involvement in neurologic and psychiatric disorders. CAN is crucial for maintaining homeostasis and modulating responses to stress, cardiovascular functions, and visceral perception. Insula and stria terminalis participate in modulating visceral functions and emotional regulation, indicating a broader influence of CAN on cognitive and emotional processing. CAN plays a critical role in maintaining homeostasis and responding to physiological changes, as evidenced by its involvement in conditions such as Alzheimer's disease, where autonomic dysfunction is prevalent. ^{[13, 14, 15, 16, 17, 18, 19]}

Afferent pathways

The afferent pathways have receptors residing in the viscera and are sensitive to mechanical, chemical, or thermal stimuli. They conduct along somatic and autonomic nerves and enter the spinal cord through the dorsal roots or the brainstem through cranial nerves. Impulses initiate local, segmental, or rostral reflexes. These pathways have a role in various physiological processes and their implications in clinical conditions. Most afferent signals converge at the solitary nucleus in the medulla before being relayed to higher brain centers, which is crucial for integrating the sensory input with autonomic responses. This integration of sensory information plays a crucial role in autonomic feedback loops, contributing to the body's adaptive responses to environmental and internal stimuli. ^{[13, 14, 15, 16, 17]}

Efferent pathways

ANS consists of the SNS and PNS. SNS descends to the intermediolateral and intermediomedial cells in the thoracolumbar regions of the spine, extending from TI to L2. Preganglionic axons exiting the spinal cord enter the white rami communicantes to join a network of prevertebral and paravertebral ganglia. These preganglionic axons are relatively short, myelinated, and cholinergic. Postganglionic axons exit the ganglia through the gray rami communicantes and extend with the peripheral nerves and blood vessels to innervate their end organs. These postganglionic axons are long, unmyelinated, and primarily adrenergic, except for the innervation of the sweat glands, which are cholinergic.

Adrenergic receptors are (1) alpha, which cause peripheral vasoconstriction; (2) beta 1, which increase heart rate and contractility; or (3) beta 2, which cause the relaxation of smooth muscle located in the peripheral vasculature, bronchi, GI tract, and GU organs. The PNS exits the CNS primarily with cranial nerves III, VII, IX, and X, as well as the sacral spinal roots. Preganglionic axons are generally myelinated and have long peripheral projections before synapsing with the postganglionic neurons in the ganglia that are located close to the end organs; preganglionic axons are also cholinergic. The postganglionic axons are short and cholinergic; cholinergic receptors are also known as muscarinic receptors because of the pharmacology that defines them. ^[20]

Nerve fibers contributing to the superior hypogastric plexus and the hypogastric nerves are currently considered to comprise an adrenergic part of the ANS located between vertebrae T1 and L2, with the cholinergic aspects originating from the sacral spinal segments S2-4. The illustrates the nature of the superior hypogastric plexus, which gives a better understanding of the urinary and sexual dysfunctions after surgical injuries. ^[21]

Dysfunctions within these networks can significantly affect conditions such as heart failure and neurodegenerative diseases. For instance, studies have shown that sympathetic hyperactivation is a common feature in patients with heart failure and can be targeted with pharmacological interventions. ^{[13, 14, 15, 16, 17]}

The concept of central integration in cardiac and vascular regulation can be better understood by knowing that any increase in blood pressure and cardiac output increases the activity of the afferent pathway, which reflexively inhibits sympathetic activity or activates parasympathetic activity or both. However, any decrease in blood pressure and cardiac output decreases afferent activity, which reflexively increases excitatory responses. Thus, cardiovascular function is controlled by a negative-feedback system and increasing activity of the afferent pathway results in decreasing activity of the sympathetic efferent pathway and/or increasing activity of the parasympathetic efferent pathway and vice versa.

In afferent pathways, arterial baroreceptors located in the carotid sinus, aortic arch, and various thoracic arteries respond to changes in the blood pressure and give rise to afferent activity, which conducts in the glossopharyngeal and vagus nerves. The cardiac mechanoreceptors are sensitive to mechanical deformation of the cardiac chambers and give rise to afferent activity, which conducts in the vagus nerve. The pulmonary stretch receptors are sensitive to lung volumes, and inhalation increases afferent activity, which conducts in the vagus nerve.

In efferent pathways, the SNS is predominantly involved in cardiac and vascular regulation, and PNS only has a little influence on the peripheral vasculature. Postganglionic sympathetic fibers innervate the atria, ventricles, and coronary arteries from the cervical ganglia as the superior, middle, and inferior cardiac nerves or from the thoracic ganglia at the TI-T4 level. Stimulation causes increased heart rate, increased myocardial contractility, and coronary vasodilation.

Postganglionic sympathetic fibers innervate the vasculature from plexi on the large proximal vessels or from the somatic nerve. The innervation is denser in resistance vessels (small arteries and large arterioles) than in capacitance vessels (venules and veins). A balance of alpha adrenergic (vasoconstricting) and beta adrenergic (vasodilating) innervation exists. Preganglionic parasympathetic fibers innervate the atria, ventricles, and coronary arteries from the vagus either by the superior and middle cardiac rami or by the recurrent laryngeal nerve as the inferior cardiac rami. Stimulation causes decreased heart rate, decreased contractility, and coronary vasoconstriction.

The window of opportunity for aggressive control of all the traditional risk factors for cardiovascular events or sudden death with intensification of therapy is with short-duration diabetes, the absence of cardiovascular disease, and a history of severe hypoglycemic events. ^[22] Autonomic dysfunction and neuropathy have become the most powerful predictors of risk for mortality.

Studies highlight increased recognition of autonomic dysfunction, particularly in patients with diabetes and cardiovascular disease, as a significant risk factor for adverse outcomes. There is evidence that managing traditional cardiovascular risk factors aggressively in patients with short-duration diabetes and no existing cardiovascular disease can substantially reduce the risk for future events. Guidelines from the American Heart Association stress the importance of managing cardiovascular risk factors aggressively, particularly in populations at high risk for sudden cardiac events. These guidelines advocate for lifestyle modifications alongside pharmacologic interventions to optimize cardiovascular health outcomes. ^{[23, 24]}

Central integration of thermoregulation is controlled primarily in the preoptic and anterior hypothalamus, where a set point is established by a balance between the activities of the thermosensitive neurons. When body temperature is below the set point, autonomic reflexes generate heat by shivering and reduce convective heat loss by cutaneous vasoconstriction and piloerection. When body temperature exceeds the set point, sudomotor activity stimulates sweating to increase evaporative heat loss and precludes cutaneous vasoconstriction and piloerection.

Afferent activity originates from thermosensitive neurons located within the hypothalamus, skin, abdominal viscera, spinal cord, and brainstem. Sleep-wake cycles, hormonal cycles, fluid balance, exercise, metabolic status, and humoral factors influence thermoregulation.

In the efferent pathway, thermoregulation is predominantly controlled by the SNS, with only a little involvement of PNS. Sympathetic sudomotor fibers, which are the only sympathetic postganglionic fibers that are cholinergic, innervate the sweat glands to regulate evaporative heat loss.

Sympathetic vasomotor fibers cause vasoconstriction of cutaneous vasculature composed of abundant arteriovenous anastomoses in the dermis, which shunts blood flow away from the surface to reduce convective heat loss. The control of pilomotor function is rudimentary in humans, but contraction reduces surface area, which reduces convective heat loss.

Thermoregulation is a complex physiological process influenced by a myriad of internal and external factors. Understanding these dynamics is crucial for identifying at-risk populations during heat stress scenarios and developing effective strategies for managing thermal health in diverse environments. ^[25]

Studies have highlighted the complexity of thermoregulation under various conditions: ^[25]

Heat stress - The human body must effectively increase heat loss through sweating and cutaneous vasodilation during heat stress. Factors such as age, biological sex, and health conditions significantly affect thermoregulatory responses. For instance, older adults and individuals with certain medical conditions may have impaired thermoeffector function, increasing their risk for heat-related illnesses during elevated temperatures. ^[25]

Physiological adaptations - Research indicates that physiological traits can alter both active and passive thermoregulatory systems. For example, elite athletes often exhibit enhanced thermotolerance due to repeated exposure to high temperatures during training, which can lead to adaptations such as increased expression of heat-shock proteins that protect cellular function under thermal stress. ^[25]

Environmental factors - The rising global temperatures and increased frequency of heat waves necessitate a better understanding of how different populations respond to thermal stress. This includes examining how intrinsic factors such as body composition and external factors such as clothing impact an individual's ability to regulate temperature effectively. ^[25]

Pupillary regulation is a finely tuned process involving intricate neural pathways that respond dynamically to various stimuli. ^{[26, 27, 28, 29]} Central integration for pupillary regulation is in the dorsal midbrain and Edinger-Westphal nucleus. These areas play a critical role in processing visual information related to pupillary responses. ^{[26, 27, 28, 29]}

The afferent pathway is along the optic nerve (CN II). Light exposure stimulates retinal ganglion cells, which send signals through the optic nerve to the pretectal area in the midbrain. From there, signals are relayed to the Edinger-Westphal nucleus. The importance of the Edinger-Westphal nucleus is not only in the pupillary reflex but also in other ocular reflexes, highlighting its central role in visual processing. ^{[26, 27, 28, 29]}

In the efferent pathway, preganglionic sympathetic innervation of the pupil is from the C8-T2 spinal segments via the superior cervical ganglion. Postganglionic fibers extend along the carotid artery to the cavernous sinus and then enter the orbit with the fifth cranial nerve. The major action is pupillary dilation and also involves Mueller's muscle of the upper lid.

The parasympathetic innervation is from the third cranial nerve and ciliary ganglion and innervates the pupillary constrictor muscles and ciliary muscle for accommodation.

Both sympathetic and parasympathetic pathways help in maintaining normal pupillary function. Disruptions along these pathways can lead to clinical conditions such as Horner's syndrome, characterized by ptosis, miosis, and anhidrosis due to sympathetic innervation loss. ^{[26, 27, 28, 29]}

Quantitative assessments of pupillary light reflex (PLR) have been shown to provide valuable prognostic information in neurologic conditions such as carbon monoxide poisoning and traumatic brain injury. Automated pupillometry has emerged as a superior method for evaluating pupillary dynamics compared to traditional penlight methods, highlighting its utility in predicting neurocognitive outcomes. ^{[26, 27, 28]}

Central integration occurs in spinal centers and the CAN, coordinating complex reflexes and voluntary control. ^{[29, 30, 31, 32]}

The GU system's afferent activity is along autonomic and somatic pathways, ensuring proper sensory feedback and motor responses. CAN, which includes brain structures like the insular cortex and brainstem regions such as the parabrachial nucleus, regulates the ANS. This network helps maintain a balance between sympathetic and parasympathetic responses, crucial for functions such as micturition (urine storage and voiding) and sexual function. ^{[29, 30, 31, 32]}

Key neural circuits extend from the cerebral cortex to the bladder, facilitating communication between various regions involved in micturition control. Specific subpopulations of neurons in the urethra release neurotransmitters such as ACh and serotonin, which can trigger bladder contractions. ^{[29, 30, 31, 32]}

Sympathetic innervation of the GU system is from the T11-L2 spinal segments, inferior mesenteric and superior hypogastric ganglia, and hypogastric nerves. It causes uterine contraction, ejaculation in males, bladder wall inhibition, detrusor and trigone muscle contraction, and urethral smooth muscle contraction. This sympathetic activation primarily releases noradrenaline, which acts on various adrenergic receptors to modulate these functions. Dysfunction in this pathway may lead to conditions such as overactive bladder and urinary retention. ^{[29, 30, 31, 32]}

Parasympathetic innervation of the GU system is from the S2-S4 spinal region and the pelvic nerves. It causes genital vasodilation, erections in males, bladder wall contraction, detrusor and trigone muscle relaxation, and internal sphincter relaxation. Parasympathetic postganglionic neurons release ACh, which stimulates M3 muscarinic receptors in the bladder smooth muscle, facilitating contraction and promoting urination. This system's balance with sympathetic input is vital for coordinated urinary and reproductive system function, with dysfunction contributing to conditions such as detrusor underactivity and erectile dysfunction. ^{[29, 30, 31, 32]}

The external sphincters are innervated by the pudendal nerve and are under somatic, not autonomic, control. This nerve allows voluntary control over urinary and fecal continence by regulating contraction of the external sphincters. ^{[29, 30, 31, 32]}

Research continues to explore these neural mechanisms, revealing potential therapeutic targets for conditions such as neurogenic lower urinary tract dysfunction following spinal cord injuries. ^{[29, 30, 31, 32]}

The regulation of the GI system uses a complex interplay between the CNS, spinal centers, and the enteric nervous system (ENS), with neuromodulation and the gut-brain axis (GBA). Studies highlight that the GBA, a bidirectional communication system linking the gut and the CNS, plays a crucial role in digestion, metabolism, immune function, and even cognitive processes such as mood and behavior. Central integration occurs primarily in the nucleus of the tractus solitarius and nucleus ambiguus, which integrate signals from vagal afferents, while efferent responses are coordinated through the ENS embedded in the GI tract walls. ^{[33, 34, 35, 36, 37, 38]}

The afferent pathways synapse locally or in the ganglia, spinal cord, and more rostral portions of the ANS, facilitating communication between the gut and CNS. ^{[33, 34, 35, 36, 37, 38]}

The efferent pathways occur through the local integrative system, called the ENS, which consists of networks of nerves and plexuses embedded in the wall of the GI tract and integrated into local circuits for a variety of operations such as secretion, absorption, peristalsis, and sphincter coordination.

The sympathetic innervation is from the thoracolumbar segments by way of the celiac, superior mesenteric, and inferior mesenteric ganglia and the splanchnic, hypogastric, and colonic nerves. The main effects are stimulation of the esophageal sphincters and relaxation of the motility and internal rectal sphincter. This sympathetic influence is essential for regulating blood flow and mucosal secretion during stress responses. ^{[33, 34, 35, 36, 37, 38]}

The parasympathetic innervation is from the vagus nerve, which innervates the esophagus, stomach, small intestines, and proximal colon, and by the sacral outflow from S2 to S4, which innervates the distal colon and internal anal sphincter. The main effects are to stimulate motility and relax the internal rectal sphincter.

The external sphincters are innervated by the pudendal nerve and are under somatic, not autonomic, control.

ENS serves as a major regulator of GI functions, coordinating motility, sensation, and secretion. It interacts with various cell types within the gut, including intestinal epithelial cells and immune cells. This interaction is vital for maintaining gut homeostasis and responding to luminal stimuli. Disruptions in the ENS function can contribute to GI disorders and other systemic diseases. For instance, early-life disturbances in ENS interactions have been linked to neurodegenerative diseases. ^{[33, 34, 35, 36, 37, 38]}

Research has highlighted potential interventions such as ENS progenitor transplantation to treat conditions such as Hirschsprung's disease, where functional ENS cells are repopulated in aganglionic bowel segments. Additionally, studies are exploring how gut microbiota influence the ENS function and overall GI health. ^{[33, 34, 35, 36, 37, 38]}

Vagus nerve stimulation (VNS) can also target regulatory systems to treat conditions such as irritable bowel syndrome and gastroesophageal reflux disease. Such treatments offer therapeutic potential by modulating the disrupted neural pathways. ^{[33, 34, 35, 36, 37, 38]}

Various tests measuring the autonomic function are available. ^{[39, 40, 41]} Both cardiovascular and sweat tests can be used to evaluate autonomic function. The sensitivity of the tests is variable according to the underlying disorder. These tests help assess conditions such as dysautonomia, Parkinson's disease, and MSA. ^{[42, 43]}

Most laboratories perform a battery of multiple tests to enhance reliability and sensitivity of various autonomic functions. A typical screening battery includes:

• Heart rate response with deep breathing - This test measures the heart rate response to controlled breathing, highlighting parasympathetic function ^{[42, 43]}
• Valsalva ratio and Valsalva maneuver analysis - It is a sensitive measure for detecting neurogenic OH and sympathetic dysfunction, though not specific enough to differentiate between conditions such as Parkinson's disease and pure autonomic failure; this test involves forced expiration against resistance, which reveals different blood pressure responses in four phases to assess autonomic reflexes ^{[42, 43]}
• Orthostatic testing - This measures blood pressure and heart rate changes upon standing to assess for OH, useful in detecting autonomic dysfunction ^{[42, 43]}
• Sweat tests - At least one test of thermoregulatory function, such as QSART or the thermoregulatory sweat test (TST), can be conducted for evaluation. Tests like TSTs remain essential for evaluating SNS function by analyzing sweat production ^{[42, 43]}

Pretesting preparation

Before testing, patients should abstain from alcohol, caffeine, and nicotine for 3 hours (preferably 12 hours). Medications with anticholinergic properties (e.g., antidepressants, antihistamines, and certain over-the-counter medications), adrenergic antagonist action (e.g., beta blockers), sympathomimetic properties, parasympathomimetic properties, and fluid-altering properties (e.g., diuretics or fludrocortisone) should be stopped. Note that consultation with the patient's primary physician may be required to stop some of these medications.

Patients should be rested and relaxed during and before the testing. Compressive dressings such as elastic stockings should be removed.

Advantages/disadvantages of autonomic testing

Autonomic testing has several advantages, including verification of the diagnosis; precise neuroanatomic localization of the abnormality to the central or peripheral nervous system levels; prognosis for the severity, staging, and monitoring of the treatment; and determination of the physiologic organ systems involved, and the predominant system involved, whether it is sympathetic, parasympathetic, or both.

Small, unmyelinated nerve fibers that control many autonomic functions are inaccessible for direct neurophysiologic recording. In addition, numerous technical and physiologic variables must be controlled. Additionally, the Valsalva maneuver, though sensitive, lacks specificity for differential diagnosis.

Quantitative sudomotor axon reflex test

QSART is the most sensitive test of distal SFN (see the images below). This test involves iontophoresis of ACh onto the skin to stimulate sympathetic C-fibers in the sweat glands. The sweat response that is evoked is quantitated using a sudomotor, which measures the humidity of the evoked sweating response.

QSART is particularly useful in diagnosing conditions such as autonomic neuropathy, complex regional pain syndrome (CRPS), diabetes-related neuropathy, multiple sclerosis, POTS, Sjögren’s syndrome, and SFN. ^{[44, 45, 46]}

Research suggests QSART is extremely sensitive for detecting the early signs of neuropathy, especially in conditions such as diabetes and Parkinson's disease. ^{[44, 45, 46]}

Generalized dysautonomias, complex regional pain syndrome, atopic dermatitis, anticholinergic medication use, and abnormalities of the skin and sweat glands can interfere with the test results. ^[47]

Thermoregulatory sweat test

Thermoregulatory sweat test (TST) assesses the body's ability to sweat in response to increased body temperature, examining a more global sudomotor function compared with QSART. ^{[44, 45, 46]} In this test, patients are placed in a warming cabinet to provoke sweating. Their sweating pattern is then assessed by the color change of alizarin powder dusted over the body, limbs, and forehead (see the following image). ^[48] Studies emphasize the TST's utility in diagnosing autonomic dysfunctions, including MSA, where it helps differentiate between preganglionic and postganglionic lesions. ^{[44, 45, 46]}

Sympathetic skin response test

The sympathetic skin response (SSR) test is based on the fact that electrodermal activity reflects sympathetic cholinergic sudomotor function, which induces changes in the resistance of skin to electric conduction. ^[49] Many modalities of stimulation suffice to elicit the potential reflexively, including electrical depolarization of a sensory nerve in the digit that startles. Other eliciting stimuli include a startling auditory sound or deep inspiratory gasps.

The potentials in the hands have larger amplitudes and shorter latencies than those in the feet. The latency is about 1.5 seconds in the hand and about 2 seconds in the foot following an eliciting stimulation. The major contributor to latency is the efferent conduction along the sudomotor pathways, which are small, unmyelinated C fibers.

Studies have demonstrated the clinical utility of SSR in assessing autonomic dysfunction across various conditions. In essential tremor disorder, SSR detects significant differences in onset latencies and amplitudes between patients and healthy individuals. It has also shown higher sensitivity than nerve conduction studies in diagnosing and monitoring diabetic peripheral neuropathy in children. SSR is widely used for evaluating disorders such as CRPS, peripheral neuropathies, Guillain-Barré syndrome, systemic sclerosis, and rheumatoid arthritis. Additionally, SSR may help monitor autonomic dysfunction and predict recovery outcomes in patients recovering from COVID-19, with dysfunction linked to disease severity. ^{[50, 51, 52]}

In addition to tests such as QSART, TST, and SSR, several other tests can assess the sudomotor function and detect autonomic dysfunction: ^{[45, 53]}

Silicone impressions - Silicone impressions are used to capture sweat patterns on the skin. This method involves applying silicone material to the skin, which collects sweat during stimulation. The impressions can be analyzed for sweat distribution and volume, providing insights into autonomic function. This test is often used in conjunction with other methods such as QSART to enhance diagnostic accuracy. ^{[45, 53]}

Sweat imprint testing - This newer technique involves creating a visual map of sweat activity by applying iodine starch or other color-indicating substances to the skin. It can help identify abnormal sweat patterns that may be due to autonomic neuropathies affecting specific areas of the body, and it is particularly useful for diagnosing localized sweat dysfunctions. ^{[45, 53]}

Sudoscan - Sudoscan is a test that evaluates the electrochemical activity of sweat glands, offering a quick assessment of autonomic neuropathy. It is increasingly used for detecting early diabetic neuropathy and can provide insight into autonomic dysfunction affecting the small nerve fibers that regulate sweat production. ^{[45, 53]}

ACh Sweat-Spot Test - The ACh Sweat-Spot Test involves administering an intradermal injection of ACh in specific skin areas to stimulate sweating. The amount of sweat produced is then measured, allowing for localized assessment of sudomotor function. This test can help identify specific areas of dysfunction and is useful in conjunction with other assessments. ^{[45, 53]}

Quantitative direct and indirect axon reflex testing (QDIRT) - QDIRT is a newer technique that evaluates both direct and indirect responses of the sudomotor system. It combines aspects of QSART with advanced imaging techniques to measure sweat production more accurately. QDIRT can help localize lesions affecting pre- and postganglionic pathways, making it a valuable tool for diagnosing autonomic dysfunction. ^{[45, 53]}

A comprehensive approach that includes multiple tests can improve the diagnostic accuracy for conditions such as diabetic neuropathy, amyloid neuropathy, and POTS. Combining QSART, TST, silicone impressions, SSR, and QDIRT allows clinicians to evaluate the different aspects of autonomic function effectively. ^{[45, 53]}

Head-up tilt table testing

Beat-to-beat blood pressure measurements formerly required invasive intra-arterial recording, but modern photoplethysmographic (Finapres) devices generate waveforms similar to intra-arterial recordings and allow noninvasive recording.

Postural physiology has been studied in the laboratory by means of a head-up tilt on a tilt table. ^[54] Upon changing from a recumbent to upright position on a tilt table, there is almost one third of a shift of venous blood from the central to the peripheral compartment; approximately 50% of the change occurs within seconds. This results in decreased cardiac filling pressures and the stroke volume is decreased by up to 40%, which decreases afferent activity from the sensory baroreceptors. The heart rate rises, first during parasympathetic activity withdrawal and then from increased sympathetic activity. Overall, the cardiac output only drops 20%, and blood pressure is largely maintained. See the image below.

Cardiovascular responses to standing and 30:15 ratio

Standing induces an exercise reflex as well as mechanical squeeze on both the venous capacitance and arterial resistance vessels. The changes stimulate the baroreceptors; a pronounced neurally mediated reflex ensues, which decreases sympathetic outflow, releases vasoconstrictor tone, decreases total peripheral resistance by up to 40%, and drops blood pressure by up to 20 mm Hg; these changes last 6-8 seconds.

The heart rate increases immediately upon standing and continues to rise for the next several seconds. ^[55] The initial cardiac acceleration upon standing is an exercise reflex that withdraws the parasympathetic tone, and subsequent changes are baroreflex-mediated changes, which enhance the sympathetic tone.

Heart rate variation with deep breathing

The variation in heart rate with respiration is known as sinus arrhythmia and is generated by autonomic reflexes (see the following image). ^[56] Inspiration increases the heart rate, and expiration decreases it. The variation is primarily mediated by the vagus innervation of the heart. Pulmonary stretch receptors as well as cardiac mechanoreceptors and possibly baroreceptors contribute to the regulation of heart rate variation. It increases with slower respiratory rates and reaches a maximum of about five or six respirations per minute.

Valsalva maneuver and Valsalva ratio

The Valsalva maneuver consists of respiratory strain that increases the intrathoracic and intra-abdominal pressures and alters the hemodynamic and cardiac functions (see the image below). The Valsalva maneuver is usually recorded by invasive monitoring of the intra-arterial blood pressure. Levin monitored the heart rate alone without monitoring blood pressure during the Valsalva maneuver and calculated a ratio of the fastest heart rate to the slowest as a way of noninvasively quantifying the procedure. ^[57] Newer photoplethysmographic monitoring devices can noninvasively record beat-to-beat blood pressures as well as heart rates, thus allowing easier evaluation of the maneuver. ^{[12, 58]}

The Valsalva maneuver has the following four phases:

• Phase I - This phase is transient and lasts only a few seconds, with increase in the blood pressure caused by increased intrathoracic pressure and mechanical squeeze of the great vessels
• Phase II - This phase has early and late components; in the early phase II, venous return decreases, which results in decreasing stroke volume, cardiac output, and blood pressure; in about 4 seconds in the late phase II, blood pressure recovers to baseline levels; this recovery stems from increased peripheral vascular resistance from sympathetically mediated vasoconstriction
• Phase III - This phase occurs with the release of strain, which results in the blood pressure decreasing for a few seconds due to the mechanical displacement of blood to the pulmonary vascular bed, which previously had been under increased intrathoracic pressure
• Phase IV - This phase occurs with further strain cessation; the blood pressure slowly increases and heart rate decreases; because the blood pressure rises to above baseline levels and the heart rate to below baseline levels, it is often called the "overshoot."

The Valsalva ratio is the ratio of the maximal heart rate in phase II to the minimal heart rate in phase IV. This may be calculated easily as the ratio of the longest R-R interval during phase IV to the shortest of phase II.

Wearable technology: Wearable devices, such as smartwatches equipped with heart rate variability (HRV) sensors and photoplethysmography, are gaining popularity for remote autonomic testing and long-term monitoring. These wearables can provide early detection of autonomic dysfunction by continuously tracking fluctuations in the heart rate, blood pressure, and respiratory patterns, contributing to more dynamic patient management and real-time data collection. However, the accuracy of these devices is questionable in many cases. ^{[59, 60]}

Shortened tilt table testing protocols: A significant advancement is the introduction of a shortened tilt table test protocol, known as the "Fast Italian protocol," which reduces the testing duration from 35 minutes to just 20 minutes. This new protocol performs comparably to the traditional method, offering a more time-efficient option for diagnosing conditions such as reflex syncope and autonomic dysfunction. This change is particularly important given the high prevalence of syncope, which can lead to serious outcomes and reduced quality of life in patients. ^{[59, 60]}

Blood pressure response tests

Blood pressure response to sustained handgrip

Persistent muscle contraction causes the blood pressure and heart rate to increase. The mechanism involves the exercise reflex, which withdraws parasympathetic activity and increases sympathetic activity. This test requires the patient to apply and maintain grip at 30% maximal activity for up to 5 minutes; the diastolic blood pressure should rise more than 15 mm Hg. ^[61]

Blood pressure response to mental stress

Mental stresses such as arithmetic, emotional pressure, and even sudden noise can cause sympathetic outflow to increase, which leads to increased blood pressure and heart rate. This test has been used as a measure of sympathetic efferent function that has the advantage of not requiring direct afferent stimulation. ^[62]

Blood pressure response to cold water immersion

In 1932, Hines and Brown noted an increase in blood pressure after submerging a patient's hand in ice water. The afferent limb of the reflex is somatic, and the efferent limb is sympathetic.

Many patients find it difficult to maintain their hand in ice water for the requisite time. This test also lacks sensitivity as many normal subjects do not show a significant rise in blood pressure.

Studies have introduced and refined other blood pressure response tests, particularly in assessing vascular reactivity and cardiovascular risk:

Critical closing pressure: It assesses the vascular tone in response to hypertension treatments. This noninvasive test is proving useful in understanding how blood vessels react to blood pressure regulation. By calculating the point at which the blood flow in the vessels ceases, it provides insight into vessel health and response to therapy. ^{[63, 64]}

Treadmill blood pressure response: Research highlights the importance of treadmill exercise testing in predicting future cardiovascular events. Blood pressure responses during treadmill tests help identify individuals at a higher risk for new-onset hypertension and cardiovascular complications. ^{[63, 64]}

Personalized drug response testing: In this test, patients undergo n-of-1 trials with different antihypertensive drugs to identify the most effective medication. This approach ensures more tailored treatments and significantly improves outcomes. ^{[63, 64]}

Plasma catecholamine levels and infusion tests

Plasma norepinephrine levels approximately double with the upright posture in view of the initiation of vasopressor responses, which are sympathetic and adrenergic. In preganglionic sympathetic disorders such as multisystem atrophy, resting supine norepinephrine levels are normal but fail to rise in the standing position owing to the lack of preganglionic drive. In postganglionic sympathetic disorders, such as progressive autonomic failure, resting supine norepinephrine levels are low and fail to rise while standing. ^{[65, 66]}

Patients with POTS often exhibit elevated upright norepinephrine levels, sometimes exceeding 600 pg/mL, suggesting a hyperadrenergic state. This condition can lead to symptoms such as tachycardia and orthostatic intolerance. ^{[67, 68, 69]}

In cases of autonomic dysfunction, the ability to respond to physiological stimuli (such as posture changes) is compromised, leading to inadequate catecholamine release. Furthermore, diagnostic testing for conditions such as pheochromocytoma now often prioritizes measuring plasma metanephrines over catecholamines due to their stability and continuous secretion during "spells." ^{[67, 68, 69]}

Neurogenic OH: In neurogenic OH (NOH), plasma norepinephrine responses are blunted. Normally, these levels should increase significantly within minutes of standing; however, in NOH, the increase may be less than 60% or approximately 150 pg/mL. This blunted response is often associated with baroreflex failure and can lead to significant clinical symptoms upon standing. ^{[67, 68, 69]}

Moreover, in cases of hyperadrenergic OH, patients demonstrate an exaggerated increase in norepinephrine levels upon standing, reaching 938 pg/mL on average. Despite this hyperadrenergic state, they may still experience impaired autonomic reflexes. ^{[67, 68, 69]}

Tests of pupillary regulation

When parasympathetic denervation exists, denervation hypersensitivity occurs, and the pupil constricts to such dilute stimulation. Similarly, epinephrine acts directly on the sympathetic adrenergic dilatory muscles to cause pupillary dilatation. However, in very dilute amounts (0.1% solution), it normally causes minimal dilatation. When sympathetic denervation exists, denervation hypersensitivity occurs, and the pupil dilates. Cocaine (4-5% solution) blocks the reuptake of norepinephrine in the sympathetic nerve terminals that innervate the pupillary dilator muscles and causes pupillary dilatation. ^[70]

Research on pupillary regulation continues to expand the understanding of the mechanisms involved, particularly in cases of autonomic dysfunction. ^{[71, 72, 28, 73]}

Parasympathetic denervation and hypersensitivity: When the parasympathetic supply to the eye is compromised, hypersensitivity occurs. This is observed when the pupil is exposed to cholinergic agents. Even dilute concentrations result in exaggerated constriction due to the heightened sensitivity of the denervated pupil. ^{[71, 72, 28, 73]}

Sympathetic regulation and epinephrine: Epinephrine, through adrenergic receptors, normally induces pupil dilation via the dilator muscle. In healthy individuals, diluted epinephrine (0.1%) causes minimal effect. However, in sympathetic denervation, hypersensitivity occurs, causing exaggerated dilation with even small amounts of epinephrine. ^{[71, 72, 28, 73]}

Cocaine's role in diagnosing sympathetic dysfunction: Cocaine (4-5%) prevents the reuptake of norepinephrine, maintaining dilation in normal individuals. In sympathetic denervation such as in Horner's syndrome, cocaine will not cause dilation due to the lack of norepinephrine, making it a useful diagnostic tool. ^{[71, 72, 28, 73]}

Parasympathetic and sympathetic pathways: The PNS constricts the pupil via the sphincter pupillae, originating from the Edinger-Westphal nucleus and synapsing in the ciliary ganglion. Sympathetic innervation dilates the pupil through the dilator pupillae, involving the superior cervical ganglion. Denervation hypersensitivity in either of the pathways leads to exaggerated responses to stimuli. ^{[71, 72, 28, 73]}

Pharmacological agents and denervation: ^{[71, 72, 28, 73]}

• Epinephrine - Normally induces minimal dilation at low concentrations but causes significant dilation in denervation cases
• Cocaine - Inhibits the reuptake of norepinephrine, enhancing dilation, but fails to work in cases of sympathetic denervation

Pupillary responses in neurologic conditions: ^{[71, 72, 28, 73]}

Studies highlight the importance of pupillary dynamics in diseases such as Parkinson's and Alzheimer's:

• In Parkinson's disease, slower peak constriction velocities and altered responses suggest autonomic dysfunction.
• In Alzheimer's disease, impaired PLR correlate with cognitive decline and may serve as early biomarkers for diagnosis.

Assessing pupillary reactions is crucial for diagnosing autonomic dysfunctions and neurodegenerative diseases. Abnormal pupillary reactions can indicate broader neurologic issues. Guidelines emphasize the need for accurate recording and analysis of pupillary measurements in both clinical and psychophysiological research, recognizing that emotional and cognitive factors also influence the pupil size. ^{[71, 72, 28, 73]}

Tests of gastrointestinal autonomic regulation

Manometry or pressure transducers placed in different portions of the GI tract help localize the sites of stasis. Sympathetic denervation may be identified by various neurochemical studies, including the norepinephrine response to edrophonium. ^[74] Parasympathetic denervation may be identified by the plasma pancreatic polypeptide response to sham feeding or hypoglycemia. In MSA, the rectal sphincter is frequently denervated from degeneration of the Onuf nucleus in the sacral spinal cord, ^[75] and EMG of the rectal sphincter may be abnormal in these patients.

Conditions such as functional dyspepsia and irritable bowel syndrome have been linked to autonomic dysfunction. HRV is being increasingly utilized as a surrogate marker for the assessment of autonomic function in these disorders, highlighting the importance of the gut-brain communication. ^{[76, 77, 78]}

Studies show that neuromodulation techniques, such as transcutaneous auricular VNS, have shown promise in improving GI motility and alleviating symptoms related to functional GI disorders. These methods enhance the vagal efferent activity and have been associated with improved gastric accommodation and rectal sensation. ^{[76, 77, 78]}

Tests of genitourinary autonomic regulation

Electrophysiologic tests such as bulbocavernosus reflexes, sensory conduction in the dorsal nerve of the penis, pudendal sensory-evoked potentials, motor latencies of the pudendal nerve, and routine and single fiber EMG of the sphincters test the somatic function and not the autonomic function. Methods of electrophysiologic study of the smooth muscle of the corpus cavernosum have been used as well. ^[79]

For a comprehensive evaluation of autonomic regulation, additional methods such as questionnaires or diagnostic tools such as the Composite Autonomic Symptom Score (COMPASS) 31 may be necessary. This questionnaire has been validated for detecting signs of autonomic neuropathy and could complement traditional electrophysiological assessments. ^[80]

Genetic testing

Yuan et al identified a novel homozygous mutation in SCN9A from Japanese families with autosomal recessive hereditary sensory autonomic neuropathy. ^[81] This loss-of-function SCN9A mutation results in disturbances in the sensory, olfactory, and ANSs.

Genetic syndromes and autonomic dysfunction: Studies advocate for genetic testing in all cases of idiopathic autonomic dysfunction, which can help establish a molecular diagnosis and assess recurrence risk in families. This proactive approach could lead to tailored preventive or therapeutic measures for the affected individuals. ^{[82, 83, 84, 85, 86]}

Spinocerebellar ataxia (SCA): Studies have explored autonomic dysfunction across various genetic subtypes of SCAs. They have confirmed that autonomic dysfunction is prevalent in these conditions, emphasizing the cerebellum's role in the ANS regulation. This highlights the importance of assessing autonomic functions in clinical practice for the better management of patients with SCA. ^{[82, 83, 84, 85, 86]}

GBA-related Parkinson's disease: Research indicates that patients with Parkinson's disease carrying GBA mutations exhibit more severe autonomic dysfunction compared with noncarriers. Instrumental assessments of autonomic function revealed that many patients with GBA mutations experience significant cardiovascular and sudomotor abnormalities, which are often under-recognized by clinical evaluations alone. ^{[82, 83, 84, 85, 86]}

Genetic testing has revealed cases of dopamine beta-hydroxylase deficiency, which impacts the ANS and leads to OH and other symptoms such as fainting. This condition can be identified through genetic panels targeting autonomic disorders. ^{[82, 83, 84, 85, 86]}

Research has linked autonomic dysfunctions to post-COVID syndromes, with particular attention to OH and POTS. These are being investigated post-COVID patients, where symptoms of dysautonomia are seen to persist even after recovery from the acute phase of infection. Genetic testing in this domain can help identify predisposed individuals by examining mutations affecting the ANS. ^{[82, 83, 84, 85, 86]}

In the context of Prader-Willi syndrome (PWS), autonomic dysfunction has been a notable finding. Studies have identified abnormalities in the autonomic function, such as impaired baroreflex sensitivity and PNS activity. Genetic mutations related to PWS, specifically those influencing the metabolism and energy regulation, have been implicated in these dysfunctions, shedding light on potential pathways for therapeutic interventions. ^{[82, 83, 84, 85, 86]}

Assessing symptom burden in postural orthostatic tachycardia syndrome

The new Malmö POTS Score (MAPS) (Malmo POTS) can be useful as a semiquantitative system to assess the symptom burden, monitor disease progression, and evaluate the pretest disease likelihood. ^[87]

Research has also explored the feasibility of incorporating the MAPS into broader treatment strategies, including individually tailored exercise programs for patients experiencing post-COVID-19 conditions related to POTS. These interventions aim to enhance physical capacity while addressing specific symptoms assessed by the MAPS and other questionnaires. ^{[88, 87, 89, 90, 91, 92]}

Assessing symptom burden in POTS have introduced various tools and methodologies beyond the MAPS. ^{[88, 87, 89, 90, 91, 92]}

COMPASS 31: A 31-question survey that measures autonomic dysfunction across six domains: orthostatic intolerance, vasomotor, secretomotor, GI, bladder, and pupillomotor functions. It quantifies symptom burden, offering valuable insights into POTS severity. ^{[88, 87, 89, 90, 91, 92]}

Vanderbilt Orthostatic Symptom Scale (VOSS): A validated tool designed to evaluate the range and severity of symptoms such as fatigue, dizziness, and cognitive impairment associated with orthostatic intolerance. It complements other assessments such as MAPS. ^{[88, 87, 89, 90, 91, 92]}

Orthostatic Grading Scale: Evaluates symptom severity and the impact on daily activities in patients with POTS. It captures nuances in symptom burden and functional limitations. ^{[88, 87, 89, 90, 91, 92]}

Patient Health Questionnaire & General Anxiety Disorder Scale: These tools assess psychological factors such as depression and anxiety, which are common comorbidities with POTS, providing insights into their contribution to overall symptom burden and quality of life. ^{[88, 87, 89, 90, 91, 92]}

Long-Term POTS Outcomes Survey: An extensive self-report questionnaire tracking the quality of life, symptom severity, and treatment efficacy over time. It is useful for the longitudinal tracking of patient experiences. ^{[88, 87, 89, 90, 91, 92]}

Individually tailored exercise programs: Exercise interventions for POTS patients, monitored through tools like MAPS and VOSS, aim to reduce symptom burden and improve quality of life. ^{[88, 87, 89, 90, 91, 92]}

Tilt table test: A diagnostic tool that measures the heart rate and blood pressure response to positional changes, helping diagnose orthostatic intolerance in patients with POTS. ^{[88, 87, 89, 90, 91, 92]}

Holter monitoring: A 24-48-hour heart rate and rhythm monitoring technique to detect abnormalities missed during standard ECGs. ^{[88, 87, 89, 90, 91, 92]}

Blood volume analysis: Assesses hypovolemia, common in patients with POTS, providing insights into symptom severity. ^{[88, 87, 89, 90, 91, 92]}

Orthostatic Hypotension Questionnaire: Adapted for patients with POTS, it focuses on symptoms such as dizziness and fatigue during standing to quantify symptom severity. ^{[88, 87, 89, 90, 91, 92]}

POTS Disability Index: Assesses how POTS impacts daily activities and physical function, offering a broader view of the disorder's effect on quality of life. ^{[88, 87, 89, 90, 91, 92]}

Patient-Reported Outcome Measures: A variety of questionnaires, including the Short Form-36 Health Survey and EuroQol 5-dimensions, which patients complete to report symptoms and their impact on daily life. They help track their mental health, physical functioning, and social interactions over time. ^{[88, 87, 89, 90, 91, 92]}