We have reviewed a number of tests in this column, most of which we all learned in optometry school but perhaps don’t use on a daily basis. For this month’s offering, we thought we would look at a test that is less well-known—one that you may not have seen outside of a lab course at some point—but one that is incredibly easy to run and highly useful: the Modified Thorington test.

Thorington Use

The main purpose of the Modified Thorington test is to measure ocular deviation. While most of us perform cover tests or perhaps phorias as a routine part of our comprehensive eye exams, we often use these tests to look only for horizontal deviations. Cebrian et al. compared distance heterophoria on two separate occasions by two examiners in 110 subjects aged 18 to 32 years using four different tests: cover test, von Graefe, Maddox rod and Modified Thorington.¹ The Modified Thorington showed good intra-examiner repeatability and the best inter-examiner reliability. 

Here, we will also discuss how the Modified Thorington can be used not only to measure horizontal misalignments, but also subtle vertical ones, which often have a large impact on patients’ quality of life.  

The set-up for the Modified Thorington is as easy as it gets. You only need a few items to run it: the Modified Thorington card itself (Figure 1), a Maddox rod (Figure 2) and your transilluminator (a penlight will also work, but the transilluminator fits nicely into the card). The patient holds the Maddox rod in front of their right eye, with the striations oriented appropriately for the direction of deviation that you want to measure. Recall that when the patient looks through the Maddox rod, they will see a line oriented perpendicular to the direction of the striations, so hold the striations horizontally to measure lateral phorias and vertically to measure vertical ones. Holding the card at 40cm from the patient, shine the light through the hole in the center of the card (Figure 3). Since they are only seeing the line with one eye, they are performing a monocular fixation in a binocular field type of test; thus, the eyes’ natural deviation will manifest. Ask the patient to report where the line formed by the Maddox rod crosses the scale printed on the card. Whatever they report is their deviation at that moment. The direction of deviation (eso, exo, hyper, hypo) is conveniently printed on the card.

Fig. 1. Modified Thorington cards. On left, near card for use at 40cm. On right, distance card for use at 10 feet. Click image to enlarge.

While we can measure ocular deviation in multiple ways in the exam room, the Modified Thorington is quick and simple, as well as allowing for a relatively unrestricted measurement of phorias or tropias. This is extremely useful for patients who may not be comfortable in the phoropter or for whom you want a more precise measurement than your cover tests provide. 

The following case illustrates the use of the Modified Thorington in a patient we saw recently. Shout-out to our fourth-year intern, Payton Lock, who thought of the Modified Thorington test for this patient and uncovered some subtle findings that made all the difference.

Fig. 2 (left). Maddox rod. With this orientation, a vertical line will appear to the patient, allowing testing of horizontal deviation. Fig. 3 (right). Set-up for testing with the Modified Thorington card. Click image to enlarge.

Case Report

A nine-year-old girl presented to the Vision Therapy & Rehabilitation Service with a history of an eye turn diagnosed at an examination in the Pediatric Service earlier the same month. She was in fourth grade but was reading at a third grade level. Her grades were As and Bs except in reading, in which she was getting a C. She and her mother reported that she turned her head to the left and had daily frontal headaches. She skipped words when reading, but the patient reported that this was due to comprehension. She also reported being clumsy and bumping into things often, stating that she “bumps her head all the time.” Her exam from five years prior, which was the last time she was seen, did not show the presence of an eye turn.

At the primary care pediatrics exam, she was given +0.50D OU for full-time use, so all testing performed was completed through those lenses. Her visual acuity was 20/20-2 OD, OS, OU at distance and near. Extraocular motilities, fields by confrontation and pupils were normal. Stereo was measured at 30 sec of arc. The cover test was 6^ʌ intermittent alternating exotropia at distance and 18^ʌ intermittent alternating exotropia at near. The control score, a measure of the turn that takes into account the patient’s ability to “control” the intermittent exotropia, was 1. This indicated that there was “no exotropia unless dissociated, recovers in one to five seconds.” The near point of convergence was 7/10cm, 8/11cm and 8/11cm, and accommodative amplitudes were 16.00D OD, OS. The quality-of-life questionnaire was a 37. As a reminder, anything over a 20 is a red flag for a binocular vision issue.  

Based on the patient’s control of the intermittent exotropia and the severity of the headaches, Payton decided to pull out the Modified Thorington, as she had a hunch that there might be a small vertical deviation present. Sure enough, the distance measurement showed a 4^ʌ exophoria and a 2^ʌ right hyperphoria. The near showed a 16^ʌ exophoria and a 5^ʌ right hyperphoria. To assess the impact of the convergence stress on the hyperphoria at near, we trialed 2^ʌ base-in prism in each eye and retested. This time, the hyperphoria was reduced to 1^ʌ right hyperphoria.       

With the use of the Modified Thorington test, we were able to assess this patient’s vertical deviation appropriately. Since we were planning to start the patient in vision therapy to address the intermittent alternating exotropia and the small amount of base-in prism had such a huge impact on the hyperphoria, we decided to hold off on prescribing the vertical prism. As we move through the therapy program, we will address both the horizontal and vertical issues to enable the patient to look at near targets without the headache burden.

This case resulted in a great outcome from such a quick check. The Modified Thorington can help identify subtle vertical deviations that evade even the most experienced clinician on cover test and, as a bonus, this test can easily be performed by a paraoptometric technician. We use the test fairly often in clinical care and can recommend both its ease of use and its benefits for your patients.

Dr. Taub is a professor and co-supervisor of the Vision Therapy and Pediatrics residency at Southern College of Optometry (SCO) in Memphis. He specializes in vision therapy, pediatrics and brain injury. Dr. Schnell is a professor at SCO and teaches courses on ocular motility and vision therapy. She works in the pediatric and vision therapy clinics and is co-supervisor of the Vision Therapy and Pediatrics residency. Her clinical interests include infant and toddler eye care, vision therapy, visual development and the treatment and management of special populations. They have no financial interests to disclose.