OCT Magic Trick

Over the past several years, I have been interested in using OCT to assess ocular torsion in neuro-optometric conditions, specifically to aid in the diagnosis of trochlear nerve (CN IV) palsies and skew deviations.^1,2

Multiple studies have shown the most common cause of vertical diplopia to be CN IV palsy.^3,4 However, there are other important neuro-optometric conditions that can also cause vertical diplopia and mimic CN IV palsy on the Parks-Bielschowsky three-step test, such as skew deviation, thyroid eye disease, or myasthenia gravis.^5,6 Therefore, accurately identifying abnormal torsion can mean the difference between a correct and incorrect diagnosis.^5-7

A MAGIC TRICK WITH OCT

I have spent a considerable amount of my professional time studying ocular torsion in these neuro-optometric presentations over the past several years. Using the trick described below, I have been able to diagnose acquired CN IV palsies in distinct cases before ever stepping foot into the examination room.

The Secret Revealed

CN IV innervates the superior oblique muscles. In primary gaze, the primary and secondary functions of the superior oblique muscle are incyclotorsion and depression, respectively. So, in a patient with a unilateral CN IV paresis or palsy, the involved eye is expected to have increased excyclotorsion and a hyperdeviation.^6,7 In CN IV palsies, the hyperdeviation typically worsens in contralateral gaze and ipsilateral head tilt.⁶

Given that the most common cause of hyperdeviation is some form of CN IV weakness,^3,4 I specifically observe the torsion on OCT scans and look for the eye with the most excyclotorsion. At this point, prior to performing the examination, I guess that this eye will most likely be the hyperdeviated eye. Then, I go into the examination room to perform a cover test or single Maddox rod test to prove the eye is, in fact, the hyperdeviated eye.

If a vertical deviation is present, I follow up with Parks-Bielschowsky three-step testing, double Maddox rod testing, and the upright-supine test to confirm other signs of the underlying neuro-optometric finding.^6,7 If the eye with the increased excyclotorsion is the higher eye, then CN IV palsy is the most likely diagnosis. However, if the eye with the increased excyclotorsion is the lower eye, then I go down the skew deviation route for differential diagnosis.

I feel it is important to remind the reader to use this “magic trick” within the context of the clinical exam. Just because there is more excyclotorsion than expected on the OCT scans does not necessarily guarantee that patient has a CN IV palsy—or even another neuro-optometric issue. For example, there is a fairly large range of normal excyclotorsion values in normal patients (-3.8° to -9.4°, average -6.6°), as tested with the Spectralis OCT platform (Heidelberg Engineering).⁸ Another study that also looked at normal ocular torsion values with OCT showed a broader range of normal torsion, but similar average torsion (+2.5° to -17.5°, average -6.7°).⁹ A third, recent study compared torsion in healthy controls versus CN IV palsies with OCT and found that torsion in CN IV palsies ranged from -7.7° to -14.9° with an average of -11.3°.¹⁰ This same study also found that normal ranges of ocular torsion were -4.3 to -8.5° with an average of -6.4°.¹⁰

Using these three studies as my foundation,^8-10 I developed general guidelines for assessing ocular torsion on OCT:

• Average torsion of healthy patients: -6.5° (Figure 1)
• Average torsion of patients with CN IV palsies: -11.5° (Figure 2)

In addition, head position could give the false impression of abnormal ocular torsion. For example, if the patient’s head is not perfectly vertical while obtaining their OCT scans, then the tilted position of the head might induce certain amounts of ocular torsion that could mimic neuro-optometric disorders. Therefore, proper patient instruction and technician assurance that the patient keeps their head in a normal, vertical position is imperative to assess ocular torsion accurately.

Consider Other Findings

Although one should be wary of making diagnoses based on this magic trick alone, if there is a hyper deviation in the eye with the increased amount of excyclotorsion found on the OCT, the Parks-Bielschowsky three-step test confirms superior oblique involvement (ie, the hyper deviated eye worsens in contralateral gaze and ipsilateral head tilt), the overall extraocular muscle pattern shows inferior oblique overaction upon extraocular muscle testing, and the upright-supine test is negative, then those multiple findings taken together in context would indeed strongly suggest CN IV palsy.^6,7

It bears repeating: Always interpret ocular torsion in the context of other findings. Still, ocular torsion observation can be a strong clue to heighten your suspicion that additional clinical testing may be necessary.

Educational Benefit

To my students, this process does appear to work like magic, especially when I predict what I would find before even examining the patient’s eyes. Because my students are smart, curious, and motivated, they ask how I did it. Being a jokester, I tell them it’s magic before I walk into the exam room to perform cover testing, show the hyper-deviation, and finish up the first half of the examination. Then, I tell them how it was done while the patient’s eyes are dilating.

This delay in answering my students encourages them to critically think about the patient’s presentation while I confirm my suspicions before I give them the correct answer. I’ve lectured on this topic in several venues recently, and each time I share this trick, I get excellent questions, and a few chuckles, from my colleagues.

IF OCULAR TORSION IS ABNORMAL

If you note the presence of abnormal torsion with your visual inspection of OCT scans, then perform some kind of vertical posture testing (ie, cover testing or single Maddox rod testing) to confirm what you are seeing. You might be surprised by how many people in your chairs have asymptomatic or subtle symptomatic CN IV palsies. If patients are asymptomatic, treatment will most likely only require simple observation. But further treatment may be necessary in some symptomatic cases, which may include occlusion, prism, vision therapy, and possible surgery.

What if you uncover an acute or chronic finding that affects the patient’s ocular or systemic health in some manner? For example, using the outlined trick above, I recently was able to diagnose a completely asymptomatic skew deviation, and neuroimaging resulted in finding a cerebellar stroke that the patient didn’t know he had.¹¹ By simply paying attention to the torsion of his OCT scans for his chronic glaucoma, we identified a neurologic issue that needed to be addressed.

A SIMPLE FINDING CAN HAVE A BIG EFFECT

I realize that the ocular torsion trick I’ve described here is not novel, nor is it inherently difficult to identify, but, oftentimes in clinic, I find that there are clues all around us that we can overlook as practitioners. Perhaps the next time you look at an OCT scan from a glaucoma patient, you will take care to glance for asymmetric or abnormal ocular torsion, which may even lead to uncovering a serious neurologic condition.

I hope this article helps raise your awareness of looking for abnormal torsion in your OCT patients moving forward and further enhances the high quality of care you already provide.