Fixation Disparity and Stereopsis in mTBI

Just had the occasion to re-read the superb book chapter by Drs. Eric Singman and Patrick Quaid on Vision Disorders in Mild Traumatic Brain Injury that was published last year.

The section on fixation disparity and impaired stereopsis is so well done, that I wanted to share it in its entirety here. I have removed the reference numbers and references to streamline it, but you can access the full chapter with its references at the link provided above. Regarding abbreviations, DH is dissociated heterophoria, and AH is associated heterophoria.

Fixation Disparity and Impaired Stereopsis

Recognizing that saccades, pursuits, vergences, and accommodation can be disrupted in mTBI, it is not surprising that visual fusion suffers as well. In this context, deficits in visual fusion refer to FD slips, or AH. FD slips represent a small misalignment of the visual axes of either eye under binocularly fused conditions such that there is a lack of bi-foveal fixation, but maintenance of normal retinal correspondence. This can occur because the disparity is still within Panum’s area of fusion, which is approximately 100 arc at fixation in normal subjects. This small heterophoria, as low as 0.5PD, is too subtle to be measured by those tests use for dissociated heterophoria (DH), such as the cover-uncover test or Maddox-rod test.

Whereas AH is a deviation from orthophoria that occurs when fusional contours are absent only from the central visual field, DH is a deviation that occurs when neither central nor peripheral fusional contours are offered, such as during cover-uncover- or Maddox-rod testing. AH has been shown to be a much better indicator of symptomatology compared to DH as the alignment of the visual axis under binocular conditions is more relevant to the habitual oculomotor status. In addition, AH is more useful in determining who might be uncomfortable using 3D-viewing technology. It has been reported that patients with mTBI often demonstrate vertical heterophorias and that correction with prism aids in reducing symptomatology. Devices to measure AH, that is, the minimum prismatic correction required to attain alignment recorded in arc minutes or prism diopters, include the Mallett unit, Sheedy Disparometer, and Wesson Fixation Disparity card.  These tests offer suppression checks, that is, polarized filters over either eye so that different targets can be viewed binocularly, allowing the examiner to determine whether patients are avoiding diplopia through suppression of one visual field, that is, becoming functionally monocular.

Another aspect of visual fusion is stereopsis (depth perception), which can be described as local or global in nature. Stereopsis is measured by standardized tests and reported in minutes/seconds of arc; normal values have been published. Global stereoscopic targets (i.e., randot displays), which require a larger visual area in order to be seen, can better reveal symptomatology compared to local stereoscopic targets. Notably, stereopsis normally declines as the visual target moves from the fovea to the retinal periphery.  In addition, perception of depth appears to require both retinal and extra-retinal inputs during motion of the scene or the observer. Finally, it is noteworthy that studies employing fMRI suggest that depth perception appears to be subserved by the dorsal visual stream in normal subjects, in particular to visual cortical areas V3A, V7, and MT 1/V5.

Patients with mTBI demonstrate reduced stereopsis at near and, to a lesser degree, at distance. However, these impairments seem insufficient to explain their relatively common complaint of reduced depth perception. Furthermore, patients suffering concussion report decreased tolerance to aniseikonia (different retinal image sizes due to differing refractive errors between the two eyes) which may play some role in their reduced ability fuse. Although the primary visual cortex (V1) of adult patients suffering from blast-related mTBI demonstrate abnormalities with fMRI, there are no published reports specifically exploring the changes in integrity or connectivity of the ventral and dorsal visual pathways.

Anecdotally, patients with mTBI frequently report intolerance to viewing 3D movies (personal experience, personal communications). Even in normal individuals, observing 3D movies is known to elicit symptoms of imbalance, headache, eyestrain, and motion sickness. In addition, these symptoms tend to be more common and more severe in individuals with a history of susceptibility to motion sickness or migraine. Although there are no studies of the responses of mTBI patients to watching 3D movies, it is likely they would fall into the category of susceptible patients. However, it is unclear whether their reduced stereopsis would play a role in that susceptibility; a large study of normal patients who reported untoward effects of watching 3D movies indicated that there was no correlation between degree of stereopsis and symptoms.

Reduced stereoacuity also seems to a problem shared by patients with mTBI and those with ADHD, further supporting the idea that mTBI is associated with abnormal vision information processing. While further research is certainly needed, it seems reason- able to hypothesize that reduced depth perception and binocular fusion may result from impaired integration of central (ventral) and peripheral visual (dorsal) information.

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