What’s on BV – Episode 3 – How to get more from your occluder – Part 3

When is a tropia not a tropia?

The previous episodes have covered the topics of asymmetric phoria and the importance of recovery in latent squint. This episode aims to highlight the importance of observing both the covered and uncovered eye on the cover test.

I tested an 11 year old girl recently who has a long history of orthoptic assessments during her childhood. At the age of 18 months she was noticed to have a LEFT convergent squint, and has worn a hyperopic Rx since then, the current prescription being approx +5.00DS in both eyes. Other than a brief spell of occlusion at the age of four, she has had no other intervention. With specs, acuities measured R 6/6 L 6/6 with crowded optotypes (i.e. logMAR chart) and stereoacuity measured 80� arc with Wirt.

Cover test at near without specs, unsurprisingly, showed a large (35 dioptre) LEFT esotropia, and at first glance, there was a 10 dioptre esophoria with spectacles, suggesting a classical fully accommodative esotropia (FAET) or Donder’s squint. The BV anoraks who correctly calculated the AC/A ratio to be 5:1 can give themselves another point! (As promised in Episode 1, AC/A ratio calculations will be covered properly later in the series.)

However, on closer inspection, the cover-uncover test with spectacles showed a very small LEFT esotropia, measuring approximately 4 dioptres, which then increased to 10 dioptres on alternate cover test. On removal of the occluder, the eyes made a rapid recovery movement to their original position.

In summary, with spectacles, cover test with spectacles found a 4 dioptre esotropia, increasing to a 10 dioptre esophoria, recovering to a 4 dioptre esotropia, as per demonstrated in the following animation:

But how is this possible? Performing an alternate cover test on a conventional heterotropia, for example the 35 dioptre esotropia this girl has without specs, does not lead to any further increase in deviation (as the alternate cover test dissociates the eyes in order to exhibit the maximum deviation, and by definition, the eyes are already dissociated when there is heterotropia). Additionally, as was shown in the previous episode, when there is a recovery movement, the eyes are recovering to BSV, but in this case, the visual axes are still not aligned after recovery.

Under normal circumstances, retinal points in each eye correspond with each other and have a common visual direction. For example, in the case of normal retinal correspondence, the fovea of the right eye corresponds to the fovea of the left, and both are directed towards the object of regard. Under certain circumstances, for example foveal suppression, this relationship may become disrupted, and the fovea of one eye may develop an abnormal association with an extra-foveal point of the fellow eye. This phenomenon is known as abnormal retinal correspondence (ARC), and is said to be able to exist in small-angled deviations, up to 20 dioptres.

It is important to stress at this point that ARC is a binocular phenomenon, i.e. it relates to the eyes working as a pair. As soon as the fixing eye is occluded, foveal suppression ceases thus normal foveal fixation can take place. (This is not the same as eccentric fixation where the fovea remains suppressed on monocular fixation). The small-angled nature of ARC allows the characteristic elements of BSV to occur, namely fusion and stereopsis.

In this case, under binocular conditions, the LEFT eye is slightly deviated, but there is measurable stereopsis, confirming the presence of (abnormal) BSV. On dissociation, the angle of deviation increases from 4 to 10 dioptres with either eye achieving foveal fixation. On removal of the occluder the eyes recover to their original, slightly deviated position, as a result of the fusion system restoring (abnormal) BSV.

This type of small-angled deviation with ARC is referred to as microtropia without identity and may occur in conjunction with another binocular vision anomaly, for example FAET as in this example, or in isolation.

The differences between microtropia with and without identity will be covered in a later episode.

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