What’s on BV – Episode 2 – How to get more from your occluder – Part 2

What do we mean by recovery?

In the last episode, we saw that concomitant deviations do not have to be symmetrical, and that in the case of anisometropia, one may see an asymmetrical movement of the eyes on the alternate cover test.  This episode highlights the importance of the recovery movement seen on cover test.

In my time teaching BV in various clinical settings, it seems that the majority of people are quite happy with terminology used for manifest strabismus (tropia), whereas latent strabismus (phoria) is somewhat less well understood.

One term in particular is ‘recovery’.  When describing the characteristics of a patient’s phoria, it is common to write, for example:

‘Cover Test @ 1/3m: slight exophoria with rapid recovery’.

But what exactly do we mean by recovery?

Take for example the patient used above.  The cover test result implies that when fixating a target at 1/3m, both eyes are aligned, but on dissociation with an occluder, the covered eye deviates outwards. 

As the occluder is moved to the other eye, the previously covered eye has to move inwards in order to retain fixation on the target.  This movement is simply a ‘fixation’ movement and should not be confused with recovery.

On removal of the occluder, the eyes return to their straight-ahead position, in other words, binocular single vision (BSV) is restored.  This is the recovery movement which one can therefore consider to mean ‘recovery to BSV’.  This means that observation of the eyes the instant the occluder is removed is critical in order to assess recovery. 

 

The speed of recovery can provide useful information as to the patient’s ability to control their phoria, and is arguably a much more useful indicator than the actual size of deviation.   A patient may present with a 45-dioptre exophoria with very fast recovery and be totally asymptomatic, whereas a two-dioptre esophoria with slow recovery may cause asthenopic symptoms.

The grading of speed of recovery is largely arbitrary, and to my knowledge there are no definitive scales, such as those used in other areas of optometry, to which to refer.  Personally, I use the terms ‘rapid’, ‘good’, ‘fair’, ‘slow’ and ‘delayed’, which in effect act as a five-point grading scale (grade 0 = no action, grade 4 = requires management) in the same way as the Efron grading scales in contact lens practice.

As we can now consider recovery to mean ‘recovery to BSV’, it should naturally follow that the mechanism used to restore BSV is motor fusion, the extent of which is determined by the fusional reserves.  Motor fusion itself relies on the presence of sensory fusion, and the stimulus for sensory fusion is diplopia.  As a result, for recovery to occur, the visual system must appreciate diplopia, and there must be sufficient fusional reserves in order to make the necessary to join the images.

Slow or delayed recovery on cover testing is therefore indicative of a disruption to the fusion system, whether by suppression/reduced visual acuity (i.e. there is a failure to appreciate diplopia, thus preventing sensory fusion, and by definition, motor fusion) or reduced fusional reserves (i.e. diplopia is appreciated, but the motor system is unable to efficiently realign the visual axes).  In either case, it is likely to be appropriate to investigate the control of the phoria in more detail, by assessing the degree of suppression, extent of motor fusion reserves and presence of fixation disparity.

Incidentally, the speed of fixation on cover test can be a useful clue as to the level of vision, and is particularly handy when testing young children with suspicions of strabismus.  A slow fixation movement in one eye is often associated with reduced visual acuity and may be indicative of amblyopia.

This episode should hopefully clarify the difference between the fixation and recovery movements seen on cover test, and demonstrate the importance of assessing the speed of recovery on removal of the occluder.

Tune in again soon for Part 3 of How to get more from your occluder.

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