What’s on BV – New Series

Welcome to the first episode of the new ‘What’s on BV’ series, where I hope to highlight some examples of BV tests that I find wobv_logo_125.gifparticularly useful in clinical decision making.  Unfortunately there is no free binder with part one, but I hope they prove a useful collection in the long run.  If there are any specific subjects you would like to see, please do not hesitate to contact me by using the contacts page – Simon

How to get more from your occluder – Part 1:

I often joke about my occluder being my best friend, but there are times it can be exceptionally useful and give subtle, but useful clues as to a patient’s binocular status, and much more besides.

I recently tested a 35 year-old gentleman who was reporting symptoms of gradually worsening eyestrain when reading for long periods.  He explained that he had worn ‘reading’ glasses as a child, but had stopped at the age of 8.

On examination, acuities measured 6/6 and N5 in both eyes.  Smooth pursuits were full with no diplopia in any position of gaze, and convergence was normal.  Cover test showed an esophoria at near, but was asymmetrical, i.e. the angle of deviation differed depending on the fixing eye.  In this case, the deviation measured 4 dioptres when fixing with the right eye and 12 fixing with the left.

Asymmetric phorias, or tropias for that matter, tend to point towards an incomitant deviation, however in this case, there was no apparent imbalance of the extraocular muscles.  So what else could explain the asymmetry in the cover test measurement?

If we consider why esophoria occurs in the first place, it tends to arise in cases of uncorrected hyperopia (although not exclusively).  In such cases excessive accommodation is required to overcome the ametropia which in turn leads to excessive convergence, due to the relationship between accommodation and accommodative convergence via the AC/A ratio.  Therefore the greater the degree of hyperopia, and thus accommodation, the greater the angle of deviation.

In this case, the deviation is larger when fixing with the left eye, in other words, when moving the cover from eye to eye during the alternate cover test, the movement of the right eye as it is uncovered (i.e. the left eye was fixing) is larger than the movement of the left eye as it is uncovered (i.e. when the right was fixing).  I chose to measure this with a prism bar, increasing the base out prism before one of the eyes until the smaller movement was neutralised, and then continuing to measure the larger movement.

In the absence of an incomitant deviation, the anomalous cover test result may then be explained by an asymmetry in accommodation.  The most common cause of a difference in accommodation between the eyes, in my experience at least, is anisometropia.

My suspicions were confirmed with this gentleman by performing retinoscopy, which found R +1.50 L +3.50.  Binocular subjective refraction (‘binocular’ being essential in cases like this, in my opinion, in order to control accommodation) gave a final Rx of R +1.00 L +3.00.  Cover test at near with this Rx eradicated the esophoria completely in both eyes.  The BV anoraks amongst you will probably have deduced that this chap’s AC/A ratio is 4:1 (If not, don’t worry as I’ll cover AC/A ratio calculations another time).

I prescribed him spectacles to use as he felt necessary, which would include both distant and near tasks, and so far he seems to have adapted well.

This example should hopefully demonstrate that comparing the size of deviation when fixing with either eye can give a hint as to the possible refractive error.

Tune in next time for Part 2 of How to get more from your occluder.

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