There is an interesting article on fixation disparity in the clinical section this week.
Fixation disparity is a concept I have always had some trouble with. During my undergraduate days, we were taught how binocular vision relied on the images at the back of each of the eyes corresponding. In many cases, the muscles of the eye need to strain to maintain the binocular lock and, when they relax or are dissociated, they adopt a position of non-alignment. This is a phoria and is described as decompensated when there are symptoms caused by maintaining the binocular lock.
The theory goes that for every retinal point in one eye, there may be a corresponding point in the other over an area described as Panum’s area. As long as the corresponding point falls within this area there is full compensation. Where a phoria is decompensated, the corresponding point falls outside this area though still with a disparity that the extraocular muscles may fuse, albeit with resultant symptoms. This is fixation disparity. The references for this part of the syllabus were the famous ‘Mallett papers’ – considered seminal yet never having passed through the peer-review publication route.
This all seemed very neat to me until my pre-reg year. At the hospital, orthoptists dismissed fixation disparity entirely, relying instead on measures of fusional reserve. Conversely, in my fortnightly visits to the soon to be renamed London Refraction Hospital, Mallett ruled and practical application of his theory seemed to work. Roll forward a few years into high street practice, and I regularly came across prescriptions having been issued with one or even 0.5Δ in front of one eye, sometimes in children, apparently as a means of addressing symptoms much more likely to have been resolved by simple lifestyle advice.
There is a place for prism – the new technique we report on this week needs our consideration.