Friday, December 7, 2007

3.3.2 Squint (noun)

Childhood squint, i.e., strabismus or tropia is different from phoria. The latter refers to the eyes assuming a natural position when they are artificially dissociated. They resume fixation at the same point when both eyes now see at the same time. The angle of deviation, in prism diopters, can be measured quite accurately, e.g., with rotating prisms. And frequently, small amounts of prisms are prescribed to relieve extraocular-muscular strains from the need to actively maintain the binocular alignment.

In squint, the two eyes remain dissociated and misaligned, also known as crossed- or walled-eyes when the affected eye turns in or out, respectively. The deviation can be horizontal, vertical, or mixed. It can be constant or it can vary with gaze. And it can involve one or both eyes. The main cause can be either central involving the brain, or a local neuromuscular problem. So the diagnosis is best left to a professional. The deviated eye either has already developed amblyopia or has the potential to become so. Proper therapy, either patching or atropine eyedrops, must be initiated in conjunction with orthoptic re-alignment of the eyes. Often strabismus requires surgery to re-arrange the positions of the extraocular muscles; although mostly for cosmetic reasons.

There is a special case, accommodative esotropia. It is from relatively high hyperopia and when the child tries to see even at distance, accommodation kicks in and the eyes converge (cross) as a result. A pair of properly prescribed glasses or contacts can re-align the eyes without the need for surgery or orthoptics.

Also in Asian babies, because of the flat nose bridge and the thick epicanthal skin covering more of the nasal part of the eyes, there is an optical illusion of esotropia or pseudo-esotropia. You can shine a penlight about 3m away into the baby's eyes and observe the reflexes on the corneas. If the reflexes appear at the same position in each eye, then the eyes are aligned.

You may have heard of the term “vision therapy” and its putative efficacy. Historically, “vision therapy” is not very well-defined and in fact quite confusing. This much we do know: In two 2005 studies, comparing intensive office-based therapy and in-home pencil push-up exercise, the former seems to better resolve symptoms from convergence insufficiency, e.g., headaches, eye fatigue, blurred and double vision from near work. This conclusion is not without controversies, though. The claim of remedying learning disability, on the other hand, is unsupported.

More recent versions of “vision therapy” include training of re-focusing and visual perception, improvement of oculo-motor coordination and eye tracking. In fact, some professional athletes are enthusiastic supporters of this type of therapy.

As in other fields of medicine, the effectiveness of any treatment must be evidence-based. Let’s just say, much more needs to be done in the field of vision therapy.

1 comment:

Narinder Mehta said...

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