General EYE ADVICE

1) Meridional Accomodation & Latent Astigmatism

As mentioned above, persons with latent astigmatism who undertake much close work may suffer from eyestrain related to the unconscious, intensive use of meridional accommodation (focusing mechanism that compensates for corneal astigmatism). The normal methods of testing show little or no evidence of astigmatism and thus the practitioner either cannot explain the eyestrain symptoms or in some cases may attribute them to other reasons such as a mild coincidental eye muscle imbalance. Knowledge of MA would enable the practitioner to find a suitable astigmatic spectacle prescription, especially designed for the task that causes eyestrain.

Part of the corneal astigmatism becomes manifest in some individuals with latent astigmatism when focusing at near objects e.g. reading etc. The reason for this is that the effectiveness of MA in compensating for the corneal astigmatism is normally reduced as the amount of focusing (involving the lens in the eye) is increased. Thus at near, more MA is needed to produce the required ATR astigmatism of the lens in order to compensate for the corneal astigmatism.

As most eyes normally have a small degree of latent astigmatism, the human eye is designed in such a way that additional MA is automatically stimulated when focusing at near objects, to counteract the reduced effectiveness of MA for near focusing. I have termed this automatic type of MA as "reflex" MA. In some individuals in whom there is insufficient additional MA stimulated automatically, some of the latent astigmatism becomes manifest as WTR astigmatism, resulting in slightly blurred vision at near. Those individuals, in whom an excess of MA is stimulated when focusing at near, may show ATR astigmatism. Over longer periods of time, this type of astigmatism can lead to more permanent ATR astigmatism of the cornea, with consequent adverse effects on distance vision as well.

The practitioner thus needs to also be aware of the type of astigmatic changes that can accompany the normal focusing of the eye. Having this knowledge, he/she can then be in a better position to prescribe proactively and in the best interests of the patient. By recognising the effects of MA and by performing a separate test for astigmatism at near, the practitioner is better able to prevent a range of problems that can accompany the sub-optimal correction of astigmatism at near. (Refer to Part I (B) for discussion of these problems.)

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