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