Meridional Accomodation & Correction of WTR Astigmatism in Children
Prescribing for the most common type of astigmatism in children (WTR) is of particular importance, as the astigmatic lens that is prescribed can influence the progression of the child's astigmatism. For greatest long-term benefit to infants, it is important to consider the questions of when to intervene with a refractive correction and how much of the astigmatism to correct. These are both very challenging to the practitioner. Certainly, there are no simple answers that will work well with all children. The answers to these questions lie in advanced knowledge of the physiology of MA and its clinical implications, especially involving amblyopia (lazy eye) and astigmatism.
A third question arises, especially in children who are old enough to perform a subjective test for their astigmatism. In this test, the patient is asked questions about which is the clearest lens and the final prescription may be based on this result. The practitioner can also use objective tests to determine the refraction (powers of the eye). There is often a difference in the results, depending on whether a subjective or an objective testing procedure is utilised. The main reason for the difference in the results is related to the amount of tonic (or continuous) MA, which may be in a different direction to the astigmatism of the cornea. If the practitioner is not aware of the MA mechanism, he/she cannot be in a position to understand or to account for the difference in results between the two modes of testing.
Furthermore, the correcting toric lens that is prescribed (especially in relation to its axis) can influence the progression of WTR astigmatism in children. This is particularly likely to occur when the final correction is based solely on subjective measurements. The axis of the astigmatism that is found on subjective testing, is usually at a small angle to that of the corneal astigmatism. By prescribing lenses based on the subjective result, the axis of the corneal astigmatism is forced to change in time towards the prescribed axis. This can lead to a significant angling of the axis of the flattest corneal meridian after a few changes of prescription, which can make the correction of astigmatism more difficult. Objects around the patient can appear to be distorted when wearing a pair of toric spectacle lenses which are fitted with their axes at an angle to each other.
MA is the physiologic mechanism that is responsible for regulating the shape
of the cornea. This is achieved via muscular force that is applied
on the cornea. MA is most effective in controlling and moulding
the corneal shape in the first 2-3 years of life. However, a healthy
lifestyle and active eye use can help to change the curve of the
cornea for better vision even in adult age. In the case of WTR astigmatism
in children, a lens that is pro-actively prescribed, based
on knowledge of the physiology of MA, will direct the force of MA
on the cornea in a way that will act on reducing the magnitude,
and not change the axis (angle) of the astigmatism. Thus pro-active
prescribing will not only help to prevent the angling of the corneal
astigmatism and the consequent problems of distortion but will also
help a child to achieve better long-term vision.