General EYE ADVICE

How does the Eye Normally Prevent the Formation of Astigmatism?

To understand the means, by which our visual system prevents the formation of any astigmatism and produces normal eyesight, one needs to understand the mechanism that is involved in controlling the shape of the optical structures of the eye. As noted above, astigmatism of an optical surface denotes that its shape is "warped". If the degree of astigmatism of the lens is not precisely equal and opposite to that of the cornea, the astigmatism of the cornea is not fully neutralised. The residual astigmatism thus produced is what is normally termed the astigmatism of an eye. If the visual system had no control over the shape of the optical structures of the eye, very few people would be free of astigmatism, as fine-tuning of the optical structures would not be possible. A control mechanism has been postulated to exist over a hundred years ago and is known as Meridional Accommodation. It has also been termed Astigmatic Accommodation.

Unfortunately, due to its very subtle presence, demonstrating the existence of such a physiological mechanism in the human being's visual system has never been easy. However, without postulating its presence, one cannot explain how the visual system normally operates to be free of the problem of astigmatism. I have presented the essential evidence for meridional accommodation in Part I of the scientific paper presented at the 1997 APOC convention.

As many schools for eyecare professionals do not teach the physiology and clinical implications of Meridional Accommodation, the practitioner nowadays is restricted to prescribing for astigmatism in a reactive manner. The lens that gives the sharpest vision is the one that is usually prescribed as a first attempt. Experience may guide him or her to modify this result and achieve more successful prescribing; however, the basis on which the result is modified relates more to art than science.

The practitioner who is not taught the physiology and clinical implications of the mechanism that can fine-tune the eye's astigmatism, cannot be expected to confidently predict how the visual system will cope with any intervention and whether or not any eye changes are expected. Furthermore, if a patient complains of visual difficulties with prescribed glasses, knowledge of the mechanism that attempts to control the eye's astigmatism is essential if the practitioner is to understand why the patient is having difficulties. Only when the reasons for the difficulties are clearly understood can the practitioner have a high level of confidence in the solutions that he/she may offer.

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