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
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.