EYE ADVICE: Professional Papers

B. Corneal changes due to Ciliary Muscle Action

Yamanaka (1961) demonstrated corneal toricity changes upon atropinization. The evidence shows that MA acts not only on the crystalline lens, but also on the corneo-scleral meshwork. This latter ability enables MA to affect corneal toricity. Indeed, I believe that the most important function of MA is its ability to produce emmetropisation of astigmatism in neonates, through its ability to influence corneal toricity. The ATR corneal changes which occur in older age are due to over-action of PMA. (See Part II)

The trabeculae in the eye form a circumferential ring which extends into the ciliary muscle (Wolf 1976, Pg. 70). The trabeculae have attachments predominantly with circular muscle fibres but also with radial muscle fibres, which have been associated with sympathetic innervation by several workers. (Olmstead and Morgan 1941). Tripathi and Tripathi (1982) note that those trabeculae which have direct ciliary muscle attachments, generally terminate below the scleral spur in the human. The connective tissue, however, in which the muscle fibres are embedded, becomes organised into trabecular sheets which can extend further forward up to the termination of Descemet's membrane. This would imply that circular muscle fibres can exert a force on both the scleral spur and the peripheral cornea. The action of these circular fibres would be expected to direct the force radially inward towards the pupil centre.

It is likely that PMA is brought about by the action of specific circular muscle fibres with the majority of these lying within 45 degrees of the horizontal in the temporal sector. PMA circular muscle action which is restricted beyond 45 degrees from the horizontal is unlikely as this would eliminate the sum total ATR accommodative effect, giving a vector force which would lead to more WTR lenticular and corneal changes.

Several authors have documented a cessation of increasing WTR corneal curvature changes by age 20 and a tendency for ATR refractive corneal changes after age 30. Saunders (1988) describes refractive changes with age and Lyle (1971) and Hayashi et al (1995) describe the corneal toricity changes. Lyle found a 3.7% ATR corneal toricity in the 30-40 year age group compared with 13.7% for the average of the over 50 years age group. Saunders shows that there is a reversal in preponderance of ATR compared with WTR refractive astigmatism around the 40 to 50 years age group. In the 50-60 years group of 101 patients Saunders notes - ATR: n=49, WTR: n=32 oblique: n=20. The work of these authors and others, suggests that the refractive state of the lens is the first to change towards ATR astigmatism followed by the corneal changes.

Some workers have suggested that lid tension may determine the corneal toricity. This however, could not explain the preceding lenticular changes. Furthermore, Vihlen and Wilson (1983) and Thorn et al (1987), conclude that lid tension is not likely to determine corneal toricity. An explanation for the ATR lenticular and corneal changes observed with presbyopia is given in Part II.

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