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Assistant Professor Alexis Tsorbatzoglou and Professor Andras Berta discuss the various theories presented to define the mechanism of accommodation.
What did Helmholtz say?
In spite of large clinical investigations, the exact nature of human accommodation is not fully understood. Helmholtz provided the first comprehensive description of the accommodative mechanism in 18551 which, until recently, remained the most accepted theory by ophthalmologists. He proposed that the ciliary muscle is responsible for accommodation, which is relaxed during far fixation, and is contracted during accommodation. Ciliary muscle contraction causes zonular relaxation all around the lens equator during near fixation. As a result of the zonular tension release, lens diameter decreases, and the curvature of the anterior lens surface increases and moves forward, resulting in increased refractive power of the eye. Fincham observed that the increase in lens thickness is greater than the decrease in anterior chamber depth during near fixation.2 Consequently, he suggested that the lens posterior surface moves backward with accommodation.
What does pressure have to do with it?
In contrast to the capsular theory, Tscherning4 and Coleman5 thought that a pressure difference between vitreous body and aqueous, which is caused by a simultaneous small vitreous pressure increase and a small aqueous pressure decrease during the accommodative effort, can change the position of the lens, thus increasing the refractive power of the eye.
More recent studies, however, suggest that changes in corneal curvature also influence the accommodative response. This hypothesis has been verified by a keratometer6 and corneal topography,7 but it was not supported by videokeratoscope testing.8 The original assumption postulates that the centripetal force of ciliary muscle contraction during accommodation acts on the limbus as a "sphincter" pulling the limbus centripetally, which shortens the cornea's diameter and steepens its shape, thus producing an increase in corneal refractive power. This dioptric change is relatively small compared with the entire refractive power alteration during accommodation in young subjects. However, it can be significant in older eyes, where the accommodating function of the lens is diminished.
Using the partial coherence laser interferometry method, Drexler9 and Mallen10 proved that transient elongation in axial length of the eye occurs during near fixation. This phenomenon may result from ciliary muscle contraction, which causes an inward pull force to a region of the choroid and sclera adjacent to the ciliary body. This force induces a decrease in the circumference of the sclera, and thereby an elongation of the posterior pole of the globe to maintain a constant ocular volume. The increase in axial length is small, but can be measured. Therefore it cannot be excluded from the mechanism of accommodation.