Theories of Accommodation
The Helmholtz hypothesis or capsular theory of accommodation states that during distance vision the ciliary muscle is relaxed and the zonular fibers that cross the circumlental space between the ciliary body and the lens equator are in a state of “resting” tension. With accommodative effort, an anterior movement of the ciliary muscle annular ring and a release of tension on the zonules occur, increasing the accommodative power of the lens. An anterior movement of the ciliary muscle annular ring also takes place during accommodation. The reduced zonular tension allows the elastic capsule of the lens to contract, causing a decrease in equatorial lens diameter and an increase in the curvature of the anterior and posterior lens surfaces. This “rounding up” of the lens yields a corresponding increase in its dioptric power, as is necessary for near vision (Fig 9-1). When the accommodative effort ceases, the ciliary muscle relaxes and the zonular tension on the lens equator increases to its resting state. This increased tension on the lens equator causes a flattening of the lens, a decrease in the curvature of the anterior and posterior lens surfaces, and a decrease in the dioptric power of the unaccommodated eye.
In the Helmholtz theory, the equatorial edge of the lens moves away from the sclera during accommodation and toward the sclera when accommodation ends. In this theory, all zonular fibers are relaxed during accommodation and under tension when the accommodative effort ends. According to Helmholtz, presbyopia results from the loss of lens elasticity with age. When the zonules of an older lens are relaxed, the lens does not change its shape to the same degree as a younger lens does; therefore, presbyopia is an aging process that can be reversed only by changing the elasticity of the lens or its capsule.
Diametrically opposed to the Helmholtz hypothesis is the Schachar theory of accommodation. Schachar suggested that during accommodation ciliary muscle contraction leads to a selective increase in equatorial zonular tension—rather than to the uniform decrease (anterior, equatorial, and posterior) proposed by the Helmholtz theory—with a subsequent pulling of the equatorial lens outward toward the sclera (Fig 9-2). Schachar postulated that accommodation occurs through the direct effect of zonular tension (as opposed to the passive effect proposed by Helmholtz), causing an increase in lens curvature. In this theory, the loss of accommodation with age is a result of the continued growth of the lens, leading to an increase in lens diameter and a decrease in the lens–ciliary body distance, which in turn cause a loss of zonular tension. Anything that increases resting zonular tension (eg, scleral expansion) should restore accommodation.
Schachar proposed that the mechanism for functional lens shape change is equatorial stretching by the zonules, which would decrease the peripheral lens volume and increase the central volume, thereby causing central steepening of the anterior central lens capsule (Fig 9-3). During accommodation and ciliary muscle contraction, tension on the equatorial zonular fibers increases, whereas tension on the anterior and posterior zonules decreases. These actions would allow the lens to maintain a stable position at all times, even as it undergoes changes in shape. Schachar theorized that the anterior and posterior zonules serve as passive support structures for the lens but that the equatorial zonules are what actively determine the optical power of the lens.
Evidence from recent studies on human and nonhuman primates contests Schachar’s theories of accommodation and presbyopia. Investigations in human tissues and with scanning electron microscopy reveal no zonular insertions (equatorial or otherwise) at the iris root or anterior ciliary muscle. Various imaging techniques have consistently demonstrated that the diameter of the crystalline lens decreases with accommodation so that the equator moves away from the ciliary body. In vitro laser scanning imaging shows that the crystalline lens does not change focal length when increasing and decreasing radial stretching forces are applied. This evidence thus contradicts Schachar’s proposal that the lens remains pliable with age and that presbyopia is due solely to lens growth and crowding that prevents optimum ciliary muscle action.
Using model-based reasoning, Goldberg proposed another theory of accommodation with the help of a computer-animated 3-dimensional (3-D) model of the eye and the accommodative system. Goldberg’s theory of reciprocal zonular action describes 3 components of the zonules and posits that a synchronized movement among the ciliary body, zonules, and anterior hyaloid complex leads to a shift in the posterior lenticular curvature and refractive power (Video 9-1).
VIDEO 9-1 Theory of reciprocal zonular action.
Courtesy of Daniel B. Goldberg, MD. Access all Section 13 videos at
www.aao.org/bcscvideo_section13.
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Glasser A, Kaufman PL. The mechanism of accommodation in primates. Ophthalmology. 1999; 106(5):863–872.
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Goldberg DB. Computer-animated model of accommodation and theory of reciprocal zonular action. Clin Ophthalmol. 2011;5:1559–1566.
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Schachar RA. Cause and treatment of presbyopia with a method for increasing the amplitude of accommodation. Ann Ophthalmol. 1992;24(12):445–447, 452.
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Strenk SA, Strenk LM, Koretz JF. The mechanism of presbyopia. Prog Retin Eye Res. 2005; 24(3):379–393.
Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.