Historical Background

It is at the beginning of the 17th century that the first theories of accommodation were developed by several scientists. Briefly, Scheiner (1619) demonstrated that the eye includes a specific apparatus that can adjust focus and provide near vision. Kepler (1611) demonstrated that the backward and forward movements of the crystalline lens provide accommodation. Later, Descartes (1637), in his Traite de l’homme , demonstrated that it is a change in the shape of the lens that enables accommodation. He thought that the fibers suspending the lens were responsible for the change in lens shape.

During the 18th century, William Porterfield (1759) confirmed that accommodation occurs by a change in the crystalline lens and Albrecht Von Haller (1763) showed the constriction of the pupil during accommodation.

During the 19th century, Helmholtz (in 1855 and later, in 1909, in his Treaty of Physics ) demonstrated that the contraction of the ciliary muscle triggers an increase in curvature of anterior and posterior surfaces of the lens as well as an increase of its thickness. He stated that when the eye accommodates, there is a contraction of the ciliary muscle, enabling a relaxation of the zonule that leads to a change in the shape of the lens (increased curvatures and thickness). Tscherning (1895) disagreed with Helmholtz’s theory and claimed that the ciliary muscle contraction triggers an increasing zonular tension that can induce a change in lens shape. Fincham (1937) suggested that the change in the shape of the lens during accommodation can be related to the capsule. He noticed that the thickness of the capsule was greater at the equator than at the poles, thus explaining the anterior and posterior bulging of the lens at the poles and its relative flatness at the equator during accommodation when the zonule relaxes.

Current Understanding of Accommodation

Helmholtz’s theory has been widely accepted by the scientific community to explain the mechanisms of accommodation. According to Helmholtz, after accommodating stimulus, ciliary muscle contracts, which relaxes the anterior and posterior zonules connected to the capsule and the lens. The relaxation of the zonule enables the crystalline lens to retrieve its original shape thanks to capsular elasticity ( Fig. 34.1 ). During accommodation, the equatorial diameter decreases while the lens thickness increases with steepening of anterior and posterior radius of the lens. In a relaxed or unaccommodated state, all the zonular fibers are taught. Currently, many studies have reported data supporting Helmholtz’s theory. The steepening of the anterior and posterior radius of the lens, as well as the thickening of the lens and the decrease in anterior chamber depth during accommodation, have been assessed during multiple in vivo studies using different imaging techniques. Dubbelman et al., in 2005, reported a steepening of the anterior radius greater than the steepening of the posterior radius, a lens thickening of approximately 300 µm during accommodation in a young 29-year-old subject, using the Scheimpflug camera ( Fig. 34.2 ). Similar results were found by Strenk et al. using magnetic resonance imaging (MRI) and Croft et al. using ultrasound biomicroscopy (UBM).

Schematic of Helmholtz’s theory of accommodation. During accommodation, the ciliary muscle contracts, which relaxes the anterior and posterior zonular fibers connected to the capsule and the lens. The equatorial diameter of the lens decreases during accommodation.

Lens thickness, anterior chamber depth (ACD) and radius changes of the crystalline lens during accommodation. During accommodation, lens thickness increases while ACD decreases. Both the anterior and posterior radii of the lens steepen during accommodation.

The major opponent to Helmholtz’s theory is Ronald Schachar, who published a new theory in 1992 to explain accommodation. In his theory, based on Tscherning’s theory, the zonule is divided into the anterior zonule, equatorial zonule, and posterior zonule ( Fig. 34.3 ) and each part has a different role in the accommodation process. In a relaxed state, the zonule is taut. After contraction of the ciliary muscle with the radial fibers pulling outward and posteriorly, the equatorial zonule increases its tension while the anterior and posterior zonules are relaxed, thus explaining the central bulging of the lens and the relative flatness of its periphery during accommodation. However, no experiments presented by Schachar have been able to be reproduced by any other scientific teams.

Schematic of Schachar’s theory of accommodation. After contraction of the ciliary muscle, the equatorial zonule increases its tension while the anterior and posterior zonules are relaxed, thus explaining the central bulging of the lens and the relative flatness of its periphery during accommodation. The equatorial diameter of the lens increases during accommodation.

In the 1970s, Coleman was one of the first scientists to point out the extralenticular aspect of accommodation. The difficulty in studying these extralenticular structures lies on their location, which is deep inside the eye and therefore difficult to access through in vivo imaging. In his theory, also called the catenary theory owing to its relation with the mathematical catenary model, Coleman described the role of a diaphragm, including crystalline lens, capsule, vitreous, and zonule ( Fig. 34.4 ). After contraction of the ciliary muscle, the aforementioned diaphragm is pushed forward because of the change in pressure gradient.

Schematic of Coleman’s theory of accommodation. This theory is also called the catenary theory because of its relation to the mathematical catenary model. After contraction of the ciliary muscle, the diaphragm—including the crystalline lens, capsule, vitreous and zonule—is pushed forward because of the change in pressure gradient between the posterior and anterior segments.

More recently, due to the development of imaging technology enabling access to micro intraocular structures, scientists have developed interest in the extralenticular aspect of accommodation. Using UBM technology, Croft et al. studied the role of the vitreous, zonule, and sclera. The architecture of the zonule fibers being more complex than previously assumed, Lütjen-Drecoll et al. showed that some zonule fibers were directly connected to the vitreous membrane and the posterior part of the lens, thus questioning the exact role of the sclera, choroid, and vitreous during accommodation.