Age-Related Lens Changes
As the lens ages, it increases in mass and thickness and decreases in accommodative power. As new layers of cortical fibers form concentrically, the lens nucleus compresses and hardens (a process known as nuclear sclerosis). Chemical modification and proteolytic cleavage of crystallins (lens proteins) result in the formation of high-molecular-mass protein aggregates. These aggregates may become large enough to cause abrupt fluctuations in the local refractive index of the lens, thereby scattering light and reducing transparency. Chemical modification of lens nuclear proteins also increases opacity, so that the lens becomes increasingly yellow or brown with advancing age (Fig 5-1). Other age-related changes include decreased concentrations of glutathione and potassium and increased concentrations of sodium and calcium in the lens cell cytoplasm.
A frequent cause of visual impairment in older adults is age-related cataract, the pathogenesis of which is multifactorial and not completely understood. There are 3 main types of age-related cataracts: (1) nuclear, (2) cortical, and (3) posterior subcapsular. In many patients, components of more than 1 type are present. (See also BCSC Section 4, Ophthalmic Pathology and Intraocular Tumors.)
Some degree of nuclear sclerosis and yellowing is normal in patients older than 50 years. In general, nuclear sclerosis interferes only minimally with visual function, at least until the condition becomes severe. The central opacity causes an increased amount of light scattering, which the ophthalmologist observes as a yellow-brown central lens nucleus. A nuclear cataract (Fig 5-2) is best evaluated by using a slit-lamp biomicroscope with off-axis illumination through a dilated pupil.
Figure 5-1 Increasing yellow-to-brown coloration of the human lens from age 6 months (A) to 8 years (B), 12 years (C), 25 years (D), 47 years (E), 60 years (F), 70 years (G), 82 years (H), and 91 years (I). J, Brown nuclear cataract in a 70-year-old patient. K, Cortical cataract in a 68-year-old patient. L, Mixed nuclear and cortical cataract in a 74-year-old patient.
(Reproduced with permission from Lerman S. Phototoxicity: clinical considerations. Focal Points: Clinical Modules for Ophthalmologists. American Academy of Ophthalmology; 1987, module 8.)
Nuclear cataracts are slowly progressive. They are usually bilateral but may be asymmetric. Nuclear cataracts typically cause greater impairment of distance vision than of near vision. In the early stages of cataract development, the progressive hardening of the lens nucleus frequently causes an increase in the refractive index of the lens and a myopic shift in refraction (lenticular myopia). In hyperopic or emmetropic eyes, the myopic shift enables individuals to have improved distance vision or near vision without the use of spectacles, a condition referred to as “second sight.” A change in astigmatism and, in rare instances, a hyperopic shift can occur as the nucleus matures. Occasionally, the abrupt change in refractive index between the sclerotic nucleus (or other lens opacities) and the lens cortex can cause monocular diplopia. Progressive yellowing or browning of the lens causes patients to have poor color discrimination, especially at the blue end of the visible-light spectrum. In bilateral cases, patients are frequently unaware of their altered color discrimination.
Figure 5-2 Nuclear cataract viewed with diffuse illumination (A) and with a slit beam (B). C, Schematic of nuclear cataract.
(Part C illustration by Mark Miller.)
Visual dysfunction in low light often occurs with advancing nuclear cataract. In the most advanced cases, the lens nucleus becomes increasingly opaque and brown and is called a brunescent nuclear cataract. On histologic examination, it is difficult to distinguish the nucleus in a nuclear cataract from the nucleus of a normal, aged lens. Investigations by electron microscopy have identified an increased number of lamellar membrane whorls in some nuclear cataracts. The degree to which protein aggregates or these membrane modifications contribute to the increased light scattering of nuclear cataracts is unclear.
Excerpted from BCSC 2020-2021 series: Section 11 - Lens and Cataract. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.