The lens is the normally clear structure that is located just posterior to the iris. The primary function of the lens is to focus light rays on the retina. The lens is attached to the ciliary body by zonules. These zonules produce tension in response to contraction of the ciliary muscle. The ciliary muscle is circular, so that when it contracts its diameter becomes smaller (Figure 30–1). This decreases the tension on the lens zonules, and the lens becomes more spherical. This is known as accommodation, which is what allows the eye to focus at near. As people age, the lens becomes progressively stiffer. Because of this, the ability to focus at near gradually deteriorates, to the point that almost all people require reading glasses during the fourth decade of life.

The lens consists of the anterior and posterior lens capsules, which are basal lamina produced by the lens epithelium. The lens epithelium lines the inner capsule. The epithelial cells at the equator of the lens continue to divide and produce lens fibers throughout life. The central portion of the lens (the nucleus) is formed by birth, and the surrounding portion of the lens (the cortex) is produced postnatally (Figure 30–2). The cytoplasm of lens cells contains crystallins, which produce the light-focusing properties of the lens.

Cataracts are present if the lens is not clear. Cataracts may range from mild to severe, and their effect on vision can range from minimal to profound. Cataracts are very common in adults as they age, but they are uncommon in children. However, they do represent a significant cause of visual morbidity in children, accounting for approximately 10% of childhood vision impairment.

In addition to cataracts, rare lens disorders include aphakia (absence of the lens), microspherophakia (an abnormally small, round lens), ectopia lentis (decentration of the lens), and lens coloboma (a partial defect in the periphery of the lens due to absence of adjacent zonules, usually associated with colobomas of the iris and retina).

Embryology

The lens initially develops as a thickening of the surface ectoderm overlying the optic vesicle. This tissue invaginates and separates to form the circular lens vesicle. The outer portion of this vesicle becomes the lens capsule. The cells on the posterior surface of the vesicle elongate and fill the vesicle, which becomes the embryonic nucleus (Figure 30–3). Epithelial lens cells at the equator then divide and produce secondary lens fibers, which gradually move toward the center to form the Y-sutures. These cells continue to proliferate throughout life.

The embryological formation of the lens is analogous to rings on a tree, with continuous layering of lens fibers. Because of this, the timing of intrauterine disturbances of lens growth can be inferred by the location of the abnormality: early disturbances create central lens opacities and later disturbances affect the peripheral lens.

Cataracts are considered visually significant if they substantially interfere with ability of the lens to focus light. There are a wide variety of etiologies and appearances of cataracts. They may be unilateral or bilateral. They may be present at birth or develop later in life. Unilateral infantile cataracts are usually sporadic and often have no identifiable etiology. Bilateral infantile cataracts are more likely to be associated with other disorders. Approximately one-third of bilateral infantile cataracts are hereditary, approximately one-third are associated with systemic diseases, and approximately one-third are idiopathic. Cataracts in older children may be idiopathic, but are more likely to have an identifiable cause, such as trauma, external beam radiation, or medications (such as corticosteroids).

Heritable cataracts are most commonly inherited as an autosomal dominant trait, but autosomal recessive and X-linked forms also occur. A large number of genetic mutations have been identified in patients with familial cataracts. Currently there are over 350 entities associated with cataracts in the Online Mendelian Inheritance in Man database. The age at which inherited cataracts appear varies from infancy to young adulthood.

Many systemic disorders are associated with cataracts, particularly those that present in infancy. These include metabolic diseases, chromosomal aberrations, and congenital infections (Table 30–1). Infants with bilateral cataracts require a medical evaluation to search for these problems (discussed below) (Table 30–2).

Unilateral ectopia lentis is often related to ocular trauma. Bilateral ectopia lentis may occur as an isolated finding, but is often associated with one of several specific systemic disorders (discussed below) (Table 30–3).

In addition to their direct effects on vision, cataracts and other lens disorders in children may also cause visual loss due to amblyopia. This is particularly true in young infants with unilateral cataracts. The onset of amblyopia in this setting is rapid and profound, and early intervention is necessary to maximize the chance for a good visual outcome.

Cataracts are most commonly noted in infants and young children by their interference with the red reflex. Dense cataracts may appear as white opacities in the normally black pupil, which may be directly visible to the parents (Figure 30–4). Cataracts are usually best detected by an abnormality of the red reflex (Figure 30–5).

If cataracts are not visualized directly, they may cause other problems that ultimately lead to their detection. Infants with bilateral significant cataracts cannot fixate normally, and nystagmus develops at 2 to 4 months of age. Infants and young children with unilateral cataracts generally ignore the poor vision in the affected eye, and they can function normally by using only the good eye. They may develop strabismus due to the poor vision, which is sometimes the first abnormality that brings the cataract to attention.

The appearance of cataracts is highly variable. The opacities can occur anywhere from the front surface of the lens to the back of the lens. The appearance of these various morphologies may assist in the differential diagnosis.

Beginning at the front of the lens and moving posteriorly:

Anterior Polar Cataracts

Anterior polar cataracts are small, white opacities on the anterior surface of the lens (Figure 30–6). They are present at birth and may be unilateral or bilateral. They are not progressive and are usually small enough that they do not directly interfere with vision. However, affected patients have an increased risk of amblyopia due to asymmetric refractive errors.

Nuclear Cataracts

These cataracts affect the central portion of the lens, usually resulting from a perturbation of lens formation early in its embryological development. These may be inherited as an autosomal dominant trait (Figure 30–7A and B). The effect on vision depends on the density of the opacity.

Lamellar Cataracts

Lamellar cataracts affect one or more layers in the midperiphery of the lens, and are therefore larger than nuclear cataracts. These develop during a later stage of lens formation than nuclear cataracts.

Posterior Subcapsular Cataracts

These appear as irregular white opacities on the back surface of the lens. These may be present at birth, but are more frequently acquired. Traumatic cataracts and corticosteroid-induced cataracts are most commonly of this type. External beam radiation may also cause posterior subcapsular opacities (Figure 30–8).