Figure 23-8 Flattening of the lens equator (with dislocation), which may be referred to as lens coloboma.
Dislocated Lenses in Children
When the lens is not in its normal anatomical position, it is said to be dislocated, subluxed, subluxated, luxed, luxated, or ectopic. Luxed or luxated lenses are completely detached from the ciliary body; they are free in the posterior chamber (Fig 23-9), or they have prolapsed into the anterior chamber. The amount of dislocation can vary from slight displacement with minimal iridodonesis (tremulousness of the iris) to severe displacement, with the lens periphery not visible through the pupillary opening. Lens dislocation can be familial or sporadic, or it can be associated with multisystem disease or an inborn error of metabolism (Table 23-4). Lens dislocation can occur with trauma, usually involving significant injury to the eye, but this is not common. Spontaneous lens dislocation has been reported rarely both in aniridia and in buphthalmos associated with congenital glaucoma.
Figure 23-9 Lens dislocation into vitreous.
Table 23-4
Isolated Ectopia Lentis
In simple ectopia lentis, the lens is displaced superiorly and temporally. The condition is usually bilateral and symmetric. Most commonly, it is inherited as an autosomal dominant trait. Onset can be at birth or later in life. Glaucoma is common in the late-onset type.
Ectopia Lentis et Pupillae
Ectopia lentis et pupillae is a rare autosomal recessive condition in which there is bilateral displacement of the pupil, usually inferotemporally, and dislocation of the lens in the opposite direction (Fig 23-10A). Affected patients have small, round lenses (microspherophakia), miosis, and poor pupillary dilation with mydriatics. The condition may be the result of a membrane extending from a posterior origin to attach to the proximal pupil margin (Fig 23-10B). Traction from the membrane may cause iris distortion and lens subluxation in the opposite direction due to disruption of zonules. Some family members may have subluxation only and no pupillary displacement. The condition is nonprogressive.
Figure 23-10 A, Ectopia lentis et pupillae. B, This 50-MHz ultrasonography scan shows a membrane posterior to the iris
attaching at the pupil margin. (Part A reproduced with permission from Byles DB, Nischal KK, Cheng H. Ectopia lentis et pupillae. A hypothesis revisited. Ophthalmology. 1998;105(7):1331–1336. Part B courtesy of Ken K. Nischal, MD.)
Marfan Syndrome
Marfan syndrome is the systemic disease most commonly associated with subluxated lenses. The syndrome consists of abnormalities of the cardiovascular, musculoskeletal, and ocular systems. It is inherited as an autosomal dominant trait, but family history is negative in 15% of cases. Marfan syndrome is caused by mutations in the fibrillin gene on chromosome 15, which is the major constituent of extracellular microfibrils. Affected patients are characteristically tall, with long limbs and fingers (arachnodactyly); loose, flexible joints; scoliosis; and chest deformities. Cardiovascular abnormalities are a source of significant mortality and manifest as enlargement of the aortic root, dilation of the descending aorta, dissecting aneurysm, and floppy mitral valve. The life expectancy of patients with Marfan syndrome is about half that of the normal population.
Ocular abnormalities occur in more than 80% of affected patients, with lens dislocation being the most common. In approximately 75% of cases, the lens is subluxated superiorly. Typically, the zonules that are visible are intact and unbroken. Examination of the iris usually shows iridodonesis and may reveal transillumination defects that are more marked near the iris base. The pupil is small and dilates poorly. The corneal curvature is often decreased. The axial length is increased, and affected patients are frequently myopic. Retinal detachment can occur spontaneously, usually in the second and third decades of life.
Homocystinuria
Homocystinuria is a rare autosomal recessive condition that occurs in approximately 1 in 100,000 births. The classic form is caused by an abnormality in the enzyme cystathionine β-synthase, but it can be caused by other enzyme defects. This abnormality causes homocysteine to accumulate in the plasma and to be excreted in the urine.
The clinical manifestations of homocystinuria vary and can affect the eye, skeletal system, central nervous system, and vascular system. Most of the abnormalities develop after birth and become progressively worse with age. The skeletal features are similar to those of Marfan syndrome.
Affected patients are usually tall, with osteoporosis, scoliosis, and chest deformities. Central nervous system abnormalities occur in approximately 50% of patients; intellectual disability and seizures are the most common.
Vascular complications are common and secondary to thrombotic disease, which affects large or medium-sized arteries and veins anywhere in the body. Partial or complete vascular obstruction is present in various organs. Hypertension and cardiomegaly are common. Anesthesia carries a higher risk for patients with homocystinuria because of thromboembolic phenomena; thus, this disorder should be identified before patients undergo general anesthesia.
The main ocular finding is lens subluxation, most frequently inferiorly, but the direction of subluxation can vary and is not diagnostic. Subluxation typically begins between the ages of 3 and 10 years. The lenses may dislocate into the anterior chamber, a finding suggestive of homocystinuria (Fig 23-11). The lens zonules are frequently broken, in contrast with those in Marfan syndrome.
Figure 23-11 Homocystinuria. The lens may dislocate into the anterior chamber with acute pupillary block glaucoma.
Diagnosis is confirmed by the detection of disulfides, including homocystine, in the urine. The medical management of homocystinuria is directed toward normalizing the biochemical abnormality. Dietary management (low methionine and supplemental cysteine) has been attempted, and coenzyme supplements (pyridoxine) decrease systemic problems in approximately 50% of cases.