The development of techniques for identifying the genetic bases for disease has dramatically increased our understanding of eye diseases. The prevalence of genetic disease as the basis for significant visual loss is uncertain and a function of the population, healthcare resources, and cultural values. Genetic factors may influence development of the eye but also may cause a progressive deterioration over time. Although a specific mutation of a gene may result in a consistent phenotype, variation is common and may reflect the effect of other genes or environmental factors. Complex diseases, such as myopia and strabismus, probably have both genetic and environmental bases. Ascertainment bias, inconsistent data collection, or cultural factors influence population-based studies. Autosomal recessive disorders are more common in cultures with high rates of consanguinity (Middle East or Southeast Asia). Other disorders, such as the group of diseases called retinitis pigmentosa, have a more uniform geographic prevalence rate of 20 to 40 per 100,000 worldwide. With the completion of the Human Genome Project (http://www.ornl.gov/sci/techresources/Human_Genome/home.shtml) and the rapid development of improved analytical tools, a new understanding of the genetic bases of eye diseases offers the hope of understanding causes and identifying cures.
Mutations of genes expressed primarily or exclusively in the eye cause a multitude of diseases. Some disorders are multisystemic. A genetic basis should be identified for visual loss or malformations that are not attributable to infections or trauma to optimize both treatment of the patient and counseling of the family. The evaluation should be initiated with a careful family history and pedigree. The parents should be queried about consanguinity. It is helpful to identify the anomalies commonly encountered in genetic eye diseases and syndromes (eg, kidney abnormalities and general dysmorphic features such as abnormal interpupillary distance, epicanthal folds, and palpebral fissure slant). The ophthalmologist increasingly depends on the primary care physician to make an accurate diagnosis, and the primary care physician/geneticist depends on the ophthalmologist for information that may be helpful in rendering a diagnosis and prognosis. Consultation with a board-certified geneticist is wise. The European Union requires referral of patients with possible genetic diseases to a board-certified geneticist (Oviedo Convention, 1997, Treaty of Lisbon, 2009) in countries with such certification.
This chapter will deal with some genetic diseases of the eye. Cataract, glaucoma, and corneal diseases often have a genetic basis and are discussed in Chapter 580. Section 12 includes multisystem genetic disorders that have ocular manifestations (eg, neurofibromatosis in Chapter 176).
Major structural anomalies occur in 2% to 3% of all newborns, and minor anomalies occur in 15%. The eye begins as a groove in the forebrain at 22 days of gestation. Histologically, the lens placode is evident by 4 weeks with a lens vesicle in the optic cups by 5 weeks. The groove on the ventral surface of the optic cup closes by 7 weeks, closing the optic nerve ...