Pigmentary retinal degeneration, as seen through the ophthalmoscope, was first described by van Trigt in 1853 and named retinitis pigmentosa by Donders in the Netherlands.⁴⁰ The term retinitis pigmentosa (RP) has traditionally included a group of hereditary retinal degenerations with characteristic features. These features include night blindness, progressive field constriction with relative preservation of macular function, and pigmentary disturbances within the posterior pole (figure 69.1). The prevalence of RP in different countries varies from one in 3000 to one in 4000 individuals.¹⁰⁰ The number of affected individuals in the United States is estimated to be between 50,000 and 100,000. Approximately 20% of these cases are autosomal-dominant, 10% are X-linked, 20% are autosomal-recessive, and the remainder are isolated (simplex RP; no known family history).³⁰ Most patients with RP are nonsyndromic; that is, they do not have any other associated systemic disease. The most common exception is Usher syndrome, which accounts for approximately 10–15% of RP and is associated with either profound (Type I), partial (Type II), or, extremely rarely, progressive (Type III) hearing loss. Other syndromic conditions with associated RP include Bassen-Kornzweig syndrome (abetalipoproteinemia), Refsum disease, Laurence-Moon-Bardet-Biedl syndrome, neuronal ceroid lipofuscinosis (Batten disease), Alström disease, and Kearns-Sayre syndrome.

Figure 69.1.  

Fundus photograph showing the posterior pole of a 42-year-old patient with XlRP. Note the “waxy disk,” the attenuated retinal vessels, and the bone spicule–like pigmentary deposits throughout the midperiphery.

The term retinitis pigmentosa has been retained for historical reasons even though it is merely descriptive and inappropriate, since it implies an inflammatory condition. In fact, this descriptive term for the entire category of diseases will gradually be replaced by specific mechanistic disease names reflecting the disease-causing mutation. At the present time, we have a transitional situation, in which we are still trying to force patients into descriptive categories such as retinitis pigmentosa, cone-rod dystrophy, and pattern dystrophy. In many cases, the fits do not work, and even within a single family with a RDS-peripherin mutation, for example, we may find individuals with different “diagnoses.”^64,112 As knowledge evolves, it will become increasingly more informative to describe patients with names related to their mutations, such as RP1 or RDS/peripherin, than just RP. When the specific mutation is known, the patient can be counseled more accurately with respect to rate of progression. The number of known locations and specific cloned genes is constantly growing (see http://www.sph.uth.tmc.edu/retnet). In the summary that follows, the current state of knowledge with regard to genes that cause retinitis pigmentosa will be presented. In the second part of the chapter, a summary will be given of some of the ERG protocols that are available for characterizing phenotype. It should become clear that one current challenge is to broaden the scope of the ERG and ancillary functional techniques to enrich the description of phenotype.