CLINICAL SUBTYPES OF CONE-ROD DYSTROPHY

Objective.-To determine possible distinct phenotypic subtypes of cone- rod dystrophy. Patients.-Thirty-three patients with cone-rod dystrophy (from 25 families) were assessed prospectively on electroretinography , visual field testing, psychophysical threshold profiles, and fundus features. The clinical records of an additional 150 patients with cone -rod dystrophy were examined retrospectively in terms of the classific ation schema derived from the prospective study. Results.-Based on ele ctroretinographic recordings, two major types of cone-rod dystrophy we re differentiated. In type 1, cone amplitudes were reduced to a greate r degree than were rod amplitudes on electroretinography, while in typ e 2, cone and rod electroretinographic amplitudes were reduced in equa l proportion. These two types were further subdivided on the basis of patterns of visual field loss and threshold elevation. In type la, the re was a central or paracentral scotoma, and cone thresholds were more elevated centrally than peripherally. In type lb, there was no centra l scotoma, and cone thresholds were more elevated peripherally than ce ntrally. In type 2a, there was a central scotoma, cone thresholds were more elevated centrally than peripherally, and rod thresholds were mo re elevated peripherally than centrally. in type 2b, a partial or comp lete ring scotoma was present, cone thresholds were more elevated peri pherally than centrally, and rod thresholds were more elevated in the midperipheral than in either the central or far peripheral region of t he retina. Of the 150 additional patients with cone-rod dystrophy, dat a sufficient for classification were available for 95 patients, and al l but two had findings that were consistent with classification into o ne of these four subtypes. Conclusion.-Our results identify four funct ionally distinct subtypes of cone-rod dystrophy that may be useful for patient counseling and future molecular genetic studies.