Peripheral cone dystrophy: a variant of cone dystrophy with predominant dysfunction in the peripheral cone system

To determine the phenotype of 3 patients from 2 pedigrees with an unusual form of cone dystrophy in which the peripheral cone system is more affected than the central cone system, and whose rod system is relatively normal.

Three observational case reports with electrophysiologic and psychophysical studies.

Three patients underwent fundus photography, fluorescein angiography, color vision testing, Goldmann visual field testing, full-field electroretinograms (ERGs), focal macular cone ERGs, and rod-cone perimetry, in addition to routine ophthalmologic examinations. Multifocal ERGs also were recorded from 2 patients.

The fundus examination and fluorescein angiogram results essentially were normal except for a mild temporal pallor of the optic disc in 2 patients. The corrected visual acuity ranged from 20/16 to 20/100. Color vision was normal in 1 patient, but was abnormal in 2 patients. A relative paracentral scotoma was detected in 2 patients. Full-field ERG cone responses were reduced significantly, but rod responses were normal in all patients, as in patients with typical cone dystrophy. However, the focal macular cone ERGs were well preserved in all patients. Psychophysical rod-cone perimetry demonstrated that the peripheral cone system was impaired, whereas the rod sensitivity was completely normal. The results of the multifocal ERG in 2 patients supported the findings made by the full-field and focal macular ERGs.

These findings demonstrate that there is a subgroup of patients with cone dystrophy where the peripheral cone system is more affected than the central cone system.