Myopic shift of refractive error in retinitis pigmentosa

Purpose. Retinitis Pigmentosa (RP) is characterized by a significant myopic shift of the mean spherical refractive error. The goal of this study was to calculate the distribution of the myopic refractive error in a group of a pure RP patients, and to assess the influence of myopia on morphological aspects of RP. Methods. 244 subjects (471 eyes) with non syndromic, typical RP were studied on the basis of distinct refractive classes: hyperopia; emmetropya; low, medium and high myopia. Sex, age, visual acuity and typical myopic alterations at the posterior pole were tabulated for each class. Associations between the tabulated categories were calculated and statistically evaluated by the c2 test. Results. 179 eyes were emmetropic, 64 eyes were hyperopic and 228 (48.4%) eyes were myopic. 65.6% of the total RP group was emmetropic or with low myopia. In all groups, but one, no significant associations between refractive defect and ophthalmoscopic changes were found. Sclerectasy and peripapillary atrophy were significantly found in eyes with myopia greater than 8 diopters (p=.005). Conclusions. In most of the patients no myopic changes were ophthalmoscopically detected, but for the higher myopic group. Two hypotheses, optical and metabolic, were formulated to explain the high prevalence of emmetropic and low myopic eyes in this sample. From an optical point of view, RP patients could require a myopic correction to better define objects in the constricted visual field. Metabolic hypothesis originated from evidences of the experimental literature which suggested that dopamine release defects and/or growth factors anomalies are involved in the deprivation myopia. These metabolic alterations were also found in degenerative retinas. Could these two aspects linked to explain the high prevalence of myopia in RP?