Rod and rod mediated function in patients with beta-thalassemia major.

An electroretinographic (ERG) study was undertaken to test the hypothesis that scotopic retinal function is altered in transfused thalassemics on chronic Deferoxamine (DFO). ERG a- and b-wave responses and dark adapted visual thresholds were obtained from 11 patients with beta-thalassemia major, ages 7 to 38 (median 17) years. A quantitative model of the activation of phototransduction was fitted to the a-waves to estimate the gain of the transduction processes and the saturated amplitude of the rod photoresponse. From b-wave stimulus/response functions. the saturated b-wave amplitude and an index of b-wave sensitivity (log sigma ) were calculated. The patients' data were compared to those of normal subjects. The relations of the ERG parameters to age. average ferritin level, and duration of transfusion without DFO as well as other clinical parameters were examined. Longitudinal measures of b-wave responses and dark adapted visual thresholds. available for nine of the patients, were examined for significant change over time. For all patients both the gain and saturated amplitude of the rod response are normal. In two patients log sigma is below the 99% prediction interval for normal. One has low scotopic visual sensitivity. The duration of transfusion therapy unprotected by DFO chelation therapy was correlated with log a. These results suggest iron accumulation rather than DFO toxicity underlies scotopic dysfunction in older thalassemics. some of whom may have had extended periods of transfusion without the protection of chelation. Thus, monitoring of retinal function is recommended in such patients.