Scotopic thresholds and plasma retinol in cystic fibrosis.

Patients with cystic-fibrosis (CF) often have low plasma concentrations of vitamin A. We have measured dark-adapted scotopic thresholds of 56 patients with CF, ages 4 to 34 years, either with a two-alternative forced-choice procedure or as the final threshold after a full dark-adaptation curve. Fasting plasma vitamin A alcohol (retinol) was measured in 34 of the 56 patients. The average thresholds were higher and retinol values lower in patients than in controls. In two patients with very low (less than 7 micrograms/dl) initial retinol levels and elevated thresholds, decreased rhodopsin densities were observed; rhodopsin density and thresholds returned to normal after treatment with oral vitamin A. Rhodopsin density and log sensitivity were linearly related. Only marked decreased in plasma retinol were associated with elevations of dark-adapted threshold and decreases in rhodopsin density, suggesting that the tissues of patients with CF sequester vitamin A to maintain retinal function.