SUPPRESSION OF MELATONIN SECRETION IN SOME BLIND PATIENTS BY EXPOSURETO BRIGHT LIGHT

Background. Complete blindness generally results in the loss of synchr onization of circadian rhythms to the 24-hour day and in recurrent ins omnia. However, some blind patients maintain circadian entrainment. We undertook this study to determine whether some blind patients' eyes c onvey sufficient photic information to entrain the hypothalamic circad ian pacemaker acid suppress melatonin secretion, despite an apparently complete loss of visual function. Methods. We evaluated the input of light to the circadian pacemaker by testing the ability of bright ligh t to decrease plasma melatonin concentrations in 11 blind patients wit h no conscious perception of light and in 6 normal subjects. We also e valuated circadian entrainment over time in the blind patients. Result s. Plasma melatonin concentrations decreased during exposure to bright light in three sightless patients by an average (+/-SD) of 69+/-21 pe rcent and in the normal subjects by an average of 66+/-15 percent. Whe n two of these blind patients were tested with their eyes covered duri ng exposure to light, plasma melatonin did not decrease. The three bli nd patients reported no difficulty sleeping and maintained apparent ci rcadian entrainment to the 24-hour day. Plasma melatonin concentration s did not decrease during exposure to bright light in seven of the rem aining blind patients; in the eighth, plasma melatonin was undetectabl e. These eight patients reported a history of insomnia, and in four th e circadian temperature rhythm was not entrained to the 24-hour day. C onclusions. The visual subsystem that mediates the light-induced suppr ession of melatonin secretion remains functionally intact in some sigh tless patients. The absence of photic input to the circadian system th us constitutes a distinct form of blindness, associated with periodic insomnia, that afflicts most but not ail patients with no conscious pe rception of light.