Nyctalopia with normal rod function: a suppression of cones by rods.

Twenty-nine patients with exaggerated rod-cone interaction are described. All were referred because they appeared to suffer from night blindness. ERG and EOGs were performed but were normal. However, investigation with a modified dark-adaptometry technique showed that in these patients cone flicker thresholds rise considerably more during dark adaptation than is normal, and this is sufficient to explain the symptoms. In one case, the condition appears familial. Many patients report their symptoms begin in early adult life and slowly get worse, but we have no objective evidence of progression.