Hemeralopia, or day blindness, is worsening of vision in bright light (cf. nyctalopia). This phenomenon may reflect severe impairment of blood flow to the eye, such that photostressing the macula by exposure to bright light is followed by only slow regeneration of the bleached photopigments.
If due to retinal ischemia, hemeralopia may be accompanied by neovascularization of the retina. Impoverished perfusion pressure may be demonstrated by pressing on the eyeball (e.g., with the thumb) during ophthalmoscopy ("digital ophthalmodynamometry") and observing the collapse of retinal arteries: thumb pressure greater than diastolic retinal artery pressure causes intermittent collapse; thumb pressure greater than systolic pressure leads to a cessation of pulsation.
Hemeralopia may also occur in retinal diseases, such as cone dystrophies.
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