Condition in which the patient sees badly in poor light, caused by inadequate retinal rod function. Some time is always needed for accommodation when passing from light to darkness; the rods may need an hour to achieve maximum night vision. The rods may be absent congenitally, but the condition can also be caused by vitamin A deficiency, the vitamin from which chemicals in the rods are formed. Night blindness also occurs as a complication of other conditions, and as a precursor of some retinal disorders. Poor nutrition can also cause night-blindness. If night-blindness is a complication of another condition, that condition must be treated. Only if vitamin A deficiency is the cause is treatment with that vitamin effective.