The subjective examination of the sensitivity of the retina to light creates many problems, more especially in tropical countries where the pattern of clinical practice as seen in the West is made more intricate by additional diseases. Although a primary deficiency in vitamin A remains the most important cause of night blindness, there are other possibilities, even beyond the growing recognition of the fact that the relation of avitaminosis A to hemeralopia is not as straightforward as was at first believed. It has been shown in America, for example, that exposure to a bright sun for long periods by a cumulative effect raises the threshold for many hours afterwards (Hecht, Hendley, Ross, and Richmond, 1948). Debilitation, common in the tropics, especially when the harvest is poor, might itself lead to a primary reduction in light sensitivity or alternatively interfere with the subject's concentration in the course of what
RODGER FC, DHIR PK, HOSAIN ATMM. Night Blindness in the Tropics. AMA Arch Ophthalmol. 1960;63(6):927–935. doi:10.1001/archopht.1960.00950020929006
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