Night blindness is a visual defect caused by lack of rhodopsin in the rods of the retina located in the back of the eyeball due to inadequate vitamin A.
Q: How does inadequate vitamin A cause a lack of rhodopsin in the eyeball?
A: Rhodopsin is a visual pigment that requires vitamin A as a component.
Night blindness is characterized by decreased ability to see at night or in the dark because rhodopsin is a chemical required for the retina to distinquish light and dark. It is an early feature of vitamin A deficiency and also decreased content of oxygen in the blood such as anemia.
With increasing vitamin A deficiency, damage to the rods and to the eyeball itself occurs because these tissues also require vitamin A for structure and function.
Night blindness can be brought on by poor diet, gastroenteritis and fever due to increased requirements for vitamin A if stores in the liver are low, malabsorption, and systemic disease that interferes with storage in the liver or transport in the blood.
Breastfeeding infants of mothers who are vitamin A deficient are at high risk since they will not have adequate liver stores and cannot get enough from mother’s milk.
In teenagers it can be the beginning symptoms of retinitis pigmentosa, a group of retinal degenerations that primarily affect the rod photoreceptors.1 
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Gordon K. Klintworth, MD, PhD. The Eye Pathologist.com ↩