Night blindness or Nyctalopia is inability or difficulty in seeing in the night or poor light. While it is congenital in some, in others it is caused due to injury, certain drugs or other causes. This causes problems especially during driving at night. Persons suffering from night blindness tend to notice reduced contrast vision; as their eyes take more time to adjust from brightly lit areas to dim ones. Some causes for night blindness are cataract and nearsightedness. An ophthalmologist might conduct tests such as retinal exam, color vision testing and pupil light reflex.
Hemeralopia is the opposite of night blindness where a person is unable to see in bright light or suffer acute discomfort in the eyes. Hemeralopia can occur owing to many conditions - Cone dystrophy (genetic condition in which cones are lost in the retina), Cohen Syndrome and even Cataracts. Some drugs can cause temporary Hemeralopia in some cases. ( eg.Trimethadione) Wearing dark glasses or neutral density filters in the correcting lens could mitigate the pain.
Electroretinography or ERG is an eye test used to detect the abnormal function of the retina. The rods, cones and the ganglion cells of the eyes are examined during this test. An electrode is placed on the cornea to measure the electrical response to the light in the retina and the back of the eye. This test helps identify any defects in the retina and can help in identifying if retinal surgery is required.
Patient's eyes are dilated and anesthetic drops are placed on the eyes. Eyes are kept open using a speculum and an electrode is placed in the eye. Another electrode is placed on the skin. The patient is made to watch a standardized light stimulus or flash ERG. The signal received is measured according to its amplitude. The readings are taken when the room is normally lit and when the room is dark. If the tests are normal, it will display a normal A and B pattern for each flash.
Abnormal results can indicate:
Vitamin A is a fat soluble vitamin which is derived from beta carotene which plays a significant role in the process of vision and other important metabolic pathways pertaining to cell division and genetic expression. The significant forms of vitamin A include retinol, retinal, retinoic acid and retinyl esters. There are approximately six hundred derivatives of beta carotenes and the most important form is retinol.
Functions of Vitamin A
Vitamin A is essential for many metabolic pathways in the body. It is the chief requirement for the function of the rhodopsin protein located in the retina to absorb light and to differentiate functions of the cornea and the conjunctival membranes. Apart from this, vitamin A plays a significant role in immune system functions, cell signaling and cellular communication and reproduction. The functions and pathways associated with vitamin A are directly related to the functionality of vital organs such as heart, brain, lungs, liver and kidneys. Hence vitamin A is also known as an important antioxidant.
Food sources and recommended dietary allowance
Vitamin A is naturally available in dairy products such as milk, cheese, curd, cream. Meat products, especially fish and leafy vegetables are excellent sources of vitamin A. Other sources include pumpkin, potatoes, broccoli, cereals, beans and cow peas. Studies indicate that the intensity of the fruit or vegetable color is directly proportional to the amount of vitamin A present in it. The recommended intake of vitamin A per day for children 500 micrograms, males 1000 micrograms and females 800 micrograms respectively.
Deficiency of Vitamin A
Pregnant and lactating women, premature children, children living in rural areas of developing countries and patients who have a history of liver diseases such cirrhosis and cystic fibrosis are most susceptible to Vitamin A deficiency. The predominant form of vitamin A deficiency is nyctalopia or night blindness. This occurs as result of retinol imbalance which is the chief derivative of vitamin A. Xerophthalmia or dry eyes is due to lacrimal gland dysfunction. Other associated conditions include keratomalacia and follicular hyperkeratosis.
Bibliography / Reference
Collection of Pages - Last revised Date: September 22, 2018