Expansion
Vision, epithelium and differentiation, with teratogenicity in excess
Expansion
Three functions
- Vision: retinal combines with opsin to form rhodopsin in rod cells. This is the most sensitive function, so night blindness is the earliest deficiency symptom
- Epithelial integrity and mucus secretion
- Cell differentiation and growth, through retinoic acid acting on nuclear receptors
Deficiency, a leading cause of preventable childhood blindness worldwide
- Night blindness (nyctalopia), the earliest
- Xerophthalmia, conjunctival and corneal drying
- Bitot’s spots, foamy conjunctival plaques
- Keratomalacia, corneal ulceration and perforation
- Follicular hyperkeratosis and increased infection, particularly measles mortality
Toxicity, because it is fat soluble and stored
- Acute: nausea, vomiting, raised intracranial pressure, and desquamation
- Chronic: dry skin, hair loss, hepatotoxicity, bone pain and hypercalcaemia
- Teratogenicity: craniofacial, cardiac and central nervous system defects
“Retinoids are teratogenic, which is the whole reason for the isotretinoin pregnancy prevention programme”, requiring two forms of contraception and monthly pregnancy testing, with pregnancy avoided for a month afterwards. Acitretin requires 3 years.
Liver and cod liver oil are so rich in vitamin A that they are avoided in pregnancy.
Therapeutic uses: all-trans retinoic acid in acute promyelocytic leukaemia, isotretinoin in acne, topical retinoids in photoageing.