Learning Objectives
- Describe the biochemical functions of Vitamin A in vision and cellular differentiation.
- Recognize the clinical presentation of Vitamin A deficiency, including ocular findings.
- Contrast acute and chronic toxicity and understand their teratogenic mechanisms.
- Identify therapeutic uses in Dermatology, Hematology, and Infectious Disease.
1. Biochemistry and Function
Vitamin A is a group of unsaturated nutritional organic compounds that includes retinol, retinal, and retinoic acid.
- Vision: 11-cis-retinal is a constituent of visual pigments (rhodopsin) in the retina.
- Differentiation: Essential for the normal differentiation of epithelial cells into specialized tissues (e.g., mucus-secreting goblet cells).
- Prevention: Prevents squamous metaplasia (the transformation of specialized epithelium into keratinized squamous epithelium).
2. Deficiency: The Ocular Sequence
Deficiency is a leading cause of preventable blindness worldwide. The symptoms typically progress in a specific order:
| Stage | Clinical Finding | Description |
|---|---|---|
| Early | Night Blindness | Nyctalopia: inability to see in low light. |
| Intermediate | Bitot Spots | Keratin debris appears as foamy, triangular patches on the conjunctiva. |
| Advanced | Keratomalacia | Corneal softening and liquefaction, leading to irreversible blindness. |
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3. Therapeutic Uses
- Measles: Supplementation in deficient children reduces morbidity and mortality (specifically respiratory and diarrheal complications).
- Dermatology: Oral Isotretinoin is used for severe cystic acne; Retin-A (topical) for wrinkles and acne.
- Hematology: All-trans retinoic acid (ATRA) treats Acute Promyelocytic Leukemia (APL) by inducing differentiation of leukemic promyelocytes.
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4. Toxicity & Teratogenicity
Because Vitamin A is fat-soluble, it accumulates in the liver and adipose tissue, leading to potential toxicity.
- Acute: Nausea, vomiting, and increased intracranial pressure (blurred vision, vertigo).
- Chronic: Alopecia, hepatic toxicity/enlargement, and idiopathic intracranial hypertension (pseudotumor cerebri).
- Teratogenicity: Interferes with Homeobox (HOX) genes during embryogenesis.
- Risks: Cleft palate, cardiac abnormalities.
- Protocol: Must have a negative pregnancy test and use two forms of contraception before starting Isotretinoin.
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