Vitamin A deficiency (hypovitaminosis A) is a pathological condition caused by a lack of retinol due to dietary restriction, malabsorption, transport defects, or hereditary increased metabolic demand. The pathology leads to impaired night blindness, epithelial metaplasia of the respiratory tract, skin lesions, and systemic disorders of bone formation.
RetیناNyctalopia develops due to rod dystrophy and rhodopsin deficiency
MetaplasiaReplacement of simple columnar epithelium with stratified squamous epithelium
Supporting SystemInhibition of osteogenesis and chondrogenesis with bone tissue destruction
GeneticsIn Darier's disease, a genetic defect creates a high demand for retinol
Etiological Forms and Pathogenetic Mechanisms
Retinol deficiency is divided into two main categories: genetically determined and acquired forms.
Genetically determined forms are characterized by a hereditary increased physiological demand for retinol. A prominent example is Darier's disease (follicular dyskeratosis). Due to a genetic defect, patients require high doses of vitamin A. This condition is characterized by the following symptoms:
Skin: pronounced dryness and formation of thickened crusts.
Mucous membranes: dryness of the oral mucosa.
Skin appendages: longitudinal ridging and notches of the nail plates.
Neuropsychiatric status: predisposition to psychoses and decreased intellectual development.
Acquired forms are common and clinically significant. Their etiology includes:
Dietary deficiency: lack of $\beta$-carotene or retinol in the diet.
Malabsorption: gastrointestinal absorption disorders. Bile acids are critical for the emulsification and assimilation of this fat-soluble vitamin.
Transport disorders: disruptions in the delivery system of retinol to target cells.
Clinical Manifestations of Retinol Deficiency
Vitamin A deficiency leads to lesions of the visual organ, respiratory system, and skin:
Nyctalopia ("night blindness"): decreased visual acuity in dim light. The pathogenesis is based on dystrophic changes in the rods of the retina caused by impaired rhodopsin synthesis.
Metaplasia of the respiratory epithelium: normal simple columnar epithelium is replaced by stratified squamous epithelium. This leads to a loss of the barrier function of the respiratory tract and frequent infectious diseases.
Dermatological disorders: dryness and roughness of the skin (hyperkeratosis) develop. The lesions are localized mainly on the extensor surfaces of the elbow and knee joints.
Impact on Systemic Metabolism and Bone Tissue
At the general pathological level, retinol deficiency drives profound disorders in the structure of the osteocartilaginous system and proteins:
Proteosynthesis (protein synthesis) processes are inhibited.
Osteogenesis and chondrogenesis are suppressed.
Destruction of bone and cartilage tissue progresses.
Osteoporosis develops, accompanied by calcification of internal organs.
Chronic Vitamin A Toxicity (Hypervitaminosis A)
Excessive long-term intake of retinol causes hypervitaminosis affecting most body systems:
Skin and hair: pigmentation, peeling, and skin cracks appear (especially pronounced on the palms and soles). Nails and hair become dry and brittle, and alopecia (hair loss) develops.
Musculoskeletal system: joint and bone pain occur, worsening with walking.
Visceral organs:hepatomegaly and splenomegaly (enlargement of the liver and spleen), loss of appetite, weight loss, nausea, and vomiting are observed.
Neurological status: increased irritability and insomnia are characteristic.
Mnemonic
Remember the primary targets of vitamin A deficiency using the "V-E-B" rule: Vision (nyctalopia), Epithelium (metaplasia), Bones (suppressed osteogenesis).
Frequently asked questions
Which specific transport proteins deliver retinol to target cells in blood plasma?
The provided sources describe transport disorders in vitamin A deficiency as defects in vitamin delivery to target cells, but the specific set of proteins providing precisely retinol delivery in blood plasma is not detailed.
It directly follows from the sources that acute-phase transport proteins include: prealbumin, albumin, orosomucoid, lipocalins, haptoglobin, transferrin, mannose-binding and retinol-binding proteins, etc. Their function is the transport of metabolites, metal ions, and physiologically active factors.
Why do signs of vitamin A deficiency develop in biliary tract pathology?
Vitamin A is fat-soluble. Bile acids are required for its emulsification and successful intestinal absorption; their deficiency leads to malabsorption.
What is the mechanism behind the development of night blindness?
Nyctalopia occurs due to impaired synthesis of the visual pigment rhodopsin, leading to dystrophic changes in the retinal rods and a drop in scotopic vision.
What are the skin manifestations of follicular dyskeratosis (Darier's disease)?
Pronounced skin dryness with thickened crusts, dryness of the oral mucosa, and longitudinal ridging and notches of the nail plates are observed.
How does the toxic effect of excess vitamin A manifest on the nervous and visceral systems?
Chronic intoxication leads to insomnia, increased irritability, nausea, vomiting, loss of appetite, weight loss, as well as hepatomegaly and splenomegaly.
Go deeper
Pathogenesis of clinical manifestations in Darier's disease
Role of bile acids in the emulsification and absorption of fat-soluble vitamins
Mechanisms of columnar epithelium metaplasia into stratified squamous epithelium
Disorders of osteogenesis and chondrogenesis in retinol deficiency
Differential diagnosis of vitamin A deficiency and chronic retinol intoxication