Write a short note on osteoporosis, osteomalacia and vitamin D deficiency.

Osteoporosis

Definition: a skeletal disorder of reduced bone mass and microarchitectural deterioration, leading to bone fragility and increased fracture risk. Bone is normal in quality but reduced in quantity — the key contrast with osteomalacia. Defined by WHO as a bone mineral density T-score of −2.5 or below at the spine or hip; osteopenia is a T-score between −1 and −2.5.

Causes and risk factors:

• Primary: postmenopausal (type 1) — oestrogen deficiency accelerating bone resorption; senile (type 2) — age-related.
• Secondary: corticosteroid therapy (the commonest secondary cause), hypogonadism, thyrotoxicosis, hyperparathyroidism, Cushing's syndrome, malabsorption and coeliac disease, chronic kidney and liver disease, rheumatoid arthritis, myeloma, and drugs (heparin, anticonvulsants, aromatase inhibitors, androgen deprivation therapy, proton pump inhibitors).
• Lifestyle: smoking, alcohol excess, immobility, low body weight, poor calcium and vitamin D intake, and low sun exposure — the last being unexpectedly common in urban India.

Clinical features:

• asymptomatic until fracture — "the silent disease". Presents with fragility fracture (a fracture from a fall from standing height or less) at the vertebrae, hip, distal radius (Colles') and proximal humerus
• vertebral fractures cause loss of height, thoracic kyphosis ("dowager's hump") and chronic back pain, and are frequently asymptomatic and detected incidentally.

Investigations:

• DEXA scan of hip and lumbar spine — the standard for diagnosis and monitoring
• X-rays show reduced density and vertebral wedging or crush (insensitive until 30% of bone is lost)
• FRAX score for 10-year fracture probability. Screen for secondary causes: calcium, phosphate, alkaline phosphatase, renal and liver function, 25-OH vitamin D, PTH, thyroid function, testosterone in men, serum and urine protein electrophoresis, and coeliac serology.
• In pure osteoporosis, calcium, phosphate and alkaline phosphatase are all NORMAL — abnormal values point to another diagnosis.

Management:

• Lifestyle: weight-bearing and resistance exercise, smoking cessation, reduced alcohol, adequate protein, and falls prevention — home hazard assessment, vision correction, review of sedating drugs. Preventing falls prevents fractures as effectively as drugs.
• Calcium and vitamin D supplementation as the base of all treatment.
• Bisphosphonates (alendronate, risedronate, zoledronic acid) — first-line antiresorptive. Oral agents must be taken fasting with a full glass of water, remaining upright for 30 minutes, to avoid oesophagitis. Rare risks: osteonecrosis of the jaw (dental assessment before starting) and atypical femoral fracture with prolonged use, prompting consideration of a "drug holiday".
• Denosumab (RANKL inhibitor) — potent antiresorptive; must not be stopped abruptly, as rebound vertebral fractures occur.
• Teriparatide / abaloparatide (anabolic) for severe osteoporosis or treatment failure.
• Romosozumab; raloxifene (SERM); hormone replacement therapy in selected younger postmenopausal women.
• Prevention of glucocorticoid-induced osteoporosis — assess and treat anyone expected to take significant doses for 3 months or more.

Osteomalacia and Vitamin D Deficiency

Definition: defective mineralisation of osteoid, producing soft bone. In children, before epiphyseal fusion, the same process is called rickets. The bone is abnormal in quality, with normal or increased osteoid volume — the contrast with osteoporosis.

Vitamin D physiology: cutaneous synthesis of cholecalciferol under UVB, then hepatic 25-hydroxylation and renal 1-alpha-hydroxylation to the active 1,25-dihydroxyvitamin D (calcitriol), which increases intestinal calcium and phosphate absorption.

Causes:

• Vitamin D deficiency — the commonest: inadequate sun exposure (purdah and covering clothing, indoor occupation, urban living, air pollution, dark skin), inadequate dietary intake, and malabsorption (coeliac disease, pancreatic insufficiency, post-bariatric surgery). Vitamin D deficiency is extremely prevalent in India despite abundant sunshine — a paradox worth stating in an answer.
• Impaired hydroxylation: chronic liver disease, chronic kidney disease (renal osteodystrophy), anticonvulsants (phenytoin, phenobarbitone induce catabolism).
• Phosphate deficiency: renal tubular disorders — X-linked hypophosphataemic rickets, Fanconi syndrome, renal tubular acidosis; oncogenic osteomalacia (FGF-23).
• Rare enzyme defects: vitamin D-dependent rickets types 1 and 2.

Clinical features: diffuse bone pain and tenderness (classically pelvis, ribs, spine and proximal long bones), proximal myopathy with a characteristic waddling gait and difficulty climbing stairs or rising from squatting, fractures and pseudofractures, and hypocalcaemic symptoms — paraesthesiae, tetany, positive Chvostek's and Trousseau's signs, and seizures. In children: bowing of legs, rickety rosary, widened wrists, delayed fontanelle closure, craniotabes and growth retardation.

Investigations: low or low-normal calcium and phosphate, RAISED alkaline phosphatase, LOW 25-OH vitamin D, and raised PTH (secondary hyperparathyroidism). X-rays show generalised osteopenia and Looser's zones (pseudofractures) — translucent bands perpendicular to the cortex, classically at the medial femoral neck, pubic rami, ribs and scapula. Bone biopsy with tetracycline labelling is definitive but rarely required.

Management:

• vitamin D replacement — high-dose loading followed by maintenance, using cholecalciferol for simple deficiency
• active analogues (calcitriol, alfacalcidol) where 1-alpha-hydroxylation is impaired, as in renal disease. Add calcium supplementation, correct phosphate where deficient, treat the underlying cause (malabsorption, drug review), and advise sensible sun exposure and dietary sources. Symptoms and biochemistry improve within weeks
• radiological healing takes months. Monitor calcium and renal function during high-dose treatment.