Osteomalacia

Overview

Darker skin (Fitzpatrick IV-VI) - reduced UVB penetration

Limited sun exposure - housebound, cultural dress

Elderly - reduced skin synthesis

Malabsorption - coeliac disease, IBD

Chronic kidney disease - impaired renal activation

Chronic liver disease - impaired hepatic hydroxylation

Long-term anticonvulsants - accelerated catabolism

Pregnancy and lactation - increased demand

Presentation

•Bone pain - diffuse, dull, aching; spine, pelvis, hips, lower limbs; worse with weight-bearing and direct pressure

•Proximal muscle weakness - characteristic; caused by calcitriol deficiency and impaired calcium-mediated muscle contraction

•Waddling gait - due to proximal lower limb myopathy

•Insufficiency fractures - minimal trauma; femoral neck, pubic rami, ribs, metatarsals

•Features of hypocalcaemia (severe cases) - perioral paraesthesia, tetany, Chvostek's sign, Trousseau's sign

💡

Bone pain + proximal muscle weakness in an adult should always prompt consideration of osteomalacia - this combination is far more typical of osteomalacia than osteoporosis (which is usually asymptomatic until fracture).

Investigations

•Serum 25-OH vitamin D - primary diagnostic test; low confirms vitamin D deficiency

•Serum calcium - low or low-normal

•Serum phosphate - low (PTH promotes renal phosphate excretion)

•ALP - raised (overactive osteoblasts attempting mineralisation)

•PTH - elevated (secondary hyperparathyroidism)

•Bone X-ray - Looser zones (pseudofractures), generalised demineralisation with 'fuzzy' trabecular pattern

•Renal function and LFTs - identify CKD or liver disease as underlying cause

•DXA scan - low BMD (cannot distinguish from osteoporosis without biochemistry)

•Bone biopsy with tetracycline labelling - gold standard; widened osteoid seams and reduced mineralisation front; rarely required when biochemistry is clear

🎯

Looser zones are the pathognomonic X-ray finding - bilateral, symmetrical, transverse lucencies with sclerotic borders at the medial femoral neck, pubic rami, ribs, and axillary border of the scapula.

Management

•Identify and treat underlying cause - gluten-free diet in coeliac, review anticonvulsants, manage CKD-mineral bone disorder

•Loading dose: colecalciferol ~300,000 IU total over 6-10 weeks (weekly or daily doses) - rapidly replenishes stores

•Maintenance: colecalciferol 800-2000 IU daily after loading - combined with dietary and sun exposure advice

•Calcium supplementation - if dietary intake insufficient

🥈 Second-line

•alfacalcidol - used in CKD where renal 1-alpha hydroxylase is impaired; bypasses renal activation step

•Phosphate supplementation - in renal phosphate wasting (X-linked hypophosphataemia, Fanconi syndrome)

⚠️

Bone pain may temporarily worsen in the first few weeks of treatment - warn patients. It reflects increased metabolic activity at mineralisation sites as healing begins and does not indicate treatment failure.

Follow-up

•Recheck calcium, phosphate, ALP, PTH, and 25-OH vitamin D at ~3 months to confirm biochemical response

•Bone healing (including Looser zone resolution) typically occurs within 6 months

•Recheck vitamin D levels annually in those on maintenance therapy or with ongoing risk factors