On a lateral skull X-ray, a 'hair on end' trabecular pattern is seen in the diploic space of the skull vault. Which condition is classically associated with this radiological sign?

• A Paget's disease of bone
• B Fibrous dysplasia — ground glass matrix
• C Multiple myeloma — 'punched out' lytic lesions
• D Thalassaemia major — expanded marrow due to compensatory erythropoiesis ✓

Explanation

The 'hair on end' (or 'sun ray') pattern on skull X-ray results from expanded diploic space due to marked medullary erythroid hyperplasia driving the outer table outward with perpendicular trabecular striations. This is classically seen in thalassaemia major and sickle cell disease, where chronic hemolysis drives compensatory bone marrow expansion. The same pattern can occur in severe iron-deficiency anemia and congenital hemolytic anemias. Paget's shows 'cotton wool' appearance. Fibrous dysplasia shows ground-glass density. Myeloma shows multiple discrete lytic 'punched-out' lesions without a hair on end pattern.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.