Hair-on-end | Radiology Signs

Answer:

The hair-on-end appearance is a fine perpendicular striation of the skull diploë, in which long thin trabeculae stand at right angles to the inner table within a widened diploic space, while the outer table is thinned or lost. It is a marker of chronic marrow hyperplasia, classically in thalassaemia major and sickle cell disease, and it spares the occipital squama below the internal occipital protuberance because that region contains little marrow. Other causes include iron deficiency anaemia, hereditary spherocytosis, G6PD deficiency, chronic cyanotic congenital heart disease, long-term G-CSF therapy, and metastatic neuroblastoma. It differs fundamentally from a sunburst periosteal reaction: sunburst spicules are new bone laid down outside the cortex by an aggressive lesion such as osteosarcoma, whereas hair-on-end is remodelling of pre-existing trabeculae within the diploic space, driven from inside by expanding marrow.

Why is it called so?

The fine parallel vertical trabeculae standing off the inner table look like hair standing on end.

Pathophysiology

Chronic haemolysis or ineffective erythropoiesis drives compensatory expansion of red marrow. In the skull, the marrow-containing diploic space is bounded by two rigid cortical tables, so the expanding marrow widens the diploë and remodels the trabeculae within it. The outer table, being under the least mechanical constraint, is progressively thinned and may disappear; the surviving trabeculae are reoriented and thickened along the axis of expansion, that is perpendicular to the inner table. The characteristic sparing of the lower occiput reflects its lack of haematopoietic marrow, and the same marrow expansion produces the facial bone changes and the chipmunk facies of thalassaemia major.

Alternative names: Hair-on-end sign, crew cut appearance, brush skull

Other associated named signs: Chipmunk facies of thalassaemia, rodent facies, bone-within-a-bone, and H-shaped vertebrae

References

1. Walor DM, Berdon WE, Westra SJ. ‘Hair-on-end’ skull changes resembling thalassemia caused by marrow expansion in uncorrected complex cyanotic heart disease. Pediatr Radiol. 2005;35(7):698-701. PMID: 15776229.