Hair-on-End Sign
A 35-year-old man with a history of sickle cell disease presented with a sickle cell crisis and headaches. Noncontrast computed tomography (CT) of the head revealed a thickened calvaria with perpendicular proliferation of the trabeculae, a finding termed the hair-on-end sign (Panels A and B). The hair-on-end sign was historically described on plain films (although it can also be seen on CT and magnetic resonance imaging) in patients with severe anemias, primarily thalassemia major and, less commonly, sickle cell disease. It has more rarely been described in patients with hemolytic anemia, spherocytosis, chronic iron deficiency, or cyanotic congenital heart disease. The appearance is due to anemia causing red marrow proliferation and a widened diploic space with thinning of the outer table of the skull. Destruction of the vertical trabeculae occurs in conjunction with thickening of the residual trabeculae. When the outer table is disrupted, new spicules also form, which results in this alternating pattern of new spicules amid nonradioopaque marrow. The patient’s headache resolved quickly in response to treatment with pain medication and did not recur during his hospital stay.