The hair on end sign refers to a radiographic appearance of the diploic space of the skull vault which results from a thickening of trabeculae as the diploic space expands. These trabeculae are perpendicular in orientation, interspersed by radiolucent marrow hyperplasia along with skull vault.
This appearance of skull is also known as crew cut. This is classically seen in diseases which require increased erythropoiesis. These diseases can be remembered by mnemonic HI NEST
H= Hereditary Spherocytosis
I= Iron deficiency anemia
E=Enzyme Deficiency (G6PD Deficiency which leads to hemolytic anemia
S= Sickle cell anemia
T= Thalassaemia Major
Apart from Neuroblastoma, all hyperplasia are caused due to increased erythropoiesis to compete the red cell demand