Which condition is associated with a hair-on-end or crew-cut appearance on an X-ray?

The "hair-on-end" or "crew-cut" appearance on X-ray is primarily associated with conditions that result in increased erythropoiesis or bone marrow hyperplasia, which occurs in response to chronic hemolytic anemia. Both β-thalassemia and sickle cell anemia lead to extramedullary hematopoiesis due to the failure of effective erythropoiesis in the context of anemia. This compensatory response causes the marrow to expand and can lead to characteristic changes in the skull's bone structure, resulting in the hair-on-end appearance.

In β-thalassemia, there is an imbalance in the production of hemoglobin chains, resulting in chronic anemia and the body’s effort to compensate by producing more red blood cells. Similarly, in sickle cell anemia, the sickling of red blood cells causes hemolysis and a resultant increase in erythropoietic activity in the bone marrow. Both conditions thus manifest with this prominent X-ray finding.

Megaloblastic anemia, while it presents with macrocytic red blood cells and could potentially cause some marrow expansion, does not typically produce the hair-on-end appearance because the underlying mechanism involves ineffective erythropoiesis due to vitamin B12 or folate deficiency