The persistence of blood in the subdural space incites a tissue reaction to organize and resorb the hematoma. Most of the organization and resorptive activity in a persistent hematoma takes place on the outer, or dural aspect where proliferating fibroblasts and capillaries permeate the clot to a uniform depth of several millimeters. The development of this layer proceeds at a relatively predictable rate, and is useful in dating the age of the hematoma. Concurrent with the maturation of the outer membrane, a reaction ensues on the arachnoidal or inner aspect of the hematoma. The inner membrane lacks the capillaries so conspicuous in the outer membrane.
Some hematomas undergo extensive calcification or even ossification.
Inner membrane is a delicate lamina of loose fibrous tissue of low cellularity. Microscopic Features of SDH Time Clot Dural Side Arachnoid Side 0-36 hours RBCs intact. 36-60 hours RBCs intact. A few fibroblasts at dural junction. 4-5 days RBCs lysing and pigment laden macrophages.