it is not clear why the collection enlarges in some cases and resolves in other. The leakage of CSF from the subarachnoid tear may be influenced by a pressure gradient between the subarachnoid space and the subdural space. it has been postulated that the return of CSF to the subarachnoid space is blocked by compression of the subarachnoid space due to brain edema or to pressure in the subdural space from the fluid itself. Hasegawa et al and lusins and levy showed that in certain cases the enlargement of posttraumatic hygroma was not due solely to the flap-valve mechanism, but rather to bleeding into the hygroma collection. Hasegawa et al reported five cases in which a subdural hygroma grew gradually in spite of the absence of radiological or visible evidence of an arachnoid tear, and in which brain scans showed an accumulation of intravenously injected radioisotope in the subdural hygroma. The postulated that there is an efflux of fluid from vessels in the neomembrane of a subdural hygroma. Gitlin also suggested that the subdural fluid came from the capillaries. it has also been postulated that the subdural fluid originates from transudation of plasma due to a degenerative process involving the vessels of the duramater. According to Gutierrez et al, stretching and narrowing of the cortical veins traversing the subdural space could result in a disturbance of cerebral hemodynamics, with leakage of fluid from the veins. Hygromas may also occur without trauma and have been reported to develop following craniotomy for tumor removal or clipping of aneurysm, ventriculo peritoneal shunt and ventricular catheter placement, and lumbar puncture. Subdural hygromas may also result from meningitis, extradural infection, sinus thrombosis, or dural metastasis. They may occur in the prenatal period secondary to congenital malformation of the arachnoid membrane. Some hygromas may evolve froma chronic subdural hematomas after degradation of blood and proteins. Shrinking of the brain by dehydration, hypertonic solutions, hyperventilation and hypocarbia ad diuretics may enhance extra cerebral fluid accumulation. Coughing, sneezing, ad straining, and pre existing cerebral atrophy in elderly patients may also increase the risk of hygromas formation.