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WHEN cerebrospinal fluid progressively accumulates over the surface of one or both cerebral hemispheres, the condition is known as external hydrocephalus, or subdural hydroma. It results from a tear in the arachnoid membrane, through which the fluid pours into the subdural space, where it cannot absorb. Moreover, when a peripheral branch of the subarachnoid space is involved the fluid is trapped in the subdural space as by a valve, the compression of the arachnoid by the fluid closing the opening and preventing its return into the subarachnoid space, although the fluid can and does continue to pass into the subdural space. It is now known that cerebrospinal fluid can be absorbed only in the subarachnoid space and from the capillaries of the pia-arachnoid. It cannot be absorbed in the subdural space (except in minimal amount). The end result of this sequence of events is a large quantity of fluid over
WALTER E. DANDY. TREATMENT OF AN UNUSUAL SUBDURAL HYDROMA (EXTERNAL HYDROCEPHALUS). Arch Surg. 1946;52(4):421–428. doi:10.1001/archsurg.1946.01230050428003