External hydrocephalus has received scant mention in the medical literature of the past decade which contains so much of value concerning the nature and causes of the internal form. Blackfan1 even questions the advisability of considering it a clinical entity since its pathogenesis is so imperfectly understood. He said: "It is an exceedingly rare condition closely associated with and in all probability secondary to internal hydrocephalus." During recent years, however, several papers have appeared dealing with internal hemorrhagic pachymeningitis and chronic subdural hematoma in which mention is made of the presence of considerable quantities of yellow or bloody fluid in the subdural space with a widening of the cranial sutures and enlargement of the head. Penfield2 described a case in which there was moderate dilatation of the ventricles and also a large collection of fluid in the subdural space but no communication between the subdural and subarachnoid spaces.
SMITH EH. EXTERNAL HYDROCEPHALUS DUE TO STENOSIS OF THE LONGITUDINAL SINUS AND INTERNAL HEMORRHAGIC PACHYMENINGITIS. Am J Dis Child. 1935;49(1):147-152. doi:10.1001/archpedi.1935.01970010156016