External hydrocephalus in small children. 
Confusion exists in literature regarding the entity of external hydrocephalus which has been addressed by different terms like subdural effusion, hygroma, benign infantile hydrocephalus, etc.  In this report, the author has tried to define the clinical and radiological criteria for reaching the diagnosis of external hydrocephalus.  Five children 9 months to 2 years of age with subdural fluid collection were studied prospectively for a period of 8 years.  The radiological and clinical criteria sparsely described in literature were followed to study the profile of suspected cases.  Four children presented with craniomegaly; the fifth case had tense, wide, and bulging fontanelle.  CT of all five children showed hemispheric extracerebral fluid collection with enlarged subarachnoid spaces.  Three children had mild to moderate ventriculomegaly without significant periventricular lucency, while the other two had borderline ventricular enlargement.  All were managed conservatively.  All five children were asymptomatic at follow-up from 12 weeks to 2 years.  The possible conclusions from this study of rare cases are: external hydrocephalus needs differentiation from other conditions of subdural fluid collections.  The radiological criteria such as (a) extracerebral fluid collection (relatively more in anterior convexity than posterior convexity), (b) dilatation of subarachnoid spaces, and (c) mild to moderate ventriculomegaly without (or with questionable) periventricular lucency may be helpful in making a radiological diagnosis of external hydrocephalus.  Conservative treatment with acetazolamide helped in ameliorating the clinical features of the cases studied.