External hydrocephalus associated with dural sigmoid sinus arteriovenous fistula: a case report

External hydrocephalus (EH) is a recognised sub-type of hydrocephalus associated with macrocephaly in infancy. EH is characterised by the enlargement of subarachnoid spaces (so-called subarachnomegaly) with a normal ventricular system on brain imaging. EH is traditionally considered benign and self-limiting, yet its pathophysiology remains puzzling. Mounting evidence for an association between EH and hydrovenous disorders reshapes our understanding of this condition and its management. To our knowledge, we report the first association between EH and dural arteriovenous fistula (dAVF) in a 17-months-old boy. As dAVF may be a life-threatening condition, early diagnosis and optimal treatment are critical. This case epitomises the intricacies of EH's aetiology and associated conditions requiring careful management. Therefore, we recommend considering MR angiography in EH's workup and long-term follow-up. Our experience supports the ongoing reconsideration of EH's presumed benignity.