Helmetlike skull deformity with a large arachnoid cyst

Background: It is not difficult to find localized skull ballooning or macrocrania in patients with intracranial arachnoid cysts. However, there have been no previous reports regarding large localized skull protuberant deformities resembling a war helmet. The authors report with a review of literature a case of an adult with helmetlike skull deformity resulting from a large supratentorial arachnoid cyst. Case Description: A 35-year-old man presented with a large head deformity since his early childhood that had been the result of gradual progression from infantile macrocrania. He also had mental retardation, sixth cranial nerve palsy with recent aggravation of headache, reduced activity, poor voiding control, and walking disturbance. Magnetic resonance imaging of the head showed hydrocephalus with a large supratentorial arachnoid cyst located in the bilateral parietooccipital area compressing the hemisphere anteriorly, and the tentorium and cerebellum inferiorly. Magnetic resonance venogram demonstrated low-lying short transverse and lateral sinuses, and the superior sagittal sinus and falx were displaced to the right side. Radioisotopic cisternogram showed nonfilling of the isotope in the bilateral parietooccipital area. Cerebrospinal fluid pressure measured by lumbar puncture was 17 cm H₂O. We tentatively diagnosed the condition as normopressure hydrocephalus with a large supratentorial arachnoid cyst. His headache, reduced activity, poor voiding control, and walking disturbance improved after a cystoperitoneal shunt. Conclusions: This might suggest that large arachnoid cysts found in childhood should be treated for prevention of skull deformity and late aggravation of increased intracranial pressure.