The strange association of pneumosinus dilatans and arachnoid cyst: case report and review of the literature.

BACKGROUND

The authors present the case of a 71-year-old man with dramatic pneumosinus dilatans adjacent to a large, symptomatic, fronto-temporal arachnoid cyst.

METHOD

The literature on pneumosinus dilatans and its association with arachnoid cyst is reviewed.

FINDINGS

Pneumosinus dilatans may be either idiopathic, a reaction to an adjacent meningioma, or an 'ex-vacuo' response to cerebral volume loss and intracranial hypotension. It is also found with large arachnoid cysts and is probably under-recognised in this context. The co-existence of an expansile intradural lesion with changes in the skull base that tend to reduce the intracranial volume is puzzling, and has not yet been fully explained. Differences in the relative timing of paranasal sinus and arachnoid cyst growth, and the 'temporal agenesis' theory of arachnoid cyst formation have been proposed but do not account for all the features of this unusual association.

INTERPRETATION

Pneumosinus dilatans is a useful and under-recognised indicator of the presence and chronicity of a variety of intracranial pathologies. Its association with arachnoid cyst is paradoxical, and a new explanation is offered as to how this may arise.