title: The strange association of pneumosinus dilatans and arachnoid cyst: case report and review of the literature.
abstract: 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.
  
  
  METHODS
  The literature on pneumosinus dilatans and its association with arachnoid cyst is reviewed.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Neurosurgery, King's College Hospital, London, UK.
authors.forename: A J
authors.initials: AJ
authors.lastname: Martin
authors.forename: J M
authors.initials: JM
authors.lastname: Jarosz
authors.forename: N W
authors.initials: NW
authors.lastname: Thomas
mesh_terms: D000368:Aged; D016080:Arachnoid Cysts / Q000150:complications* / Q000473:pathology; D002908:Chronic Disease; D003937:Diagnosis, Differential; D006801:Humans; D008297:Male; D010254:Paranasal Sinus Diseases / Q000209:etiology* / Q000473:pathology; D019291:Skull Base / Q000473:pathology