title: How often is a perimesencephalic subarachnoid haemorrhage CT pattern caused by ruptured aneurysms?
abstract: BACKGROUND
  Angiography is usually recommended in perimesencephalic subarachnoid haemorrhage (PM SAH) to rule out a basilar artery aneurysm. However it is not known how often aneurysms are found among patients with a CT pattern of PM haemorrhage or the frequency of this CT pattern after rupture of posterior circulation aneurysms.
  
  
  METHODS
  CTs of all SAH caused by posterior circulation aneurysms admitted from 1/85 to 12/92 where reviewed by two examiners. Late (> 72h) examinations were excluded. The remaining CTs were classified in perimesencephalic (PM) or non PM.
  
  
  RESULTS
  81 posterior circulation aneurysms were collected. Only one PM-like CT pattern was found, due to ruptured posterior communicating artery aneurysm (1.3%; 95% Cl (confidence intervals) = .03-6.7%). During the same period 37 PM SAH with negative angiographic results were admitted. The likelihood of finding an aneurysm in a patient with an early CT showing a PM distribution of haematic densities was 2.7% (95% Cl = .07-.14%).
  
  
  CONCLUSIONS
  Although the probability of finding an aneurysm in a SAH patient with a PM CT pattern is low, a complete 4-vessel angiogram must be obtained.
authors.affiliation: Department of Neurology, Hospital St. Maria, Lisbon, Portugal.
authors.forename: A N
authors.initials: AN
authors.lastname: Pinto
authors.forename: J M
authors.initials: JM
authors.lastname: Ferro
authors.forename: P
authors.initials: P
authors.lastname: Canhão
authors.forename: J
authors.initials: J
authors.lastname: Campos
mesh_terms: D017542:Aneurysm, Ruptured / Q000000981:diagnostic imaging* / Q000601:surgery; D002533:Cerebral Angiography; D005260:Female; D006801:Humans; D002532:Intracranial Aneurysm / Q000000981:diagnostic imaging* / Q000601:surgery; D008636:Mesencephalon / Q000000981:diagnostic imaging* / Q000601:surgery; D008875:Middle Aged; D009460:Neurologic Examination; D011183:Postoperative Complications / Q000000981:diagnostic imaging; D013345:Subarachnoid Hemorrhage / Q000000981:diagnostic imaging* / Q000601:surgery; D014057:Tomography, X-Ray Computed