title: The negative predictive value of CT angiography in the setting of perimesencephalic subarachnoid hemorrhage.
abstract: BACKGROUND
  Perimesencephalic subarachnoid hemorrhage (PMSAH) is only rarely associated with a ruptured cerebral aneurysm and CT angiography (CTA) has very good sensitivity and specificity for aneurysm detection. The necessity for invasive imaging with digital subtraction angiography (DSA) is therefore debatable. We chose to assess the negative predictive value (NPV) of CTA in a series of patients with PMSAH treated at our institution over a 9-year period.
  
  
  METHODS
  We retrospectively assessed the diagnostic yield of DSA after initial negative CTA in patients with a PMSAH pattern defined as blood centered anterior to the midbrain and/or pons within the pre-pontine or interpeduncular cistern with possible quadrigeminal or ambient cistern extension; possible extension into the basal parts of the sylvian fissures but not the lateral sylvian fissures; possible extension to the cisterna magna but not centered on the cisterna magna; and possible extension into the fourth ventricle and occipital horns of the lateral ventricles.
  
  
  RESULTS
  Using this definition of PMSAH, of 72 patients, one patient showed a potentially significant finding on DSA that was not demonstrated on initial CTA (NPV 98.61% (95% CI 92.47% to 99.77%)). However, when cisterna magna extension was excluded from the definition of PMSAH, no false negative CTAs in 56 patients were encountered (NPV 100% (95% CI 93.56% to 100.00%)).
  
  
  CONCLUSIONS
  The NPV of normal CTA for an arterial abnormality in patients with PMSAH is high and our results therefore question the role of invasive imaging. The findings also suggest that a prospective study designed to clarify the necessity of performing DSA in this population would be feasible.
authors.affiliation: Department of Neuroradiology, Southmead Hospital, Bristol, UK.
authors.forename: Alex M
authors.initials: AM
authors.lastname: Mortimer
authors.affiliation: Department of Neuroradiology, Southmead Hospital, Bristol, UK.
authors.forename: Auke Pa
authors.initials: AP
authors.lastname: Appelman
authors.affiliation: Department of Neuroradiology, Southmead Hospital, Bristol, UK.
authors.forename: Shelley A
authors.initials: SA
authors.lastname: Renowden
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D015901:Angiography, Digital Subtraction / Q000379:methods*; D002533:Cerebral Angiography / Q000379:methods*; D000072226:Computed Tomography Angiography / Q000379:methods*; D005260:Female; D006801:Humans; D008297:Male; D008636:Mesencephalon / Q000000981:diagnostic imaging*; D008875:Middle Aged; D011237:Predictive Value of Tests; D012189:Retrospective Studies; D013345:Subarachnoid Hemorrhage / Q000000981:diagnostic imaging*
keywords: Aneurysm; Angiography; CT; CT Angiography; Subarachnoid