Benign perimesencephalic subarachnoid hemorrhage (pSAH)

  • Subarachnoid hemorrhage confined to the basal cisternae around midbrain.
  • Relatively rare, about 5-10% of all SAH.
  • Vast majority are non-aneurysmal and are probably caused by hemorrhage from small veins from the venous plexa around the brainstem.
  • Patients with non-aneurysmal pSAH usually present with minimal clinical symptoms (Hunt & Hess scale 1)
  • Non-aneurysmal pSAH has excellent prognosis and outcome. Hydrocephalus and vasospasm are very rare and should prompt re-evaluation and repeat imaging.
  • 95% of cases will have a normal angiogram
  • 5% will either be aneurysmal, or (rarely) due to arteriovenous malformation or AV-fistula.

Criteria for the diagnosis of pSAH

  1. SAH in perimesencephalic cisterns anterior to midbrain.
  2. May extend to the medial but not the lateral part of the Sylvian fissures.
  3. May extend to the anterior part of the interhemispheric fissure but not the entire length of the fissure.
  4. If there is layered blood in the ventricles frank intraventricular hemorrhage should be excluded.
  5. No intraaxial hemorrhage or extension of hemorrhage in parenchyma.

For patients meeting the above criteria and having a negative CT angiogram:

  • Neither DSA nor repeat CTA is needed as follow-up unless the clinical picture is inconsistent with pSAH.
  • The patients have excellent prognosis and outcomes and no risk of re-bleeding.


References

  • Perimesencephalic hemorrhage: a nonaneurysmal and benign form of subarachnoid hemorrhage Neurology. 1985 Apr;35(4):493-7. doi: 10.1212/wnl.35.4.493
  • Current status of perimesencephalic non-aneurysmal subarachnoid hemorrhage Front Neurol. 2022 Sep 1:13:960702. doi: 10.3389/fneur.2022.960702
  • Perimesencephalic Subarachnoid Hemorrhage Is Not Always a Benign Condition: Hemorrhage Volume as a Predictor for Complications and Clinical Outcome Biomedicines. 2025 Apr 27;13(5):1061. doi: 10.3390/biomedicines13051061
  • Recurrent Perimesencephalic Nonaneurysmal Subarachnoid Hemorrhage: Case Report and Review of the Literature World Neurosurg. 2017 Nov:107:877-880. doi: 10.1016/j.wneu.2017.08.079
  • Use of Follow-Up Imaging in Isolated Perimesencephalic Subarachnoid Hemorrhage A Meta-Analysis. Stroke. 2015 46:401-406. doi: 10.1161/STROKEAHA.114.007370