Perimesencephalic Nonaneurysmal Subarachnoid Hemorrhage (pnSAH)

KEY FACTS

Terminology

• Subarachnoid hemorrhage (SAH) centered immediately anterior to midbrain ± pons

• No source demonstrated at CTA/DSA/MRA

Imaging

• NECT: Hyperdense prepontine, perimesencephalic cerebrospinal fluid (CSF)

  • Often involves interpeduncular, ambient, quadrigeminal cisterns

  • ± thin extension into posterior suprasellar, proximal sylvian/interhemispheric fissures

  • Does not extend into distal sylvian, interhemispheric fissures

• CTA used to exclude basilar tip aneurysm

• MR

  • T1: Iso- to hyperintense

  • T2 variable (iso- to hyper-) intensity compared with CSF

  • FLAIR: Hyperintense prepontine, perimesencephalic CSF

Top Differential Diagnoses

• Aneurysmal SAH

• Traumatic SAH

• Artifact (incomplete CSF suppression on FLAIR)

Pathology

• Most likely from ruptured perimesencephalic/prepontine vein

  • More common if basal vein of Rosenthal small, drains into veins other than Galen

• 5% of pnSAHs have other etiologies

  • Basilar bifurcation aneurysm, dissection most common

  • Other: Trauma, dural arteriovenous fistula, spinal cord vascular malformation, vascular neoplasm

Clinical Issues

• Benign course: Rebleed rare (< 1%); no vasospasm

TERMINOLOGY