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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

(Left) Axial graphic shows a

classic pnSAH. Hemorrhage is

confined to the

interpeduncular fossa and

ambient (perimesencephalic)

cisterns st. The source is

usually venous in pnSAHs,

unlike in aneurysmal SAHs.

(Right) Axial NECT of a 49-

year-old woman in the ER with

a "thunderclap" headache

shows SAH around the

midbrain in the

perimesencephalic cisterns ſt.

Note the absence of blood in

the sylvian fissures and

anterior suprasellar

subarachnoid space.

(Left) Axial NECT in the same

patient shows that the

subarachnoid blood ſt

extends inferiorly in front of

the pons. (Right) Coronal CTA

in the same patient with

reconstructed view through

the basilar artery bifurcation

shows no evidence for

aneurysm. The high density in

the vessels is easily discernible

even with the presence of

significant SAH. DSA (not

shown) confirmed the absence

of aneurysm. This case

illustrates classic findings in

pnSAH.

Trauma, and Stroke

Brain: Pathology-Based Diagnoses: Malformations,

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