SAH posterior fossa

Case contributed by Prof Peter Mitchell


Headache and altered conscious state - SAH?

Patient Data

Age: 66
Gender: Male

Subarachnoid blood of heterogeneous density is seen around the cervicomedullary junction, inferior cerebellar vermis and basal cisterns. There is intraventricular blood is evident. Moderately severe hydrocephalus is seen. There is mild effacement of the cortical sulci.

Pattern suggests dissecting aneurysm or aneurysm of PICA.


No aneurysm, dissection or focal narrowing of vertebral areteries shown.


Normal. Remain suspcicious for dissection - close imaging follow up with multiple modalities recommended.


Partially shown on this MRA of tthe neck vessels is narrowing in the V4 segment of the vertebral artery. Vasospasm or dissection are the differential diagnoses.

DSA (angiography)


DSA (angiography)

Note marked progressive narrowing of the V4 segments has developed bilaterally, crossing the PICA origins. Changes are worse on the right. No vasospasm in the anterior circulation. Close inspection reveals a small aneurysm at the junction of the right PICA and vertebral artery. Likely diagnosis is dissecting aneurysm of vertebral / PICA junction - bilateral dissections, major differential diagnosis is dissection of PICA origin and vasospasm explaining the vertebral stenoses.

Case Discussion

Posterior fossa pattern of SAH is shown - the differential diagnosis is between PICA aneurysm and vertebral dissecting aneurysm.  PICA aneurysms are usually evident on initial imaging - if initial imaging is negative remain suspicious of underlying dissection.  Repeat imaging may be required over several days to make the diagnosis.  In this case the likely diagnosis is dissecting aneurysm of the PICA/Vertebral junction, with dissection involving the vertebral with associated vasospasm in both V4 segments.  The alternative diagnoses are initially occult PICA berry aneurysm and now vasospasm, or dissecting aneurysm with bilateral V4 dissections.

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Case information

rID: 34421
Published: 22nd Feb 2015
Last edited: 14th Aug 2019
Inclusion in quiz mode: Included

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