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.