Seventy aneurysms of the posterior inferior cerebellar artery: anatomical features and value of computed tomography angiography in microneurosurgery.

OBJECTIVE: Aneurysms at the junction of the vertebral artery (VA) and posterior inferior cerebellar artery (PICA) are relatively rare. Their treatment is challenged by the diverse anatomy of the VA and PICA, close involvement of the PICA in the aneurysm neck, and scant space deep in the posterior fossa next to the cranial nerves, brain stem, and obstructing structures of the skull base. Computed tomography angiography (CTA) visualizes bony structures in addition to the vasculature, and being noninvasive and easily available, it can serve for planning microsurgical treatment. We analyzed the anatomy of PICA aneurysms imaged by CTA to promote planning the treatment.

METHODS: We examined the CTA images of 70 consecutive patients with a saccular PICA aneurysm who were all treated in the Department of Neurosurgery, Helsinki, Finland, 2001 to 2011.

RESULTS: Each of the 70 patients had 1 aneurysm at the VA-PICA junction. The proportion of small aneurysms (<7 mm) was high, 67%. The dome-to-neck ratio was <1.2 in 24 (34%) of the aneurysms. In the coronal view, 58 (83%) aneurysms projected cranially, and only 2 (3%) caudally. The aneurysms were located from 1 mm below the foramen magnum to 31 mm above it, most arising at the level of the jugular tubercle. The median distance to the aneurysm from the midline was 6 mm, ranging from 7.5 mm contralateral to the origin of the parent artery to 14 mm ipsilateral. Compared with the right VA, the left VA was larger, it harbored the most aneurysms, and its aneurysms were more often ruptured.

CONCLUSIONS: Relation of PICA aneurysms to skull base structures is highly variable; the aneurysms can also be extracranial, or lie on the site of the skull contralateral to the origin of the parent artery. These anatomical variations demand meticulous study of the angiography in each individual case, especially before surgical treatment of the aneurysm.