[Distal aneurysm of the postero-inferior cerebellar artery. Anatomical and surgical considerations].

A patient with a rare peripheral aneurysm of the posterior inferior cerebellar artery (P.I.C.A.) was treated by microsurgery. Anatomical features and surgical procedures are discussed. The aneurysm was located in the retromedullary, tonsillomedullary segment of the right P.I.C.A. and formed a caudal or infratonsillar loop in the cisterna magna close to the inferior part of the tonsil. The operation was performed with the patient in the sitting position using a median suboccipital approach through a cervical 1 (C1) laminectomy. The widened neck of the aneurysm was clipped successfully and the post-operative course was completely asymptomatic. A vertebral angiogram control showed the P.I.C.A. to be patent. The surgical approach should be related to the site of the aneurysm. Those affecting the telovelotonsillar segment of a P.I.C.A., forming cranial or supratonsillar loops, and those involving its cortical segment can be effectively operated using the same median suboccipital approach. Aneurysms arising from the anterior or lateral medullary segment of a P.I.C.A. should be approached through a unilateral suboccipital craniectomy extending down to the foramen magnum. Although clipping the neck of the aneurysm is preferable, trapping is possible for lesions arising from or distal to the telovelotonsillar segment, this preserving the small arteries arising from the lateral and posterior medullary segments.