Lateral C2--C3 subarachnoid puncture for metrizamide myelography or cisternography.

Metrizamide is useful for evaluating both posterior fossa and the cervical spinal canal [1, 2]. Because the contrast material is diluted by spinal fluid enroute from the lumbar region to the cervical spinal canal, the C1-C2 puncture technique has been advocated as the most suitable approach for instillation of contrast material [3]. The procedure is safe and without great technical difficulties, although problems with a puncture at this level have been reported with lesions centered at C1-C2, [4]. There is also thepossibility of injury of a herniated cerebellar tonsil or anomalous caudal loop of the posterior inferior cerebellar artery. In patients with basilar invagination or congenital fusion deformities of the occiput and upper cervical spine, a C1C2 puncture may be impossible. Thus, a lumbar puncture remains the safest approach in patients with possible ton-