It is done by inserting a needle through the skin below the external occipital protuberance into the cisterna magna and is an alternative to lumbar puncture.
Subarachnoid hemorrhage and direct puncture of brain tissue are the most common major complications.
When CSF cannot be obtained from the lumbar space (and when its analysis is considered critical to treatment), a cisternal tap may be required.
This is technically fairly easy; however, if the needle is advanced too far it can enter the medulla, sometimes causing sudden respiratory arrest and death.
An alternative route that may be used by neurosurgeons and neuroradiologists is lateral to C-1 with penetration through the large C-1 intervertebral hiatus.