Xylocaine is typically injected in and beneath the skin to reduce local discomfort. Warming of the analgesic by rolling the vial between the palms seems to diminish the burning sensation that accompanies cutaneous infiltration. The bevel of the LP needle should be oriented in the longitudinal plane of the dural fibers (see below regarding atraumatic needles). It is usually possible to appreciate a palpable “give” as the needle approaches the dura, followed by a subtle “pop.” At this point, the trocar should be removed slowly from the needle to avoid sucking a nerve rootlet into the lumen and causing radicular pain. Sciatic pain during the insertion of the needle indicates that it is placed too far laterally. If the flow of CSF slows, the head of the bed can be elevated slowly. Rarely, one resorts to gentle aspiration with a small-bore syringe to overcome the resistance of proteinaceous and viscous CSF. Failure to enter the lumbar subarachnoid space after two or three trials usually can be overcome by performing the puncture with the patient in the sitting position and then helping him to lie on one side for pressure measurements and fluid removal. The “dry tap” is more often the result of an improperly placed needle than of obliteration of the subarachnoid space by a compressive lesion of the cauda equina or by adhesive arachnoiditis. In an obese patient, in whom palpable spinal landmarks cannot be appreciated, or after several unsuccessful attempts in any patient, fluoroscopy can be employed to position the needle.
LP has few serious complications. The most common is headache, estimated to occur in one-third of patients, but in severe form in far fewer. A history of migraine headaches may increase the incidence of prolonged or severe post-LP headache. The headache becomes apparent when the patient assumes the upright posture and is presumably the result of a reduction of CSF pressure from leakage of fluid at the puncture site and tugging on cerebral and dural vessels. Prolonged recumbency immediately after the procedure has not been shown to prevent headache, but is often implemented nonetheless. Strupp and colleagues have found that the use of an atraumatic needle almost halved the incidence of headache. Curiously, headaches are twice as frequent after diagnostic LP as they are after spinal anesthesia. Severe headache can be associated with vomiting and mild neck stiffness. Unilateral or bilateral sixth nerve palsy occur rarely after LP, even at times without headache, and rare cases of hearing loss, facial numbness, or facial palsy have been reported. The syndrome of low CSF pressure, its treatment by “blood patch,” and other complications of LP are considered further in Chap. 29.
Bleeding into the spinal meningeal or epidural spaces after LP can occur in patients with abnormal coagulation, as discussed earlier. Treatment of bleeding complications is by reversal of the coagulopathy and, in rare cases, surgical evacuation of the clot. Purulent meningitis and disc space infections rarely complicate LP.