It is advisable to determine that the patient’s coagulation function is adequate for safe LP. In general, it is safe to perform LP on patients without history or overt signs of coagulopathy and those who are not taking anticoagulant medications. An international normalized ratio (INR) less than or equal to 1.4 and platelet count greater than 50,000/mm3 are generally acceptable, as is the use of aspirin in conventional doses. Individuals with impaired platelet function from diseases such as alcoholism or uremia may have bleeding complications. For patients receiving heparin by continuous intravenous infusion, the LP is best performed after the infusion has been discontinued for a period of time, and if possible, the partial thromboplastin time has been determined to be in a safe range. There are circumstances, however, where these provisions are not practical.
LP carries some risks if the CSF pressure is very high (evidenced mainly by headache and papilledema), for it increases the possibility of a fatal cerebellar or transtentorial herniation. The risk is considerable when there is an intracranial mass that distorts and displaces brain tissue, particularly asymmetric mass lesions near the tentorium or foramen magnum. The risk is much lower in patients with subarachnoid hemorrhage, in hydrocephalus with communication among all the ventricles, or with pseudotumor cerebri. Indeed, these are conditions in which repeated LPs may be employed as a therapeutic measure. In patients with purulent meningitis, there is also a small risk of herniation, but this is outweighed by the need for a definitive diagnosis and the institution of appropriate treatment at the earliest moment. With this last exception, LP should generally be preceded by computed tomography (CT) or magnetic resonance imaging (MRI) whenever an elevation of intracranial pressure is suspected.
If imaging procedures disclose a mass lesion that poses a risk of herniation, yet it is considered essential to have the information yielded by CSF examination, the LP may be performed—with certain precautions.
If the pressure proves to be very high, one should obtain the smallest necessary sample of fluid, adequate for the diagnosis of the suspected disease, administer mannitol or another hyperosmolar agent, and ideally observe a fall in pressure on the manometer. Dexamethasone or an equivalent corticosteroid may also be given in an initial intravenous dose of 10 mg, followed by doses of 4 to 6 mg every 6 h in order to produce a sustained reduction in intracranial pressure. Corticosteroids are particularly useful in situations in which the increased intracranial pressure is caused by vasogenic cerebral edema (e.g., tumor-associated edema).
Cisternal (foramen magnum) puncture and lateral cervical subarachnoid puncture are infrequently performed, but are safe in the hands of an expert. LP is preferred except in obvious instances of spinal block requiring a sample of cisternal fluid or for myelography above the lesion. In critical care practice, CSF is often obtained from external ventricular drain, and care is taken to maintain a closed drainage system and antiseptic technique.
Technique and Complications of LP
Experience teaches the importance of meticulous technique and proper positioning of the patient. LP should be done under locally sterile conditions. The patient is placed in the lateral decubitus position, preferably on the left side for right-handed physicians, with hips and knees flexed, and the head as close to the knees as comfort permits. The patient’s hips should be vertical, the back aligned near the edge of the bed. The puncture is usually easiest to perform at the L3-L4 interspace, which corresponds in many individuals to the axial plane of the iliac crests, or at the interspace above or below. In infants and young children, in whom the spinal cord may extend to the level of the L3-L4 interspace, lower levels should be used.