Lumbar puncture (LP) is a frequently performed procedure by hospital-based physicians. Although its description may sound frightening to parents or other laypersons, in actuality, it is a fairly simple and straightforward procedure in most patients.

LP is performed whenever cerebrospinal fluid (CSF) is needed for evaluation.¹ The most typical scenario for a pediatric hospitalist is a febrile patient in whom meningitis is a concern, and a majority of such patients are neonates or infants with nonspecific signs and symptoms of meningitis. Other indications include evaluation of suspected central nervous system (CNS) bleeding, measurement of intracranial pressure (ICP), as in pseudotumor cerebri, and investigation of suspected inflammatory conditions of the CNS. In patients with confirmed pseudotumor cerebri, removal of CSF via LP is sometimes done to acutely lower ICP, providing symptom relief and preventing complications that may result from untreated increased ICP.

The practitioner should always consider whether it is safe to perform LP. If a patient has increased ICP, such as from a cerebral mass, release of pressure during LP may potentially precipitate shifting of intracranial contents from an area of high pressure to one of low pressure (i.e. cerebral herniation). If this is at all a possibility, the patient should have CNS imaging performed before a spinal tap is done. The risk is significantly less in infants who have an open fontanelle. The same is true for significant intracranial bleeding. In general, if the patient does not have focal neurologic findings on physical examination or demonstrate signs or symptoms of increased ICP by history or on physical examination, it is safe to perform LP before imaging. LP is relatively contraindicated in patients with underlying bleeding diatheses to avoid the formation of a hematoma around the spinal column or within the surrounding soft tissues. If this problem is suspected, laboratory studies such as a complete blood count and coagulation studies should be performed before LP is attempted.

CSF is produced in the choroid plexuses of the lateral ventricles. It circulates between the lateral ventricles and around the spinal cord and cerebrum in the subarachnoid space. The goal of LP is to obtain a sample of CSF in the safest possible manner, which is achieved by puncturing the dura mater in the lumbar region, below the termination of the spinal cord itself, where only the cauda equina is found. In young children, the inferior termination of the spinal cord is at the level of L3, and in older children it is even higher. Thus LP can nearly always be performed safely, barring any anatomic anomaly, which is usually obvious, by inserting the spinal needle between the spinous processes of L4 and L5. To locate this interspace, palpate the posterior superior iliac crests; the L4–L5 interspace is located at that level. On the patient’s back you will feel the spinous processes of the vertebrae. When the needle is inserted, it will penetrate the skin and then the supraspinous ligament, the interspinous ligament, and the ligamentum flavum before puncturing the dura mater to enter the subarachnoid space, where CSF is found (Figure 187-1).

Virtually all hospitals will have a commercially made LP tray that contains most of the equipment needed to perform LP (Table 187-1). Although these kits all contain local anesthetic, most do not contain topical anesthetic (e.g. EMLA, ELA-Max). One will also need to obtain an appropriately sized spinal needle based on the age or size of the patient (Table 187-2). It should also be noted that some pediatric LP kits have no manometer, so to measure opening pressure, a manometer must be obtained from an adult LP kit.