What Is a Lumbar Puncture—and Why Is It Performed?

Lumbar puncture—commonly known as a spinal tap—is a diagnostic and therapeutic procedure in which a thin, hollow needle is inserted into the lower lumbar region of the spine to access the subarachnoid space. This space contains cerebrospinal fluid (CSF), a clear, colorless fluid that cushions the brain and spinal cord, delivers nutrients, and removes metabolic waste.

The primary purpose of lumbar puncture is to obtain CSF for laboratory analysis. Testing the fluid can help diagnose or rule out serious neurological conditions—including bacterial, viral, or fungal meningitis; encephalitis; autoimmune disorders such as multiple sclerosis or neuromyelitis optica; subarachnoid hemorrhage; and certain malignancies like leptomeningeal carcinomatosis. In some cases, CSF pressure is measured during the procedure to assess for conditions such as idiopathic intracranial hypertension.

Lumbar puncture also serves therapeutic functions—for example, relieving elevated CSF pressure in patients with pseudotumor cerebri or performing CSF drainage in selected cases of hydrocephalus. Additionally, it provides a route for administering medications directly into the CSF, such as chemotherapy agents or antibiotics for central nervous system infections.

The procedure is typically performed under local anesthesia, with the patient positioned either laterally in the fetal position or seated and leaning forward. Imaging guidance is rarely required but may be used in patients with anatomical challenges or prior spinal surgery. While generally safe, potential complications include post-dural puncture headache, transient back pain, infection, bleeding, or—rarely—nerve root irritation or herniation in patients with significantly elevated intracranial pressure.