Lumbar puncture

A lumbar puncture (LP), also called a spinal tap, involves inserting a needle into the spinal canal, usually to obtain cerebrospinal fluid (CSF) for lab tests. The primary goal is to diagnose central nervous system disorders affecting the brain and spine, such as meningitis or subarachnoid hemorrhage. It can also serve a therapeutic purpose in certain cases. The procedure is considered safe, though a headache after the puncture is a common side effect, especially if a small atraumatic needle is not used.

The procedure is done under local anesthesia with sterile technique. A hypodermic needle accesses the subarachnoid space to collect fluid, which may be analyzed biochemically, microbiologically, or cytologically. Using ultrasound to guide the needle can improve success rates. Lumbar punctures were first performed in 1891 by German physician Heinrich Quincke.

**Medical Uses**

Lumbar punctures are performed for diagnosis or treatment.

**Diagnosis** The main diagnostic use is collecting CSF to check for infectious, inflammatory, or neoplastic diseases of the central nervous system. It is most commonly done when meningitis is suspected, as no other reliable test can rule out this life-threatening but treatable condition. It can also determine whether someone has Stage 1 or Stage 2 *Trypanosoma brucei* infection. Young infants often undergo lumbar puncture as part of a routine fever workup because they have higher meningitis rates than older individuals and do not reliably show classic symptoms like neck stiffness or headache. In any age group, the test can help confirm or rule out subarachnoid hemorrhage, hydrocephalus, benign intracranial hypertension, and other conditions. It can also detect malignant cells in CSF, as seen in carcinomatous meningitis or medulloblastoma. For subarachnoid hemorrhage workup, a "negative" tap has fewer than 10 red blood cells per cubic millimeter, while a "positive" tap has 100 or more.

**Treatment** Lumbar punctures can inject medications into the CSF (intrathecally), such as for spinal anesthesia or chemotherapy. Repeated lumbar punctures may temporarily treat idiopathic intracranial hypertension (IIH), a condition of high CSF pressure causing headache and potential vision loss. Although medication is the main treatment, multiple punctures can improve symptoms in some cases, but this is not recommended as a routine approach due to discomfort, procedural risks, and short-lasting benefit. In normal pressure hydrocephalus—marked by urinary incontinence, gait changes, and dementia—some people get symptom relief after CSF removal.

**Contraindications** Lumbar puncture should be avoided in these situations: - Idiopathic increased intracranial pressure (ICP), because it risks uncal herniation; an exception is therapeutic LP to reduce ICP, but only after ruling out obstruction (e.g., in the third ventricle). - Precautions: CT brain scan may be needed, especially in people over 65, those with reduced Glasgow Coma Scale, recent seizure, focal neurological signs, abnormal breathing, or hypertension with bradycardia and declining consciousness. Ophthalmoscopy for papilledema is also advised. - Bleeding tendency (relative): coagulopathy or platelet count below 50 billion per liter. - Skin infection at the puncture site. - Vertebral deformities (scoliosis or kyphosis) when performed by an inexperienced physician.

**Adverse Effects**

**Headache** Post-dural-puncture headache with nausea is the most common complication. It often responds to pain relievers and intravenous fluids. Lying flat for two hours after the procedure was long thought to prevent it, but modern studies with large groups do not support this. Performing the puncture with the person on their side may lower the risk. Intravenous caffeine is often effective for stopping these headaches. A persistent headache that occurs only when sitting up may indicate a CSF leak from the puncture site; this can be treated with more bed rest or an epidural blood patch, where the person's own blood is injected to clot and seal the leak. Using atraumatic needles greatly reduces the risk of headache and the need for pain medication or blood patch, without affecting procedure success. Despite similar cost and difficulty, adoption was only about 16% as of around 2014. These headaches may result from accidentally puncturing the dura mater.

**Other** If the needle touches a spinal nerve root, it can cause temporary tingling or strange sensations (paresthesia) in a leg during the procedure. This is harmless, and patients can be warned in advance to reduce anxiety. Serious complications from a properly performed lumbar puncture are extremely rare.