A lumbar puncture, also known as a spinal tap, is a medical procedure involving the insertion of a thin needle into the lower back to collect cerebrospinal fluid (CSF). This clear fluid surrounds and cushions the brain and spinal cord, providing protection and circulating nutrients. The procedure aids in diagnosing various conditions affecting the brain, nerves, and spinal cord, and also serves therapeutic purposes, such as administering medications or relieving pressure.
How a Lumbar Puncture is Performed
A healthcare provider typically performs a lumbar puncture with the patient either lying on their side with knees drawn to the chest or sitting and leaning forward. These positions flex the back, widening spaces between vertebrae for easier needle insertion. The skin over the lower back is cleaned with an antiseptic, and a local anesthetic is injected to numb the area, minimizing discomfort. Once numb, a thin, hollow needle is carefully inserted between two lower spinal bones into the subarachnoid space, where CSF is located.
Cerebrospinal fluid is collected into tubes. Several small samples are typically gathered for laboratory analyses. The procedure usually takes 15 to 30 minutes. After fluid collection or medication administration, the needle is withdrawn, and a dressing is applied.
Diagnosing Medical Conditions
Lumbar punctures diagnose neurological conditions by analyzing CSF. The fluid is tested for components like red and white blood cells, protein, glucose, bacteria, viruses, or abnormal cells. Identifying specific markers in the CSF can point to different diseases.
For instance, in suspected meningitis, a lumbar puncture identifies infection markers. Bacterial meningitis often shows elevated white blood cells (leukocytes), particularly neutrophils, and altered glucose and protein levels. Viral meningitis also has increased white blood cells, but typically with lymphocytic predominance and different glucose and protein profiles.
The procedure also diagnoses autoimmune disorders like multiple sclerosis (MS) and Guillain-Barré Syndrome (GBS). For MS, CSF analysis often reveals oligoclonal bands, specific proteins indicating central nervous system inflammation, and elevated IgG levels. In GBS, a characteristic finding is albuminocytologic dissociation: elevated protein levels without a significant increase in white blood cells. This pattern reflects nerve root inflammation without direct CSF infection.
When subarachnoid hemorrhage (bleeding around the brain) is suspected but not seen on a CT scan, CSF analysis is important. Xanthochromia, a yellowish discoloration of CSF due to red blood cell breakdown products like bilirubin, strongly suggests a subarachnoid hemorrhage that occurred hours before the lumbar puncture. A lumbar puncture can also identify cancer cells that have spread to the central nervous system, such as in certain leukemias or lymphomas.
Therapeutic Applications
Beyond diagnosis, a lumbar puncture delivers treatments directly to the central nervous system. This direct approach is beneficial because many oral or intravenous medications cannot effectively cross the blood-brain barrier, which protects the brain and spinal cord.
One therapeutic use is administering chemotherapy drugs directly into the CSF, known as intrathecal chemotherapy. This method treats cancers that have spread to the brain and spinal cord or prevents such spread, particularly in certain leukemias and lymphomas. Common agents include methotrexate and cytarabine.
Lumbar punctures can also relieve pressure from excess cerebrospinal fluid. This is useful in conditions like normal pressure hydrocephalus or idiopathic intracranial hypertension. By temporarily removing CSF, the procedure alleviates symptoms such as headaches or vision changes associated with elevated intracranial pressure.
After the Procedure
After a lumbar puncture, patients are advised to rest quietly and lie flat for 1-2 hours to minimize complications. Staying well-hydrated is also recommended. Patients can usually resume normal activities, including showering, after 24 hours, but strenuous activities should be avoided for a day or two.
The most common side effect is a post-lumbar puncture headache, affecting up to 25% of individuals. This headache typically starts hours to two days after the procedure and worsens when sitting or standing, improving when lying down. It occurs due to a small CSF leak, leading to reduced fluid pressure around the brain.
Conservative management includes pain relievers like acetaminophen, increased fluid intake, and caffeine, which can help by constricting blood vessels. If a severe or persistent headache does not improve, an epidural blood patch may be performed. This involves injecting a small amount of the patient’s own blood into the space outside the dura to seal the leak, often providing prompt relief. Patients should contact their healthcare provider if they experience a worsening headache, fever, neck stiffness, or new numbness or weakness.