When a Lumbar Puncture Is Needed
Testing the Fluid That Surrounds the Brain
Why Would We Even Consider This?
A lumbar puncture, abbreviated LP and commonly called a spinal tap, is not routine. We consider it when we are seriously concerned about what is happening around the brain. In the emergency department, the two most urgent reasons are suspected meningitis, an infection of the protective layers around the brain, and concern for bleeding such as a subarachnoid hemorrhage, especially in someone with a sudden severe headache.
Imagine a patient describing the worst headache of their life that came on abruptly. A CT scan is often the first step, but it has limitations. Or consider someone with fever, neck stiffness, and confusion. In those moments, imaging alone is not enough. We need to examine the fluid that surrounds the brain and spinal cord. That fluid often tells the truth.
What Is Cerebrospinal Fluid, Exactly?
Cerebrospinal fluid, abbreviated CSF, is a clear, colorless fluid that bathes the brain and spinal cord. It is produced continuously and circulates in a closed system deep within the body. Under normal conditions, CSF is crystal clear. Not cloudy. Not bloody. Not yellow. It is remarkably pure.
When we collect CSF during an LP, even subtle changes matter. Cloudiness can suggest infection. Red blood cells may indicate bleeding. Elevated protein or abnormal glucose levels can signal inflammation or bacterial involvement. What makes this procedure powerful is that CSF should look pristine. When it does not, we pay attention.
Where Does the Needle Actually Go?
Despite the name spinal tap, we are not puncturing the spinal cord itself. The spinal cord typically ends around the level of the first or second lumbar vertebra. An LP is performed lower than that, usually between the L3–L4 or L4–L5 vertebrae. At this level, the cord has already transitioned into individual nerve roots floating in fluid, which significantly reduces the risk of cord injury.
The patient is either lying on their side in a curled position or sitting upright leaning forward. After sterilizing the area and numbing the skin with local anesthetic, a thin needle is carefully advanced into the fluid space. We collect small vials of CSF for laboratory analysis and measure the opening pressure, which gives us information about intracranial pressure.
What Are We Measuring Besides Infection?
An LP allows us to measure opening pressure, the pressure of the CSF at the moment we enter the space. Elevated opening pressure can be seen in infections, bleeding, and conditions such as idiopathic intracranial hypertension, where pressure builds without a visible mass or obstruction.
We also analyze protein levels, glucose concentration, and specific markers that help differentiate bacterial from viral infections. In some cases, removing a small amount of CSF can temporarily relieve symptoms in patients with high pressure disorders. In the emergency department, however, the primary goal remains diagnosis of infection or bleeding.
Is It Dangerous?
Like any procedure, lumbar puncture carries risk. Infection, bleeding, and nerve irritation are possible but uncommon when performed carefully. We avoid the procedure in patients with severe bleeding disorders or very low platelets because the spinal canal is a confined space with little room for extra blood.
One known complication is the post lumbar puncture headache, a positional headache that occurs if a small amount of CSF continues to leak after the procedure. It often improves when lying flat and is treatable if persistent. While the idea of a spinal tap can sound intimidating, when done thoughtfully and for the right reason, it is a controlled and deliberate procedure.
THE BOTTOM LINE
• A lumbar puncture, or spinal tap, analyzes cerebrospinal fluid to diagnose meningitis, bleeding, and abnormal brain pressure
• Cerebrospinal fluid is normally crystal clear, and subtle changes in its appearance or composition can signal serious disease
• An LP is performed below where the spinal cord ends and is used when critical answers cannot wait
Written by a Board-Certified Emergency Medicine Physician