Lumbar puncture is also known as spinal tap. The examination is most commonly performed in neurological and infection wards. The aim of the spinal tap is to obtain a sample of cerebrospinal fluid. Cerebrospinal fluid is a clear, slightly yellowish liquid that is produced in the brain ventricles, flows through these brain ventricles, surrounds the brain and protects it. The cerebrospinal fluid is absorbed into venous blood. The fluid protects the brain from mechanical damage and it takes waste products from the brain tissue. The cerebrospinal is not only present in and around the brain, but it also flows into the spinal canal. We use this fact when performing the spinal tap.
The procedure aims to enter the spinal canal and take a sample of the cerebrospinal fluid. The best access is from the back lumbar area. The spinal cord is relatively short and in the majority of people, it ends above this level. The lumbar portion of the spinal canal therefore contains only nerve fibers leaving the spinal cord and the risk of damaging spinal cord is minimal.
The patient usually lies on the side and makes rounded back. The doctor performs the puncture in the middle line of the lumbar area between the vertebrae. The hollow needle perforates the skin, subcutaneous tissue and deeper structures. It goes between the vertebrae until it reaches the spinal canal. When this happens, the fluid begins to flow from the hollow needle, so it can be sampled. The sample is sent for the biochemical, microbiological and cytological examination. The actual sting is almost painless, but local anesthetic can be applied.
The patient does not need any special preparation. However, when possible, it is advisable to discontinue any anticoagulant agents to decrease the risk of bleeding. The spinal tap should not be performed in people with intracranial hypertension and we should exclude it by computed tomography of brain or by ophthalmoscopy (in intracranial hypertension, the papilla of the optical nerve is swollen).
Cerebrospinal fluid is very important, because it is in direct contact with the central nervous system. Its attributes and composition (color, chemical composition, cytology) is affected by many pathological processes and diseases affecting the brain and spine. Increased number of white blood cells is typical for encephalitis and meningitis and in addition, microbiological examination can help to find the causative microorganism. Blood in cerebrospinal fluid may mean acute subarachnoid hemorrhage and tumor cells in the fluid are present in cases of spinal and brain tumors. A sample of cerebrospinal fluid is important for detecting neuroborreliosis (Lyme disease affecting the central nervous system) and autoantibodies typical for multiple sclerosis.
The most common complications is bleeding from the puncture site and irritation of a nerve located in the spinal canal. When a nerve is hit with the needle, it causes a sharp pain, which usually radiates into a lower extremity. The most serious complications is a direct damage of the spinal cord, but this is very rare. It could happen when the examiner punctures the canal too high, or when the spinal cord is longer and reaches to the lumbar area. Spinal cord injury may cause paralysis of the lower extremities or sphincter disorders.
In patients with present intracranial hypertension, spinal tap may be life-threatening, because the procedure decreases local pressure in the spinal canal and this may push the already compressed brain towards the spinal canal and damage vital brain centers.
The lumbar puncture is a very informative examination method that can quickly inform us about the situation in central nervous system and helps to confirm the diagnoses of many serious diseases.