🔬 Lumbar Puncture

Lumbar puncture (LP) is an invasive procedure used to sample cerebrospinal fluid (CSF) for diagnostic purposes. It is essential for investigating meningitis, subarachnoid haemorrhage, and various inflammatory neurological conditions. It involves inserting a needle into the subarachnoid space of the lumbar spine under local anaesthesia.

Lumbar puncture is primarily indicated for the diagnosis of suspected central nervous system infections (meningitis, encephalitis) and subarachnoid haemorrhage. It is also used to measure intracranial pressure in suspected idiopathic intracranial hypertension (IIH) and to identify inflammatory or autoimmune conditions such as multiple sclerosis. Therapeutic indications include the administration of spinal anaesthesia, intrathecal chemotherapy, or the drainage of CSF in normal pressure hydrocephalus.

The procedure is performed under aseptic conditions, typically with the patient in the lateral recumbent position (fetal position) or sitting upright. A spinal needle is inserted into the subarachnoid space, usually between the L3/L4 or L4/L5 vertebrae, which is below the level at which the spinal cord terminates (L1-L2). Once the space is entered, opening pressure is measured using a manometer, and CSF is collected into sterile tubes for biochemistry, microbiology, and cytology.

A normal result shows a clear, colourless fluid with an opening pressure between 10-20 cmH2O in adults. Biochemistry should show a glucose level >60% of the simultaneous plasma glucose and a protein level <0.45 g/L. Microbiology should reveal a white cell count of <5 cells/uL (predominantly lymphocytes/monocytes), with no organisms seen on Gram stain and no growth on culture. Red blood cell count should be near zero.

Interpretation involves comparing CSF results against Serum samples (especially for glucose and protein). A 'clear and colourless' fluid with normal pressure and biochemistry usually excludes acute infection. Bacterial meningitis typically presents with high neutrophils, very low glucose, and high protein. Viral meningitis often shows high lymphocytes with normal glucose. Multiple sclerosis interpretation requires the presence of oligoclonal bands in the CSF that are not present in the serum.

Abnormal findings include an elevated opening pressure (>25 cmH2O), suggesting idiopathic intracranial hypertension or meningitis. Turbid appearance indicates high white cell count, typically neutrophils in bacterial meningitis or lymphocytes in viral or tuberculous meningitis. Low glucose relative to plasma glucose (<40%) is a hallmark of bacterial or fungal infection. Elevated protein levels are seen in inflammation, infection, or subarachnoid haemorrhage. The presence of xanthochromia confirms older subarachnoid blood.

The lumbar puncture is a critical diagnostic tool in neurology and emergency medicine. It is the gold standard for diagnosing meningitis and encephalitis, and for excluding subarachnoid haemorrhage when a CT head is negative but clinical suspicion remains. It also plays a role in the diagnosis of inflammatory conditions like Guillain-Barré syndrome (showing albuminocytologic dissociation) and multiple sclerosis (showing oligoclonal bands).

A 'traumatic tap' can introduce red blood cells into the tubes, potentially mimicking a subarachnoid haemorrhage; comparing the RBC count in the first and last tubes can help (a falling count suggests a traumatic tap). Failure to check coagulation status or platelet count beforehand can lead to the devastating complication of a spinal epidural haematoma. Post-lumbar puncture headache is a common complication, usually managed with fluids and analgesia.

A lumbar puncture cannot be performed safely in patients with signs of raised intracranial pressure (e.g., papilloedema, reduced GCS, focal neurology) due to the risk of brainstem herniation. It is also limited by patient habitus; obesity or spinal deformities can make the procedure technically difficult. It may yield 'non-diagnostic' results if performed too early in the course of an illness or after the administration of antibiotics, which can 'decapitate' bacterial growth.

Crucial for MLA exams: Always check for contraindications (coagulopathy, infection at the site, signs of raised ICP/space-occupying lesion) before proceeding. Know the CSF patterns for bacterial vs. viral meningitis. Understand that CT must precede LP in suspected subarachnoid haemorrhage, but LP is mandatory if CT is negative after 6 hours.