Lumbar Puncture (Spinal Tap)

A child has a high fever and a neck that hurts to bend. An adult has the worst headache of their life, yet the CT scan is normal. In both cases the answer may lie in the fluid around the brain and spinal cord.

A lumbar puncture, often called a spinal tap, collects a small sample of that cerebrospinal fluid (CSF). After numbing your lower back, a doctor passes a fine needle between two vertebrae into the fluid-filled space below where the spinal cord ends. A few millilitres drip into sterile tubes, and your body replaces them within hours.

It can show whether meningitis is bacterial, viral or tuberculous, or whether there has been bleeding around the brain, which scans alone can't always answer. The main downside is a headache afterwards, which usually settles within days.

Our neurology team treats meningitis, encephalitis and multiple sclerosis, and samples are tested in our pathology lab, which runs microbiology and biochemistry.

Normal spinal fluid is crystal clear with very few cells. Cloudy fluid suggests infection, and blood-stained or yellowish fluid can mean bleeding. Opening pressure in adults is normally about 10 to 25 cm of water.

Cells, protein and sugar come back within hours. Many white cells with low sugar suggest bacterial meningitis, while a moderate rise with normal sugar fits viral meningitis. Low sugar, high protein and mainly lymphocytes raise concern for TB. High protein with normal cells is typical of Guillain-Barré syndrome. Cultures take 2 to 3 days and TB cultures several weeks, so if infection is likely, treatment usually starts before they're back.