HEALTH / Common Procedures: Lumbar puncture

THE diagnostic test called lumbar puncture or spinal tap is commonly feared by patients, but the anticipation is usually worse than the reality. The brain and spinal cord are surrounded by a crystal-clear watery solution of salts and proteins, and the purpose of a lumbar puncture is to obtain a small sample of this cerebrospinal fluid for laboratory examination.

The procedure is quite straightforward. The spinal cord is well protected within the bony tunnel of the spinal column, but it does not reach all the way to the bottom. The lumbar puncture needle is inserted between two of the lower bones in the lumbar region of the back, well below the end of the cord. The joints between the bones open up when the spine is curved forward. So for the test to be done, the patient lies on one side on a firm surface and pulls the knees up into the chest, making the spine curve as much as possible. The skin is cleaned and a little anaesthetic injected beneath the target spot. A long, thin, hollow needle with a sharp inner core is then inserted between the bones and advanced into the spinal canal. As the needle passes through the tough dura mater, the outermost of the membranes (meninges) that protect the cord, there is a distinct feeling of change in resistance. The central core is then removed from the needle and cerebrospinal fluid will flow out.

The pressure of the fluid is measured and a sample is taken and sent to the laboratory. This specimen of fluid gives some important information that cannot be obtained in any other way. If meningitis is suspected, a lumbar puncture removes any doubt: usually there will be enough white immune cells in the fluid to make it look obviously cloudy, and laboratory tests will allow precise identification of the bacteria causing the infection and the most suitable antibiotic to treat it.

Tests on the spinal fluid are also essential for the diagnosis and treatment of late syphilis and parasitic infections such as sleeping sickness. If a blood vessel on the surface of the brain has burst, then blood will be seen in the fluid. Tests may also be done which will help in the diagnosis of many other brain disorders, including tumours and multiple sclerosis. Drugs may be injected into the spinal fluid through the needle.

What about the hazards? There is a small risk of brain damage if the test is done in someone in whom the fluid is at very high pressure, but checks should be done to rule out this possibility. In practice, the only common problem is that some patients develop a severe headache which may last for hours or even days. The cause of this headache is thought to be leakage of the fluid through the puncture hole made in the spinal membranes. The risk of headache seems to be reduced if the patient lies flat for several hours after the test has been done.

Fewer lumbar punctures are done nowadays than in the past. The advances made in brain imaging techniques have made it possible to diagnose many brain disorders without needing a sample of spinal fluid. But a lumbar puncture does give direct access to the central nervous system, and it remains essential in the precise diagnosis of meningitis - though antibiotics may sometimes be given before the test is done on a child who is seriously ill.