Ask Dr Gary: Peace of mind from small needle

Meningitis and brain aneurysms can be diagnosed through spinal taps.

Lumbar punctures are probably one of the least welcome procedures in the emergency department - after urinary catheters of course. This week we'll talk about how lumbar punctures work, when they're necessary, and what the risks are.

To clinch a neurological diagnosis, sometimes there's just no substitute for looking at the spinal fluid, which bathes the brain and spinal cord. In the case of the teenager with fever and a stiff neck, the fluid might be crawling with meningitis-causing bacteria. Or in someone who's just experienced the worst headache of their life, the spinal fluid might lead us to the critical diagnosis of a ruptured brain aneurysm.

Both diseases are uncommon, but not exactly rare. A small New Zealand emergency department might see one or the other every few months. To catch those cases, which can be devastating or even fatal if missed, we have to do dozens of spinal taps on people who only may have these conditions.

To perform a tap, doctors insert a thin needle deeply into the back, about 5cm deep in thin people, sometimes much further in very large individuals. The doctors can't see their target, which is where it gets tricky.

Using a combination of bony landmarks felt on the spine and experience, they pass the needle between the bones of the spine, at an angle and through spinal ligaments, until they hit clear fluid that looks just like water. Things like old age, spinal curvature, excess weight, and arthritis all make the process more challenging.

Contrary to what a lot of people think, the spinal cord doesn't actually run down to the tailbone, but ends in the middle of the back, continuing on as a bunch of little nerves.

We tap in the lower back, where there is no spinal cord and thus no chance of the needle tip injuring it. In studies, about a third of spinal tap patients have transient back pain, a worse headache, or leg pain and tingling after the procedure.

These are non-serious, and typically go away within hours to a few days. One in a thousand can end up with long-lasting leg pain or tingling.

And of course, just like with getting any injection or an IV, any time the skin is broken there is a tiny chance of infection or bleeding which, in extremely rare cases, can cause real problems.

Blood or pus can accumulate, pushing on nerves. For those on blood thinners, the bleeding risks can be serious. For those not on blood thinners, the risks related to bleeding are exceptionally small.

Like any intervention, the benefit of the test has to outweigh the risk.

The doctor can share their understanding of the potential risk of both the disease and the spinal tap, but it's up to the patient to process that information and give their informed consent.

In general when a tap is being considered, the disease we're considering is potentially deadly, and the decision to get tested is usually not a hard one. Peace of mind counts for a lot.