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.