Glycerol injections offer a low-risk surgical option to patients suffering from trigeminal neuralgia. Glycerol injections are one example of a rhizotomy; that is, a surgical procedure during which selected nerves are damaged to stop pain. Typically, the outpatient procedure is very straightforward: a needle is inserted through the skin (in the cheek, next to the mouth) into the trigeminal nerve near the base of the skull. This is the place where the three parts of the nerve come together and exits the skull. Doctors use an x-ray to confirm that the needle is in place, then they inject a small amount of glycerol, which damages the nerve and thus blocks the pain signal. Throughout the short process (it takes only a few minutes), the patient is sedated intravenously.
One very positive aspect of glycerol injections is their almost immediate effects; the glycerol acts quickly and stops nerve activity within a matter of hours. In a study published with the Journal of Neurosurgery by Harold Wilkinson, eighteen patients with trigeminal neuralgia were given sixty injections of 10% phenol, and "eighty-seven percent of injections brought marked or total relief initially." The downside to this procedure, however, is that the pain often returns for many patients. About half of the patients who benefit from glycerol injections will have recurrent pain within three or four years; glycerol injections can be performed more than once, but they are less effective over time (or with increasing number of procedures). In Wilkinson's study, relief lasted for an average of nine months and most patients requested a repeat procedure. Also, "Of those injections that provided initial relief, 37% still provided relief after 1 year and 30% after 2 years."
The logistics of the procedure are obviously very straightforward, and because it is so non-invasive, there are relatively few side effects associated with the surgery. Adverse effects to anesthesia are minimal (because the treatment is administered within such a short time), and other major problems include damage to the wrong nerve, or a failure to access the trigeminal nerve, as well as bleeding from the puncture site. The goal of the x-ray, of course, is to help target the correct nerve and prevent administering the glycerol to the incorrect site. More commonly, most patients experience mild facial numbness and/or tingling as a result of the damaged nerve, but this fades over time (unfortunately, numbness can persist for up to six months, which is not much shorter than the time frame for which the injection may be effective).
But how does the procedure really work? According to the National Institute of Neurological Disorders and Stroke, "The glycerol injection bathes the ganglion (the central part of the nerve from which the nerve impulses are transmitted) and damages the insulation of trigeminal nerve fibers." Glycerol and phenol (glycerol more commonly) are used as sympathetic blocks for nerve signals. On one hand, phenol dissolves nerve fibers and kills the nerve tissue prematurely (a process called "necrosis,") during which the dying cells do not signal to the immune system as normally-dying cells would do, leading to a build-up of dead tissue. This is why the injection is only administered in small amounts and to very specific regions of the nerve. Glycerol also prevents the nerve from producing signals by breaking up the nerve fibers. Over time, the pain may return as the nerve repairs and re-myelinates (coats) itself, although studies on rats indicate that examination of phenol- or glycerol- affected nerves can be morphologically distinguished from healthy nerves.