We inject Botox or Myobloc under EMG guidance, selecting four points in the muscle that should reach different myoneural junctions (see Fig. 5). Each site receives either 75 U of Botox or 3125 U of Myobloc, adding up to 300 U of Botox or 12,500 of Myobloc. We use a 75- or 80-mm monopolar Teflon-coated 23 gauge injectible monopolar needle (e.g., of the type made by Chalgren Enterprises; see Figs. 5 and 6).
We palpate the buttock for the most tender spot, then judge by the greater trochanter and the sciatic foramen where the piriformis muscle's outlines are located. After one has a little experience, the rostral and caudal edges of the muscle are usually identifiable,
because it lies just beneath the gluteus maximus, and often close to 1 in. below the skin at its lateral extreme (23).
We start the series of four injections by positioning the patient in lateral decubitus position with the hips and knees flexed 90°. The first needle is inserted just medial to the musculo-tendinous junction, inserting the needle approximately 1 in.
Then we go through two maneuvers to be sure the EMG-guided needle is located in the piriformis muscle. First, the patient is asked to abduct the thigh. If we see no interference pattern, then we insert the needle 0.25 in. further and repeat the process until we see a reasonable interference pattern. The interference pattern assures us that we are either in the gluteus maximus or the piriformis muscle.
Second, we ask the patient to extend the leg. If we are in the piriformis muscle, there will be electrical silence, and we can proceed with injecting the BTX. If there is an interference pattern, then the needle tip is in the gluteos maximus; we continue to advance the needle another 1/4 inch and repeat the entire process, beginning with abduction, until we see an interference pattern with abduction, but not with extension.
There are a couple of fine points. In the first maneuver, if the first injection point is particularly lateral, then it is possible that the needle will encounter the gluteus medius or minimus. In that case, one must use external rotation to distinguish these muscles from the piriformis. We ask the patient to keep his or her feet together and raise just the knee in that case, externally rotating instead of abducting the entire leg.
In the second maneuver, when a patient is asked to extend the leg, he or she will almost invariably lift it. This will engage the piriformis muscle as well as any of the glutei. So we place one hand between their knees, and encourage them to squeeze their knees together while extending the hip. We often hold the sole of the affected leg's foot and ask the patient to push that hand away while squeezing our other hand between the knees. This is easy for them to do. Once in a while the patient will try to lift just the ankle, which also has an obscuring effect on the test. We therefore also ask these patients to squeeze their ankles together while extending the upper leg.
After completing both maneuvers and the injection in all four locations, we stress to the patients that it will take anywhere from 5 days to 2 weeks to actually feel the benefits of the injection, and that physical therapy is absolutely necessary if the relief is going to last. We encourage them to learn a short series of yoga poses as well (see www.sciatica.org).
A few patients may experience reduced or no responsiveness to BTX following repeated injections. Factors including severity, improper muscle selection, dosing, and genetic characteristics can contribute to ineffectiveness or increased resistance. Although not commonly done in our practice, numerous tests are available to detect antibodies against BTX if resistance is suspected. The simplest and most cost-effective tests are the frontalis antibody test and the unilateral brow injection test. Other tests include mouse protein assay, Western blotting, and enzyme-linked immunosorbent assay. Antibody formation can be limited by using the lowest possible dose in concordance with the longest possible hiatus between injections.
Botox and Myobloc are immunologically distinct. Therefore, if the patient has had Botox, then he or she is still very unlikely to have a reaction to Myobloc unless there is a history of botulism earlier in life. Alternating Botox and Myobloc over time is a wise course if multiple injections are necessary.
However, most patients with PS do not need repeat injections. It is clear from many laboratory tests that the actual neurotoxins inhibit evagination of the acetylcholine vacuoles from the neuron at the myoneural junction for only 2 to 3 months. Nevertheless, these injections frequently result in permanent or at least significantly greater periods of relief. How does this happen? We believe that once the muscle has been lengthened, at least two different mechanisms come into play.
The first mechanism involves the continued denervation of a subset of those muscle fibers initially affected by the injection. We have seen EMG evidence of denervation many months and even years after a single BTX injection. Second, after patients have improved, they incorporate greater movements into their daily lives. It is similar to a frozen shoulder where one works and works until a good range is obtained, but then, upon cessation of therapy, the patient rarely redevelops a frozen shoulder. This commonly happens because the patients' newly acquired extra range is used reaching something on a high shelf, passing the sugar, or playing handball. The same occurs with PS: after stretching the muscle, the patient will elongate that muscle more naturally in getting up from sitting, walking up a flight of stairs, and in other activities.
The injection of Botox for PS is high-dose relative to many of its other uses. Nevertheless, it has resulted in no side effects to date, in more than 500 different instances of these multiple injection patterns.
Myobloc is safe, but we have seen a few side effects with it including dry mouth, rare blurry vision that may last a week or two (2 cases out of 100) and constipation in three cases.
One unexpected beneficial side effect from Myobloc was seen in a patient with severe asthma that frequented emergency rooms four times per year with exacerbations. She surprisingly reported that she had suffered no asthma attacks for nearly 1 year following a Myobloc injection. This may be a parasympathetic effect in that respect similar to the constipation and the dry mouth that are more commonly seen with Myobloc.
Putting no monetary value on patient suffering, loss of work days, and ability to fill societal roles such as parent or spouse, the cost of underdiagnosed and virtually untreated PS, including 1.5 unnecessary MRIs, 0.4 unnecessary surgeries, and visits to 6.5 clinicians per patient is well above $8000 per patient and rising daily (14).
The average cost of proper diagnosis and BTX injection is less than $3000 per patient, but is also susceptible to increases.
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