Myobloc in The Treatment of Piriformis Syndrome
A Dose-Finding Study
Abstract
Piriformis syndrome (PS) occurs when the piriformis muscle compresses the sciatic nerve as it leaves
the buttock just below the greater sciatic foramen. Clinical signs for PS include positive straight leg
raise, weakened abduction of the flexed thigh, and tenderness at the intersection of the muscle and
nerve.
Electrophysiological diagnosis is made by comparing posterior tibial and peroneal H-reflexes elicited in the anatomical position with those obtained in flexion adduction and internal rotation (FAIRtest). Mean prolongation seen in normals is .01 milliseconds, (standard deviation = .62 milliseconds).
Taking three standard deviation prolongation of the H-reflex as the electrophysiological criterion for diagnosis, along with two of the three clinical conditions cited above, four groups of five patients with piriformis syndrome were identified. Each patient agreed to an IRB-approved protocol in which initially five patients were injected with 5000 units of Myobloc in four separate locations of the affected piriformis muscle under EMG guidance. Adverse effects and FAIR-tests were studied at weeks 0, 2, 4, 8, and 12. Patients received physical therapy twice weekly during that period.
After one month surveillance, the second, third and fourth group were injected with 7500, 10,000 and 12,500 units of Myobloc in successive months. Side effect profile for dry mouth showed the most significant increase, from 1.2 to 2.4 units of severity as dosage rose from 10,000 units to 12,500 units of Myobloc. Patients' visual analogue scale estimates of pain fell from an average of 6.7 to 2.3 over the period, with H-reflex response in the FAIR-test showing parallel decline. These responses were more dramatic at higher doses of Myobloc.
This study suggests that physical therapy and 10,000 units of Myobloc are a safe and effective treatment for PS, and secondarily that the FAIR-test is an effective means of diagnosing PS and assessing its clinical improvement.
Figure 1. Anatomy of the Piriformis Syndrome
The sciatic nerve passes under the
piriformis muscle, and above the gemellus superior in 80-85% of legs.
Figure 2: Signs and Symptoms of Piriformis Syndrome
1. Fifteen-degree or greater discrepancy in straight leg raise, or less than 65-degrees in bilateral cases.
2. Tenderness in the region of intersection of the piriformis muscle and the sciatic nerve.
3. Weakened abduction of the flexed thigh.
Figure 3. The H-reflex
First investigated by Hoffmann in the early 20th Century,
the H-reflex is essentially the Achilles Tendon reflex electrically elicited by
stimulating IA intrafusal fibres at the popliteal fossa, with responses recorded
electronically at the soleus muscle.
Figure 4a. The FAIR-Test
Placing the leg in Flexion Adduction
and Internal Rotation tightens the piriformis muscle against the fibres of the
sciatic nerve. In 88 normal patients, this maneuver did not prolong the H-reflex,
mean prolongation = .01ms, standard deviation = 1.86 ms.
Figure 4b. The FAIR-Test
Significant Prolongation of the H-reflex with Flexion, Adduction and Internal
Rotation
5a. The discriminatory power of the FAIR-test
Figure 6a.
Figure 6a. (continued)
Figure 6b.
Correct needle
placement required EMG activity with abduction (confirming that the needle tip
was either in the piriformis or the gluteus maximus) and absence of EMG activity
with extension (confirming that the needle tip was not in the gluteus maximus).
The process pictured was repeated in each of the four starred locations.
7. Physical Therapy Protocol for Patient Diagnosed with Piriformis Syndrome*
Place patient in contralateral decubitus and Flexed Adducted Internally Rotated (FAIR) position:**
1. Ultrasound 2.02.5 Watts/square cm., applied in broad strokes longitudinally along the piriformis muscle, from the conjoint tendon to the lateral edge of the greater sciatic foramen X 1014 minutes.**
2. Wipe off ultrasound gel.***
3. Hot packs or cold spray at the same location x 10 minutes.
4. Stretch the piriformis muscle for 1014 minutes by manually applying horizontal pressure to the muscle's inferior border, being careful not to press downward, rather directing pressure tangentially, toward the ipsilateral shoulder.****
5. Myofascial release at lumbosacral paraspinal muscles.
6. McKenzie exercises.
7. Use lumbosacral corset when treating patient in the FAIR position.*****
________________________________
Duration: 23 times weekly for one to three months.
* Patients usually require 23 months of biweekly therapy for 6070% improvement.
** Because it is painful, patients often subtly shift to prone. This must be avoided because it works to place the affected leg in abduction, not adduction, greatly reducing the stretch placed on the piriformis muscle.
*** Cavitation is unreported in more than 20,000 treatments.
**** Unless explicitly stated, therapists may tend to knead or massage the muscle, which is useless or worse. The muscle must be stretched perpendicular to its fibres, in a plane that is tangent to the buttock at the point of intersection of the piriformis muscle and the sciatic nerve, but approximately 11.5 inch deep to the buttock, (i.e., just below the gluteus maximus).
***** This is particularly important to avoid inducing lumbar hypermobility in patients with histories of laminectomy, fusion, or spondylolisthesis. Physical Therapy was given twice weekly. This protocol was available for outoftown patients and is now at ``Sciatica.org''.
Figure 8a. Pain Relief with Myobloc and Physical Therapy
Figure 8b. Onset and Decline of Beneficial and Adverse Effects of Myobloc
Figure a.
Dramatic pain-reduction and FAIR-test value reduction were strongly correlated
with each other.
Figure b.
Dramatic pain-reduction and FAIR-test value reduction
were strongly correlated with the appearance and resolution of adverse effects.
Figure 9. Clinical Course of Reinjected Patients
Figure 10. Synaptic strengths
Conclusions
Patients diagnosed with piriformis syndrome
via the FAIR-test are successfully and innocuously treated with Myobloc and physical
therapy. The present study presents optimal efficacy without adverse effects with
10,000 units given in four divided doses given at four sites in the affected piriformis
muscle at the time of diagnosis, and twice weekly physical therapy. The FAIR-test
provides an objective independent measure of remission of the syndrome, and is
well-correlated with visual analogue estimates of pain. For further details, please
see ``sciatica.org''.
