Dear Editor, We present findings from a longitudinal, prospective study that examined the long-term efficacy of ultrasound-guided injection of incobotulinumtoxinA in six patients with piriformis syndrome. Characterized by buttock pain corresponding to the anatomical location of the piriformis muscle that varies according to patient position or activity, piriformis syndrome is a rare and poorly defined disorder—often still included in the broad definition of lower back pain . While etiology is not well established, the most accepted pathophysiologic factor in piriformis syndrome is compression of the sciatic nerve, which passes below and through the piriformis muscle. In the absence of a specific test, piriformis syndrome is primarily diagnosed on the basis of clinical symptoms, physical examination, and a positive response to local injection of anesthetic into the muscle . Conservative treatment of piriformis syndrome involves the use of analgesics, anti-inflammatory drugs and muscle relaxants, physical therapy, and intramuscular injection of corticosteroid and/or local anesthetics, guided by various imaging techniques. However, the operational complexity of guidance techniques such as fluroscopy and electromyography can limit their clinical usefulness . Ultrasound-guided injection offers process simplification in tandem with efficient, localized delivery of the injected drug , although clinical evidence to support its efficacy, safety, and precision for performing piriformis injections is currently lacking. Recently, injection of botulinum toxin into the piriformis muscle has shown promise in the management of piriformis syndrome [5–9]. We look to build upon this evidence base by establishing the long-term efficacy of incobotulinumtoxinA treatment, extending the duration of follow-up from the 12 to 16 weeks used in clinical trials to date [6,8,10] to six months. Our study, conducted at the Physical Medicine and Rehabilitation Department of the Hospital Virgen Macarena, Seville, Spain, assessed the efficacy, safety, and precision of a single ultrasound-guided injection of 100 U incobotulinumtoxinA (Xeomin; Merz Pharmaceuticals GmbH, Frankfurt am Main, Germany; in 2 mL saline) into the piriformis muscle of adults with chronic buttock or sciatic pain clinically compatible with piriformis syndrome. The study received relevant ethical approval. IncobotulinumtoxinA dose selection was based on previous studies of botulinum toxin for piriformis syndrome, which have used doses of 40 U incobotulinumtoxinA (case study), 50 to 100 U onabotulinumtoxinA, 150 to 200 U abobotulinumtoxinA, and 5,000 to 12,500 U rimabotulinumtoxinB . Patients were recruited on the basis of treatment-refractory buttock pain in the anatomical area of the piriformis muscle on selective finger palpation and at echopalpation, a pain score of 5 or higher out of 10 on a visual analog scale (VAS), and over three months of pain progression. Patients undergoing treatment with anticoagulants, with neurologic focality at the lower homolateral limb diagnosed as impairment of osteotendon reflexes or loss of strength, and the presence of other pathologies causing muscle weakness were excluded. In total, 24 patients (mean age = 57.0 years, SD = 12.6 years, 83.3% female) were recruited. Of these, only six (25.0%) attended with a correct diagnosis of piriformis syndrome, with sciatica (41.7% of patients) and chronic lower back pain (33.3%) accounting for misdiagnoses. Localization of the injection site was determined as described previously . The piriformis muscle appeared as a hypoechoic band-shaped mass on a hyperechoic image that corresponded to the sciatic recess (Figure 1). Poor needle imaging due to depth and inclination, an acknowledged difficulty with ultrasound-guided injection techniques, and limited visualization of the sciatic nerve (six cases, 25.0%) were the main technical difficulties encountered. Complications associated with the injection procedure were acute, self-limiting sciatica (one patient) and postinjection pain (four patients). Figure 1 View largeDownload slide Ultrasound image of the piriformis muscle. A Mindray M–7 ultrasound with a convex transducer at 5 to 7.5 MHz frequency was used. The piriformis muscle appears as a hypoechoic band on a hyperechoic image corresponding to the iliac fossa. GM = gluteus maximus; GSN = great sciatic notch; PM = piriformis muscle; SCT = subcutaneous tissue; SN = sciatic nerve. Figure 1 View largeDownload slide Ultrasound image of the piriformis muscle. A Mindray M–7 ultrasound with a convex transducer at 5 to 7.5 MHz frequency was used. The piriformis muscle appears as a hypoechoic band on a hyperechoic image corresponding to the iliac fossa. GM = gluteus maximus; GSN = great sciatic notch; PM = piriformis muscle; SCT = subcutaneous tissue; SN = sciatic nerve. Patients perceived an improvement in chronic pain intensity and quality of life (QoL) for up to six months after treatment with 100 U incobotulinumtoxinA, as evidenced by statistically significant reductions in VAS pain scores and significant improvements in pain-related QoL based on the Lattinen Index (LI; validated in Spanish ) at one and six months following treatment (P < 0.05, Bonferroni test for comparison of means) (Figure 2). At six months, all patients achieved predefined responder thresholds (≥50% score reduction from baseline) for both VAS and LI scores, with a significant correlation between the two (Spearmann correlation coefficients of 0.80 and 0.90 for months 1 and 6, respectively). The LI items that showed the greatest reduction in scores were degree of disability and sleep duration, suggesting that these were directly related to patient-perceived improvement in QoL. Six months after incobotulinumtoxinA injection, half of the patients were asymptomatic and the remainder reported feeling better; no patients reported unchanged or worsening symptoms. IncobotulinumtoxinA injections were well tolerated, and no adverse reactions were reported. Figure 2 View largeDownload slide Pain and QoL improvements after botulinum toxin treatment. A) Mean (SD) VAS score, range from 0 (no pain) to 10 (severe pain). B) Mean (SD) LI score of QoL in chronic pain, range from 0 (no impairment) to 20 (severe effect on QoL by pain). *Significant reductions (P < 0.05 Bonferroni test for comparison of means) in VAS and LI scores were found between baseline and one month post-treatment and baseline and six months post-treatment. LI = Lattinen Index; QoL = quality of life; VAS = visual analog scale. Figure 2 View largeDownload slide Pain and QoL improvements after botulinum toxin treatment. A) Mean (SD) VAS score, range from 0 (no pain) to 10 (severe pain). B) Mean (SD) LI score of QoL in chronic pain, range from 0 (no impairment) to 20 (severe effect on QoL by pain). *Significant reductions (P < 0.05 Bonferroni test for comparison of means) in VAS and LI scores were found between baseline and one month post-treatment and baseline and six months post-treatment. LI = Lattinen Index; QoL = quality of life; VAS = visual analog scale. The use of ultrasound guidance for incobotulinumtoxinA injection into the piriformis muscle reduced the technical complexity of the injection procedure in comparison with other conventionally used guidance techniques without increasing the incidence of complications or reducing the overall effectiveness of the procedure. That the procedure required only a single consultation without any need for irradiation has positive implications with respect to resource utilization in clinical practice. However, such promise must be countered by the fact that accurate localization of the piriformis muscle requires expertise in order to avoid injury to the sciatic nerve. Given the higher cost of botulinum toxin compared with that of local anesthetics  and the risk of muscle atrophy and fat generation that has been reported following botulinum toxin injection in piriformis syndrome , we acknowledge that this treatment approach may be most relevant for those patients with a level of disease that has proven refractory to previous treatment approaches. In conclusion, these results support the use of ultrasound-guided injection of 100 U incobotulinumtoxinA for the long-term management of treatment-refractory piriformis syndrome, suggesting that, with adequate clinical examination, the diagnostic need for an initial conservative injection of anesthetics may be eliminated. However, given the limited sample size, we acknowledge that further investigation is warranted before firm conclusions can be drawn, particularly regarding the lowest effective dose of incobotulinumtoxinA and the cost-effectiveness of such an approach. Acknowledgments The authors would like to thank the study patients and investigators. This study and the editorial support were funded by Merz Pharmaceuticals GmbH, Frankfurt am Main, Germany, with an unrestricted grant. Editorial support in the preparation of this manuscript (editing for English language) was provided by Claire Cairney (PhD) of Complete Medical Communications. References 1 Niu CC, Lai PL, Fu TS, Chen LH, Chen WJ. Ruling out piriformis syndrome before diagnosing lumbar radiculopathy. Chang Gung Med J 2009; 32: 182– 7. Google Scholar PubMed 2 Misirlioglu TO, Akgun K, Palamar D, Erden MG, Erbilir T. Piriformis syndrome: Comparison of the effectiveness of local anesthetic and corticosteroid injections: A double-blinded, randomized controlled study. Pain Physician 2015; 18: 163– 71. Google Scholar PubMed 3 Fishman SM, Caneris OA, Bandman TB, Audette JF, Borsook D. Injection of the piriformis muscle by fluoroscopic and electromyographic guidance. 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Pain Medicine – Oxford University Press
Published: Feb 1, 2018
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