Description

Piriformis syndrome is due to entrapment of the sciatic nerve by the piriformis muscle. Buttock pain is exacerbated by activities that promote internal rotation of the hip, as the muscle promotes external hip rotation. This entity is unusual, sometimes even controversial, and is difficult to diagnose. Piriformis syndrome should be suspected when more common conditions, such as S1 radiculopathy, sacroiliitis, facet arthropathy and lumbar disk disease, have been excluded by imaging.1 Moreover, MRI of the pelvic girdle is extremely helpful to assist the diagnosis, because it can show structural changes in the muscle,2 as in the present case.

A 35-year-old man started suffering from right buttock and pelvic pain and in the right leg without any history of trauma. Clinical examination was significant for positive Freiberg’s, Pace’s, Beatty’s and flexion, adduction, internal rotation (FAIR) manoeuvres, suggesting piriformis syndrome.1 Electrodiagnostic studies ruled out lumbar radiculopathy or plexopathy. MRI demonstrated asymmetric hypertrophy of the right piriformis muscle (figure 1A) and was unremarkable in the lumbar spine.2 Conservative treatment with anti-inflammatory analgesic therapy, physical therapy and pregabalin 300 mg/day for 4 months did not lead to pain improvement. We treated the patient with three CT-guided botulinum toxin A injections (100 UI for the first injection and 50 UI for the others, in 2 mL saline solution, injected in equally divided doses into two locations of the muscle) within the first year (figure 1B), causing selective amyotrophy (figure 1C) and denervation (figure 1D) of the right piriformis muscle, with dramatic pain improvement.3 Three years later, the same pain relapsed together with reappearance of asymmetric right piriformis muscle hypertrophy (figure 2). We therefore repeated botulinum toxin injection in the right piriformis, obtaining sustained (more than 24 months) pain remission.3

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Figure 1

(A) MRI at diagnosis. Axial T1-w images show size asymmetry of piriformis muscles, with right (white arrow) much thicker than left (white arrowhead). (B) CT-guided botulinum toxin injection in the right piriformis muscle. (C, D) MRI 5 months after the first injection. Axial T1-w images (C) show amyotrophy of the right piriformis muscle (white arrow), while axial T2-w fat sat images (D) show muscle denervation.

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Figure 2

MRI at relapse (3 years later). Axial T1-w images show recurrent asymmetric hypertrophy of the right piriformis muscle (white arrow).

The prolonged follow-up in this case allowed us to find a direct correlation between piriformis muscle hypertrophy and clinical symptoms. Muscle MRI can be used to identify muscle hypertrophy, as a target for botulinum toxin injection.