Epidemiology and etiology of secondary piriformis syndrome: A single-institution retrospective study

Piriformis syndrome (PS) is a rare etiology of extra-spinal sciatica in which pathologies associated with or around the piriformis muscle (PM) irritate the adjacent sciatic nerve (SN), however, there is scarcity in the literature regarding its exact etiologies, thus, we performed a retrospective study to elucidate the epidemiology of PS and assess various causes of the syndrome. Our study included patients assessed at our institution who presented with sciatica of non-spinal origin between May 2014 and December 2015. Radiology reports of all patients who received pelvic MRI were examined for positive findings involving PM and SN. Of the 143 patients recognized with sciatica and negative lumbar pathology, 24 patients (17%) exhibited positive PM and SN findings. Average patient age was 50.0 ± 15.1 years (range: 21-75), and 17 were female. Seven patients (5%; 4M/3F) presented with tumor, seven patients (5%) had chronic inflammatory changes, one patient had SN adhesions to obturator muscle, three patients (2%, 3F) had aberrant anatomy, and the remaining patients had positive MRI findings, such as nerve atrophy or PM hypertrophy without identifiable cause. Seven patients received steroid injections in the peri-sciatic fossa, and four displayed poor response. Our findings suggested possible trends in extra-spinal sciatica. Affected males appeared more likely to present with tumor, while affected females were more likely to present younger, but with aberrant anatomy. Steroid injections appeared to be suboptimal in most cases. Pelvic MRI is helpful in patients with sciatica and negative spine imaging to rule out neoplastic involvement.