First described in 1937 (1), piriformis syndrome is caused by abnormal piriformis bundles compressing the sciatic nerve, leading to pain in the buttocks, hips, and/or lower limbs and lower urinary tract symptoms. Beaton and Anson reported six possible relations between the sciatic nerve and its subdivisions to the piriformis muscle. Cadaver studies have found that these anatomical variations range 6.4 – 15.8% (1,2). More recently, intrapelvic entrapment of sacral nerve roots by abnormal bundles of the piriformis muscles have been described as a possible cause of pudendal neuralgia, lower urinary tract symptoms and sciatica (3)(Figure). However, the intrapelvic portions of the lumbosacral plexus and sacral nerve roots involved in anatomic variations of the piriformis muscles have not yet been reported.

A study of 32 female cadavers donated to an academic institution for medical education was undertaken. Laparoscopic pelvic dissections were performed, using standard laparoscopic techniques including Veress entry with closed intraperitoneal insufflation. Lateral and suprapubic ports were placed. Retroperitoneal dissection was undertaken using sharp and vessel-sealing instruments. Dissection was performed by gynecology faculty, fellows, and residents, under the supervision of faculty selected for their experience in minimally invasive gynecologic surgery. The quality of visualization was similar to an intraoperative dissection.  
	Of 32 cadavers, 29 had bilateral dissections, thus providing 61 total sites for analysis. Data was collected in native pairs. After dissecting the presacral space, the piriformis muscle was identified and followed to determine if any sacral nerve roots were entrapped by abnormal muscles bundles. In particular, we identified the specific nerve roots affected.

Anatomical variations were seen in 12 of 32 cadavers (37.5%, 95% CI 
20.73, 54.27). None had bilateral variants. Of the twelve variations, five were on the left, and seven on the right. The nerve root most involved was S3 (9/12), followed by S2 (11/12), S1 (5/12), and S4 (2/12).

This cadaver study reveals that intrapelvic piriformis anatomical variations that could lead to piriformis syndrome exists in the general population at a prevalence of 37.5%, which is much higher than most reports. This may be due to limitations in the six types described by Beaton and Anson (1).

Knowledge of the possible anatomical variations can greatly aid diagnosis of sciatica and pudendal neuralgia, neurogenic pelvic floor dysfunction and lower urinary tract symptoms.

Beaton LE and BJ Anson. The relation of the sciatic nerve and of its subdivisions to the piriformis muscle. The anatomical record. 1937(70): 1-5.
Natsis K, Totlis T, Konstantinidis GA et al. Anatomical variations between the sciatic nerve and the piriformis muscle: a contribution to surgical anatomy in piriformis syndrome. Surg Radiol Anat. 2014(36): 273-80.
Lemos N, Papillon-Smith J, Moretti-Marques R, Fernandes G, Girao M, Solnik J. Intrapelvic nerve entrapment as a cause of pelvic floor dysfuction and refractory pudendal pain: a review of 50 cases. IN: 47th Annual Meeting of the International Continence Society, 2017, Florence. Neurourology & Urodynamics. Wiley, 2017. v.36(S3). p.S212 – S213