Piriformis revisited

In regret triggered by the title of Probst et al 2019.[1]

This diagram illustrates the anatomical location of piriformis relative to the pelvis, and the characteristic pain referral patterns that form part of the myofascial syndrome. The blue dots on the leg represent tingling that might occur if there is nerve entrapment in piriformis syndrome.

Whenever I see a paper on piriformis syndrome (PS), I recall with regret that when Acupuncture in Medicine achieved Medline listing in 2001 (hurrar), the archive was not included in the listing. I had just published an extensive review article on PS in the issue immediately prior to the first issue listed – issue 1 of volume 19. My review paper was in issue 2 of volume 18.[2] The current review, 19 years on, has 24 fewer references than mine, hence regret that my review is not easily found. When writing a contemporary paper, you really want to add to what has already been published not subtract!

Anyway, I got over it, and decided to search the recent literature to see if anyone had come up with anything new of interest.

I should note at this point that most papers on PS refer to sciatic nerve entrapment at the pelvic outlet where the muscle piriformis and the sciatic nerve both squeeze through the greater sciatic foramen – an unyielding rim of bone and sacrospinous ligament. It is somewhat controversial because it is hard to diagnose, and it is not a common cause of ‘sciatica’. By contrast, the myofascial syndrome from piriformis in the absence of sciatic nerve entrapment is very common.

Myofascial pain from piriformis is common

Sciatic nerve entrapment by piriformis is uncommon

The first paper that caught my eye was from a radiology journal in 2018. It had a catchy title: Is it painful to be different? Sciatic nerve anatomical variants on MRI and their relationship to piriformis syndrome.[3] 783 MRI studies were included, and of them 150 hips (19.2%) showed anatomical variants according to the Beaton and Anson anatomical types.[4] Clinically diagnosed piriformis syndrome was present in 11.3% of variant hips compared with 9.0% of normal hips (p=0.39). The percentage of anatomical variants in this retrospective review was somewhat higher than in Beaton and Anson’s original cadaveric study of 2250 limbs.[4] In the latter, the percentage of variants was 10.7%, but selection criteria are different of course, and MRI studies in a retrospective review are done for a reason, therefore they do not sample the population in the same way as a cadaveric study.

This graphic illustrates the six different anatomical variants of the sciatic nerve in relation to piriformis and the greater sciatic foramen. The nerve is illustrated with two components – peroneal and tibial. The top left is the most common arrangement, and is seen in around just under 90% of hips. The variant below it, with the peroneal component piercing piriformis is next most common at around 10%. The others are much rarer, and make up less than 1% of hips. All the figures from different anatomical studies are listed in my original review.

The next paper that attracted me was a quite recent one on ultrasound (US) diagnosis of piriformis syndrome (PS).[5] I looked at it because it was published in Muscle & Nerve, a journal with a very good reputation. They studied 36 patients with piriformis syndrome and 25 healthy volunteers. PS was diagnosed clinically using criteria defined by Kirschner et al,[6] and scored using criteria defined by Michel et al.[7] Piriformis muscle thickness and cross-sectional area (CSA) were measured with US and MRI. US proved reliable in identifying the symptomatic side in PS patients and demonstrated a significant increase in muscle thickness and CSA in PS compared with healthy controls.

I noted an editorial in the same issue of Muscle & Nerve, and I recognised the name of the first author, so I next looked at that.[8] The name I recognised was Lauren Fishman. I had sited a prior paper of hers in my review 19 years ago. She had found that the H-reflex latency is lengthened in patients with PS when they are placed in the FAIR position. This refers to hip Flexion Adduction and Internal Rotation.

The H-reflex or Hoffmann reflex in simple terms is the electrophysiological equivalent of a stretch reflex, usually the ankle jerk. I had to really get to grips with it for an editorial in Acupuncture in Medicine 10 years ago.[9] Lars (the current president of IASP) was excited about the fact that the H-reflex stimulated from the same muscle was different when created by a needle within a trigger point band (with SEA – spontaneous electrical activity) compared with muscle outside the band (with no SEA).[10] When he spoke at the BMAS meeting in Leeds in 2007, I remember him saying that the H-reflex is a hard electrophysiological measure, and certainly not subject to modification by expectation. For him it put trigger points on the map I think.

The editorial by Fishman and Hosseini covered three diagnostic methods for PS.[8] The H-reflex latency change in FAIR, the new kid on the block ie US, and MRN. What is MRN I hear you say? Yes I had not heard of that one either. It is Magnetic Resonance Neurography, that is MRI tuned to focus on the water inside nerves. It is not in widespread use as yet, and no doubt quite expensive.

Well most of us will not have access to any of those diagnostic tests, so we will have to continue to rely on clinical examination and the diagnostic probe that is the filiform acupuncture needle. Is that your pain? Or is it just another one in the same area as yours?


1         Probst D, Stout A, Hunt D. Piriformis Syndrome: A narrative review of the anatomy, diagnosis, and treatment. PM&R 2019;:pmrj.12189. doi:10.1002/pmrj.12189

2         Cummings M. Piriformis Syndrome. Acupunct Med 2000;18:108–21. doi:10.1136/aim.18.2.108

3         Bartret AL, Beaulieu CF, Lutz AM. Is it painful to be different? Sciatic nerve anatomical variants on MRI and their relationship to piriformis syndrome. Eur Radiol 2018;28:4681–6. doi:10.1007/s00330-018-5447-6

4         Beaton LE, Anson BJ. The relation of the sciatic nerve and of its subdivisions to the piriformis muscle. Anat Rec 1937;70:1–5. doi:10.1002/ar.1090700102

5         Zhang W, Luo F, Sun H, et al. Ultrasound appears to be a reliable technique for the diagnosis of piriformis syndrome. Muscle Nerve 2019;59:411–6. doi:10.1002/mus.26418

6         Kirschner JS, Foye PM, Cole JL. Piriformis syndrome, diagnosis and treatment. Muscle Nerve 2009;40:10–8. doi:10.1002/mus.21318

7         Michel F, Decavel P, Toussirot E, et al. The piriformis muscle syndrome: an exploration of anatomical context, pathophysiological hypotheses and diagnostic criteria. Ann Phys Rehabil Med 2013;56:300–11. doi:10.1016/j.rehab.2013.03.006

8         Fishman LM, Hosseini M. Piriformis syndrome – a diagnosis comes into its own. Muscle Nerve 2019;59:395–6. doi:10.1002/mus.26417

9         Cummings M. Myofascial trigger points: does recent research gives new insights into the pathophysiology? Acupunct Med 2009;27:148–9. doi:10.1136/aim.2009.001289

10       Ge H-Y, Serrao M, Andersen OK, et al. Increased H-reflex response induced by intramuscular electrical stimulation of latent myofascial trigger points. Acupunct Med 2009;27:150–4. doi:10.1136/aim.2009.001099

Declaration of interests MC