This article is divided into several pages for easier reading:

4.0     What Can Cause Piriformis Syndrome?

The aetiology of piriformis syndrome is not completely understood, but anatomical variations of the sciatic nerve in orientation to the piriformis muscle have been suspected as a possible cause.

“This is one of the rare causes of sciatic pain that is not spinal in origin. Although it has been established as a genuine compression neuropathy originating from the passage of the sciatic nerve through the infrapiriform foramen, there are various potential aetiologies for this compression. It may be due to inflammation, trauma, a tumour, a malformation, but most often it is muscular…” (Al-Al-Shaihk, 2015, p.41).

The primary mechanisms of piriformis syndrome include:

  • Initially, a person may have a piriformis strain which “may be caused by excessive external rotation and abduction when the thigh is being flexed.” (Hamil, Knutzen & Derrick, 2015, p.193).
    • This can result in pain in adduction, flexion, and internal rotation of the thigh and, consequently, piriformis syndrome can develop.
  • Inflammation (Al-Al-Shaihk, 2015).
  • Trauma (Al-Al-Shaihk, 2015), for example:
    • Haematoma (bleeding) in the area of the piriformis muscle.
    • Swelling of the piriformis muscle, due to injury or spasm.
    • Tightening of the muscle, in response to injury or spasm.
    • Long-term micro-trauma causing scarring (Knudsen, Mei-Dan & Brick, 2016).
    • Haematoma, swelling and scarring can reduce the space in the area leading to compression of the sciatic nerve.
    • “…macrotrauma to the buttocks, leading to inflammation of soft tissue, muscle spasm, or both, with resulting nerve compression.” (Boyajian-O’Neill et al., 2008, p.659).
    • “Microtrauma may result from overuse of the piriformis muscle, such as in long-distance walking or running or by direct compression. An example of this kind of direct compression is “wallet neuritis” (ie, repetitive trauma from sitting on hard surfaces).” (Boyajian-O’Neill et al., 2008, p.659).
  • Tumour (Al-Al-Shaihk, 2015).
  • Malformation, for example (Jankovic, Peng & van Zundert, 2013; Al-Al-Shaihk, 2015):
    • Asymmetry/anatomical variation of the piriformis muscle(s). However, Russell et al. (2008) demonstrated asymmetry in piriformis muscle thickness ranging from 3-8 mm, and cases of piriformis syndrome where the piriformis was of normal size.
    • Course variants of the sciatic nerve.
    • Research suggests that 8-11% could have an anatomical variation. This means certain individuals may be at an increased risk of injury to the nerve with specific activities, for example, bike riding.
    • Course variants of the posterior cutaneous femoral nerve, inferior gluteal nerve, and the superior gluteal nerve, can predispose to piriformis syndrome.
    • Having a long versus short piriformis muscle (Horschig, 2017).
  • Muscular, for example (Al-Al-Shaihk, 2015, p.42):
    • Hypertrophy or atrophy of the piriformis muscle.
    • “…a functional compression of the sciatic nerve during muscle contraction.”
    • Overuse and/or vigorous activity, for example, marathon training.
      “…prolonged or excessive contraction of the piriformis muscle (PM).” (Jankovic, Peng & van Zundert, 2013, p.1004).
  • Muscle spasm in the piriformis muscle, either because of irritation in the piriformis muscle itself, or irritation of a nearby structure such as the sacroiliac joint or hip.
  • A tight piriformis muscle may compress the sciatic nerve:
    • Nerve impingement may result from overuse or hypertrophy of the piriformis muscle.
    • Nerve impingement may cause features such as radicular pain, numbness, or weakness of the lower extremity.
  • Piriformis syndrome can also manifest as a consequence of a functional short leg that lengthens the piriformis and then stretches it as the pelvis drops to the shorter leg (Hamil, Knutzen & Derrick, 2015).
  • Improper use or prolonged disuse of the pelvis that results in a piriformis that is short and tight, for example, a sedentary (desk) job.
  • Hyperlordosis (abnormally exaggerated forward curvature of the lumbar and cervical regions of the spine) of the lumbar spine, as seen with pregnancy, excessive abdominal weight and/or muscular imbalance. The exaggerated spinal curve increases the tension of the pelvic-femoral muscles, including the piriformis, as the muscles try to stabilise the pelvis and spine in the new position.
  • Other causes identified in a review by Jankovic and colleagues (2013, p.1006) included:
    • Intragluteal injection.
    • Femoral nailing.
    • Myositis ossificans of the piriformis muscle.
    • Klippel-Trénaunay syndrome.
    • Abscess.
    • Bursitis of the piroformis muscle.
    • Colorectal carcinoma.
    • Episacroiliac lipoma.

Despite the numerous potential causes for piriformis syndrome noted above, Jankovic and colleagues (2013, p.1005) state “In most patients, there is no identifiable cause.” However, they do go on to state that previous gluteal trauma can cause sciatica-like pain, and this is probably the most common cause.

4.1     Why Is It A Problem?

“Failure to identify the cause of pain in a timely manner can increase pain perception, and affect mental control, patient hope and consequently quality of life.” (Martin et al., 2015, p.99).

A piriformis strain can result in pain in adduction, flexion, and internal rotation of the thigh. Consequently, piriformis syndrome can develop which is an impingement of the sciatic nerve aggravated by internal and external rotation movement of the thigh during walking.

The irritation of the sciatic nerve causes pain in the buttock area that can travel down the posterior surface of the thigh and leg.

It is a problem because it can cause/mimic:

  • Pain when sitting or undertaking activity;
  • Radicular pain of the lower back or hip;
  • Muscular weakness;
  • Paraesthesia of the affected leg (a sensation of pricking, tingling, or creeping on the skin that has no objective cause);
  • Neurological symptoms of abnormal reflexes or motor weakness; and/or
  • A hamstring tear or intra-articular hip pathology such as aching, burning sensation, or cramping in the buttock or posterior thigh.

“Through compensatory or facilitative mechanisms, piriformis syndrome may contribute to cervical, thoracic, and lumbosacral pain, as well as to gastrointestinal disorders and headache.” (Boyajian-O’Neill, 2008, p.659).

4.2     What Are The Complications?

  • Some invasive surgical techniques can provoke postoperative scarring tissue formation which can increase the incidence of the syndrome recurring, although minimally invasive techniques are being developed which should minimise scarring tissue formation (Knudsen, Mei-Dan & Brick, 2016).
  • “Some authors have suspected that contraction of the PM is an often overlooked cause of coccygodynia [pain in the coccyx and adjacent regions].” (Jankovic, Peng & van Zundert, 2013, p.1005). Swelling in the affected leg and disturbances of sexual function are observed (dyspareunia in women, 13-100%, and disturbances of potency in men are very often present as accompanying symptoms). Intense pain will occur when the patient sits or squats (39-95%).

4.3     Arguments for Piriformis Syndrome

In their review Miller and colleagues (2012, p.578) suggest a number of arguments for piriformis syndrome:

  • Piriformis syndrome is a reasonable explanation for at least a significant proportion of the vast majority of patients with sciatic-like symptoms whose pain is not explained by other, more accepted diagnoses.
  • The anatomic location of the piriformis muscle corresponds exactly with the area of focal tenderness observed in these patients.
  • The course of the muscle relative to the sacral nerve roots explains the results of a host of provocative tests that often are positive in these patients.
  • A variety of imaging and neuro-diagnostic tests now confirm the presence of piriformis pathology.
  • Numerous patients have responded very well to either focal injections or surgical manipulation of the piriformis, thereby implicating it as the cause of symptoms in those cases.

4.4     Arguments against Piriformis Syndrome

In their review Miller and colleagues (2012, p.578) suggest a number of arguments against piriformis syndrome:

  • There is no convincing evidence that the piriformis is anything more than a rare to very rare cause of sciatic nerve entrapment.
  • The evidence that exists suggesting otherwise is based upon flawed studies and/or reasoning.
  • Studies on patients who have undergone surgery for other reasons, as well as on cadavers, have demonstrated that piriformis-induced sciatic nerve compression is either uncommon or highly non-specific.
  • Electrophysiologic and imaging studies suggesting pathology generally are non-specific and, consequently, potentially misleading.
  • Numerous other causes of the symptoms are at least as likely.
  • The label ‘piriformis syndrome’ is therefore misleading, and should be changed to a more general term that does not implicate any particular anatomic structure. Linked to Martin and colleagues (2015) term of deep gluteal syndrome (Section 2.2).
  • Injections and surgical manipulations of the piriformis muscle are being performed too commonly and usually without adequate justification.
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