Lewis AM, Layzer R, et al. “Magnetic Resonance Neurography in Extraspinal Sciatica” Arch Neurol. 2006;63:1469-1472
When it comes to low back pain with associated radiating lower limb pain (sciatica), lumbar disc herniation and/or stenosis are the usual suspects with regard to causation. However, the clinical picture gets more complicated when the former and latter are not present on imaging, yet the patient suffers debilitating sciatica.
Besides other possible nonpathological causes (i.e., discogenic referred pain versus biochemical induced sciatica), abnormality within the piriformis and/or gemellus musculature have been proposed as a possible cause of the relatively rare extraspinal sciatica. More specifically, since the sciatic nerve passes between the piriformis and gemellus musculature, it has been speculated that hypertrophy in those muscles (as well as involvement of a adjacent fibrous band of connective tissue ) may compress the sciatic nerve in the pelvis, which in turn results in the patient's sciatica.
There is a problem, however, with making the diagnosis of piriformis-induced sciatica and that is the fact that we do not have a way to see what is going on other than exploratory surgery. Although that may be about to change:
In 2006, Kaiser’s Lewis et al, published the results of their retrospective investigation into the diagnostic value of MR neurography for making the diagnosis of extraspinal sciatica – i.e. radiating lower limb pain which did not have the typical disc herniation and/or stenosis ideology. This study could also be used to look at the correlation between extraspinal sciatica, piriformis/gemellus musculature abnormalities, and focal sciatic nerve inflammation (the authors didn't go this far, but I will).
The cohort (group of patients) was created from a retrospective (done after the fact) review of medical records of 14 patients that suffered unexplained unilateral sciatica – i.e., these patients had no disc herniation, stenosis or any other obvious cause of the sciatica. All of these patients had previously undergone MR neurography as part of a further diagnostic workup. The subsequent images from MR neurography were interpreted by one of the researchers, all of whom specialized in reading MR neurograms.
Unfortunately this was not a well written paper and the results were confusing. The authors reported that in 12 of patients (86%), MR neurography demonstrated sciatic nerve abnormalities within the pelvis, which could lead one to believe that MR neurography was fairly accurate for diagnosing piriformis syndrome. However, this statement did not encompass all the facts. More specifically, of those 12 patients, only 8 of them (57%) had signs of sciatic inflammation exactly adjacent to the sciatic nerve. Confounders (findings that did not support the theory) included the following:
#1) the MR neurography in one patient (7%) demonstrated inflammation in the sciatic nerve on both sides: the sciatic-affected leg, as well as the normal leg.
#2) another patient had a completely normal MR neurography, yet he had unexplained sciatica. To make things more confounding, this patient ultimately underwent a successful piriformis release-- the patient's sciatica got better from releasing the sciatic nerve from entrapment within the piriformis muscle (so MR neurography completely missed the boat here).
#3) another patient had positive MR neurography on the ipsilateral side, but the inflammation in the sciatic nerve was not at the level of the piriformis muscle – it was all the way down at the level of the ischial tuberosity (far away from the piriformis muscle).s
This was a very small retrospective study aimed at investigating the sciatic nerve in patients with unexplained sciatica. Of the 14 patients, 12 (86%) had focal inflammation within the ipsilateral sciatic nerve. Only 8 of the 14 (57%) showed signs of piriformis syndrome as confirmed by MR neurography. There were two confounders (14%), which did not relate MR neurography as a useful tool for investigating extraspinal sciatica: one patient, who was ultimately surgically cured by having a piriformis release (this piriformis muscle was constricting the sciatic nerve), had negative MR neurography. In another patient, the affected sciatic nerve demonstrated positive MR neurography, but so did the unaffected sciatic nerve.
Based on these results, it is fair to conclude that MR neurography is a useful tool for the diagnosis of extraspinal sciatica (i.e., 87% of the patients demonstrated abnormalities within the ipsilateral sciatic nerve within the pelvis). It is not, efficacious for the diagnosis of piriformis syndrome specifically (i.e. only 57% of patients demonstrated piriformis syndrome that was confirmed both by MR neurography).
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