Piriformis syndrome: muscle thickness or volume does not correlate with response to CT-guided injection

To determine whether size of the piriformis muscle, as characterized by either the coronal width or a segmented volume, correlates with outcomes after CT-guided injections.

A consecutive series of 81 patients with piriformis syndrome received CT-guided injections of the sciatic nerve and piriformis muscle. Volume and thickness measurements of the piriformis were taken from T1W and T2W pre-injection images by two readers. A logistic regression was used to test volume and size effect on first injection response. A cox proportional hazards model was used to evaluate pain-free survival. Identical analyses were performed to test the effects of muscle mass abnormality, nerve abnormality, body mass index, and presence of a split sciatic nerve.

There were 15/94 negative responses, 31/94 possible positive responses, and 48/94 positive responses to CT-guided injection. The average pain-free survival time was 38.91 ± 64.43 days. There was no significant correlation of first injection responses with muscle thickness or volume. There was no significant correlation in pain-free survival for muscle thickness or volume. There was no significant correlation in first injection response or pain-free survival with body mass index, muscle abnormalities, nerve abnormalities, or split sciatic nerves. The intraclass correlation was excellent between the two readers for both muscle volume (0.95-0.98) and thickness (0.92-0.97).

Piriformis muscle volume or thickness did not significantly correlate with post-injection outcome (first injection response and pain-free survival). Thus, if the patient has clinical symptoms of piriformis syndrome, the size of muscle should not determine whether injection is advisable.