Botulinum toxin (BT) injection is a new treatment for piriformis syndrome (PS). The main purpose of our study was to use MRI to evaluate changes in piriformis muscle morphology after treatment with BT injections.

Twenty patients presenting with PS who had undergone an MRI were included retrospectively: 12 patients treated with BT injections and eight untreated patients. The following parameters were assessed and compared to a normal contralateral muscle: maximum thickness, volume, and Goutallier's classification grade of fatty infiltration of the piriformis and internal obturator muscles. Pain was assessed through a visual analogue scale (VAS).

The untreated patients had no significant difference in the volume (P=1.0) or thickness of the piriformis muscle (P=0.61). The treated patients showed a significant reduction in the thickness (−4.2mm; P<0.001) and volume (−74.4mm³; P<0.001) and an increase in the fatty infiltration (P<0.001) of the piriformis muscle treated by BT injection. Muscular atrophy was correlated with the number of BT injections and with the time until an MRI was performed. There was also significant pain relief after BT treatment.

BT leads to atrophy and fatty degeneration of the piriformis muscle that can be quantified by MRI and these factors explain why BT injections are effective in the treatment of PS.