Guidelines for botulinum neurotoxin injections in piriformis syndrome

Background: The piriformis muscle is normally involved in piriformis syndrome and can be treated with botulinum neurotoxin using several different injection methods. However, definitive injection guidelines for the muscle have not been reported previously. Aims: This study aimed to determine the ideal area for injections based on the intramuscular nerve distribution as obtained using a modified Sihler's staining technique. Materials and Methods: A modified Sihler's method was applied to the piriformis muscle in 15 specimens. The intramuscular arborization areas were identified based on two anatomical landmarks: (a) the lateral border of the sacrum bone and (b) the greater trochanter. Results: The nerve entry point for both piriformis muscles was found in the area between the lateral border of the sacrum and one-fifth of the distance toward the greater trochanter. The intramuscular nerve distribution for the piriformis muscle had the largest arborization patterns between one-fifth and two-fifths of the distance from the sacrum to the greater trochanter. The piriformis muscle was tendinous from two-fifths of the distance to the greater trochanter. Discussion: This study has yielded suggested optimal injection locations for the piriformis muscle relative to external anatomical landmarks. Conclusion: Clinicians can use these guidelines to ensure the effectiveness of not only botulinum neurotoxin injections but also other agents such as steroids, anesthetics, and normal saline. These guidelines will also help to avoid adverse outcomes of injection treatments.