Standardized measurement of the piriformis muscle and the proximal portion of the sacral nerve roots using transvaginal ultrasound

Abstract

Objective

To identify the sonomorphological appearance and to measure the thickness of the piriformis muscle (PM) and the proximal portion of the sacral nerve roots S1–S3 in healthy premenopausal women.

Materials and Methods

This prospective multicentric observational study included a consecutive series of women undergoing transvaginal sonography (TVS) at 2 tertiary gynecological referral centers. Standardized assessment of the pelvic organs was performed, followed by an attempt to visualize the right and left PM and sacral nerve roots S1–S3 at their origin in proximity to the sacral neuroforamen. Visualization rates, diameters of the muscle and nerve thickness, and the time needed to identify the PM were recorded.

Results

305 patients were included in the study. In 293 women (96.1%), the PM was identified bilaterally. The median diameter of the PM was 18.3mm on the right side and 18.4mm on the left side. S1 nerve roots were successfully identified bilaterally in 224 (73.4%) patients. Their right and left median diameters were 4.8mm. Both S2 nerves were successfully identified in 215 (70.5%) patients. Their median diameter were 4.3mm on both sides. S3 nerve roots were successfully identified in 203 (66.6%) patients. Their median diameter were 3.2mm on both sides.

Conclusion

We describe methods which allow consistent and rapid identification of the PM and the S1–S3 sacral nerve roots using TVS. Visualization of the PM and the proximal portion of the sacral plexus may be useful regarding identification of pathological changes in PM thickness and could help to distinguish perineural cysts from other gynecological pathologies.

Zusammenfassung

Ziel

Beschreibung der sonomorphologischen Merkmale wie auch die standardisierte Messung des Durchmessers des Musculus piriformis (PM) und der proximalen Anteile der Sakralnerven-Wurzel (SNW) S1–S3 bei prämenopausalen Frauen.

Material und Methoden

Diese prospektive Beobachtungsstudie umfasste konsekutiv untersuchte Patientinnen an 2 tertiären gynäkologischen Referenzzentren. Nach standardisierter Beurteilung der Beckenorgane wurde versucht, beide PM, deren Durchmesser und die proximalen Anteile der SNW S1–S3 an ihrem Ursprung in der Nähe des Sakral-Neuroforamens transvaginalsonografisch (TVS) darzustellen.

Ergebnisse

Von 305 Patientinnen wurde der PM bei 293 Patientinnen (96,1%) beidseitig detektiert. Der mediane Durchmesser des rechten PM betrug 18,3mm und des linken PM 18,4mm. Die S1-Nervenwurzel (NW) wurde beidseitig bei 224/305 (73,4%) Patientinnen erfolgreich identifiziert. Der mediane Durchmesser der S1-NW betrug an beiden Seiten 4,8mm. Beide S2-NW wurden bei 215/305 (70,5%) Patientinnen erfolgreich identifiziert. Der mediane Durchmesser der S2-NW betrug 4,3mm an beiden Seiten. Schließlich wurden bei 203/305 (66,6%) Patientinnen S3-NW erfolgreich identifiziert. Der mediane Durchmesser der S3-NW betrug 3,2mm an beiden Seiten.

Schlussfolgerungen

Die Identifizierung des PM durch TVS ist allermeist möglich; mit obig angegebenen Referenzwerten der PM-Dicke. Die SNW S1–S3 können bei der Mehrheit der Patientinnen mittels TVS an ihrem Ursprung identifiziert werden. Die Visualisierung des PM und des proximalen Anteils des Plexus Sacralis ist bei pathologischen Veränderungen des PM oder der Differenzialdiagnose von Adnex-Tumoren und Perineural-Zysten von Bedeutung.

Keywords

piriformis muscle - sacral nerve roots - deep endometriosis - transvaginal sonography - pelvic anatomy

Publication History

Received: 15 December 2024

Accepted after revision: 21 January 2025

Accepted Manuscript online:
21 January 2025

Article published online:
04 March 2025

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