Unilateral Accessory Slip of the Splenius Capitis Muscle: A Case Report

Abstract

Introduction: Anatomical variations in the muscles of the posterior neck are relatively uncommon, especially involving the serratus posterior superior muscle.

Case Description: This case report describes an unusual unilateral variant of the serratus posterior superior muscle found during routine dissection of a formalin-fixed male cadaver. On the right side of the neck, an accessory muscle slip was discovered, originating from the mastoid process of the temporal bone close to the typical attachment site of the splenius capitis muscle. This abnormal muscle travelled superficially over the native splenius capitis and splenius cervicis muscles, then continued deep to the rhomboid minor, and inserted into the fascia on the serratus posterior superior muscle near the medial attachment of the serratus posterior superior muscle. No similar variation was seen on the opposite side, making this a unilateral finding.

Conclusion: This variant was named the "Accessory serratus posterior superior muscle" based on its origin and insertion pattern. Recognizing such muscular variations is clinically important because they may mimic soft tissue masses in imaging studies or contribute to myofascial pain syndromes. Awareness of these anomalies is essential for clinicians, radiologists, and surgeons involved in head and neck diagnostics and interventions, as it helps prevent misdiagnosis and guides appropriate treatment planning.