Case Review: Third Head of the Sternocleidomastoid Muscle — An Uncommon Cervical Variant

The sternocleidomastoid (SCM) is a key muscular landmark of the neck, separating the anterior triangle of the neck from the posterior triangle and overlying critical neurovascular structures, including the internal jugular vein and cervical plexus branches.⁶ Classically, it arises from two heads—sternal and clavicular—converging to insert on the mastoid process and superior nuchal line. However, numerous reports now document supernumerary or "third" heads, additional clavicular slips, and fascial origins that challenge this simplified model.^{1, 10}

These variants may remain clinically silent, but when robustly developed they can distort the minor and major supraclavicular fossae, alter the available window for central venous catheterization, and compress or mask structures within the lower deep cervical fascia compartment.^{3, 8} On cross-sectional imaging they may mimic soft-tissue masses or lymphadenopathy, particularly when asymmetrical.^{4, 11}

This case-review article synthesises all available case reports and cadaveric observations describing a third head or additional independent head of the SCM, with emphasis on origin, insertion, innervation and spatial relationships within the cervical region.

A structured search strategy was applied to PubMed, Scopus and Google Scholar using combinations of the terms sternocleidomastoid, third head, accessory head, supernumerary and muscle variation, restricted to full-text human anatomical reports in English or with English abstracts.^{6, 7} Eligible publications included single cadaveric case reports, small case series, radiological case reports and classification studies in which at least one discrete additional head of SCM was documented.

For each included case, data were extracted on demographic profile, laterality, number of heads, precise origin of the additional head (sternal, clavicular or fascial), insertion pattern, innervation, impact on the minor and major supraclavicular fossae and any reported clinical or radiological manifestation.^{1, 3} Reports in which the additional fascicle clearly belonged to other named muscles (for example, cleidocervical or levator claviculae variants) were excluded unless the authors classified them within the SCM variation framework.⁷

Across this corpus, eleven discrete cases were identified in which a third or additional independent head contributed directly to the main SCM belly, permitting a qualitative synthesis of origin patterns and clinical relevance.

Most third-head configurations arose from the middle or medial third of the clavicle, coursing superiorly and medially to fuse with the cleidomastoid portion of the main muscle belly in the mid-cervical region.^{1, 5} Cherian and Nayak described a unilateral third head arising from the middle third of the clavicle and joining the normal two heads at the mid-neck without altering insertion or nerve supply.¹ Sirasanagandla et al. reported a third head originating from the investing layer of the cervical fascia in the roof of the subclavian triangle, again merging with the main belly before mastoid insertion.²

Raikos and co-workers described bilateral supernumerary heads that critically narrowed both the minor and major supraclavicular fossae, leaving limited access to the subclavian vessels and lower trunk of the brachial plexus during open procedures.³ More recent reports from Mbwambo and Sakthivel et al. confirm that accessory clavicular heads can be encountered incidentally during routine dissections, sometimes coexisting with variants of the omohyoid or other superficial neck muscles.^{4, 5}

Across pooled cases, all supernumerary heads ultimately inserted with the main SCM on the mastoid process or superior nuchal line, and none showed independent insertions beyond the classical attachments.^{1, 2}

In all detailed reports, the additional head shared the same innervation as the main SCM, receiving motor branches from the accessory nerve (CN XI) with variable contributions from the cervical plexus (C2–C3).^{1, 6} No case described independent weakness or focal atrophy attributable solely to the supernumerary head. Rather, clinical relevance arose from bulk effects and altered topography.

Raikos et al. and Kim et al. showed that multiple additional clavicular and sternal heads can substantially diminish the space within the minor and major supraclavicular fossae, reducing access corridors for vascular and nerve procedures in the lower neck.^{3, 12} Mbwambo’s cadaveric report and the series by Sakthivel et al. further highlight that accessory heads may overlie or partially obscure the submandibular region and proximal great vessels, with potential implications for both open and ultrasound-guided approaches.^{4, 5}

On imaging, hypertrophied fascial-origin heads—such as those described alongside variants of the sternalis muscle—may simulate ill-defined soft-tissue thickening unless the continuous fibre trajectory to the mastoid is recognised.¹¹ Radiologists should therefore consider supernumerary SCM heads in the differential diagnosis of unusual longitudinal muscle bundles coursing obliquely across the lower neck.

Third-head and supernumerary SCM variants fit within a broader spectrum of proximal attachment variability, most frequently affecting the clavicular head.^{6, 7} Embryologically, incomplete fusion or splitting of occipital and upper cervical myotomes within the superficial lamina of the deep cervical fascia likely accounts for accessory heads that arise from fascial sheets rather than bone.¹⁰ Large classification and morphometric studies confirm that while the majority of SCMs follow the classical two-headed pattern, up to 10–12% show some form of abnormal origin or additional slip, most commonly on the clavicular side.^{7, 8}

From a surgical perspective, these variants matter because the SCM is routinely retracted or partially divided to access deeper structures in carotid, thyroid and parathyroid surgery and during central venous catheterization. Unanticipated supernumerary heads can obscure the expected window between the muscle and the underlying great vessels, particularly within the supraclavicular fossa, and may mislead surgeons who rely on standard surface landmarks.^{3, 12}

For anatomists and educators, these reports underscore the need to present SCM anatomy as a spectrum rather than a single canonical pattern. Integrating images and schematics of third-head and multi-headed variants into teaching material can better prepare trainees for the range of morphologies encountered in the dissection room and operating theatre.⁶

Supernumerary and third heads of the SCM are rare but anatomically coherent variations, most often arising from the clavicle or investing cervical fascia and converging with the main muscle belly before mastoid insertion. They share the same neurovascular supply as the classical SCM, yet can significantly remodel the supraclavicular fossae and alter operative corridors in the lower neck.^{1, 3}

Routine awareness of these variants among surgeons, anesthetists and radiologists—supported by careful inspection of muscular contours and fascial planes—can reduce iatrogenic risk and prevent misinterpretation of accessory heads as pathological masses. As additional case reports and morphometric series accumulate, a more granular classification of SCM variants should further refine risk stratification for procedures that traverse the lower cervical region.^{7, 12}