The SCM is a unique muscle, in terms of variations at its origin.^4,5,6 Also, it has a variable innervations arrangement, the “ classical anastomotic pattern “ being observed in 50 % of the cases⁷.These anatomical details have a pivotal role in the planning of pedicle muscle flaps in reconstructive surgeries.

In many animals, the cleidomastoid belly is distinctly separate from the sternomastoid belly. (Fig2d) This condition when present in humans is considered to be a variation from normal². The two separate sternomastoid and cleidomastoid bellies further subdivide the anterior triangle into a supernumerary triangle. This extra triangle can also be considered as an extended lesser supraclavicular fossa which normally separates the sternal and clavicular heads of origin of SCM. The occurrence of such a variation can be explained by fusion failure or abnormal mesodermal splitting during development. In this regard we may refer to Sinohara's law of fusion which states that a muscle supplied by two different nerves is formed by fusion of two separate muscle masses⁸.

Some studies have indicated a supernumerary cleido-occipital muscle more or less separate from the sterno-cleido-mastoid muscle⁹. (Fig 2e) The frequency of cleido-occipital muscle occurrence has been reported up to 33 %^{10, 11}.

Occasionally, the SCM fuses with the trapezius, leaving no posterior triangle. (Fig2b) Such a phenomenon describes Sinohara's law of separation which states that two muscles( SCM and trapezius ) having common nerve supply ( accessory nerve ) are derived from a common muscle mass⁸. Some authors regard such fusions to be a normal developmental feature , due to their common derivation from the post- sixth branchial arch².

There are reports of a broad clavicular head splitting into multiple small muscular slips. ^9,12,13 (Fig2h) The number of these extra clavicular slips may vary and such occurrence may be unilateral or bilateral. They cause formation of supernumerary lesser supraclavicular fosse. Developmentally, these additional muscle slips indicate abnormal mesodermal splitting in posterior sixth branchial arch. They may not cause any functional advantage or disadvantage in neck movement but might be physically interfering during invasive procedures. However, they can be effectively utilized for muscle flap harvests.

There are also cases presenting with extra sternal and clavicular heads of origin in SCM.^6,9,14 These additional heads, may be unilateral or bilateral and cause significant stenosis of the lesser supraclavicular fossa, imposing complications for anesthesiologists during the anterior central venous catheterization approach.

Benign fibrosis, hypoplasia or aplasia of SCM is the most common cause of congenital torticolis. Abnormal head positioning in utero or difficult birth can lead to development of the compartment syndrome and congenital muscular torticollis sequela¹⁵.Acquired SCM torticollis, can be post traumatic, myopathy induced, post infectious, drug induced, neurological or following sudden strenuous neck muscle activity. Studies report that morphometric and cross-sectional area a-symmetry between SCM of two sides result from unequal growth in utero and play an important role in the genesis of tension type headache¹⁶.

About a dozen cases have reported complete unilateral absence of the muscle^17,18,19. Such cases represent the developmental defect of muscular agenesis and are diagnosed by Ultrasound or Magnetic Resonance Imaging scans. The absence of SCM cover may lead to complicated congenital neck hernias in children, in addition to functional limitations^{20 .}

A coexisting unilateral absence of SCM with the ipsilateral absent trapezius is an extremely rare variation and till date, only about three such reports are present in literature ^{21 .}Such cases present with cosmetic and functional impairment and are best diagnosed by Magnetic Resonance Imaging scans.

Occasionally, the lower portion of the SCM muscle is intercepted by tendinous intersections which indicate the origin of this muscle from different myotomes ² .The organizational pattern of the SCM can be arranged into five distinct topographical parts, namely the superficial sternomastoid, profound sternomastoid, sterno occipital ,cleidomastoid and cleidooccipital parts which are arranged in superficial and deep layers. The muscle fibers of all these layers lie within a common fascial sheath and traverse in the same direction ^{2,5 .}Knowledge of this layered arrangement and the changes in cases of muscle variations is helpful during muscle flap harvesting procedures.

Figure 3

Table 1:– Variations of the sternocleidomastiod muscle

SCM has a keen role in the physiology of head - neck balance. It acts as a check rein to prevent the head from falling backwards. The clavicular division specially contributes to the sense of equilibrium. The head position is characteristic of the action of the SCM and is suggestive of contraction or shortening of the muscle; if involved. Significant spasm of the SCM can be seen and tenderness can be elicited in the body of the muscle in various medical conditions. The SCM may develop myofascial trigger points, presenting a wide spectrum of symptoms ranging from headache, facial pain and lacrimation to dizziness and blurred vision. .The condition is referred to as the “Sternocleidomastoid syndrome”. Both the clavicular and sternal divisions of SCM produce referred pain if activated. The chiropractic approach, combined with appropriate rehabilitation tools and neuro-emotional techniques, can prove a valuable asset in the corrective treatment of such conditions.

The SCM has many vital structures lying under its cover, like the common carotid artery, accessory nerve, brachial plexus roots, cervical plexus nerves and lymph nodes. It can be rightfully called a Pandora box. The utility of SCM extends far beyond mere neck flexion or as covering agent of neck structures. It is useful for reconstructive procedures of mandible, oral floor, myocutaneous cervical oesophageoplasty, suture line for carotid artery protection, anterior venous catheterization, facial deficits etc. ^3,13,22Reports indicate that parathyroid tissue transplanted into the SCM muscle rather than other sites has a high success rate because of easy accessibility, single operative site and excellent blood supply²³.The use of SCM flaps in superficial parotidectomy may have benefits other than cosmetic improvement, like reducing the incidence of Frey's syndrome and Facial nerve dysfunction although the exact mechanism of this action is yet unclear²⁴.

When trying to understand the biodynamics of the SCM and its morphologic variations, it is imperative to elucidate the derivation of this muscle. Developmentally; SCM shares a common origin with the trapezius from the posterior sixth branchial arc¹³ .The muscle is said to develop from several myotomes².Anthropomorphology studies of SCM on human fetuses indicate that the SCM muscle most frequently consists of sternal and the clavicular heads which terminate in a very wide attachment on the cranium - from the external acoustic foramen to the occiput ¹² .Very frequently the muscle heads exhibit additional bands. The Wood's cleido-occipital muscle is present in several cases during the intrauterine phase. ²⁵During the developmental period, the muscle attachments significantly increase; between the 4 ^th and 5 ^th months²⁵.Dimorphism and bilateral a-symmetry in the dimensions of muscle are statistically non significant ^{26 .}The collagenous fibers of the perimysium of SCM form complex structures from 24 weeks of gestation in contrast to fibers of the endomysium²⁶. The cross-sectional area of the bundle of muscle fibers and the cross-sectional area of the individual muscle fibers of the SCM increase during development from 12 to 32 weeks gestation, in a process linked to the development of the perimysium²⁶.

Unlike the sternalis muscle, where the incidence varies according to sex and ethnicity, there is yet no reported influencing factor on the incidence of SCM variations²⁷. We can assume these variations to be an independent factor unless proven otherwise. However, the above reported presentations of SCM variations could possibly have suffered from troublesome neck positioning ,active myofascial trigger points, tension type headache, shoulder-neck-strain syndromes or functional limitation of neck flexion .