Classification of Sternocleidomastoid Muscle Abnormalities and Comparison of Their Incidence among Populations

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Classification of Sternocleidomastoid Muscle Abnormalities and Comparison of Their Incidence among Populations

https://doi.org/10.21203/rs.3.rs-1508017/v1

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We classified sternocleidomastoid muscle abnormality and calculated its incidence in a Thai population. Such abnormality should be considered to prevent complications from central venous catheterization.

This study aimed to classify and investigate abnormalities of the sternocleidomastoid (SCM) muscle, which is commonly used as an anatomical landmark to indicate the correct position for central venous catheterization, in a Thai population. Thirty-five embalmed cadavers from northeast Thailand (19 females and 16 males) were systemically dissected to reveal the SCM muscle anatomy. Variations in SCM origin and insertion were recorded. The prevalence of abnormality was approximately 11.4% (4 of 35 cadavers) and did not differ by sex. Abnormalities were classified into 5 types based on origin, insertion, and presence of additional heads, as follows: type I (n=31; 88.6%), type II (n=1; 2.85%), type III (n=1; 2.85%), type IV (n=1; 2.85%), and type V (n=1; 2.85%). As the incidence of abnormality was more than 10%, surgeons must seriously consider this before insertion of a central venous catheter to avoid complications.

Sternocleidomastoid muscle

Abnormalities

Variation

Cadavers

Classification

The sternocleidomastoid (SCM) is the largest and longest muscle located superficially within the anterior compartment of the cervical muscles. Anatomically, the SCM is associated with many vital neurovascular structures of the neck and is an important surgical landmark for identifying the precise location for central venous catheter insertion^4,11. This muscle originates from the manubrium sterni (sternal head) and the medial third of the clavicle (clavicular head). Its insertion is the lateral surface of the mastoid process of the temporal bone and laterally to the nuchal line ^4,18. The terminal end of the SCM inserts at the mastoid process. The SCM contributes to various modalities of head and neck movement such as flexing of the neck and rotation of the head to the opposite side. Additionally, it also supports the normal breathing system⁷.It is possible that variations or abnormalities in SCM origin and insertion may cause abnormal neck and head movements and complications during central venous catheterization. Abnormality of the SCM – especially in terms of its origin – has been reported in numerous populations including those in Brazil, Greek, India, Australia, Soult Korea, Turkey, Colombia, Pakistan, and the US^{1,2,3,5,6,8,9,10,12,13,14,15,16}. Most of these previous studies were case reports using human embalmed cadavers. An incidence of abnormality is greater than 10% in an individual population and demands clinical consideration. Previous studies in West Bengal and Columbia have shown the rates of 27.8% 11.7%, respectively^3,15. As that in the Thai population has yet to be documented, we systemically investigated the types and incidence of SCM abnormality in cadavers from northeast Thailand.

The sample consisted of 35 embalmed cadavers from northeast Thailand – 16 male (45.7%) and 19 female (54.3%). Thirty-one (88.6%; 17 male [48.6%] and 14 female [40.0%]) were normal, while the remaining four (11.4%; 2 [5.7%] of each male and female) exhibited abnormalities (Table 1). We classified the specimens into five types based anatomical origin and insertion patterns of the SCM (Table 2 and Fig. 1). The heads of the left and the right SCM were clearly separated in all cadavers. Normal SCM muscles were classified as type I, and abnormal patterns were categorized into 4 types (II, III, VI, and V). As shown in Table 2, the type I group consisted of 31 (88.6%) samples while types II-V consisted of one (2.85%) each. The anatomical features of each type observed are demonstrated in Fig. 1 and each side described in Table 3.

Table 1

Incidence of sternocleidomastoid abnormality observed in Northeastern Thais.
Parameter Sex	Sternocleidomastoid muscle patterns observed from 35 specimens
Parameter Sex	Normality (%)	Abnormality (%)	Total (%)
Male	14 (40.0)	2 (5.7)	16 (45.7)
Female	17 (48.6)	2 (5.7)	19 (54.3)
Total	31 (88.6)	4 (11.4)	35 (100%)

Table 2

Classification of sternocleidomastoid muscle types observed in Northeastern Thais.
Classifications (Types)	Numbers found (%)
I	31 (88.6%)
II	1 (2.85%)
III	1 (2.85%)
IV	1 (2.85%)
V	1 (2.85%)
Total	35 (100%)

Table 3

Descriptions of sternocleidomastoid muscle (SCM) types by side found in Northeastern Thais based on origin and insertion of the muscle (also shown in Fig. 1).
Types of SCM	Origins	Insertions
I (Fig. 1A)	Left side: one head, anterior surface of the manubrium sterni bone and one third of superior surface of medial clavicular bone Right side: one head, anterior surface of the manubrium sterni bone and antero-superior surface of the medial third of the clavicular bone	Left side: mastoid process Right side: mastoid process
II (Fig. 1B)	Left side: one head, anterior surface of the manubrium sterni bone and one-third of the superior surface of the medial clavicle bone Right side: two heads, 1^St origin: anterior surface of the manubrium sterni bone 2nd origin: superior surface of the middle part of the clavicular bone	Left side: mastoid process Right side: mastoid process
III (Fig. 1C)	Left side: two heads, 1^St origin: anterior surface of the manubrium sterni bone 2nd origin: superior surface of the middle part of clavicular bone Right side: two heads, 1^St origin: anterior surface of the manubrium sterni bone 2nd origin: superior surface of the middle part of the clavicular bone	Left side: mastoid process Right side: mastoid process
IV (Fig. 1E)	Left side: two heads, 1^St origin: anterior surface of the manubrium sterni bone 2nd origin: one-third of the superior surface of the medial area of the clavicular bone Right side: three heads. 1^St origin: anterior surface area of the manubrium sterni bone 2nd origin: one-third of the superior surface of the medial area of the clavicular bone 3rd origin: superior surface of the middle part of the clavicular bone	Left side: mastoid process Right side: mastoid process
V (Fig. 1E)	Left side: three heads, 1^St origin: anterior surface of manubrium sterni bone 2nd origin: one-third of the superior surface of the medial area of the clavicular bone 3rd origin: superior surface of the middle part of the clavicular bone Right side: three heads. 1^St origin: anterior surface of the manubrium sterni bone 2nd origin: one-third of the area in the superior surface of the medial area of the clavicular bone 3rd origin: superior surface of the middle part of the clavicular bone	Left side: mastoid process Right side: mastoid process

The initial part or origin of the sternocleidomastoid muscle (SCM) most commonly has two regular heads: the “medial” or “sternal” head, connected to the upper part of the anterior surface of the manubrium sterni bone, and the “lateral” or “clavicular” head, attached to the superior surface of the medial side of the clavicle bone. These long bilateral muscles commonly insert to the mastoid process of the temporal bone¹⁸. The triangle formed between the base of the clavicular bone and the sterno-clavicular heads of the SCM is used as a surgical landmark for identifying the precise location at which to perform central venous catheterization because it is related to the neuro-vascular structures of the neck. Variations in SCM origin must thus be addressed to avoid complications during surgery. As summarized in Table 4, SCM abnormalities have been reported in many countries around the world including Greece, Brazil, India, Australia, Turkey, Columbia, Pakistan, the US, and Korea^{1,2,3,5,8,9,10,12,13,14,15,16}. However, most were case reports observed during lessons on gross anatomy for medical or paramedical students, and few systematic studies have been published. SCM variation has been documented in India (27.8%) and Columbia (11.7%)^3,15.

Table 4

Previously documented abnormalities of the sternocleidomastoid muscle by population (N.D., not determined).
References	Populations	Observation (male: female)	Percentage of abnormality reported
Natsis et al. (2009)	Greek	A case report (1:0)	N.D.
Amorim et al. (2010)	Brazilian	A case report (1:0)	N.D.
Mehta et al. (2012)	Indian (New Delhi)	A case report (1:0)	N.D.
Raikos et al. (2012)	Australian	A case report (1:0)	N.D.
Saxena et al. (2013)	Indian (Haryana)	A case report (1:0)	N.D.
Saha et al. (2014)	Indian (West Bengal)	Research article (5:0)	27.8%
Goswami et al. (2014)	Indian	A case report (male)	N.D.
Kim et al. (2015)	Soult Korean	A case report (1:0)	N.D.
Anil et al. (2016)	Turkish	A case report (1:0)	N.D.
Arquez (2017)	Colombian	Research article (0:2)	11.7%
Mansoor and Rathore (2018)	Pakistani	A case report (1:0)	N.D.
Dupont et al. (2018)	American (US)	A case report (1:0)	N.D.
Oh et al. (2019)	Soult Korean	A case report (0:1)	N.D.
Present study (2022)	Thai (Northeastern region)	Research article (2:2)	11.4%

This study was the first of its kind conducted in a Thai population and revealed an incidence of 11.4%, comparable to that found in Columbia mentioned above (Table 4). Interestingly, previous studies have found higher incidences of SCM abnormality in males than in females^{1,2,3,5,8,9,10,12,14,15,16}. Moreover, abnormalities observed in a female specimen from South Korea differed from those in the female cadavers in our study¹³. In our study, the number of SCM muscles with abnormalities did not differ by sex (2 cases each), but each was of a different type (Fig. 1). There were two cases of additional muscular heads observed in our study, both on the right side. The first was found in a female cadaver classified as SCM type IV (Fig. 1D) and the second in a male cadaver classified as type V (Fig. 1E). These findings were similar to those of previous studies^{2,3,5,7,8,10,12,13,15,17}. The incidence of unilateral abnormality in our study was also similar to those previously reported^10,15,17. The unilateral muscle abnormality we observed in two female cases was mostly on the right side of the neck. One was classified as SCM type II originating from right sternal head and right clavicular head. The origins were clearly separated by the middle triangle and the major supraclavicular fossa, as shown in Fig. 1B. The other was SCM type IV and originated from the right sternal head clavicular head and additional head. The right sternal and clavicular heads were separated by the medial minor supraclavicular fossa, but its clavicular and additional heads were separated by the medial minor supraclavicular fossa, as shown in Fig. 1D. The incidence and patterns of SCM bilateral abnormalities observed in this study were also similar to those previously described^{2,3,5,7,8,12,13,15}. Such abnormalities were observed in two males. One was SCM type III with the right and left sternal heads separated from the right and left clavicular head by the middle triangle or major supraclavicular fossa. In addition, the right middle triangle was larger than its left counterpart. The other was classified as SCM type V and consisted of three heads: the right and left sternal heads, the clavicular head, and an additional head. Notably, the right and left sternal heads were separated from the right and left clavicular heads by the medial minor supraclavicular fossa.

The presence of both major and minor supraclavicular fossa can cause confusion when attempting to determine an anatomical landmark for central venous catheterization. Such an imprecise surgical landmark may also lead to errors in surgical planning for operations on the anterior neck area^9,14. In addition, SCM abnormalities have been reported to cause torticollis in adults⁹.

In conclusions, we reported on abnormalities of the sternocleidomastoid muscle, a crucial surgical landmark of neck, in Thai cadavers and classified them into 5 types based on the origin of muscles with same insertion to the mastoid process. The incidence of such variation was approximately 11.4% and did not differ by sex.

Specimens. Our study was performed based on the Human Tissue Authority (HTA) guidelines on the handling of human materials. Soft tissues dissected and removed from neck specimens were disposed with respect to prestige of the body donors who have signed with their relatives on the Informed consent of body donation form before death. After decease, close relatives have singed again to consent the body for medical study and researches. This study has followed the Tenets of the Declaration of Helsinki. In addition, this research has also been approved by Ubon Ratchathani University’s Ethics of Human Research Committee (code: UBU-REC-61/2562).

This cross-sectional descriptive study was conducted from 2019 to 2020 at the anatomical laboratory of the College of Medicine and Public Health at Ubonratchathani University in Thailand. The population size was calculated based on the comparative ratios described previously¹⁹ and a previous study reported by Saha and coworkers (2014)¹⁵, which found a rate of approximately 27.8% in West Bengal. We determined that 35 embalmed cadavers would be suitable for the investigation. All cadavers were from residents of northeast Thailand who had voluntarily donated their bodies to be used for medical/gross anatomy education, including research conducted at the Khon Kaen University Faculty of Medicine Department of Anatomy.

The head and neck regions of 35 embalmed cadavers (19 female and 16 male) were dissected by second year medical students enrolled in an anatomy course. Skinning was performed to expose the superficial fascia on the anterior compartment of the neck. Before approaching to the sternocleidomastoid muscle (SCM), different layers of tissues consisting of the superficial cervical fascia, deep cervical fascia, and the platysma muscle were systemically dissected and removed as previously described Moore and coworkers (2014)⁹. Before classification, the SCM origins and insertions at each side (right and left) of all cadavers were recorded and photographed.

Acknowledgements

We would like to acknowledge Dr. Dylan Southard for editing the manuscript via the KKU Publication Clinic (Thailand).

Author contributions statement

C.M. and N.M. wrote the first draft of manuscript. C.J and C.C. prepared and manipulated the figure and tables. SI put the critical thought into the discussion. All authors reviewed the manuscript.

Competing interests

The authors declare no competing interests.

Additional information

Correspondence and requests for materials should be addressed to C.M. and S.I.

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request

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Classification of Sternocleidomastoid Muscle Abnormalities and Comparison of Their Incidence among Populations

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