Abnormalities of the anterior belly of the digastric muscle are rare but have
received increased attention by radiologists in recent years in an attempt to
avoid confusion with submental cysts or enlarged submental lymph nodes on
CT or MR images. We present a case of bilateral accessory digastric muscles
which fuse (partially) with the midline raphe of the mylohyoid muscle. Fibres
from the right accessory anterior digastric muscle proceeded to decussate and
join the mylohyoid muscle and the contralateral insertion of the digastric muscle.
Embryological development and possible clinical consequences are discussed

Kapos, Theodoros

Kwiatkowska, Magdalena

Louis, Robert G.

Loukas, Marios

English

Via Medica Journals

Folia Morphol. Vol. 64, No. 3, pp. 233–236Copyright © 2005 Via MedicaISSN 0015–5659www.fm.viamedica.plC A S E  R E P O R T233Address for correspondence: Dr. Marios Loukas, MD, PhD, Associate Professor, Department of Anatomical Sciences,St George’s University, School of Medicine, Grenada, West Indies, tel: (473) 444 4175 ext. 2887, e-mail: edsg2000@yahoo.comINTRODUCTIONThe anterior belly of the digastric muscle lies uponthe inferior surface of the mylohyoid muscle and di-vides the region between the hyoid bone and themandible into 3 triangles: 2 submandibular trian-gles laterally and medially between them, separatedby the anterior bellies of the digastric muscles, thesubmental triangle [7]. The anterior belly arises fromthe intermediate tendon of the digastric muscle andcourses anteromedially to insert on the digastric fossaof the mandible, lateral to the midline [11]. It is gen-erally agreed that the function of the digastric mus-cle is to stabilise and regulate the position of thehyoid bone and to assist in jaw movements, primari-ly opening [2, 3, 18]. The two bellies of the digastricmuscle arise from two different embryological pre-cursors and are therefore supplied by two differentnerves.  The anterior belly arises from the first pha-ryngeal arch, and is therefore supplied by a branchA case of a bilateral accessory digastric muscleMarios Loukas1, 2, Robert G. Louis3, Theodoros Kapos4, Magdalena Kwiatkowska3, 51Department of Anatomical Sciences, St George’s University, School of Medicine, Grenada, West Indies2Harvard Medical School, Boston, MA, USA3Department of Anatomy, American University of the Caribbean, School of Medicine, Sint Maarten, NetherlandsAntilles4Harvard School of Dental Medicine, Boston, MA, USA5Warsaw Medical University, Department of Anatomy, Warsaw, Poland[Received 6 January 2005; Revised 13 May 2005; Accepted 13 May 2005]Abnormalities of the anterior belly of the digastric muscle are rare but havereceived increased attention by radiologists in recent years in an attempt toavoid confusion with submental cysts or enlarged submental lymph nodes onCT or MR images. We present a case of bilateral accessory digastric muscleswhich fuse (partially) with the midline raphe of the mylohyoid muscle. Fibresfrom the right accessory anterior digastric muscle proceeded to decussate andjoin the mylohyoid muscle and the contralateral insertion of the digastric mus-cle. Embryological development and possible clinical consequences are discussed.Key words: accessory digastric muscle, anterior belly, trigastric,of the trigeminal nerve (CN V), the nerve to the my-lohyoid. In contrast, the posterior belly arises fromthe second pharyngeal arch and thus derives its nervesupply from the facial nerve (CN VII). Abnormalitiesof the pharyngeal arches during development canlead to multiple malformations of the head and neckwith varying clinical consequences.Variations in the origin, insertion, and locationof the anterior belly of the digastric muscle have rarelybeen described in the literature [4–6, 11, 14]. Noanatomical or embryological textbooks contain dataon such anomalies, although Gray’s Anatomy makesreference to the anterior belly crossing the midlineand fusing with the mylohyoid muscle [22]. Wepresent a case of bilateral occurrence of an accesso-ry head of the anterior digastric muscle (AcADM)which arose from the digastric fossa and insertedupon the midline raphe (on the right) and on themylohyoid muscle.234Folia Morphol., 2005, Vol. 64, No. 3CASE REPORTThe anomaly was found in a 73-year-old malecadaver during a routine student dissection at Har-vard Medical School, during the Human Body Coursein the fall of 2003. The dissection of the anteriortriangle of the neck began with a midline incisionfrom the mental protuberance inferiorly to the ster-nal notch. Bilateral incisions were then made fromthe mastoid processes along the anterior border ofthe sternocleidomastoid to terminate at the sternalnotch. The resulting triangular flap was reflectedsuperiorly, including skin, superficial fascia, platys-ma muscle and underlying connective tissue.Upon exposure of the anterior triangle, twoAcADMs (right and left) were observed as originatingfrom the digastric fossa on the postero-inferior sur-face of the body of the mandible (Fig. 1). The leftAcADM took its origin from a separate tendon me-dial to the main belly of the anterior digastric mus-cle and proceeded posteromedially to insert uponthe midline raphe of the mylohyoid muscle.The right AcADM arose similarly, as describedabove, medial to the origin of the main belly of theanterior digastric muscle. In contrast, the rightAcADM immediately bifurcated into two heads (lateraland medial). The lateral head rejoined with the mainbelly of the anterior digastric muscle, while the me-dial head continued to insert upon the midline rapheof the mylohyoid muscle. Fibres from the rightAc-ADM then proceeded to decussate and join themylohyoid muscle and the contralateral (left) inser-tion of the digastric muscle. The intermediate ten-don of the digastric was found bilaterally in its usualposition, being anchored by a sling to the hyoid boneand connected to the posterior belly of the digastricmuscle, which was also found in its normal anatom-ical position bilaterally.The dimensions of the muscles were measuredwith the aid of the Lucia program (digital imageanalysis software), as previously described [9]. Thetendon of the AcADM was measured at the originand found to be 6 mm in width for both the left andright AcADMs. The left AcADM reached a maximumwidth of 11 mm just before its insertion onto themidline raphe. The maximum width of the right Ac-ADM was 8 mm, measured immediately proximal toits bifurcation into medial and lateral heads, whichhad maximum widths of 5 mm and 4 mm, respec-tively. The length of the left AcADM was measuredfrom its origin to its insertion upon the midline rapheand found to be 3.5 cm. The length of the rightAcADM was measured from its origin to the point ofbifurcation and found to be 1.3 cm. The medial headof the right AcADM (measured from the bifurcationto its insertion on the midline raphe) was found tobe 2.2 cm in length. The lateral head of the rightFigure 1. Bilateral accessory anterior digatric muscles are shown here with assymetrical insertions. The left AcADM inserts upon themidline raphe of the mylohyoid muscle. The right AcADM bifurcates into medial and lateral heads.235Marios Loukas et al., Bilateral accessory digastric muscleAcADM (measured from the bifurcation to its rejoin-ing with the main belly of the anterior digastricmuscle) was found to be 1.6cm in length.The nerve supply was identified bilaterally, as aris-ing from the nerve to the mylohyoid. This nerve wasfound to supply both the normal anterior belly ofthe digastric muscle and the AcADM.DISCUSSIONThe majority of reported variations of the digas-tric muscles are found in French and Italian litera-ture from the late 19th and early 20th centuries[10, 13, 16, 19]. A few recent reports have been pub-lished, which describe various abnormalities of theanterior belly of the digastric muscle [1, 4, 6, 11, 15,20, 21, 24]. Norton [11], in 2000, reported a case ofbilateral occurrence of accessory digastric muscles,which inserted upon the midline raphe, decussated,and continued to rejoin the contralateral anteriorbellies of the digastric muscles before their transi-tion into the intermediate tendons. The anomaly re-ported in that case was symmetrical bilaterally. Fur-thermore, Uzun et al. [21] presented a case in which3 anterior and posterior bellies of the digastric mus-cle had their normal origin and course and werejoined by an intermediate tendon. The accessoryanterior bellies originated from the digastric fossa,and inserted to the hyoid bone with a commonfibrous band. To the best of our knowledge, this isthe first reported case of an AcADM arising bilater-ally but with an asymmetric course and insertion.The complexity of the sequential development ofthis region naturally gives rise to potentially count-less variations. The anterior belly of the digastricmuscle, along with the other muscles of mastica-tion, arises from the first pharyngeal arch [23]. Thepharyngeal arches begin to develop early in the4th week of development as neural crest cells migrateinto the developing head and neck. By the end ofthe 4th week, well defined pairs of pharyngeal arch-es are visible externally [12]. Both the mylohyoidmuscle and the anterior belly of the digastric muscleare derived from this first pharyngeal arch. Converse-ly, the posterior belly of the digastric muscle arisesfrom the second pharyngeal arch. It is possible thatduring embryological development some of the neu-ral crest cells composing the 1st pharyngeal arch un-dergo an aberrant migration leading to the devel-opment of an AcADM. Another possibility is that theproximity of the pharyngeal arches allows a minimalamount of fusion between the contralateral corre-sponding arches, thereby leading to the developmentof accessory muscle fibres which fuse with and crossthe midline.Because the discovery of this anomaly was madepost-mortem, it is unknown whether the presence ofthe bilateral AcADM had any clinical consequences inthis case. The importance of this anomaly lies in its po-tential for confounding diagnosis in both radiologicaland surgical assessments of the floor of the mouth. Forthe surgeon the anterior belly of the digastric muscleserves as an important landmark to aid dissection dur-ing certain procedures. As a boundary of the submentaltriangle the anterior belly of the digastric muscle servesas a guideline for the removal of diseased submentalnodes and adipose tissue during malignant disease. Thepresence of AcADM could potentially give false bound-aries of the submental triangle, resulting in incom-plete removal of the malignant tissues. Other compli-cations could include mistaking AcADM for malignanttissue and subsequent removal of muscle tissue dur-ing radical suprahyoid neck dissections [17, 20].The presence of AcADM may also lead to confusionin radiological diagnostic procedures. The importance ofrecognising similar anomalies was highlighted in the pa-per of Larson and Lufkin, which described the risk of con-fusion of anomalies of the floor of the mouth during CTand MR imaging techniques [8]. Increased muscle bulkin the midline may be falsely identified during CT scan-ning as a pseudomass of the mylohyoid muscle. In addi-tion, accessory muscles such as AcADM might be mistak-enly identified as an enlargement of the submental nodes,unless the surgeon or radiologist is aware of the possibil-ity of these variations [11, 15].The presence of an AcADM could lead to mis-diagnosis, particularly when using CT or MR imagingtechniques [1, 11, 15, 24].CONCLUSIONSWe present a unique case of bilateral accessorydigastric muscles which fuse (partially) with the mid-line raphe of the mylohyoid muscle. Fibres from theright accessory anterior digastric muscle proceededto decussate and join the mylohyoid muscle and thecontralateral insertion of the digastric muscle.Although several cases studies have been publisheddescribing various anomalies of the anterior digas-tric muscle and their clinical significance, there havebeen no reports to date which have provided statis-tical data with regard to the common variations andtheir incidence. A thorough investigation of the de-tailed anatomy of the region, including the occur-rence of muscular variations, might prove very use-ful to radiologists, surgeons and anatomists alike.236Folia Morphol., 2005, Vol. 64, No. 3REFERENCES1. Aktekin M, Kurtoglu Z, Ozturk AH (2003) A bilateraland symmetrical variation of the anterior belly of thedigastric muscle. Acta Med Okayama, 57: 205–207.2. Berzin F (1995) Electromyographic analysis of the ster-nohyoid muscle and anterior belly of digastric in jawmovements. J Oral Rehabil, 22: 463–467.3. Berzin F (1995) Electromyographic analysis of the ster-nohyoid muscle and anterior digastric in head andtongue movements. J Oral Rehabil, 22: 825–829.4. Celik HH, Aldur MM, Ozdemir B, Aksit MD (2002)Abnormal digastric muscle with unilateral quadrifica-tion of the anterior belly. Clin Anat, 15: 32–34.5. Fujimura A, Onodera M, Feng XY, Osawa T, Nara E,Nagato S, Matsumoto Y, Sasaki N, Nozaka Y (2003)Abnormal anterior belly of the digastric muscle: A pro-posal for the classification of abnormalities. Anat SciInt, 78: 185–188.6. Guelfguat M, Nurbhai N, Solounias N (2001) Medianaccessory digastric muscle: radiological and surgicalcorrelation. Clin Anat, 14: 42–46.7. Hollinshead WH (1982) Anatomy for Surgeons. 2nd Ed.Harper and Row, Maryland, pp. 364–366.8. Larson SG, Lufkin RB (1987) Anomalies of digastricmuscles: CT and MR demonstration. J Com Ass Tom,11: 422.9. Loukas M, Hullet J, Wagner T (2005) The clinical anatomyof the inferior phrenic artery. Clin Anat, 18: 357–365.10. Morestin (1894) Anomalies du muscle digastrique.Bulletin Societe Anatomique de Paris. 1, xix: pp. 653.11. Norton MR (1991) Bilateral accessory digastric mus-cles. Br J Oral Maxillofac Surg, 29: 167–168.12. Peker T, Turgut HB, Anil A. (2000) Bilateral anomaly ofanterior bellies of digastric muscles. 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A case of a bilateral accessory digastric muscle

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A case of a bilateral accessory digastric muscle

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