The accessory muscle of ABDM is considered the most common variation in the submental region and many forms of such variations have been reported. The supernumerary muscle can be attached to different structures of the submental region, such as the anterior belly itself, the intermediate tendon, the hyoid bone, the mandible, the mylohyoid raphe or even the mylohyoid muscle . Due to the highly variable anomaly, some classification systems had been made by researchers. As shown in the Anderson’s study, the variation of ABDM could be classified into six types based on its origin and insertion, including atavistic, origin, insertion, mixed, complex and deletion types . Kim and Loukas also used some pattern diagrams to describe the variations of the digastric muscle . What's more, Fernandes compiled a database with the reported cases to classify the variations, the accessory muscle of ABDM occurred unilaterally or bilaterally, symmetrically or asymmetrically .
Variations of ABDM have been described more frequently as unilateral than bilateral and the symmetrical ones are more rare [1, 10]. The variation in the present study was both bilateral and symmetrical. The anomaly was somewhat similar to a few variations reported before[3, 4, 12]. However, the pattern, attachment, especially the fibers direction (almost horizontal) of the accessory muscles in this case resembled none of the variations reported before [6, 8]. So this unique variation could provide a new example for the current classification system. Besides the variation of accessory muscle, the intermediate tendons of digastric muscle had no fibrous sling and were a little far from the hyoid bone on both sides in this case. Absent fibrous sling is very rare, there is only one case report to date in the literature .
In view of this extreme accessory muscle structure, this instance might be an important material to study the function of the accessory muscles. These accessory muscles covered nearly all the area between the two anterior bellies, which might imply these muscles could lift the floor of the mouth and help stabilize the mandible during swallowing. And according to the structures of the accessory muscle and the intermediate tendon, a hypothesis could also be proposed. Because the intermediate tendon was not attached to the hyoid bone in this cadaver, the digastric muscle could not play a role in raising the hyoid bone through the intermediate tendon. Therefore, the contraction of accessory muscle could possibly raise the mylohyoid muscle and in turn raise the hyoid bone, rendering the compensatory effect of the intermediate tendon.
In anatomy teaching, such symmetrical accessory muscle could mislead the medical students even the teachers to be the mylohyoid muscle, which was almost completely covered by this large accessory muscle. However, the fibers direction and the attachment of the accessory muscle were obviously different from those of the mylohyoid muscle, which could help distinguish the mylohyoid muscle from the accessory digastric muscle.
Asymmetrical unilateral or bilateral variations of the ABDM have great clinical significance as they can cause an asymmetrical image in the submandibular region. It is essential to differentiate between tumours, metastatic lymph nodes and muscular variations [1, 11]. In recent years asymmetrical variations of the ABDM have received increased attention to avoid confusion with abnormal lesions during CT or MR imaging examinations and surgical procedures. However, the completely symmetrical variation of digastric muscle is more likely to be ignored in CT and MR imaging, which might lead to a confusion and interfere with surgical procedures in the head and neck. Also, unlike asymmetrical variations, such a symmetrical variation could cause an inexperienced surgeon to misinterpret radiological findings. If mistaken for malignant masses, their removal would lead to unnecessary ablations.
Another discovery in this case was that the stylohyoid muscle was not perforated by the intermediate tendon of digastric muscle on either side. The anatomical relationship between the intermediate tendon and stylohyoid muscle is almost drawn or described in anatomical texts that the tendon perforates the stylohyoid muscle near its insertion. But variations had been found. In 69.7% of Chinese population , the stylohyoid muscle existed on the medial side of the posterior belly and intermediate tendon of digastric muscle, not perforated by the tendon. This difference with Americans and Europeans was possibly related to the nationality.