The stylohyoid muscle, also known as musculus stylohyoideus in Latin, is one of the suprahyoid muscles of the neck as it extends between the base of the skull and the hyoid bone. It is a slender muscle present along the superior border of the posterior belly of the digastric muscle. Along with other suprahyoid muscles (digastrics, mylohyoid, and geniohyoid), the stylohyoid muscle functions during chewing, swallowing, and phonetics. Together with the infrahyoid and other suprahyoid muscles, it contributes to the fixation of the hyoid bone. The stylohyoid muscle, along with two other styloid muscles, i.e., the styloglossus muscle and the stylopharyngeal muscle, form the ‘bunch of Riolanus.’
The stylohyoid muscle connects the hyoid bone to the base of the mandible and the skull. It pulls the hyoid bone upward and backward, resulting in elevation of the base of the tongue and elongation of the floor of the mouth. This movement helps in deglutition. This muscle functions in association with other suprahyoid muscles. When the infrahyoid muscles stabilize the hyoid bone, these suprahyoid muscles depress the mandible and help with the wide opening of the mouth. They also play an active role in improving the flexion movement of the neck and in the production of treble sounds.
The stylohyoid muscle develops from the mesoderm of the second pharyngeal arch. It appears along with other suprahyoid muscles in the eighth week of gestation.
The stylohyoid muscle receives its vascular supply from branches of the external carotid artery. It is the occipital branch of the external carotid artery which gives branches to the stylohyoid. Lymphatic drainage from this part of the neck goes into the submental and submandibular group of lymph nodes. All these lymph nodes finally drain into the deep cervical group of lymph nodes.
Nerve supply derives from the second pharyngeal arches (facial nerve). It is the stylohyoid branch of the facial nerve (cranial nerve VII) that innervates this muscle. Sometimes, the nerve towards the posterior belly of the digastric (branch of the facial nerve) also gives a branch to the stylohyoid muscle.
During the facial nerve's extracranial course (after its exit from the stylomastoid foramen), the first branch to be given off from the nerve is the posterior auricular nerve. Immediately distal to this branch, two branches emerge from the facial nerve, which provides motor innervations to the stylohyoid muscle and the posterior belly of the digastric muscle.
The origin of the stylohyoid muscle is from the middle of the posterior surface of the styloid process of the temporal bone near its base. The styloid process is a thin pointed bone just below the ear projecting downward and forward from the inferior surface of the petrous temporal bone. It runs in an anteroinferior direction and inserts at the junction of the body and greater cornu of the hyoid bone immediately superior to the omohyoid muscle. This muscle connects to the posterior belly of the digastric muscle in its path.
The muscle lies anteromedial to the superior margin of the posterior belly of the digastric muscle. The styloglossus muscle lies medially and anterior to the stylohyoid muscle. The stylopharyngeal muscle is located between the stylohyoid and the styloglossus muscle. Near its insertion at the hyoid bone, the intermediate tendon of the digastric muscle runs in the center of the tendon of the stylohyoid muscle. This muscle confines the posteroinferior boundary of the digastric or submandibular triangle. After arising from the external carotid artery, the facial artery travels upwards on the superior constrictor muscle deep towards the stylohyoid muscle. The posterior auricular artery runs superficially towards the stylohyoid.
There lies an intermuscular space between the stylohyoid muscle and the styloglossus muscle. This space gives passage to the external carotid artery superiorly and the facial artery inferiorly as it courses from the retrostyloid area to the prestyloid area.
The stylohyoid muscle may be absent in some individuals. Physiological variation in its insertion area may also exist. It may insert into the mylohyoid muscle or omohyoid muscles.
In the lesion of the marginal mandibular nerve, which results in asymmetrical lips, surgical intervention is required to improve the aesthetic and functions. Stylohyoid muscle transfer is a possible surgical option when the digastric muscle is not an option.
The stylohyoid muscle is a part of a significant anatomical structure known as the stylopharyngeal septum/styloid diaphragm. This septum is considered an important surgical landmark when attempting to enter the pharyngeal spaces. The septum divides the space into two compartments (prestyloid and retrostyloid compartment) based on its relationship with the septum. Partial ossification of the ligament of the stylohyoid muscle may lead to Eagle syndrome, also known as the stylohyoid syndrome. This condition results in sharp shooting pain unilaterally in the jaw. The pain may radiate into the throat, tongue, or ear, which causes difficulty in deglutition, sore throat, and tinnitus. Surgical resection of the ligament is required as it compresses the underlying structures.
The facial nerve has a significant contribution to the oropharyngeal phase of deglutition through various muscles. It innervates the stylohyoid muscle in association with other muscles, such as the buccinator muscle, perioral muscles, and the posterior belly of the digastric muscle. In patients with peripheral facial palsy, there are reports of difficulty in swallowing. Paralysis or weakness of the stylohyoid may occur if the facial nerve gets damaged. An intracranial lesion of the facial nerve (before its exit from the stylomastoid foramen) may occur due to some middle ear pathology (tumor or infection). An extracranial lesion of the facial nerve can occur due to an infection of the nerve such as herpes virus, compression during forceps delivery (since incomplete development of the neonatal mastoid process leaves the nerve unprotected), or it can be idiopathic. Pathologic conditions associated with the stylohyoid muscle may result in cervical and pharyngeal symptoms such as neck pain laterally in the area of the angle of the mandible, submandibular space, and anterior upper neck. The pain may get exacerbated by movements such as speaking, swallowing, yawning, or head-turning. Myofascial pain syndrome can result in severe pain in the stylohyoid muscle; treatment includes nonsteroidal anti-inflammatory drugs to reduce pain and inflammation. Severe cases may require injections to numb the affected area.
In children, the stylohyoid muscle is nearly horizontal. With age, there occurs a change in length and shape of the muscle and the stylohyoid process, and thus, in adults, the course of the stylohyoid becomes more vertical.
Stylohyoid muscle can be palpated by placing the fingers along the direction of the muscle fiber under the chin and superiorly to the hyoid bone. Palpation is done laterally from the hyoid bone towards the ear lobe along the anterior border of the sternocleidomastoid muscle while the patient is asked to swallow or place the tongue against the roof of their mouth and push against it.
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