The styloid process is a cylindrical, slender, needle-like projection of various length averaging 2 to 3 cm. The styloid process projects from the inferior part of the petrous temporal bone and offers attachment to the stylohyoid ligament, and the stylohyoid, stylopharyngeus, and styloglossus muscles. Through these structures, the styloid process facilitates the movement of the tongue, pharynx, larynx, hyoid bone, and mandible. Significant vessels and nerves surround the styloid process. The internal jugular vein, internal carotid artery, and glossopharyngeal nerve (CN IX), vagus nerve (CN X), and accessory nerve (CN XI) lie on its medial side, while the occipital artery and hypoglossal nerve (CN XII) run along its lateral side. Originating as a part of Reichert's cartilage forming from the second pharyngeal arch, it undergoes endochondral ossification in the late stages of pregnancy that continues over to the first decade of life. The structure shows a variance between individuals in length, angulations, and other morphological features. Although these physiological differences are often asymptomatic, some patient might develop non-specific orofacial pain, and other complaints relating to the pharynx, face, neck, or ear known as the Eagle syndrome, which occurs secondary to changes in the styloid process. The diagnosis of the syndrome depends on clinical presentation, radiological investigation utilizing X-ray or CT scans, and lidocaine infiltration test. Management is usually either conservative using steroids injections or long-acting anesthetic injections or surgical.
As already stated, the styloid process is a cylindrical, slender, needle-like projection with a thickness that gradually tapers forming the apex of the process, the length of the process varies, averaging at 2 to 3 cm. Although the process is usually straight, it varies and might be found to be curved. The styloid process projects from the inferior part of the petrous temporal bone, lying inferior-anterior to the external auditory meatus, anteromedially to the mastoid process, and anterior to the stylomastoid foramen. It consists of two parts; proximal and distal - the proximal portion consists of the base of the process, which is contained within the vaginal process from the tympanic part of the temporal bone, the distal part consists of the shaft of the process and gives attachment to three muscles, the stylohyoid, stylopharyngeus, and styloglossus. Also attached are two ligaments, the stylohyoid and stylomandibular ligaments; the stylohyoid ligament's proximal attachment lies at the apex of the styloid process, while the distal end of the ligament attaches to the lesser cornu of the hyoid. Both ligaments facilitate in the movement of the tongue, pharynx, larynx, hyoid bone, and mandible.
Several important vessels, muscles, nerves, and other anatomical structures lie in the vicinity of the styloid process. Two significant vessels, the internal jugular vein, and the internal carotid artery and three cranial nerves, CN XI, CN X, and CN XI lie on its medial side. Moreover, the superior constrictor muscle and the pharyngobasilar fascia neighbor the tonsillar fossa on the medial side of the apex of the styloid process. Along the lateral border of the styloid process runs CN XII and the occipital artery, also found is the posterior belly of the digastric muscle. Furthermore, the facial nerve (CN VII) passes lateral to the styloid process as it pierces the parotid gland after emerging from the stylomastoid foramen posteriorly.
The styloid process originates as a part of Reichert's cartilage, which forms from the second pharyngeal arch during embryological development . Reichert's cartilage divides into four parts, the tympanohyal part, the stylohyal part, the ceratohyal part, and the hypohyal part. The tympanohyal part develops antenatally, attaches to the petrous portion of the temporal bone, and gives rise to the base of the styloid process which is ensheathed by the vaginal process of the tympanic part. The stylohyal part appears post-natally, and it gives rise to the shaft of the styloid process and the proximal portion of the stylohyoid ligament. The stylohyal part might unite with the tympanohyal after puberty, but in some cases they never do. The ceratohyal and its fibrous sheath regress, giving rise to the stylohyoid ligament. The hypohyal part gives rise to the lesser cornu of the hyoid bone.
The styloid process undergoes endochondral ossification that begins at the final stages of pregnancy and is carried on over the first 8 years of life. However, the ossification process and duration shows significant variance.
The styloid process gives origin to three muscles, the styloglossus, stylohyoid, and stylopharyngeus. The styloglossus receives innervation from CN XII, attaches to the apex of the tongue, and acts by retracting and elevating the tongue. Innervation of the stylohyoid is by CN XII; it attaches to the greater cornu of the hyoid bone with its distal tendon perforated by the intermediate tendon of the digastric muscle and acts by elevating the hyoid bone mainly during swallowing. The stylopharyngeus muscle is supplied by CN IX, attaches to the thyroid cartilage, and acts on the elevation of the larynx and elevation and dilation of the pharynx during swallowing.
The length of the styloid process is not consistent across all individuals, with studies reporting ranges from 1.52 cm to 6 cm while others report ranges from 3.5 cm to 8 cm. The length of the left and right styloid processes might also be different in the same individual. Although the length of the styloid process might vary from person to person, a length of more than 3 cm is considered elongated. The prevalence of elongated styloid process is generally estimated around 4% of the general population. However, there is variance between populations and some pockets, like rural Indian populations, show a much higher prevalence. Furthermore, it is more common in women than in men.
The ossification and fusion of the styloid process also show variance. As mentioned previously, the stylohyal part might unite with the tympanohyal after puberty. If the stylohyal part successfully fuses with the tympanohyal part and the stylohyal aspect ossifies, it results in a long styloid process. However, if the stylohyal part fails to ossify, it results in a short styloid process. The duration of the ossification process and the pattern of ossification also shows significant variance.
Multiple theories are proposed to explain the variance in ossification and elongation of the styloid process. The first theory is the "theory of reactive hyperplasia" which proposes that after pharyngeal trauma, the styloid process reacts and proliferates causing elongation. The second theory is the "theory of reactive metaplasia" which is similar to the first theory in trauma being the triggering factor. However, the second theory suggests that the stylohyoid ligament is the structure responsible for the abnormal ossification as it undergoes metaplasia and partial ossification. The third theory is the "theory of anatomic variance" which suggests that the ossification of the styloid process and the stylohyoid ligament is a normal process representing an anatomical variation resulting in the elongation of the styloid process. Although these theories further the understanding of how the styloid process might have occurred, a consensus has not been established regarding the etiology of the progress of ossification and elongation of the styloid process.
As mentioned previously, approximately 4% of the general population have an elongated styloid process. Although the majority of these individuals are asymptomatic, a small percentage of those with an elongated styloid process show symptoms, and present with what is known as Eagles syndrome. The first type of Eagle syndrome is known as classic Eagle syndrome or stylohyoid syndrome which presents as a sharp pain in the neck or the ear that extends to the maxilla, face, and oral cavity. The pain might appear or be exaggerated with a rotation of the head, chewing, swallowing, extending the tongue, or yawning. It might be associated with a foreign body sensation in the pharynx, tinnitus, and vertigo. A mass might be palpable in the tonsillar fossa. Symptoms of classic Eagle syndrome are usually unilateral but could rarely present bilaterally. These symptoms are explainable by the irritation or entrapment of the nearby cranial nerves (the V, VII, IX, or X). Although there is no consensus on the exact etiology of how this irritation occurs or what structures might cause this entrapment, postulates are that due to the common observation of classic Eagle syndrome post-tonsillectomy or other pharyngeal surgery the irritation or entrapment occurs secondary to the formation of local granular tissue resulting from the trauma.
The second type of Eagle syndrome is known as stylo-carotid artery syndrome which occurs when the styloid process impinges upon the internal or external carotid artery and the nerve plexus accompanying them. It presents as pharyngeal pain, eye pain, or parietal cephalgia, resembling a migraine or a cluster headache. The irritation of the nerves or the compression of the internal carotid artery might also present with symptoms of internal carotid vascular insufficiency such as weakness, visual changes, or syncope with head movement. The elongated styloid process might also dissect into the vessel leading to transient ischemic attacks or stroke.
Eagle syndrome symptoms manifest secondary to several factors related to the styloid process such as the length, width, direction, angulations, and degree of ossification. Studies were conducted to correlate the length of the styloid process with the severity of symptoms in patients presenting with Eagle syndrome yielding results that showed no correlation between the two. However, a correlation was found between angulation, the length of the styloid process, and symptoms of Eagle syndrome. If the styloid process deviates laterally, it is likely that it will impinge upon the external carotid artery and its branches. If the styloid process deviates posteriorly, it may impinge upon CN IX, CN X, CN XI and CN XII between it and the transverse process of the atlas. Moreover, if the styloid process deviates medially or anteriorly, it may irritate the tonsillar fossa and the important structures within it.
Diagnosis of Eagle syndrome depends on the clinical presentation of the patient, radiological investigation, and lidocaine infiltration test. As shown, the clinical image of Eagle syndrome is not specific and mimics several other diagnoses, and although the palpable mass in the tonsillar fossa might allow the clinician to differentiate it from other differentials, it is not always present in symptomatic Eagle syndrome patients. For radiological investigations, lateral head and neck X-ray can be used to identify the elongated styloid process. However, the disadvantage of a lateral X-ray is the possibility of overlap of the two styloid processes. A Towne radiograph, which is an anterior-posterior skull axis view, can be utilized and allows for assessment of medial or lateral deviation of the styloid process. Computed tomography (CT) allows for evaluation of the length, angulations, and other features of the styloid process. A 3D-CT is considered the gold standard of radiological diagnosis and provides the best supplement to a plain X-ray. The importance of CT with angiography is further emphasized in stylo-carotid syndrome to assess blood flow changes. The lidocaine infiltration test is conducted by the administration of 1 ml of 2% lidocaine to the area surrounding the palpable styloid process in the tonsillar fossa. If the patient symptoms are relieved by the anesthetic, the test is considered positive, establishing the diagnosis of Eagle syndrome.
Management of Eagle syndrome is either conservative or surgical depending on the pathogenesis and severity of the presentation. The general recommendation is that management begins conservatively and moves to surgery if necessary. Conservative management of Eagle syndrome consists of steroid or long-acting anesthetic injections at the inferior portion of the tonsillar fossa or the lesser cornu of the hyoid bone for symptomatic relief. External manipulation of the elongated styloid process and fracturing it under local anesthetic has been proposed but shows unsatisfactory long term results. The elongated styloid process can be approached surgically through two approaches, an extra-oral transcervical approach or an intra-oral transpharyngeal approach. The extra-oral transcervical approach allows for better visualization, but it is considered more complex, time-consuming, and leaves a visual scar on the patient's neck. Surgery can risk damaging the marginal mandibular nerve and can result in edema that impacts speech and swallowing.
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