Trismus, from the Greek “trismus” (“gnashing,” as in teeth) refers to restriction of the range of motion of the jaws. Commonly referred to as “lockjaw,” trismus typically stems from a sustained, tetanic spasm of the muscles of mastication. Initially described in the setting of tetanus, it currently refers to restricted mouth opening due to any etiology. Although unilateral restriction may occur, by definition, trismus is a bilateral process that results from increased tone mediated by the efferent portion of the reflex arch of the trigeminal nerve. Most commonly trismus is temporary, and typically resolves in less than 2 weeks, but permanent trismus may also occur. Trismus may interfere with normal speech and eating, including the ability to swallow normally.
There are myriad etiologies implicated in the development of trismus. Some authors classify trismus in relation to the involvement of the temporomandibular junction (TMJ)or intraarticular versus extraarticular causes. Others have outlined the causes into broad categories such as infectious, traumatic, and neoplastic sources. A disease entity may cause trismus, but conversely, it may also be iatrogenic, resulting from prescribed interventions and treatments. Below are some of the conditions associated with trismus:
Head and neck neoplasms
Psychogenic (conversion disorder)
The prevalence of trismus ranges widely, partly because no clear criteria have been established. Normal jaw opening is greater than 30 to 40 mm. Trismus has been defined as mouth opening less than 40 mm; others have defined it as an opening to 15 to 30 mm, or even less than 20mm. Additionally, some authors have graded trismus according to visual assessment of mouth opening (light/moderate/severe or grades 1 to 3, again corresponding to mouth opening). Its incidence is vastly variable and dependent on the inciting etiology. Importantly, trismus is a common finding in certain narrow patient populations, such as patients with congenital micrognathia syndromes or those undergoing radiation therapy for head and neck cancers. It can also be a relatively rare complication of common conditions, such as pharyngitis.
The muscles responsible for mouth closure, namely the masseter, temporalis, and medial pterygoid muscles, exert a force 10 times greater than exerted by the muscles that open the mouth, which include the lateral pterygoid, digastric and hyoid muscles. Innervation for the majority of these muscles is provided by the mandibular division of the fifth cranial nerve. The muscle groups that control jaw opening and closure act in antagonism, as neurogenic stimulation of one group causes reflex neural inhibition of the other. While the inciting insult may be unilateral, the reflex activated is bilateral.
Patients presenting with trismus may note restricted mouth opening and sometimes pain with attempts to force open the mouth. However, patients often have complaints related to the causative condition rather than the resulting trismus. Those with odontogenic etiologies may complain of tooth or gum pain and swelling; patients with traumatic causes may note facial or mandibular pain. A review of systems should be performed to identify systemic causes. Fever may indicate an infectious source; weight loss may be notable among patients with neoplastic causes; carpopedal spasms and paresthesias may accompany trismus in patients with neurogenic or metabolic causes. A history of tobacco use, or of known cancer, could raise suspicion of a neoplastic cause. 
More commonly, trismus is a physical exam finding encountered on attempts to visualize the oral and pharyngeal structures via mouth opening. This poses a challenge to the examining clinician attempting to ascertain the cause of trismus, as by definition this restriction limits the evaluation of intraoral structures that may be implicated in its origin. As much as possible, the exam should be targeted to the teeth and gums; facial bones and TMJ; pharyngeal pillars, tonsils, and uvula; and the neck. Assessment of the patient's speech may be important; some pharyngeal infections implicated in trismus may also cause changes in the patient's voice; a "hot potato voice" may be associated with tonsillitis or peritonsillar abscess. A directed neurologic exam should be performed to evaluate for neurogenic causes. 
The diagnosis of trismus is clinical. Imaging adjuncts may be useful to determine its etiology and determine the articular involvement of the TMJ. Computed tomography may be useful to identify traumatic etiologies including hematomas or facial and mandibular fractures when suspected. Magnetic resonance imaging may also be helpful in identifying space-occupying lesions or abnormalities in the pharyngeal or oral structures.
Treatment of trismus is directed at the inciting etiology and is most commonly treated symptomatically. Symptom-directed interventions including heat therapy, analgesics such as non-steroidal anti-inflammatory agents, and muscle relaxants may be considered in the acute phase and have been described as mainstays of treatment of uncomplicated transient trismus. Patients with post-traumatic and post-operative trismus, especially when persisting longer than 1 week, may also require stretching exercises. The exercises typically consist of repeated attempts to open the mouth against applied resistance, usually divided into multiple sessions per day. Trismus may become chronic in the setting of fibrosis or ongoing radiotherapy; these cases may benefit from intensive physiotherapy, sometimes utilizing commercially-available jaw motion rehabilitation devices or microcurrent therapy, particularly in cases refractory to more conservative approaches. Some authors have also described treatment with xanthine derivatives such as pentoxifylline. 
Since trismus has been defined as a restriction in mouth opening regardless of etiology, it does not have a wide differential diagnosis. Some authors have argued that "true trismus" is mediated via the trigeminal nerve, and intraarticular causes of impaired mouth opening, such as TMJ ankylosis or fibrosis, should be considered separately. Others have outlined such intraarticular processes as subclassifications of trismus. 
Trismus is most commonly self-limited and transient, typically resolving within 2 weeks. When identified in certain patient populations, e.g., in patients who develop fibrosis from radiotherapy, the course of trismus may be prolonged and more refractory to conservative treatment. 
Many of the infectious and traumatic causes of trismus may also have associated complications; for example, an odontogenic infection causing trismus may be further complicated by facial cellulitis or mandibular osteomyelitis. Trismus may also interfere with appropriate oral intake of nutrition and hydration. Furthermore, trismus may be associated with aspiration due to impaired swallowing mechanism. It should also be noted that intubation via oropharyngeal route may be impossible in patients with significant trismus, necessitating other approaches such as nasopharyngeal intubation or tracheotomy. When its duration is prolonged, trismus may lead to fibrosis of the TMJ, necessitating directed therapy.
Consultation may be considered based on the suspected etiology. Dental or oral surgery care may be enlisted for odontogenic causes, while an otolaryngologist is at times consulted for drainage of a peritonsillar abscess. Physiatrists may also be involved in a patient's care when directed therapeutic interventions are necessary to treat refractory trismus.
Because there are many causes of trismus, the condition is best managed by a multidisciplinary team. Consultation may be considered based on the suspected etiology. Dental or oral surgery care may be enlisted for odontogenic causes, while an otolaryngologist is at times consulted for drainage of a peritonsillar abscess. Physiatrists may also be involved in a patient's care when directed therapeutic interventions are necessary to treat refractory trismus. The outcomes are good in most patients if the primary cause can be treated. However, in patients with severe trauma, scarring, and radiation therapy, trismus may be chronic and lead to a poor quality of life. (Level V)
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