What is Trismus?
Trismus is a restricted mouth opening caused by tonic contraction or spasm of the masticatory muscles, with a mouth opening of 35 mm or less considered diagnostic. 1, 2
Definition and Clinical Characteristics
The term originates from the Greek word "trismos," meaning a prolonged, tetanic spasm of the jaw muscles that restricts normal mouth opening (also called "lockjaw"). 1
While originally used exclusively for tetanus, the term now applies to restricted jaw movement regardless of underlying cause. 1
A maximal interincisal opening of 35 mm or less should be regarded as trismus, providing a quantifiable threshold for diagnosis. 2
Common Etiologies in Clinical Practice
In Head and Neck Cancer Patients
Trismus in this population has multiple potential causes that require differentiation:
Tumor invasion into masticatory muscles (particularly the medial pterygoid), which may represent a contraindication to certain surgical approaches. 3
Radiation-induced myositis and myofibrosis following radiotherapy to the masticatory muscles, which is the most common cause in oncology patients. 4, 5
Post-surgical complications including unfavorable scarring and muscle atrophy from immobilization. 5
Temporomandibular joint involvement with tumor or radiation-induced fibrosis. 3, 5
Reflectory myospasm, pain-related restriction, jaw fracture with hardware failure, or infection. 5
In General Medical Practice
Odontogenic infections (periodontal or pericoronal) are common causes, representing a classical symptom of masticatory space infections. 6
Tetanus presents with trismus as a hallmark feature, typically manifesting as lockjaw followed by generalized rigidity. 4
Temporomandibular joint disease or arthritis, particularly relevant in older adults. 7
Oral submucous fibrosis, posterior pharyngeal infections (reflectory trismus), certain medications, and minor dental procedures. 8
Clinical Significance
A critical pitfall is that trismus can be the presenting sign of malignancy in 42% of patients with oral cavity cancers before treatment, making screening for underlying malignancy crucial when no obvious dental or TMJ cause is identified, especially with associated symptoms like unexplained weight loss, dysphagia, or oral lesions. 7
In head and neck cancer patients already treated, new-onset or worsening trismus may represent the first sign of tumor recurrence rather than a treatment sequela, requiring careful oncologic evaluation. 5, 2
Impact on Patient Care
Restricted mouth opening impedes inspection of the oral cavity needed for dental care and particularly for oncologic follow-up. 2
The condition significantly affects quality of life by interfering with eating, speaking, oral hygiene, and medical/dental examinations. 5
In radiation therapy patients, mouth opening decreases on average by approximately 20% compared to pre-treatment baseline. 2