A 30-year-old woman presents with 4 days of worsening left facial pain and difficulty opening her mouth. Symptoms began the day after a difficult mandibular molar extraction that required multiple inferior alveolar nerve blocks. She describes a constant dull ache over the left cheek that is present at rest and becomes markedly worse with chewing. She notes new swelling in the region and that her teeth feel like they do not come together normally. She denies fever or dental pain.
On examination, maximal interincisal opening is 20 mm, limited by pain. There is diffuse tenderness and mild swelling over the entire left masseter muscle. The temporomandibular joints are nontender without crepitus, and there is no fluctuance or dental caries. Cranial nerve examination is otherwise normal.
Which ONE of the following is the MOST likely diagnosis?
Correct answer: E.
Explanation:
Why this option is correct:
Myositis of the masticatory musculature is an inflammatory muscle disorder, often precipitated by direct trauma (including intramuscular local anesthetic injection) or infection. It is characterized clinically by diffuse, continuous pain over the involved muscle, tenderness throughout the entire muscle belly, swelling, and a moderate to severe limitation of mandibular range of motion that worsens with muscle use. This patient has classic features: recent local trauma from injections and difficult extraction, constant dull pain localized to the masseter region, diffuse muscle tenderness and swelling, painful trismus, and no clear joint or dental source. These findings are most consistent with myositis of the left masseter.
Why the other options are wrong:
Myofascial pain dysfunction syndrome is typically a chronic condition characterized by regional dull aching pain, identifiable trigger points within taut muscle bands, and referred pain patterns, usually without overt swelling. Pain is often intermittent and may be associated with psychosocial stressors or parafunctional habits. The vignette instead describes acute-onset continuous pain with muscle swelling after local trauma, and the examination notes diffuse rather than focal tenderness, favoring myositis over myofascial pain.
Acute temporomandibular joint internal derangement with disc displacement without reduction usually presents with preauricular joint pain, joint line tenderness, and a characteristic deviation of the mandible on opening with a hard end-feel limitation, often accompanied by joint noises or a history of clicking that has stopped. In this patient the joints are nontender, there is no reported clicking or locking history, and the maximal opening is limited by muscle pain rather than a mechanical joint block, making isolated TMJ internal derangement less likely.
An odontogenic abscess from mandibular molars would typically produce severe localized dental pain, often throbbing in character, with a carious tooth, gingival erythema or fluctuance, and frequently systemic features such as fever or malaise. Examination here reveals no caries, no fluctuant swelling, and the pain localizes to the muscle rather than a specific tooth, arguing against an acute dental abscess.
Acute lateral pterygoid spasm can cause sudden onset of pain and limited jaw opening with deviation and an acute change in occlusion, but it is usually not associated with visible swelling over the masseter region. The history of post-injection onset, diffuse masseter tenderness and swelling, and constant pain at rest and with function is much more typical of myositis of the masseter than of isolated lateral pterygoid spasm.
Further Reading:
Fricton JR. Myogenous temporomandibular disorders: diagnostic and management considerations. Dent Clin North Am. 2007;51(1):61–83. (PubMed)
Wright EF, North SL. Management and treatment of temporomandibular disorders: a clinical perspective. J Man Manip Ther. 2009;17(4):247–254. (PubMed)
Fricton J. Classification, causation and treatment of masticatory myogenous pain and dysfunction. Dent Clin North Am. 2008;52(3):485–505. (PubMed)
Additional Concepts:
• Myositis in the masticatory muscles can follow direct intramuscular injection of local anesthetic, particularly into the medial pterygoid or masseter. Repeated injections, high epinephrine concentrations, and traumatic needle placement increase the risk of localized muscle injury and subsequent inflammation.
• In contrast to myositis, myofascial pain of the masticatory muscles is defined by the presence of discrete trigger points within taut bands, often with referred pain upon palpation but without visible swelling. Recognizing this distinction is important because myofascial pain tends to be more chronic and multifactorial, whereas myositis is often acute and clearly linked to a precipitating event.
• Initial management of masticatory myositis is usually conservative and includes rest of the jaw, soft diet, nonsteroidal anti-inflammatory drugs, gentle stretching as tolerated, and elimination of precipitating factors such as parafunctional habits. Most cases resolve as inflammation subsides; persistent or atypical presentations warrant evaluation for systemic inflammatory or autoimmune myopathies.
• Imaging is typically not required to diagnose simple masticatory myositis, but in refractory cases or when myositis ossificans is suspected, CT or MRI can demonstrate calcification or abnormal signal within the affected muscle. Electromyography and laboratory evaluation may be appropriate if a systemic myopathic process is considered.
