Non-infectious myositis is a condition characterized by chronic localized myalgia originating from central nervous system effects. It is also known as centrally mediated myalgia associated with neurogenic inflammation. When this condition occurs in the lateral pterygoid muscle, clinical evaluation is difficult due to its inaccessible anatomic location. In order to diagnose this rare condition, careful clinical examination and advanced imaging are necessary. The authors report herein four cases of non-infectious myositis of the lateral pterygoid muscle diagnosed by magnetic resonance or enhanced computed tomography imaging. The patients reported prolonged parafunctional habits and chronic jaw pain. In each case, clinical signs suggested the diagnosis of anterior disc displacement without reduction, but the progressive history of internal derangement did not fit this diagnosis. Limited lateral excursion was observed, and patients reported pain in the temporomandibular joint (TMJ) area without tenderness to palpation of the TMJ. Advanced imaging, including axial views, provided valuable information for accurate diagnosis and appropriate management.
Myositis is an inflammatory muscle disorder caused by infection, trauma, or prolonged muscle tension. Myositis is divided into infectious and non-infectious subtypes. Infectious myositis arises from bacterial or viral infections and is accompanied by typical inflammatory signs, such as swelling, redness, and fever. Non-infectious myositis may not show these inflammatory signs. If muscle pain is sustained for a prolonged period, repetitive nociceptive input may result in changes in the central nervous system (CNS), potentially causing an antidromic effect on afferent peripheral neurons. For this reason, non-infectious myositis is also called centrally mediated myalgia associated with neurogenic inflammation. When the masticatory muscles are affected by non-infectious myositis, the clinical manifestations may be similar to the signs and symptoms of other types of temporomandibular disorder (TMD). When jaw pain or limited mouth opening arises from a pathology in the lateral pterygoid muscles (LPMs), it is difficult to diagnose because they are nearly impossible to palpate. Moreover the pattern of referred pain from the LPM is quite diverse. Thus, in performing the differential diagnosis of such symptoms, other diagnostic modalities can be useful.
Magnetic resonance imaging (MRI) and computed tomography (CT) are used widely to diagnose muscle and joint disorders. These advanced imaging modalities are useful to distinguish TMD-mimicking pathoses from genuine TMD. The symptoms of non-infectious myositis can be similar to those of other types of TMD, such as anterior disc displacement (ADD) without reduction. Both pathologies may also present simultaneously. Therefore, advanced imaging modalities are often considered in patients with chronic TMD to improve diagnostic accuracy.
There have been several studies on the diagnosis of infectious myositis of the masticatory muscles by MRI. However there are few reports on non-infectious myositis of the masticatory muscles. Here, the authors report four cases of patients with jaw pain and limited mouth opening, which were finally diagnosed as non-infectious myositis of the LPMs. These cases demonstrate the importance of advanced imaging modalities in distinguishing this condition from other types of TMD, thus facilitating effective management.
All cases in this report relate to patients who presented to the TMJ–orofacial pain clinic of the study hospital between 2006 and 2013. The research protocol was approved by the institutional review board.
An 80-year-old woman presented with a 2-month history of left pre-auricular pain while chewing and limited mouth opening. She reported a habit of jaw clenching and had a history of trauma to the right side of the chin approximately 6 months prior. She was unable to open her mouth more than 28 mm, but had almost normal lateral range of motion of the mandible. Although she reported clicking in her left temporomandibular joint (TMJ), physical examination revealed no noise in either TMJ. No sign of swelling or local heat was exhibited. On mouth opening, the mandible deflected to the left side. She had pain on palpation of several masticatory and neck muscles bilaterally, but no pain on palpation of either TMJ capsule area. On panoramic and transcranial radiographs, no specific bony changes were detected. The patient was diagnosed clinically with ADD without reduction of the left TMJ and myofascial pain of the masticatory muscles. She was prescribed fenoprofen 300 mg three times daily for 2 weeks. At follow-up 2 weeks after beginning treatment, her symptoms persisted, so TMJ MRI was performed.
MRI revealed the right TMJ had ADD with reduction and the left TMJ had ADD without reduction. The patient was managed conservatively with the use of moist hot packs, ultrasound therapy, exercise, a stabilization splint, and prescription of non-steroidal anti-inflammatory drugs (NSAIDs). After 1 year, the left pre-auricular pain had subsided, although her mouth opening had not improved and she had developed a feeling of fullness in the left side of her face. Enhanced CT revealed swelling of the left LPM and loss of its surrounding fat plane ( Fig. 1 ). The patient was diagnosed with non-infectious myositis of the left LPM, which had possibly progressed to muscle contracture.
Even though the left pre-auricular pain had subsided, she still suffered from a limitation of mouth opening. She declined any other further treatments and has been followed up regularly for 4 years.
A 25-year-old woman with depression was referred by a clinical psychologist for symptoms of limited mouth opening, bilateral pre-auricular pain, and crepitus of the right TMJ that had begun approximately 3 years prior. She reported a habit of jaw clenching. She had undergone orthognathic surgery of the mandible 8 years prior, and had developed an anterior open bite at 3 years prior to presentation. She had undergone TMJ lavage and physical therapy, but her symptoms persisted. She was unable to open her mouth more than 27 mm. She had pain on palpation of the right TMJ capsule and several masticatory and cervical muscles. Deflection of the mandible to the right side on mouth opening and fine crepitus in the right TMJ were observed. She reported severe pain and exhibited limited range of movement on lateral excursion of the mandible bilaterally to a similar extent. Plain radiographs showed no bony change on either condyle. An initial clinical diagnosis was made of ADD without reduction of the right TMJ with myofascial pain of the masticatory muscles. MRI of the TMJ showed a normal disc–condyle relationship on bilateral TMJs, but axial views revealed a hyperintense T2 signal of the bilateral LPMs, which led to a final diagnosis of non-infectious myositis of the bilateral LPMs. Conservative management was instituted with moist hot packs, spray and stretch, exercise, and a stabilization splint. After 1 year she had recovered a normal range of mouth opening without jaw pain.
A 49-year-old woman attended the clinic with pain of the left TMJ while opening her mouth that had begun suddenly approximately 1 month prior. She reported a habit of jaw clenching as well as a clicking noise of the left TMJ 2 weeks prior to presentation. She had undergone acupuncture treatment at an oriental medicine clinic without relief, and her symptoms had worsened after a face massage in an alternative care centre. Her mouth opening was limited to 17 mm, and the mandible deflected to the left side when open. Lateral excursion of the mandible was limited bilaterally to a similar extent. Neither TMJ capsule showed pain on palpation. Plain radiographs showed no bony change on either condyle. She was diagnosed clinically with ADD without reduction of the left TMJ. TMJ MRI showed a normal disc–condyle relationship of the bilateral TMJs, but axial views revealed a hyperintense T2 signal of the bilateral LPMs. The left LPM was more swollen than the right one, and exhibited loss of its surrounding fat plane signal ( Fig. 2 ). The final diagnosis was non-infectious myositis of both LPMs. She was prescribed aceclofenac 100 mg twice daily for 3 weeks. Her pain subsided and she was able to open her mouth to 45 mm.