Rigby, a four-month-old male Cavalier King Charles spaniel, was surrendered by his original owner due to concerns about abnormalities in his jaw range of motion, and was adopted by a veterinary technician. At that time, the dog was only able to open his mouth approximately 1 cm and seemed painful on attempts to open it. Examination by a veterinary dentist revealed a vertical range of motion of 1 cm between maxillary and mandibular incisors. A CT scan was performed and interpreted by a board-certified radiologist. Results showed bilateral temporomandibular joint (TMJ) dysplasia as described with bilateral temporomandibular joint subluxation and widening of the right temporomandibular joint, scant periosteal new bone along the left mandibular ramus (ventral to the condyloid process), and bilateral thickening of the rostral tympanic bullae walls. No thickening of the calvarium was seen. No mineralized bone was seen connecting the temporomandibular joints to the tympanic bullae. There was no mention of the appearance of the masticatory muscles. The radiologist interpreted changes on the CT scan as suggestive of early craniomandibular osteopathy (CMO). Shortly after his CT scan, Rigby was diagnosed and treated for Parvovirus, and his mouth was described as opening 1.3 cm at that time. After recovering from Parvovirus, Rigby was referred to us for treatment of possible CMO/TMJ issues. He was doing very well at home. He ate dry and wet dog food well. His pet parent reported his head previously appeared larger and flatter on top, but that the shape of his head has changed to appear less swollen on top and more dome-shaped. The owner's comment made me think of another possible diagnosis. Sure, the change in head conformation could be due to disuse atrophy, but a disease called masticatory muscle myositis (MMM) could explain this and many of Rigby’s other signs. Our examination showed normal retropulsion of the eyes bilaterally, moderate atrophy of the temporal and masseter muscles, and a range of jaw motion of 3.8 cm (interincisal distance). Rigby was nonpainful on repeated opening of the jaws. He showed minimal side-to-side jaw laxity when the mandible was moved in either direction, suggesting TMJ dysplasia was less likely. Based on our findings, we recommended sending blood to be tested for MMM, specifically the 2M antibody test at the Comparative Neuromuscular Laboratory at the University of California, San Diego. Although most dogs with MMM will cry out in pain when opening their mouth, some dogs may be less painful in the chronic stages or due to the waxing/waning nature of MMM. The assay showed a result of 1:1000, consistent with a diagnosis of MMM. We recommended 0.55 mg/kg of prednisolone every 12 hours for 7 days, followed by 0.55 mg/kg per day for one month, followed by a continued slow taper after that. Rigby’s MMM was monitored by an internal medicine specialist who worked at the same hospital as Rigby’s owner, and he was doing so well at home that steroids were deemed unnecessary. A repeat MMM titer three months later was less than 1:100, and he had a normal range of jaw motion. However, two years later, Rigby developed a new problem. Even though he had a full range of jaw motion with no pain, Rigby developed bilateral exophthalmos (Figure 1A). Acute inflammation associated with MMM can cause bilateral exophthalmos, but other differentials include sialadenitis of the zygomatic salivary glands and extraocular muscle myositis. We recommended an MRI to obtain as much soft tissue detail of the area as possible. The radiologist who read the MRI found no swelling of the masticatory muscles. In fact, atrophy was noted in the muscles of mastication. The zygomatic salivary glands beneath the eye seemed normal size and symmetrical. The radiologist felt the extraocular muscles and retrobulbar space were unremarkable, with no evidence of asymmetry. Rigby’s owner asked me to review his MRI, and I concurred with the radiologist’s findings. However, I felt the extraocular muscles appeared to be the most likely cause of exopthalmos, compared with the atrophied masticatory muscles and the normal-sized zygomatic salivary glands (Figure 1B). Rigby was started on prednisolone 0.8 mg/kg twice daily with rapid and dramatic results (Figure 2). He is now controlled on a long, slow taper and is currently on 0.2 mg/kg of prednisolone once daily. Figures 1A and 1B. A) Rigby, a two-year-old Cavalier King Charles Spaniel, developed extraocular myositis two years after a bout of masticatory muscle myositis. B) Rigby’s appearance after just a few days of prednisolone treatment. Photos courtesy Dr. John R. Lewis Figure 2. MRI image of Rigby’s extraocular muscles. Photo courtesy Dr. John R. Lewis MMM is an idiopathic autoimmune inflammatory myopathy with clinical signs restricted to the muscles of mastication.1 The temporal, masseter, and medial and lateral pterygoid muscles are all composed of type 2M fibers that contain a unique myosin. The digastricus muscle, however, contains type 2A fibers and thus is not affected. Dogs with MMM have circulating autoantibodies that specifically target type 2M fibers. The autoimmune response (infiltration of inflammatory cells into muscle tissue after immune complex formation) results in necrosis, phagocytosis, and fibrosis of the affected muscles. Serum 2M antibody testing is commercially available, non-invasive, and highly sensitive (85-90 percent) and specific (100 percent) for MMM. Corticosteroid therapy up to 10-days prior to blood collection, however, may produce a false negative titer.1 Conventional CT will often show patchy uptake of contrast, likely due to variation of inflammation in different areas. Muscle biopsy allows for immunohistochemical staining of sampled muscle tissue, revealing immune complexes surrounded by inflammatory cells, which is sufficient to make a diagnosis of MMM even in the absence of a positive 2M serum titer.1 There is a report in the literature of an atypical form of MMM occurring in a litter of Cavalier King Charles spaniels. Clinical signs, including difficulty in opening the mouth (trismus), were seen in three of four 12-week-old Cavalier King Charles spaniel littermates. Diagnosis was established by 2M immunohistochemistry, supported by characteristic histopathological changes in the affected temporal muscle. Treatment using corticosteroids at immune-modifying doses resulted in resolution of clinical signs in all the affected animals.2 Extraocular muscle myositis rarely appears in the peer-reviewed literature. A case series of 10 young dogs showed unilateral or bilateral presentations across different breeds. Clinically, abnormalities were restricted to the extraocular muscles with sparing of the masticatory muscles and limb muscles. Diagnosis was supported in some cases by imaging studies, electrophysiology, and lack of antibodies against type 2M fibers. Histologically, there was variable lymphocytic plasmacytic mononuclear cell infiltration and fibrosis.3 One case report described a single case of extraocular myositis due to eosinophilic inflammation.4 This is interesting, since Cavalier King Charles spaniels have been documented to be prone to other manifestations of eosinophilic disease, including eosinophilic palatitis.5 Here’s to wishing Rigby continued success in managing his autoimmune myositis. He is the sweetest boy! John R. Lewis, VMD, DAVDC, FF-OMFS, practices veterinary dentistry and oral surgery at Veterinary Dentistry Specialists and teaches at Silo Academy Education Center, both located in Chadds Ford, Pa. References Fink L, Lewis JR, Reiter AM. Biopsy of the temporal and masseter muscles in the dog. J Vet Dent. 2013;30(4):276-80. Pitcher GD, Hahn CN. Atypical masticatory muscle myositis in three cavalier King Charles spaniel littermates. J Small Anim Pract. 2007;48(4):226-8. Allgoewer I, Blair M, Basher T, Davidson M, Hamilton H, Jandeck C, Ward D, Wolfer J, Shelton GD. Extraocular muscle myositis and restrictive strabismus in 10 dogs. Vet Ophthalmol. 2000;3(1):21-26. Mitra S. Eosinophile Myositis der extraokulären Muskeln. Ein Fallbericht [Eosinophilic myositis of the extraocular muscles. A case report]. Tierarztl Prax Ausg K Kleintiere Heimtiere. 1998;26(5):336-40. Mendelsohn D, Lewis JR, Scott KI, Brown DC, Reiter AM. Clinicopathological Features, Risk Factors and Predispositions, and Response to Treatment of Eosinophilic Oral Disease in 24 Dogs (2000-2016). J Vet Dent. 2019;36(1):25-31.
