A 4-year-old female spayed domestic shorthair cat weighing 6.9 kg was presented with a 2-week history of intermittent bilateral exophthalmos, facial swelling and serous ocular discharge. Food prehension, the ability to open the mouth and pre-referral intraoral radiographs were unremarkable. Owing to a lack of visible clinical improvement following clindamycin (Zodon; Ceva) treatment, amoxicillin–clavulanic acid (Noroclav; Norbrook) and meloxicam (Metacam; Boehringer Ingelheim), the cat was referred to the neurology and neurosurgery service at the Animal Health Trust.\nAt presentation, generalised facial swelling, extending to the neck and cheeks, mild exophthalmos, third eyelid protrusion, chemosis and bulbar conjunctival hyperaemia were detected bilaterally (). The left eye was more severely affected than the right. Aqueous flare was absent and intraocular pressure, menace response and pupillary light and dazzle reflexes were unremarkable. Fluorescein staining was negative bilaterally and the Jones test was positive in the right eye only. Performing complete physical and neurological examinations was not possible given the uncooperative nature of the cat; however, the remainder of the physical examination under general anaesthesia was unremarkable. A disease affecting the retrobulbar area or the masticatory muscles, including masticatory myositis (MM), was suspected.\nHaematology and serum biochemistry revealed mild eosinophilia (3.38 × 109/l; reference interval [RI] 0.10–2.00) and a markedly elevated creatine kinase (CK) activity (5959 IU/l; RI 70–190). Serological titres against Toxoplasma gondii were consistent with previous exposure.\nMRI of the head was conducted with a 1.5 T scanner (Signa EchoSpeed; GE Healthcare). Transverse, dorsal and sagittal T2-weighted (T2W) (repetition time [TR] 3975–6221 ms/echo time [TE] 84–87 ms; field of view 140 mm, matrix 512 × 512) sequences, transverse precontrast T1-weighted (T1W) (TR 521 ms/TE 6.6 ms) images, and transverse and dorsal postcontrast T1W sequences – after the administration of an intravenous gadolinium-based agent (Gadovist; Bayer) – were obtained. Additionally, transverse short tau inversion recovery (STIR) (TR 8344 ms/TE 39 ms/TI 150 ms) and transverse postcontrast T1W images with fat saturation (TR 732 ms/TE 6.6 ms) were acquired. Slice thickness was 3 mm.\nThe temporal, masseter and medial pterygoid muscles were diffusely thickened bilaterally and showed multifocal-to-coalescing areas of marked hyperintensity on T2W and STIR images relative to normal muscle (). There were subtle hyper- and hypointensities on T1W images, producing an ill-defined striated appearance. The affected muscles showed strong and heterogenous contrast enhancement, with multiple embedded, non-contrast-enhancing areas (consistent with fluid or necrosis) within the abnormal muscles (,). The digastricus, extraocular and cervical muscles appeared to be unaffected. Significant mandibular and medial retropharyngeal lymphadenopathy was detected. The MRI was suggestive of severe infectious or immune-mediated MM.\nAs a feline-specific assay has not been established, a canine ELISA was used to detect serological titres against type IIM muscle fibre proteins and was positive at a dilution of 1:4000 (RI <1:100 in dogs). Cytological analysis of ultrasound-guided fine-needle aspirates of the left temporal and masseter muscles, including the suspected necrotic regions on MRI, revealed eosinophilic and macrophagic inflammation. Bacterial and fungal cultures were negative. Lack of evidence supporting active infection coupled with positive serological titres against type IIM muscle fibres supported the diagnosis of immune-mediated MM.\nElectromyography (EMG) was performed using digital electrodiagnostic equipment (Medelec Synergy; Oxford Instruments). The temporal, masseter, epaxial and pelvic and thoracic limb appendicular muscles were assessed. Normal insertional activity was recorded and no electrical activity abnormalities were identified.\nOpen muscle biopsy samples were obtained from the left temporal muscle. Both fresh refrigerated and formalin fixed samples were sent to the Comparative Neuromuscular Laboratory at the University of California, San Diego, by a courier service, for complete histopathological examination. The biopsies showed a moderate variability in type IIM myofibre size with scattered atrophic fibres having a round shape. Fibre type grouping was not observed and intramuscular nerve branches appeared normal. Multifocal areas of mononuclear cell infiltrations (lymphocytes and acid phosphatase/esterase reactive macrophages) had an endo- and perimysial distribution. Peroxidase reactive eosinophils were not identified. Scattered necrotic fibres were undergoing phagocytosis. Moderate perimysial fibrosis was observed. No fibre loss, organisms or cytoarchitectural abnormalities were identified. A multifocal inflammatory myopathy (myositis) with mild perimysial fibrosis and no obvious myofibre loss was diagnosed ().\nConsequently, the diagnosis of immune-mediated MM was established. The cat was started on an immunosuppressive dose of prednisolone (30 mg; 4.4 mg/kg q24h [Prednidale; Dechra]). The dose was decreased every 6 weeks with the aim of discontinuing administration 36 weeks after the initiation of treatment. Repeating the measurement of serum CK activity and serological titres for type IIM myofibres prior to decreasing the dose of prednisolone was recommended on each occasion. However, owing to the cat’s non-compliant behaviour, this was not achieved. Frequent email and telephone communications were maintained with the cat’s owner and photographs of the cat’s face were regularly checked.\nThree months after starting treatment (prior to decreasing the prednisolone dose from 2 mg/kg q24h to 1 mg/kg q24h), the serum CK activity and type IIM myofibre titres were 387 IU/l and negative at <1:100, respectively. Ten months after initial presentation, while receiving prednisolone at 1.5 mg (0.2 mg/kg) q48h for 6 weeks, the cat showed mild lethargy and an episode of unprovoked aggression to a family member. No changes in facial expression were noticed. At this point, the serum CK activity was within the RI and type IIM myofibre titres were positive at 1:500. Repeating muscle biopsies was recommended by the authors but declined by the owner. The prednisolone dose was therefore increased to 15 mg (2.1 mg/kg) q24h. Type IIM myofibre titres returned to within the RI (<1:100) and the prednisolone dose was halved after 2 months. Titres remained negative 6 weeks later inciting a further decrease in prednisolone dose to 5 mg (0.7 mg/kg) q24h. Following unremarkable blood tests 2 months after the prednisolone dose reduction, a further decrease in dose to 2.5 mg (0.35 mg/kg) q24h was implemented. Five months later, despite the cat appearing clinically normal, type IIM myofibre titres became positive at 1:500. Until this time, no significant side effects of prednisolone had been observed. The prednisolone dose was increased to 5 mg (0.7 mg/kg) q24h and ciclosporin (Atopica oral solution; Elanco) was started at 4 mg/kg q12h and continued to date. Three months later, negative type IIM myofibre titre tests were achieved. Ensuing polyphagia and weight loss prompted the performance of blood analyses, and the cat was diagnosed with diabetes mellitus. Appropriate treatment was started by the primary care veterinary surgeon; the prednisolone dose was halved (2.5 mg; 0.35 mg/kg q24h) and gradually discontinued. Immunosuppressive treatment was continued with ciclosporin alone (for 60 days at the time of writing). No reoccurrence of neurological clinical signs has been reported to date, with a follow-up of 27 months after initial diagnosis.
A 1-year-old male neutered mixed breed cat (4.6 kg) was presented for difficulty prehending hard food. The cat was housed strictly indoors. About 2 months prior to presentation, the owner noticed the cat was not fully opening its mouth when yawning but had been able to eat slowly, managing soft food and pieces of hard food. No other behavioral or clinical signs of disease were reported. On physical examination the cat was very bright, alert, appropriate and clinically well hydrated with a body condition score of 6/9. Owing to the long haircoat palpation was necessary to appreciate the severe bilaterally symmetrical atrophy of the masticatory musculature (). Ocular retropulsion and palpation of the mandibular lymph nodes were normal.\nAn intraoral examination was impossible owing to the decreased vertical mandibular range of motion (vmROM) of 11–12 mm as measured with a metric ruler between the upper and lower incisor tips (). Visible teeth were age appropriate and normal in appearance. Occlusion was symmetrical. Limited vmROM was smooth and without discernible crepitus, but had a firm endpoint at 12 mm. The remainder of the physical and neurological examinations was unremarkable. Owing to financial constraints and anesthetic risks associated with the restricted vmROM, CT to rule out osseous abnormalities and biopsy of a masticatory muscle were declined. Blood tests and skull radiography were approved.\nComplete blood count (CBC) values were unremarkable. Serum biochemistry values, including creatine kinase activity, were within normal limits, except for an elevation of alanine transferase (214 U/l; reference interval [RI] 10–100 U/l) and aspartate aminotransferase (176 U/l; RI 10–100 U/l) activities. Dorsoventral skull radiography showed no bony abnormalities associated with the temporomandibular joints or elsewhere in the skull. Since a feline-specific assay has not yet been established for the detection of antibodies against masticatory muscle type 2M fiber proteins, an established canine ELISA system was used to detect cross-reacting antibodies against canine type 2M fiber antigens. The serum 2M antibody titer was positive at a dilution of 1:1000 (RI <1:100 in dogs, and also <1:100 using serum from five archived normal cats). Circulating antibodies against type 2M fibers are diagnostic of MM in dogs.\nMethylprednisolone (Depo-Medrol; Zoetis) was administered intramuscularly (24 mg; 5.2 mg/kg). Forced jaw opening to improve vmROM was not performed owing to the risk of iatrogenic osseous fracture or soft tissue damage. Two weeks after the initial treatment, further examination revealed no change in the extent or character of vmROM. Appetite and activity remained good. Repeat CBC and serum biochemistry values were unremarkable. Oral prednisolone (Lloyd [5 mg; 1 mg/kg q12h]) was prescribed. One month later, in a follow-up telephone conversation with the owner, it was determined the oral prednisolone prescription had not been filled. The oral prednisolone prescription was subsequently filled and administered crushed in soft food. The cat was again evaluated almost 2 months after the initial presentation. Body weight was slightly increased (4.7 kg). The vmROM, as determined with a millimeter ruler in the relaxed conscious cat, remained restricted at 11–12 mm. Severe bilaterally symmetrical masticatory muscle atrophy persisted, suggesting end-stage disease and a poor prognosis. Biopsy from a masticatory muscle would be required to determine the degree of fibrosis, population of remaining type 2M fibers, and subsequent prognosis, but this was declined.\nThe cat was lost to follow-up until 1 year after initial presentation. At this time the cat was presented to the emergency service with a recent history of vomiting, anorexia and nasal discharge. Corticosteroid therapy had been discontinued 10 months previously. The cat was alert with appropriate behavior. Body condition score was 5/9 (4.3 kg). Body temperature was 40.3°C (104.6°F) and a bilateral purulent nasal discharge was evident. vmROM remained restricted at 11–12 mm. Thoracic radiography was unremarkable. A SNAP FeLV/FIV ELISA assay (IDEXX) was negative. After 18 h of supportive treatment, body temperature remained elevated at 40.7°C (105.3°F), and severe nasal discharge and anorexia persisted. The owners elected euthanasia on welfare grounds and approved post-mortem examination to further investigate MM.\nHead CT was performed post mortem using contiguous 2.0 slice thickness with 0.1 incremental helical slides using an ultra sharp bone algorithm (Somatom, 16 slice; Siemens). It revealed pronounced bilaterally symmetrical atrophy of the masticatory muscles with no radiographic abnormalities of the temporomandibular joints or other bony structures that would restrict mouth opening ().\nSpecimens were collected from the right and left temporalis and masseter muscles, and the biceps femoris muscle, fixed in 10% neutral buffered formalin, then evaluated at the Comparative Neuromuscular Laboratory (CNL), University of California San Diego. All of the temporalis and masseter muscles exhibited similar pathological changes (). Myofiber loss and fibrosis was marked with multifocal areas of lymphocytic infiltration having an endomysial and perimysial distribution. Large nodules of lymphocytic infiltration were focally present. The biopsy from the biceps femoris muscle was normal in appearance, without obvious cellular infiltration or fibrosis, supporting the focal nature of this inflammatory myopathy and making a generalized disorder such as polymyositis unlikely. The pathologic diagnosis was severe and end-stage inflammatory myopathy myositis with extensive fibrosis in all masticatory muscles evaluated. As in dogs, inflammation restricted to the masticatory muscles and the positive serum 2M antibody titer confirmed the diagnosis of MM in this cat.