Restricted jaw mobility and masticatory muscle atrophy resulting from metastatic neoplasia in a 4-year old Pit Bull Terrier

Clinical History

A 4-year old female spayed Pit Bull Terrier presented with a one month history of rapid and progressive atrophy of the muscles of mastication and inability to open the jaw. Prior to referral the dog was treated with prednisone, doxycycline and gabapentin with no apparent response.

Before onset of masticatory muscle atrophy

At time of clinical presentation

Physical and Neurological Examination

On presentation the dog was markedly lethargic and often recumbent in the cage, but could be aroused with stimuli. Bilateral corneal ulcers and enophthalmos were noted. A stiff pelvic limb gait was present and there was mild to moderate atrophy of the appendicular muscles. Severe and bilaterally symmetric atrophy of the muscles of mastication was present and there was marked trismus with the jaw opening only about 1.5 cm.

Diagnostic Tests

CBC - Moderate anemia with PCV 31% and moderate thrombocytopenia with platelet count of 117,000 X 103/mm3 Serum biochemical analysis - Mildly elevated globulins at 3.7 g/dl. Creatine kinase activity was within the reference range. 2M antibody and acetylcholine receptor antibody titers - Both within reference range Thoracic radiographs - Unremarkable Electromyography - Marked spontaneous muscle activity (positive sharp waves and fibrillation potentials) mainly in the right temporalis muscle and mild spontaneous activity in several appendicular and lumbar muscles. A biopsy from the temporalis muscle was collected.

Case Progression

Recovery from anesthesia was uneventful and the dog was discharged later that day pending results of muscle biopsy testing. Unfortunately, the dog passed away at home that night. A limited necropsy of several muscle and nerves was performed.

Clinical Course and Outcome

Extensive metastatic carcinoma was found in the mandibular lymph nodes, masticatory muscles (see image of temporalis muscle below, H&E stain) and biceps femoris muscle. A primary site could not be determined based on examination of the muscle and nerve tissues. Possible primary sites include the anal sac glands or respiratory epithelium. This case is an example of another cause of trismus and masticatory muscle atrophy not related to masticatory muscle myositis. The 2M antibody titer was negative.