NEUROMUSCULAR CASE OF THE MONTH - DECEMBER 2005

Unilateral presentation of masticatory muscle myositis in a 1 year old FS Golden Retriever
Contributed by Dr. Robert Bergman
Carolina Veterinary Specialists
Charlotte, NC 28273

Clinical History
This young Golden Retriever had a 3 week history of left sided exophthalmia with significant periocular swelling that progressed to inability to open the jaw. A course of clindamycin did not result in clinical improvement. Based on the unilateral nature of the clinical signs and the young age of the dog, a retrobulbar abscess was suspected.

Physical and Neurological Examination
The jaw could not be opened more than a few inches. Exophthalmia was noted on the left side. No other abnormalities were identified on the physical and neurological examinations.

Diagnostic Testing
Serum creatine kinase concentration – 302 U/L (reference <200 U/L)

Serum antibody titer against masticatory muscle type 2M fibers (ELISA) – Positive at a dilution of 1:1000 (reference <1:100).

Electromyography  (EMG) – An EMG evaluation was performed on the masticatory muscles and muscles of the dorsal cervical region. Moderate spontaneous activity was noted in the muscles of the head only on the left side.

CT of the orbits and skull – 3 mm contiguous axial images were taken through the orbit and skull pre and post-intravenous contrast. Exophthalmia of the left eye was noted with an increase in size of the left pterygoid muscle as well as of the muscles surrounding the left mandible (Figs. 1,2).  There was increased contrast enhancement of these muscles when compared to the opposite side. No bony abnormalities were noted. It was concluded that there was inflammation and swelling present in these muscles with no obvious neoplasic mass effects.

Fig. 1

Fig. 2

Diagnosis
Based on the positive serum antibody titer for masticatory muscle myositis and the findings on imaging studies, a diagnosis of masticatory muscle myositis was made. While masticatory muscle myositis is a bilateral disease, there can be a unilateral clinical presentation with one side more markedly affected than the other. Biopsies of the masticatory muscles were not performed in this case; however, involvement of both sides can usually be confirmed if samples are collected from both the right and left temporalis muscles.

Treatment
Prednisone was intiated at 20 mg in the morning and 10 mg in the evening (anti-inflammatory dose). The dog responded dramatically to the therapy and normalized within 12-24 hours. The plan was to continue this dosage of prednisone for the next 2-3 months then begin a slow taper. At recheck one month later the dog was normal in appearance with normal jaw mobility and no observable exophthalmia (see image below).      

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