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
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
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.
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.
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.
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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