A 12 year-old castrated male Welsh Springer Spaniel was presented to the referring veterinarian with a 2-3 week history of difficulty opening the jaw and significant pain upon minimal manipulation and opening the mouth. Swelling of the left side of the head in the area of the temporalis muscle, and elevation of the left third eyelid, followed a few days later. Skull radiographs, biochemical profile, total T4, and CBC were performed and the results were unremarkable. The serum 2M antibody titer for masticatory muscle myositis was negative. A tentative diagnosis of masticatory muscle myositis was made and the patient started on oral corticosteroid therapy (dexamethasone SP). The dog showed moderate improvement over the first few days but then became clinically worse. At that point the dog was referred to the California Animal Hospital for further evaluation.
Physical and Neurological Exam
Significant swelling of the left temporalis muscle was evident that was painful on palpation. Severe pain was present on manipulating the jaw with opening limited to 2-3 cm. Elevation of the third eyelid was still present in the left eye. The dog was mildly lethargic. No other significant abnormalities were present on physical and neurological examination.
CBC: Leukocytosis (WBC: 33.6 - ref. range: 5.7 - 16.3 thousand/uL), with neutrophilia (segmental neutrophils: 28896 - ref. range: 3000 -11500 /uL) and monocytosis (2352 - ref. range: 150 - 1350 /uL). Neutrophils appeared slightly toxic on microscopic evaluation.
Biochemical profile: No abnormalities detected
MRI Examination: The large soft tissue mass was hyperintense on T2W images (below left) and hypointense on T1W (below middle) and FLAIR (below right) images. The mass invaded deep into the left temporalis muscle, adjacent to the vertical ramus of the mandible and immediately caudal to the left orbit. Following gadolinium injection, avid peripheral contrast enhancement with a hypointense center was present. The brain tissue appeared normal.
CT Imaging: Similar to the MRI findings, there was a large hypointense lesion following contrast enhancement (image on right below). Neither invasion of the calvarium or other osseous changes associated with the lesion was noted.
CT Imaging : Pre-contrast |
CT Imaging : Post-contrast
Surgical Treatment and Final Diagnosis:
Based on the findings of the MRI and CT examinations, an abscess was the most likely diagnosis. Surgery was performed (see image below with dog under general anesthesia. Arrow indicates mass on left side of the head) and an abscess was confirmed. Large amount of extremely malodorous, purulent material was present. Copious lavage was used to flush the affected area and significant amount of necrotic muscle tissue were removed. Bacterial culture showed anaerobic growth (Bacteroides and Fusobacterium species) without aerobic growth.
Muscle biopsy: A biopsy was submitted from the mass associated with the left temporalis muscle. The mass was composed of dense connective tissue (modified Gomori trichrome stain below left) containing numerous acid phosphatase positive macrophages (acid phosphatase positive cells stain red in below right) with extension into the perimysium of surrounding muscle fibers. Invasion of non-necrotic fibers was not observed.
Modified Gomori trichrome stain |
Acid Phosphatase Reaction
The patient was started on antibiotics (Clavamox® and enrofloxacin) following surgery. Within 12 hours after surgery remarkable improvement was seen and the dog was able to open the jaw and eat normally. Within one week the dog was back to normal. A six-week course of antibiotic therapy was completed and the patient continues to do well at 3 months after cessation of antibiotic therapy.