NEUROMUSCULAR CASE OF THE MONTH - OCTOBER 2004

Masticatory muscle myositis and myasthenia gravis in a 1.5 year old male Vizsla
Contributed by Dr. Maggie Himann
Laurelwood Veterinary Hospital
Waterloo , Ontario , Canada



Clinical History
A one and a half year old male Vizsla was referred to the Ontario Veterinary College for a five day duration of pelvic limb weakness, hunched back appearance when walking, reluctance to open the jaw, and ptyalism.  A change in bark, intermittent retching, and difficulty swallowing were also noted by the owners.

 

Physical and Neurological Examination
On presentation, the dog was in thin body condition with atrophy of the temporalis and masseter muscles. Constant urine dribbling was noted. Ptyalism was present (A) and the dog resented examination of the jaw. Cranial nerve examination revealed bilateral facial nerve paralysis, an absent palebral reflex bilaterally, a corneal ulcer in the right eye, and marked atrophy of the masticatory muscles (B). A stiff pelvic-limb gait was present. Patellar reflexes were normal and withdrawals were present bilaterally. Differential diagnosis at this time included encephalomyelitis, polyneuropathy, polymyopathy, and myasthenia gravis.

 

       

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 A

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 B

 

Diagnostic Testing

CBC – Unremarkable.
Serum Chemistry Profile – Mild hyperglycemia and low urea.
Creatine Kinase – Normal.
Urinalysis – Specific gravity of 1.014, pH7.0, protein 0.1 g/L, and 1+ bacteria.
Urine Culture – No bacterial growth.
Thoracic radiographs – Unremarkable initially, but on repeat evaluation, megaesophagus was evident and there was mild aspiration pneumonia.
CSF– Unremarkable.
Electromyography – Fibrillation potentials in masseter muscles and tongue, with normal limb muscles.
Muscle biopsy – Biopsies of the masseter muscle and tongue were submitted. Multifocal areas of mononuclear cell infiltrates composed predominantly of lymphocytes and scattered macrophages were present. Fibrosis was not observed.
Serum antibody titer for Toxoplasma Negative
2M antibodies – Positive
Edrophonium chloride challenge test – Positive with dramatic improvement in ambulation and palpebral reflexes.
Acetylcholine receptor antibody – 6.89 nmol/l (canine reference <0.6 nmol/l).

 

Diagnosis
Based on the positive acetylcholine receptor antibody titer and edrophonium chloride challenge tests, the positive 2M antibody titer and inflammatory changes within the masseter muscle biopsy specimen, a diagnosis of both immune-mediated myasthenia gravis (MG) and masticatory muscle myositis (MMM) was confirmed.

 

Treatment
Following initiation of pyridostigmine therapy for MG, muscle strength returned to normal and the dog was able to urinate on its own without overflow incontinence. Elevation of food and water seemed to control the regurgitation. A course of clindamycin and enrofloxacin was also given. Following confirmation of MMM, prednisone was added to the treatment regime.

 

Outcome
Approximately one year following the original diagnosis, the dog was doing well and maintained on pyridostigmine bromide and low dose prednisone. The dog was active with a good appetite and only occasional regurgitation. The AChR antibody titers were monitored periodically and remained positive. Clinical signs of MG returned approximately 2 years after the original diagnosis. Two weeks prior to the exacerbation of MG, a new dog was introduced into the household that had an episode of gastrointestinal disease of undetermined origin. The clinical signs of MG resolved following an increase in the pyridostigmine dosage. The dog is now 5 years of age and still requires pyridostigmine and low-dose prednisone to control clinical signs. This necessity for long term therapy is unusual for canine MG. Most dogs with immune-mediated MG that do not expire from aspiration pneumonia or have a cranial mediastinal mass, are in clinical and immune remission by this time post-diagnosis. Clinical signs of MMM have not returned, although muscle mass has not returned to normal. Periodic updates will be given on this very interesting and unusual dog.



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