Acquired myasthenia gravis and hypothyroidism in a 5 year old MC mixed breed dog
Contributed by Drs. Karen Vernau and Jason King
William R. Pritchard Veterinary Medical Teaching Hospital
University of California, Davis
Clinical History
A 5-year-old MC Dalmation cross dog was presented to the William R. Pritchard Veterinary Medical Teaching Hospital (VMTH) at University of California, Davis with a 5 day history of regurgitation and reluctance to walk. One day prior to presentation the dog fell down a few stairs, and was subsequently unable to walk on his pelvic limbs.
Physical and Neurological Examination
Abnormalities on physical examination were restricted to a dull mentation and approximately 5-8% dehydration. The dog weighed 23.2 kg with a temperature of 101.2F, pulse rate of 120 bpm and was panting. The body condition score was 5/9. Capillary refill time was 1 second. On thoracic auscultation, no murmurs, pulse deficits, crackles or wheezes were noted. Organomegaly was not appreciated. Lymph nodes were all normal in size.
Neuroanatomical Localization
Cranial nerve examination was normal. With encouragement the dog was able to sit with the pelvic limbs placed under the abdomen. When encouraged to stand or walk, the dog had a wide based stance in the pelvic limbs with stifles flexed. The dog was unable to extend the stifles to weight bear. Voluntary movement was good in all four limbs. After taking a few steps the dog would sit and then lay down (See video clip). Conscious proprioception was present in all four limbs with normal tendon and flexor reflexes. There was no apparent spinal pain. Clinical signs of weakness and exercise intolerance were localized to the neuromuscular system and differentials included myopathy, neuropathy, or disorder of neuromuscular transmission (junctionopathy). Mental dullness was attributed to dehydration secondary to regurgitation. A junctionopathy, (such as myasthenia gravis, MG) or myopathy was considered most likely. Dogs with neuromuscular disease may develop megaesophagus and subsequent regurgitation.
Diagnostic Tests
The dog was admitted to the hospital. A minimum database was done (CBC, chemistry panel, urinalysis, thoracic radiographs, abdominal ultrasound), and treatment was initiated with IV fluids to replace fluid deficits, ongoing losses and maintenance needs. Oral feeding was discontinued.
CBC – Mild left shift with moderate toxicity
Chemistry panel – Mild hypercholesterolemia
Urinalysis – Specific gravity of 1.072
Thoracic radiographs (Figures 1 and 2) - Megaesophagus, microcardia, and right middle lung lobe consolidation with air bronchograms were present, interpreted to be consistent with aspiration pneumonia and dehydration.
Figure 1
.JPG)
Figure 2
Edrophonium chloride challenge (“Tensilon Test”) – Strongly positive
Acetylcholine receptor antibody titer – Positive at 6.9 nmol/l (canine reference <0.6 nmol/l, Comparative Neuromuscular Laboratory, University of California, San Diego)
Thyroid panel: Thyroxine (T4) 0.5 µg/dl (reference 1.0-3.6), Free T4 by dialysis 0.4 ng/dl (reference 1.0-3.5), Canine TSH 2.0 ng/ml (reference 0-0.5) and canine thyroglobulin autoantibodies positive.
Diagnosis and Long-term Follow-up
A diagnosis of myasthenia gravis with megaesophagus and hypothyroidism was made. Treatment was initiated with intravenous antibiotics (ampicillin, enrofloxacin and metronidazole) and an intravenous acetylcholinesterase inhibitor (pyridostigmine bromide). After 5 days, the dog was able to tolerate oral medication and raised oral feedings. Treatment was changed to oral pyridostigmine and soloxine was added. Although there was a favorable response to treatment, the megaesophagus remained. Continued aspiration pneumonia required hospitalization every 6 weeks. While the long-term prognosis can be good for myasthenia gravis in dogs, concurrent hypothyroidism with additional effects on the neuromuscular system, may worsen the prognosis for recovery. Four months after diagnosis, severe aspiration pneumonia developed requiring mechanical ventilation. The dog was euthanized.