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contraction. Affected animals will develop a progressive shortened stride with exercise, which progresses to total fatigue and inability to walk. Strength returns with a brief rest and they are again able to ambulate for short distances. The palpebral reflex will fatigue with repeated testing and sometimes facial nerve paresis is present. Despite profound weakness, conscious proprioception and spinal reflexes are usually normal. Megaesophagus and dysphagia are common and can result in excessive salivation, regurgitation, aspiration pneumonia and death. Intravenous administration of the short-acting anticholinesterase, edrophonium chloride (Tensilon)
1-5 mg in dogs and 0.2-1 mg in cats may cause a dramatic improvement in strength during an episode of collapse. If higher doses are given, a cholinergic crisis of bradycardia, profuse salivation, dyspnea, cyanosis and limb tremors may result which can be reversed with intravenous atropine 0.05 mg/kg. Both false- positive and false-negative Tensilon tests can occur. Other causes of weakness like polymyositis commonly improve with edrophonium chloride. A definitive diagnosis can be made with serology documenting elevated AchR antibodies in the serum. As some cases may be falsely seronegative, re-testing is important in all weak animals suspected to have myasthenia gravis. The severity of clinical signs may not correspond
with the degree of elevation of AchR antibody titers. Megaesophagus and aspiration pneumonia may
be seen on thoracic radiographs. In paraneoplastic myasthenia gravis a thymoma may be seen as a cranial mediastinal mass on thoracic radiographs. A thorough physical and radiographic examine including abdominal ultrasonography should be performed to search for neoplasia. Some dogs with myasthenia gravis have concurrent hypothyroidism and weakness will not improve until both disorders are treated. The serum total T4 or free T4 levels are usually reduced and TSH levels are usually elevated in hypothyroidism. Myasthenia gravis and polymyositis may also occur concurrently and serum CK levels may be elevated. EMG is often normal except for a decremental evoked muscle response on repetitive nerve stimulation of 5/second.
Initial therapy usually consists of the administration
of oral pyridostigmine bromide (Mestinon) 0.5-3 mg/
kg every 8-12 hours with food. A liquid formulation of pyridostigmine bromide is recommended so that the dose can be easily adjusted to the level needed to control the clinical signs. With high doses, weakness may occur as a result of a cholinergic crisis and therefore a low dose of pyridostigmine is initially given then slowly increased until weakness is resolved. Oral famotidine (Pepcid AC) 5 mg/kg/day may reduce the nausea and gastrointestinal irritation from the pyridostigmine bromide. Resolution of clinical signs can be seen in many dogs on a spontaneous basis.
An Urban Experience
Exercise-induced collapse of Labrador Retrievers
Young Labrador Retrievers between 7 months and 2 years of age may present with weakness and collapse during exercise. The weakness begins in the pelvic limbs but can progress to total collapse followed by a period of confusion. The body temperature is often severely ele- vated (up to 107o F) and a severe alkalosis is present on blood gas analysis immediately following exercise. Most mildly affected dogs return to normal within 20 minutes. All remaining clinicopathological and electrodiagnostic tests and histological examination of muscle biopsies are normal.
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