44 LMN paresis and paralysis
Acquired myasthenia gravis
DIFFERENTIAL DIAGNOSIS OF ACUTE LMN SIGNS
Immune-mediated myasthenia gravis
Signs reflect skeletal muscle weakness and may be focal (facial, pharyngeal, laryngeal, oesophageal) or generalized (focal signs + appendicular/axial weakness). The most common initial signs in dogs are generalized weakness with megaoesophagus or megaoesophagus alone. In one study, 13% of seropositive dogs had generalized weakness without oesophageal or pharyngeal weakness. Cats chiefly present with generalized weakness and a decreased palpebral reflex. Extraocular muscle weakness is not reported in the small animal. Acquired myasthenia gravis is more common in the dog than the cat.
Signs may worsen with cold weather, oestrus cycles, pregnancy, concurrent infection and vaccination.
Tick paralysis
Diagnosis is aided by clinical improvement seen after tick removal. Engorged ticks may have fallen off by the time the animal is presented.
Polymyositis
Autoimmune polymyositis
is an autoimmune disease of unknown aetiology mostly affecting adult large breed dogs of either sex. Infectious myopathies caused by protozoa or tick-borne pathogens are rare but efforts must be made to rule them out with serology. Equally, drug-induced myopathies should be considered when taking a history. Paraneoplastic myopathies are rare. A generalized immune mediated disease may present as polymyositis.
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