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Myopathies are disorders with structural changes or functional impairment of muscle and can be differentiated from other diseases of the motor unit (e.g., lower motor neuron or neuromuscular junction pathologies) by characteristic clinical and laboratory findings. Myasthenia gravis and related disorders are discussed in Chap. 448; inflammatory myopathies are discussed in Chap. 365.
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The most important aspect of assessing individuals with neuromuscular disorders is taking a thorough history of the patient’s symptoms, disease progression, and past medical and family history, as well as performing a detailed neurologic examination. Based on this and additional laboratory workup (e.g., serum creatine kinase [CK], electromyography [EMG]), one can usually localize the site of the lesion to muscle (as opposed to motor neurons, peripheral nerves, or neuromuscular junction) and the pattern of muscle involvement. It is this pattern of muscle involvement that is most useful in narrowing the differential diagnosis (Table 449-1). Most myopathies present with proximal, symmetric limb weakness with preserved reflexes and sensation. However, asymmetric and predominantly distal weakness can be seen in some myopathies. An associated sensory loss suggests a peripheral neuropathy or a central nervous system (CNS) abnormality (e.g., myelopathy) rather than a myopathy. On occasion, disorders affecting the motor nerve cell bodies in the spinal cord (anterior horn cell disease), the neuromuscular junction, or peripheral nerves can mimic findings of myopathy.
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