I. Upper motor neuron disease includes brain and brainstem hemorrhages, infarctions, and neoplasms. Some demyelinating diseases may present in this category. Spasticity, increased deep tendon reflexes, pathologic reflexes, sensory abnormalities, and impaired cerebral functions may be noted.
II. Lower motor neuron disease Brainstem lesions such as progressive bulbar palsy and poliomyelitis may present with weakness but usually have other features, such as cranial nerve dysfunction, that lead to the correct diagnosis. Anterior horn cell lesions can cause muscle weakness. Segmental involvement of muscles, which are flaccid, fasciculations, and loss of deep tendon reflexes are seen. Sensory abnormalities do not occur. Muscle atrophy occurs early.
III. Nerve root disease presents with muscle weakness if the ventral root is involved. Loss of deep tendon reflexes and muscle atrophy also occur. Muscles are hypotonic. Atrophy is less pronounced than in anterior horn cell lesions. Pain and sensory loss may occur with dorsal root dysfunction.
IV. Peripheral nerve disease presents with loss of deep tendon reflexes and hypotonia. Sensory abnormalities may or may not occur. Characteristically, several peripheral nerves are involved simultaneously. Distal weakness occurs early. These diseases can easily be confused with primary myopathies, especially during later stages when a primary myopathy may have features of proximal and distal weakness, muscle atrophy, and loss of deep tendon reflexes.
V. Myoneural junction disease Myasthenic syndromes resemble myopathies more than neuropathies. Muscle weakness is often more proximal than distal, without early reflex changes or sensory abnormalities. Myasthenia gravis is characterized by increased muscle fatigue with continued exertion. Improvement occurs with rest.
Ocular muscles are frequently involved. An edrophonium test along with electromyography (EMG) can be used to confirm the diagnosis. The Eaton-Lambert syndrome may present with similar features, but repetitive electrical stimulation on EMG causes augmented muscle response at higher frequencies. Drugs that induce a myasthenic syndrome include D-penicillamine and the aminoglycoside antibiotics.
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