The LMN unit consists of the nerve cell body in the ventral gray matter of the CNS, ventral nerve root, peripheral nerve, and muscle. Disease of any component of this unit will produce LMN signs. Normal strength depends not only on functional LMN unit, but also on effective neuromuscular transmission via acetylcholine (Ach) across the neuromuscular junction (NMJ). Diseases affecting neuromuscular transmission (e.g., myasthenia gravis, tick paralysis), so-called junctionopathies, may produce LMN signs that are indistinguishable from neuropathies and myopathies.
Because NMDs may mimic one another, the neurologic examination rarely confirms the exact component of the LMN unit that is affected. The clinician may require ancillary tests (CK, AST, Tensilon testing, acetylcholine receptor antibody titer, electrodiagnostics, nerve and muscle biopsies) to further localize the problem to the nerve, muscle, or NMJ. The LMN unit may be affected diffusely or individual peripheral or cranial nerves may be affected. When cranial nerve deficits (e.g., facial paresis) are present, these must be interpreted with other neurologic findings in order to differentiate neuromuscular disease from a brainstem disorder.
Neuromuscular weakness is characterized by flaccidity and depressed or absent spinal reflexes. Postural reactions typically are normal with pure NMDs (no sensory nerve involvement), although the patient must be well supported or may fail postural testing on account of weakness. Ambulatory patients with pure NMDs typically have a “short and choppy” gait without ataxia because GP. Exercise intolerance may be the only abnormality present in some patients with NMDs.
In some NMDs (e.g., polyneuropathies), involvement of the recurrent laryngeal nerve may lead to a change in, or loss of, voice (dysphonia) and increased inspiratory noise (stridor). The development of megaesophagus in various NMDs may cause regurgitation, often with accompanying aspiration pneumonia. The NMDs also carry the risk of hypoventilation due to involvement of the respiratory muscles. Finally, changes in muscle mass may vary from severe neurogenic atrophy (e.g., brachial plexus avulsion) to “pseudohypertrophy,” which accompanies myopathies (e.g., muscular dystrophy).
Neurologic Abnormalities Associated with LMN/Neuromuscular Dysfunction
NEUROLOGIC TEST
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POSSIBLE ABNORMALITIES
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Sensorium/behavior
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Normal
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Posture/gait
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Variable: flaccid paresis in affected limb(s) through flaccid paralysis of affected limb(s); exercise may exacerbate the paresis
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Postural reactions
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Delayed to absent in affected limb(s); normal in “pure” LMN disease if patient maintains some voluntary motor function
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Muscle mass/tone
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Decreased tone of affected limb(s); neurogenic atrophy may be severe; pseudohypertrophy in certain myopathies; possible LMN bladder
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Spinal reflexes
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Variable: hyporeflexia thru areflexia of affected limb(s); possibly reduced perineal reflexes (S1-S3)
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Cutaneous sensation
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Normal in “pure” LMN disease (but if a polyneuropathy with a sensory component, hypalgesia may be present)
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Cranial nerves
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Variable: multiple cranial nerves may be affected
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Other
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Laryngeal paralysis, dysphagia, megaesophagus common with LMN disease
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LMN, Lower motor neuron.