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  • Upper motor neuron diseases usually present with asymmetrical weakness contralateral to the lesion. Lower motor neuron diseases present with symmetrical weakness that can be isolated to specific muscle groups.
  • Involvement of bulbar muscles is manifested by cranial nerve findings, facial muscle weakness, and chewing or swallowing difficulties. Bulbar involvement can occur in both upper and lower motor neuron disorders.
  • Neuropathies are disorders of nerves and tend to cause distal muscle weakness, hypesthesias or paresthesias, and decreased reflexes, especially early in the disease.
  • Myopathies are disorders of muscle and can be inflammatory or congenital. Inflammatory myopathies usually involve proximal muscles and are associated with muscle pain or tenderness.
  • Guillain–Barré syndrome often starts with nonspecific muscular pain, most often in the thighs. The pain is followed by weakness, which is most often symmetrical and distal, progresses upward, and, in some cases, results in total paralysis within 24 hours.
  • Transverse myelitis is a syndrome characterized by acute dysfunction at a level of the spinal cord. A mass lesion must be emergently excluded.
  • Tick paralysis is caused by a neurotoxin from the Rocky Mountain wood tick or the Eastern dog tick.
  • Food-borne botulism results from ingestion of toxins contained in improperly canned foods. Diarrhea and vomiting are followed by neurologic symptoms, such as blurred vision, dysarthria, and diplopia, followed by weakness of the extremities.
  • Infantile botulism is caused by colonization of the intestinal tract by spores of Clostridium botulinum. Many cases are linked to nearby construction projects, as soil harbors the spores. A prominent early manifestation is constipation.


The term “weakness” can refer to a general phenomenon that affects all or most of the body or may refer to a specific area, such as an extremity. The complaint can imply generalized fatigue, refusal to walk, increased clumsiness, loss of bowel or bladder function, or focal motor weakness. In infants, weakness can imply lethargy, poor feeding, or poor head control. Slowly progressive forms of weakness may be due to congenital disorders. The child may present at a time when the weakness is mild, yet progressive. Others may have paralysis at the time of presentation. The primary focus in this chapter is on weakness arising from neuromuscular disorders.


The pathophysiology of weakness varies with the etiology and the specific area affected. Terms that are applied to neuromuscular disorders include the following:


  • Paresis implies a complete or partial weakness.
  • Paraparesis is weakness of the lower half of the body.
  • Quadraparesis is weakness involving all limbs.
  • Hemiparesis is weakness of one side.1
  • Quadriplegia is paralysis of all limbs; it usually results from a spinal cord lesion.
  • Hemiplegia, involving one side of the body, generally results from a lesion in the brain.


Abnormalities of the neuromuscular system are further classified as arising from an upper or lower motor neuron unit. The upper motor neuron unit arises in the motor strip of cerebral cortex, traverses the corticospinal tract, and ends in the spinal cord ...

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