CHAPTER 82 ■ WEAKNESS KATIE A DONNELLY, XIAN ZHAO INTRODUCTION Weakness is defined as an inability to generate normal voluntary force in a muscle or normal voluntary torque about a joint Although often associated, hypotonia is not synonymous with weakness Neurologists define hypotonia as decreased resistance to passive motion Not all hypotonic patients are weak; for example, a patient with Down syndrome may have normal strength yet have decreased tone PATHOPHYSIOLOGY Weakness is a reflection of a disease process that may involve any component of the motor neuron unit These diseases are classically categorized as upper or lower motor unit disorders ( Table 82.1 ) Upper motor neuron disease affects structures extending from the motor strip of the cerebral cortex, through the corticospinal tracts of the spinal cord, to (but not including) the anterior horn cell Although upper motor neuron disease is generally characterized by increased deep tendon reflexes (DTRs) and spasticity, early in the clinical course there may be flaccid paralysis Lower motor neuron disease may involve the anterior horn cell, the peripheral nerves, the neuromuscular junction (NMJ), or the muscle fibers In general, lower motor neuron disease is associated with fasciculations, muscle atrophy, hypotonia, and hyporeflexia, and may ultimately lead to flaccid paralysis DIFFERENTIAL DIAGNOSIS The cerebral cortex can be damaged by cerebrovascular accidents (CVAs), which include cerebral infarctions and cerebral hemorrhages CVAs, while rare (2.1 to 13.1 per 100,000 children per year), cause some of the most catastrophic cases of weakness ( Table 82.2 ) These children usually present with sudden onset of unilateral or asymmetric weakness Cerebral hemorrhage is usually due to a ruptured arteriovenous malformation (AVM), but may also be caused by a ruptured aneurysm Most AVMs are asymptomatic until rupture, but some children complain of periodic “migraine-like” headaches Brain tumor hemorrhage may also present acutely as weakness, severe headache, and vomiting Cerebral infarctions usually occur in the setting of predisposing factors, which include sickle cell disease, homocystinuria, structural arterial disease (e.g.,