Weakness is a decreased ability to voluntarily and actively move muscles against resistance. Weakness is easily confused with hypotonia or ataxia. Hypotonia is a decreased resistance to passive range of motion. Ataxia refers to decreased muscular coordination. Weakness may arise from any portion of the motor unit, including the upper motor neuron, lower motor neuron, peripheral nerve, neuromuscular junction, or the muscles. Due to the broad differential diagnosis of weakness and the potential for life-threatening complications, the evaluation of weakness requires a careful and comprehensive approach .
The approach to the child with muscle weakness will be reviewed here. The approach to an infant or newborn with hypotonia or weakness is discussed separately. (See "Approach to the infant with hypotonia and weakness" and "Overview of peripheral nerve and muscle disorders causing hypotonia in the newborn".)
Upper motor neuron weakness — Upper motor neuron weakness arises from lesions in the cerebral cortex and corticospinal tracts down to, but not including, the anterior horn cell in the dorsal spinal cord. Although patients with acute upper motor neuron lesions may initially present with decreased spinal cord reflexes accompanied by hypotonia or paralysis on the opposite side of the central nervous system lesion, they ultimately develop spasticity, hyperreflexia, and "upper motor neuron" signs (eg, positive Babinski sign).
Lower motor neuron weakness — Lower motor neuron weakness results from lesions located in the anterior horn cell, peripheral nerve, neuromuscular junction, or muscle. Patients with lower motor neuron weakness typically display muscle weakness, hypotonia, fasciculations, and decreased spinal cord reflexes.
ACUTE MUSCLE WEAKNESS
Many of the conditions that cause acute muscle weakness are potentially life-threatening. Mortality due to acute muscle weakness is typically caused by respiratory failure or by lesions in the brain that cause both weakness and increased intracranial pressure (table 1).