Spastic arm monoparesis is characterized by increased muscle stretch reflexes in the biceps, brachioradialis, and triceps, and exaggerated Moro reflex. The weak arm moves as much as or more than the normal arm during a Moro reflex (Figure 208.1).

A	B

Figure 208.1— Neonate with spastic right arm monoparesis. [A] At rest, the right arm does not move. [B] During Moro reflex, the right arm motion is appropriate. This neonate had left hemispherectomy for the control of seizures. He was born with left hemimegalencephaly.

Spastic arm monoparesis does not develop until 1 or 2 weeks after an acute injury. The site of injury of the upper motor neuron may be at the brain, midbrain, pons, medulla, or rostal spinal cord (Figure 208.2 A-D). The distinction among the different possible sites of central nervous system injury is based on the distribution of weakness in the arm, the presence and characteristics of facial weakness, and the associated neurological and nonneurological findings.

Figure 208.2.— Sites of possible nervous system injury that can produce arm monoparesis. A: brain to midbrain; B: upper pons; C: lower pons and medulla; D: rostal spinal cord; E: brachial center; F: brachial plexus; G: peripheral nerves; V: ventricles; T: thalamus; UQ: upper quadrant; FN: facial nerve; LQ: lower quadrant; BP: brachial plexus; LSP: lumbosacral plexus. The colored rectangles indicate the location of weakness produced by damage to the different components of the somatic motor system.