The distinction between upper and lower motor neuron flaccid arm monoplegia in the first 2 weeks of life is made based on historical information and the presence of associated neurological findings. Magnetic resonance imaging of the brain and brachial plexus and electromyographic studies may further aid in localization. Flaccid arm monoparesis may be due to an upper or lower motor neuron injury. Flaccid arm weakness due to upper motor injury may occur with a lesion in the brain, brainstem, cerebellum, or spinal cord (Figure 212.1 A-D). Flaccid arm monoparesis due to a lower motor neuron injury occurs with a lesion at the brachial center or at the roots and spinal nerves of the C5 through T1 spinal segments, brachial plexus, or peripheral nerves (Figure 212.1 E-G).

Figure 212.1.— Sites of possible nervous system injury that can produce arm monoparesis. A: brain to midbrain; B: upper pons; C: lower pons and medulla; D: rostral 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.