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Flaccid Arm Monoplegia Due to Lower Motor Neuron Lesions
A lower motor neuron lesion may occur at the: (1) brachial center of the spinal cord; (2) roots and spinal nerves that arise from the spinal segments C5 to T1; (3) brachial plexus; or (4) peripheral nerves (Figure 214.1 D through G). Lesions in the brachial center, roots, and spinal nerves usually occur together with brachial plexus lesions.

Figure 214.1.— 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.

Brachial Plexus Palsy
The brachial plexus is the most common site of injury in neonates with flaccid arm weakness. A brachial plexus lesion may present as complete or segmental flaccid weakness. Complete brachial plexus injury refers to weakness of the whole arm. Segmental arm monoparesis due to brachial plexus injury in the neonate has four clinical presentations: (1) Duchenne-Erb palsy; (2) upper-middle trunk syndrome, (3) Klumpke palsy, and (4) fascicular syndromes. Brachial plexus palsy must be differentiated from flaccid arm monoparesis due to an upper motor neuron lesion and from peripheral nerve damage.

 
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brachial plexus peripheral nerves brachial center rostal spinal corrd lower pons and medulla upper pons brain to midbrain lumbosacral plexus brachial plexus fibers to the lower facial quadrant facial nerve fibers to the upper facial quadrant thalamus lateral ventricle lateral ventricle thalamus Piatt, 1991 Painter, 1983 To identify a structure pause the pointer over the abbrevations, or the structure in question; to idenyify the sites of injury pause pointer over the letters; do not click. Not all structures are labeled. Figure must be centered.