Neonates with hypotonia and decreased dynamic tone of the upper extremities and increased dynamic tone of the lower extremities are likely to have a lesion at the C5 to T1 spinal cord segments (Figure 100.1) or a combined T2 spinal cord segment lesion and bilateral brachial plexus injury.
Damage at the C5 to T1 spinal cord segments causes: (1) a lower motor lesion in the arm due to injury to anterior horn motor neurons that supply the brachial plexus, and (2) an upper motor lesion of the legs due to injury of the pyramidal tract en route to the lumbosacral anterior horn motor neurons.

Figure 100.1.— Schematic representation of the possible sites of neuromuscular damage in neonates with generalized hypotonia and decreased upper extremity dynamic tone and increased lower extremity dynamic tone.