Flaccid leg monoparesis may occur with any lesion in the central or peripheral components of the lumbosacral somatic motor system. Flaccid leg monoparesis may occur with: (1) recent lesions in the central component of the somatic lumbosacral motor system at sites similar to those that produce spastic leg monoparesis (Figure 235.1 [A-E]); (2) cerebellar lesions; and (3) lesions in the peripheral component lumbosacral somatic motor system (lumbosacral center (Figure 235.1 [F]), cauda equina (Figure 235.1 [G]), lumbosacral plexus (Figure 235.1 [LSP]), or peripheral nerves.

Figure 235.1.— Schematic representation of the cortical component of the somatic motor system and sites of possible injuries causing leg monoparesis. The colored rectangles indicate the location of weakness produced by damage to the various components of the somatic motor system. V: ventricles; T: thalamus; UQ: upper quadrant; LQ: lower quadrant; FN: facial nerve; BP: brachial plexus; LSP: lumbosacral plexus. A: brain and midbrain; B: upper pons; C: lower pons and medulla; D: upper spinal cord above the brachial center; E: lower spinal cord below the brachial center but above the lumbosacral plexus; F: lumbosacral motor center; G: lumbosacral plexus; H: lower extremity peripheral nerves.