Because cutaneous innervation of the lower extremity consists of
multiple overlapping sensory fields, it can be difficult to accurately
determine which specific peripheral nerve is inadequately
anesthetized.3,4 The four P's methodology takes advantage
of the principle that peripheral nerves consist of an outer mantle layer
surrounding an inner core layer. The mantle layer, which innervates the
proximal extremity, is generally anesthetized first.5 This
arrangement causes proximal motor fibers to become anesthetized before
distal sensory fibers.6 Thus, motor function of the
proximal lower extremity typically shows signs of conduction block before
the more distal sensory fibers. For example, a patient may show early
gastrocnemius weakness (pushing on the gas) before sensory changes in the foot.
Furthermore, selecting motor or sensory function specific to an individual
nerve will accurately show its functional integrity. Thus, adduction of the
leg (pulling the leg toward the midline) is exclusively an obturator nerve
function, whereas sensation of the proximal lateral thigh
(pinching) is only within the lateral femoral cutaneous nerve's sensory field.