Chapter 32. Clinical Assessment of Lower Extremity Nerve Blocks

The extent of anesthesia after selective blocks of the peripheral nerves originating from the lumbar and lumbosacral plexi is often assessed before surgery can begin. This assessment may be deceptively difficult if one has incomplete understanding of lower extremity innervation or if the evaluation is hindered by slow onset of sensory block. Conversely, timely evaluation of the block allows the anesthesiologist to identify inadequately anesthetized nerves, thus providing the opportunity to modify the block before incision.

A simple system for assessing the adequacy of upper extremity nerve block has been described and enjoys acceptance by anesthesiologists worldwide. The four P's (push, pull, pinch, punt) concept was likely developed during World War II as a straightforward method for medics to determine the extent of battlefield injury. The concept was later formalized and popularized by Thompson and Brown.1 Herein is described a methodology for assessing lower extremity anesthesia that is a variation of the four P's concept. For lower extremity evaluation, the P's are modified slightly to become push, pull, pinch, punt.

Several minutes after depositing local anesthetic near a peripheral nerve, the following individual assessments are undertaken to determine blockade of the four major nerves of the lower extremity.

The sciatic nerve is derived from the lumbosacral plexus and provides motor control to the posterior thigh and the entire lower leg and foot. Its sensory distribution includes the posterior thigh, the posterior knee joint, and all the lower leg except for the saphenous nerve distribution (medial lower leg and ankle). Because plantar flexion of the foot is controlled by the sciatic nerve, its function is evaluated by asking the patient to push the foot against the resistance of the examiner's hand or “step on the gas” (Figure 32–1). Anesthesia within the sciatic nerve distribution is indicated by weakness during the performance of this maneuver. Because the evaluation is performed distally, it is applicable to all sciatic nerve block approaches, including the blocks at the popliteal fossa.

The obturator nerve, a component of the lumbar plexus, provides motor innervation to the adductors of the thigh and variable sensory innervation to the proximal medial thigh. It also has small branches to the knee and hip joints. To assess obturator nerve function, the anesthesiologist abducts the patient's leg and then requests that the patient pull the leg toward the midline against resistance (Figure 32–2). The obturator nerve has been successfully blocked if the patient exhibits adductor weakness during this task.

The lateral femoral cutaneous nerve, a part of the lumbar plexus, is the only purely sensory nerve of the four major lower extremity nerves. The cutaneous innervation of this nerve includes the lateral buttock and the lateral thigh. Thus, the inability to detect a pinch on the proximal lateral thigh signals successful conduction block of the lateral femoral cutaneous nerve (Figure 32–3).

The femoral nerve provides motor innervation to the quadriceps femoris and sartorius muscles. Its sensory distribution includes the anterior thigh, the saphenous nerve, and branches to the hip joint and the majority of the knee joint. To test femoral nerve function, the anesthesiologist supports the patient's knee by placing an arm under the popliteal fossa and slightly raising the knee off the bed. The patient is then asked to punt an imaginary football against the resistance of the examiner's restraining hand (Figure 32–4). The inability to contract the quadriceps muscle and extend the lower leg at the knee indicates conduction block of the femoral nerve. If the leg is splinted, requesting the patient to contract the quadriceps muscle accomplishes the same purpose.

This chapter describes a simple and reliable method to verify lower extremity anesthesia after regional block techniques. By using push, pull, pinch, punt, the anesthesiologist can rapidly perform a qualitative assessment of conduction block of the sciatic, obturator, lateral femoral cutaneous, and femoral nerves, respectively.

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.

Because each described maneuver is specific to a peripheral nerve, early functional assessment facilitates selective supplementation of an inadequately anesthetized nerve well in advance of surgical incision. Therefore, the use of the four P's (push, pull, pinch, punt) is recommended as a simple, rapid, and reliable method for assessment of lower extremity nerve block.