Case 4

Published on 03/03/2015 by admin

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Last modified 03/03/2015

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Case 4

HISTORY AND PHYSICAL EXAMINATION

A 49-year-old woman with ulcerative colitis, status post total abdominal colectomy, underwent a proctectomy, using a combined abdominal and perineal approach, for persistent proctitis in the retained rectal stump. When she awoke from general anesthesia, she became aware of right leg weakness, and numbness in the anterior thigh and leg.

Examination revealed severe weakness of the right quadriceps (Medical Research Council [MRC] 2/5) and iliopsoas (MRC 4–/5), with normal thigh adduction and ankle dorsiflexion. The right knee jerk was absent. There was sensory loss in the right anterior thigh and medial leg.

The clinical diagnosis of a right femoral neuropathy was made. The patient had been in a lithotomy position during surgery. Three weeks postoperatively, and because of persistent weakness, the electromyographer was asked to confirm, localize, and prognosticate.

Please now review the Nerve Conduction Studies and Needle EMG tables.

QUESTIONS

1. A normal femoral compound muscle action potential (CMAP) amplitude in this patient is consistent with:

A. Axon-loss femoral mononeuropathy.
B. Axon-loss lumbar plexopathy.
C. Axon-loss lumbar polyradiculopathy (L3 and L4).
D. Demyelinating femoral mononeuropathy.

2. The clinical and EDX findings in this patient are the result of:

A. A femoral nerve lesion at the inguinal ligament.
B. A lumbar plexus lesion.
C. A saphenous nerve lesion.
D. A femoral nerve lesion in the pelvis.

3. The neurologic complications of pelvic surgery include:

A. Compression of the femoral nerve by a retracting blade.
B. Compression of the femoral nerve by an iliacus hematoma or lumbar plexus by a psoas hematoma.
C. Inadvertent femoral or obturator nerve transection.
D. Compression of the femoral nerve at the inguinal ligament during lithotomy positioning.
E. All of the above.

EDX FINDINGS AND INTERPRETATION OF DATA

The pertinent electrodiagnostic (EDX) findings in this case include:

1. Absent right saphenous sensory nerve action potential (SNAP) with a normal left saphenous SNAP. This excludes an L4 root lesion within the intraspinal canal.
2. Normal right femoral CMAP, symmetrical to the left (6.0 mV versus 5.5 mV), without significant difference in distal latencies.
3. Fibrillations and neurogenic recruitment in the quadriceps and iliacus muscles, with normal thigh adductors and tibialis anterior.

A lumbar plexus lesion is excluded by documenting normal thigh adductor muscles. Involvement of the iliacus muscle confirms that the femoral lesion is within the pelvis (i.e., proximal to the takeoff of the motor branch to the iliacus muscle), and is not the result of compression of the femoral nerve at the inguinal ligament which may follow a lithotomy positioning.

The prognosis for recovery is good because the distal femoral CMAP amplitude is normal, consistent with a predominant proximal demyelination. Note that the femoral nerve may be stimulated only at the inguinal canal distal to the location of the pelvic lesion. Some axonal loss obviously has occurred, based on the fibrillations and the absent saphenous SNAP, but these findings have no prognostic value for the outcome of motor function.

Case 4: Nerve Conduction Studies

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Case 4: Needle EMG

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This intraoperative and intrapelvic femoral nerve lesion is most likely due to compression by the surgical retractor against the pelvic wall. A retroperitoneal hematoma also is possible and must be ruled out urgently. An inadvertent femoral nerve transection is unlikely since it results in axonal loss and not segmental demyelination.

DISCUSSION

Applied Anatomy

The femoral nerve (also called the anterior crural nerve) is formed by the combination of the posterior divisions of the ventral rami of the L2, L3, and L4 spinal roots (the anterior divisions of the same roots form the obturator nerve). It immediately gives branches to the psoas muscle which receives additional branches from the L3 and L4 roots directly. Then, the femoral nerve passes between the psoas and iliacus muscles and is covered by a tight iliac fascia which forms the roof of the iliacus compartment. The iliacus muscle and femoral nerve are the main constituents of this compartment.

The femoral nerve emerges from the iliacus compartment after passing underneath the rigid inguinal ligament in the groin. About 4–5 cm before crossing the inguinal ligament, it innervates the iliacus muscle. Soon after passing under the inguinal ligament (lateral to the femoral vein and artery), the femoral nerve branches widely into (1) terminal motor branches to all four heads of the quadriceps (rectus femoris, vastus lateralis, vastus intermedius, and vastus lateralis) and sartorius muscles, and (2) three terminal sensory branches, the medial and intermediate cutaneous nerve of the thigh which innervate the skin of the anterior thigh, and the saphenous sensory nerve (Figure C4-1).

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Figure C4-1 The femoral nerve and its terminal motor and sensory branches including the saphenous nerve.

(From Haymaker W and Woodhall B. Peripheral nerve injuries: principles of diagnosis. Philadelphia, PA: WB Saunders, 1953, with permission.)

The saphenous nerve travels the thigh, lateral to the femoral artery, by passing posteromedially from the femoral triangle through the subsartorial (Hunter or adductor) canal. It gives off the infrapatellar branch that innervates the skin over the anterior surface of the patella. About 10 cm proximal and medial to the knee, the saphenous nerve becomes subcutaneous by piercing the fascia between the sartorius and gracilis muscles. Then, it crosses a bursa at the upper medial end of the tibia (pes anserinus bursa). In the lower third of the leg, it divides into two terminal branches to innervate the skin of the medial surface of the knee, medial leg, medial malleolus, and a small area of the medial arch of the foot.

Clinical Features

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