Case 8

Case 8



HISTORY AND PHYSICAL EXAMINATION


A 36-year-old man underwent mitral valve replacement for rheumatic heart disease. On awakening from general anesthesia, he noted weakness of the right foot and numbness of the dorsum of the foot. He had no pain. His footdrop did not improve, and he was referred to the electromyography (EMG) laboratory 5 weeks after the onset of footdrop.


On examination, there was a near-complete right foot and toes drop (Medical Research Council [MRC] strength 1/5). Foot eversion was markedly weak (3/5), but inversion and plantar flexion were normal. Deep tendon reflexes, including ankle jerk, were normal. There was sensory impairment to touch, and pin sensation over the dorsum of the right foot. Tinel sign was negative on percussion of the right peroneal nerve at the fibular neck. The rest of the neurologic examination was normal.


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



QUESTIONS



1. Based on the nerve conduction studies only, the lesion is likely:
A. Axonal.

B. Demyelinating.

C. Mixed, demyelinating and axonal.

D. Undetermined until the needle EMG is completed.

2. The most common precipitating factor for this type injury, when acute, is:
A. Leg crossing.

B. Weight loss.

C. Diabetes mellitus.

D. Recent general anesthesia and surgery.

E. Underlying peripheral polyneuropathy.

3. The most common precipitating factors for this type of injury, when subacute or chronic, are:
A. Weight loss.

B. Diabetes mellitus.

C. Diabetes mellitus and underlying peripheral polyneuropathy.

D. Recent prolonged hospitalization.

4. The prognosis for recovery of footdrop in this patient is:
A. Poor because of prominent fibrillation potentials.

B. Poor because of the near absence of voluntary motor unit potentials.

C. Favorable and biphasic, with anticipated early rapid improvement and some delayed recovery.

D. Poor, because of very low proximal peroneal compound motor action potential (CMAP) amplitudes recording tibialis anterior and the extensor digitorum brevis.


EDX FINDINGS AND INTERPRETATION OF DATA


Relevant electrodiagnostic (EDX) findings in this case include the following:


1. Low-amplitude right superficial peroneal sensory nerve action potential (SNAP), compared with the left (5 μV versus 20 μV).

2. Low-amplitude right peroneal CMAP stimulating at the ankle, while recording extensor digitorum brevis (1.9 mV on the right versus 8.0 mV on the left). In addition, there is a conduction block in the peroneal nerve segment between below the fibular head and knee stimulation sites (1.7 mV to 0.8 mV, respectively). The loss in CMAP amplitude (i.e., block) measures 53+ across both sites and is supported by a 48+ drop in CMAP area (Figure C8-1). Finally, there is a concomitant focal slowing of the conducting fibers across the fibular head (conduction velocity of 30 m/s proximally versus 45 m/s distally).

3. The peroneal CMAP, recording tibialis anterior, is similarly, low in amplitude with distal stimulation (below the fibular head: 3.2 mV on the right versus 5.5 mV on the left). Also, there is significant conduction block across the fibular head (3.2 mV from below the fibular head and 0.4 mV from the knee). This is equivalent to an 87.5+ amplitude loss and is supported by a significant decrease in CMAP area (not shown). There is also slowing of the conducting fibers (20 m/s on the right versus 43 m/s on the left).

4. The right sural sensory and tibial motor conduction studies as well as the bilateral H reflexes are normal.

5. The needle examination confirms that all common peroneal innervated muscles, except the short head of the biceps femoris, are abnormal, as evidenced by the presence of fibrillation potentials, decreased recruitment, and the increased duration and polyphasic motor unit action potentials. In contrast, all sampled tibial-innervated muscles (tibialis posterior, flexor digitorum longus, medial gastrocnemius), as well as all other muscles innervated by the L5 root (tibialis posterior, flexor digitorum longus, gluteus medius) and the lumbar paraspinal muscles, are normal.

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Figure C8-1 Right peroneal motor conduction studies recording extensor digitorum brevis. The distal compound muscle action potential (CMAP) amplitude is low (1.9 mV compared to 8.0 mV on the left), consistent with axon loss. In addition, there is evidence of conduction block across the fibular head, manifested by a significant drop in CMAP amplitude (53+) and area (48+) with stimulation below the fibular head and knee.




Case 8: Nerve Conduction Studies

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Case 8: Needle EMG

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This is consistent with a common peroneal mononeuropathy across the fibular head, manifested by segmental demyelination and axonal loss.


• Segmental demyelination in this case is confirmed by the identification of peroneal motor conduction block (recording tibialis anterior and extensor digitorum brevis [EDB]) across the fibular head 5 weeks after the onset of this acute lesion. It also is supported by the presence of focal slowing of the conducting fibers in the same segment of the peroneal nerve.

• Motor axonal loss is verified by the low peroneal CMAP amplitudes (recording tibialis anterior and EDB), stimulating distal to the lesion (i.e., at the ankle and below the fibular head, respectively). Sensory axonal loss also is confirmed by a low amplitude superficial peroneal SNAP, which is studied distal to the lesion. The presence of fibrillation potentials is another proof of motor fiber axonal loss, but this is a poorly quantitative measurement since it is seen in mild acute or subacute lesions and is absent in chronic lesions.

The prognosis for this patient should be relatively good and is likely to be biphasic. The initial phase of recovery would be dependent on remyelination and should be relatively rapid, occurring within 2 to 3 months. The second phase is slower and more protracted because it is dependent on sprouting and reinnervation. Sprouting should be relatively productive in this patient because the lesion is partial, and reinnervation is likely to be effective because several of the affected muscles, such as the tibialis anterior and peroneus longus, are located relatively close the site of injury.



DISCUSSION



Applied Anatomy


The common peroneal nerve (also called the lateral popliteal nerve) shares a common sheath with the tibial nerve (also called the medial popliteal nerve) to form the sciatic nerve. The common peroneal nerve innervates the short head of biceps femoris only, via a motor branch that exits the nerve close to the gluteal fold. All the other hamstring muscles (long head of biceps femoris, semitendinosus and semimembranosus) are innervated by the tibial nerve. The complete separation of the common peroneal nerve from the tibial nerve is variable, but is usually at the popliteal crease or up to 10 cm above it (Figure C8-2).


image

Figure C8-2 Anatomy of the common peroneal and tibial nerves in the popliteal fossa.


Soon after separating from the tibial nerve in the popliteal fossa, the common peroneal nerve gives off first the lateral cutaneous nerve of the calf, which innervates the skin over the upper third of the lateral aspect of the leg (Figure C8-3, top inset). It also gives the peroneal communicating nerve which joins the sural nerve in midcalf. Then, the common peroneal nerve winds around the fibular neck, where it lies in close contact with it, and passes through a tendinous tunnel between the edge of the peroneus longus muscle and the fibula, sometimes referred to as the fibular tunnel.


image

Figure C8-3 The common peroneal nerve, with its superficial branch (top) and deep branch (bottom), showing its relation to the fibular head and its terminal branches.


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


The common peroneal nerve divides into superficial and deep terminal branches usually near the fibular neck but sometimes more proximally (Figure C8-4). The common peroneal nerve around the fibular neck has a topographical arrangement where the fibers to the superficial branch are placed laterally while those destined to the deep peroneal nerve are located medially in close contact with the fibular bone. This renders the deep peroneal nerve more susceptible to compression at the fibular neck than the superficial nerve.


image

Figure C8-4 Locations of the division of the common peroneal nerve into its terminal branches, the deep (D) and superficial (S) peroneal nerves. (A) Most common site close to the fibular neck. Less common variations with more proximal divisions close to the knee joint line (C) or in the distal part of the popliteal fossa (B).


The superficial peroneal nerve innervates the peroneus longus and brevis and the skin of the lower two thirds of the lateral aspect of the leg and the dorsum of the foot (see Figure C8-3, top). The deep peroneal nerve is primarily motor; it innervates all ankle and toe extensors (the tibialis anterior, the extensor hallucis, and the extensor digitorum longus and brevis) and the peroneus tertius, in addition to the skin of the web space between the first and second toes (see Figure C8-3, bottom).


The accessory deep peroneal nerve is a common anomaly of the peroneal nerve. It is present in about 20+ of the population and sometimes bilaterally. The nerve arises as a motor branch of the superficial peroneal nerve, usually a continuation of the muscular branch that innervates the peroneus brevis muscle. The accessory deep peroneal nerve traverses along the posterior aspect of the peroneus brevis muscle, and then, accompanied by peroneus brevis tendon, passes behind the lateral malleolus near the sural nerve to reach the foot. There, it sends branches to the lateral part of extensor digitorum brevis, ankle joint, and ligaments (Figure C8-5).


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Figure C8-5 The accessory deep peroneal nerve anomaly.


(Adapted with revisions from Preston DC, Shapiro BE. Electromyography and neuromuscular disorders. Boston MA: Elsevier/Butterworth-Heinemann, 2005, with permission.)



Clinical Features


Peroneal mononeuropathy usually presents with a foot drop, defined as severe weakness of ankle dorsiflexion (extension) with intact plantar flexion. Foot drop should be distinguished from flail foot which, in contrast, is characterized by no or minimal ankle and foot movements in all directions, including severe weakness of ankle dorsiflexion, plantar flexion, and intrinsic foot muscles. Voluntary movement at or distal to the ankle occur in foot drop due to intact plantar flexion and intrinsic foot muscles, but are absent in flail foot. Table C8-1 lists the common causes of unilateral and bilateral footdrop, starting caudally and progressing cephalad along the neuraxis.


Table C8-1 Causes of Unilateral and Bilateral Footdrop






Unilateral footdrop
Deep peroneal mononeuropathy

Common peroneal mononeuropathy

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Aug 12, 2016 | Posted by in CARDIOLOGY | Comments Off on Case 8

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