Case 1

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Case 1

HISTORY AND PHYSICAL EXAMINATION

A 29-year-old white man noted an acute right footdrop. He was otherwise in excellent health, with no history of diabetes or trauma. He was referred to the electromyography (EMG) laboratory.

Neurologic examination revealed severe weakness (Medical Research Council [MRC] 2/5) of right ankle and toe dorsiflexion and moderate weakness of right ankle eversion (MRC 4/5). Ankle inversion and plantar flexion were normal. Deep tendon reflexes, including Achilles reflexes, were normal. The patient had slight impairment of pain sensation over the dorsum of the right foot. Tinel’s sign could not be induced by percussion of the peroneal nerve around the fibular neck.

EMG examination was performed 4 weeks after the onset of acute footdrop.

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

QUESTIONS

1. Based on clinical grounds only:

A. The prognosis is poor and recovery will be delayed.
B. The prognosis is excellent and recovery will be rapid.
C. The prognosis cannot be predicted.

2. The predominant pathophysiologic process involved here is:

A. Demyelination, with anticipated protracted recovery.
B. Axon loss, with anticipated rapid recovery.
C. Demyelination, with anticipated rapid recovery.
D. Axon loss, with anticipated protracted recovery.

3. A conduction block pattern on nerve conduction study accompanies the following condition(s):

A. Segmental demyelination.
B. Axonal loss preceding wallerian degeneration.
C. Axonal loss following wallerian degeneration.
D. A and B.
E. A and C.

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. Excellent because of normal distal peroneal compound muscle action potential (CMAP) amplitudes.
D. Poor because of very low proximal peroneal CMAP amplitudes.

5. Which of the following is not usually encountered in a purely demyelinating focal nerve lesion?

A. A decrease in amplitude with proximal stimulation, compared with distal stimulation.
B. Significant dispersion of motor response with proximal stimulation, compared with distal stimulation.
C. Focal slowing of conduction.
D. Fibrillation potentials in weak muscles.

EDX FINDINGS AND INTERPRETATION OF DATA

The pertinent electrodiagnostic EDX features in this case include the following:

1. Prominent right peroneal conduction block across the fibular neck, recording both the extensor digitorum brevis (EDB) and the tibialis anterior. As is shown in the Nerve Conduction Table, the right peroneal CMAP amplitude, recording the EDB, decreased from 5.3 mV to 1.4 mV (73.5+) with stimulation below the fibular neck and at the knee, respectively (Figure C1-1). This was supported by a similar drop in CMAP area across the fibular neck from 19.5 mV/ms to 4.5 mV/ms (77+). There is no definite focal slowing across the fibular neck (conduction velocities of 41 m/s versus 46 m/s). Also, the peroneal CMAP amplitude, recording the tibialis anterior, decreased from 5.0 mV to 0.5 mV (90+) with stimulation below the fibular neck and at the knee, respectively. The conduction velocity here is slowed, especially when compared with the asymptomatic left side (23 m/s on the right versus 41 m/s on the left).
2. Preservation of the distal peroneal CMAP amplitudes (recording both the EDB and the tibialis anterior) and of the superficial peroneal sensory nerve action potential (SNAP), both in absolute values and when compared with the asymptomatic side.
3. Fibrillation potentials and neurogenic recruitment of all deep and superficial peroneal innervated muscles below the knee (worse in the deep peroneal distribution), with a normal biceps femoris, short head.
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Figure C1-1 Peroneal motor nerve conduction studies recording the extensor digitorum brevis and stimulating the ankle (1), below the fibular neck (2), and the knee (3). Note the significant decrease in compound muscle action potential (CMAP) amplitude and area between the second and third tracings, without prolongation of negative peak duration. (Amplitudes are 5.6, 5.3, and 1.4 mV, respectively, and areas are 20.5, 19.5, and 4.5 mV/ms, respectively. Sensitivity, using a vertical scale, is 2 mV.)

Case 1: Nerve Conduction Studies

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Case 1: Needle EMG

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These findings imply that the predominant pathologic process is segmental demyelination (conduction block with normal distal peroneal CMAPs and SNAP), with minimal axonal loss (fibrillation potentials). The prognosis for recovery is excellent because it is dependent primarily on remyelination.

DISCUSSION

The anatomy and clinical and electrodiagnostic (EDX) presentations of peroneal mononeuropathy are discussed in detail, along with an accompanying case of peroneal nerve lesion (Case 8). The discussions here are limited to peripheral nerve injury and the electrodiagnostic findings of such injury.

Structure of Peripheral Nerve

The peripheral nerve consists of both unmyelinated and myelinated fibers and their supporting elements. The unmyelinated axons are surrounded only by the plasma membrane of a Schwann cell. The myelinated axons are engulfed by a Schwann cell that wraps around the axons multiple times, thereby insulating the axon with multiple layers of cell membrane, which is rich in lipid sphingomyelin. The myelinated axon is surrounded completely by myelin and Schwann cells, except at certain gaps. In adults, the gaps between myelin segments, called the nodes of Ranvier, measure approximately 1 μm, while myelinated segments between nodes, called the internodal segments, measure approximately 1 mm each.

The relatively thick myelin sheath has a low capacitance and a large resistance to the electrical current that attempts to escape from the axon to the extracellular space. Also, the axons contain a high concentration of voltage-gated sodium channels at the nodes of Ranvier, which are essential for the propagation of action potentials. These characteristics of myelinated fibers (myelin and Na channels at the nodes of Ranvier) result in a rapid saltatory conduction between consecutive nodes of Ranvier.

Pathology of Peripheral Nerve Injury

Transient neurologic symptoms related to minor peripheral nerve compression are extremely common and are rapidly reversible. They probably result from action potential propagation failure caused by ischemia. They are not associated with structural alteration of the axon, myelin, or supporting structure. In contrast, prolonged or severe compression, traction, laceration, thermal, or chemical injury may damage the myelin, axon, or the supporting components of the peripheral nerves and results in significant disability from which the patient may not recover completely.

Nerve injuries that are associated with focal interruption of the continuity of the axons cause significant changes in the structure of the peripheral nerve distal to the lesion (Table C1-1). The distal axons undergo a degenerative process, known as wallerian degeneration. This occurs since all the necessary building blocks needed for maintaining the axon are made in the cell body (peikaryon) and cannot reach the distal stump. The rate at which wallerian degeneration proceeds varies depending on the nerve injured, axon diameter, and the length of distal stump (the larger and the longer the distal stump the more time is needed for wallerian degeneration to be completed). Within hours of most nerve injuries, myelin begins to retract from the axons at the nodes of Ranvier. This is followed by swelling of the distal nerve segment, leakage of axoplasm, and subsequently the disappearance of neurofibrils. Within days, the axon and myelin fragment, and digestion of nerve components starts. By the end of the first week, the axon and myelin become fully digested and Schwann cells start to bridge the gap between the two nerve segments. In chronic nerve lesions, the endoneurial tubes in the distal stump shrink, the nerve fascicles atrophy distal to the lesion, and, in complete nerve transection, the severed ends retract away from each other.

Table C1-1 Consequences of Focal Axonal Injury Distal to the Lesion

Wallerian (axonal) degeneration
Myelin breakdown
Neuromuscular transmission failure
Endoneurial tube shrinkage
Fascicular atrophy
Denervation atrophy of muscles

In contrast to the severe changes that occur distal to the lesion, only minor changes occur proximally. Though most of the proximal stump survives and maintains its ability to regenerate, there is often a slight retrograde degeneration of axons, up to several centimeters from the site of injury depending on the severity of the lesion. Also, the neuron cell body reacts to the axonal injury, by revealing an eccentric nucleus and marginally placed rough endoplasmic reticulum (Nissl’s substance). These changes are worse with proximal than with distal nerve lesions.

Classification of Peripheral Nerve Injury

Many classifications of peripheral nerve injury have been suggested, but Seddon’s and Sunderland’s classifications are the most widely used in clinical practice. These are based on the functional status of the nerve and on histologic findings. They are shown in Table C1-2 and in Figure C1-2, with their corresponding electrophysiologic findings.

1. Neurapraxia (first degree nerve injury). Focal pressure on the peripheral nerve, when brief and modest, may distort the myelin near the nodes of Ranvier, producing segmental block of conduction without wallerian degeneration. The nerve conducts normally distally but not across the lesion, resulting in conduction block which is the electrophysiologic correlate of neurapraxia (see below). There are no or little changes in the muscles and recovery is usually complete following remyelination that occur within 1–3 months if the offending cause is removed.
2. Axonotmesis. With increasing compression or other physical injuries, the axons are focally damaged resulting in secondary wallerian degeneration distal to the site of injury. However, with this type of injury, there is variable disruption of the supporting structures (endoneurium, perineurium, and epineurium) that carries variable prognosis. Hence, Sunderland advocated that axonotmesis lesions be divided into three further subtypes, depending on what component of the surrounding nerve stroma is affected:

a. Second degree nerve injury, in which axon loss is associated with intact endoneurial tubes, perineurium, and epineurium. These lesions have better prognosis than other axon loss lesions since axonal regneration is well guided by the intact endoneurial tubes.
b. Third degree nerve injury, in which the axons, Schwann cell tubes, and endoneurium are damaged leaving the perineurium and epineurium intact. These lesions have poor prognosis and may require surgical intervention since axonal regeneration is often misdirected and may lead to neuroma formation.
c. Fourth degree nerve injury, where the perineurium is also disrupted, but the epineurium is intact. These lesions have very poor prognosis and surgery is often required.

3. Neurotmesis (fifth degree nerve injury). This is the most severe type of nerve injury manifesting as complete disruption of the nerve with all the supporting structures. The nerve is transected with loss of continuity between its proximal and distal stumps. These lesions have no chance for improvement without surgical repair.

Table C1-2 Classification of Peripheral Nerve Injury

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Figure C1-2 Sunderland classification of peripheral nerve injury. 1, First degree: conduction block. 2, Second degree: wallerian degeneration secondary to a lesion confined to the axon, with preservation of the endoneurial sheath. 3, Third degree: disruption of the axon and endoneurial tube with an intact perineurium. 4, Fourth degree: disruption of all neural elements except the epineurium. 5, Fifth degree: transection with complete discontinuity of the entire nerve trunk.

(Reprinted with permission from Sunderland S. Nerve injuries and their repair: a critical appraisal, Edinburgh: Churchill Livingstone, 1991.)

Diagnosis of Peripheral Nerve Injury

Injuries to peripheral nerves are highest in prevalence in young adults between the ages of 18 and 35 years and result in substantial degree of disability. They are often accompanied by other bodily injuries including fractures, dislocations, or soft tissue damage. When associated with head or spine injury, peripheral nerve lesions may be overlooked until late during the rehabilitative phase of treatment. Traumatic nerve injuries may be direct (such as with a stab wound to the sciatic nerve) or indirect (such as with radial neuropathy following humeral fracture). These lesions are much more common during wartime, but they also accompany civilian trauma that results from vehicular accidents, industrial accidents, gunshots, or knife wounds. Also, a significant percentage of peripheral nerve injuries encountered in clinical practice are iatrogenic, occurring in the setting of surgical or radiological procedures, or following needle insertion or medical therapy such as with the use of anticoagulation.

The diagnosis of peripheral nerve injury often requires a detailed history and neurologic examination, with the EDX studies and surgical findings playing important roles in diagnosis and management. The history and physical examination are extremely important in predicting the location, type, and severity of the nerve lesion. For example, a stab wound injury to a nerve is often associated with axonal interruptions and grade three to five nerve injuries, while intraoperative nerve compression distant from the site of surgical field is usually a grade one (neurapraxic and demyelinating) or two (axonal) nerve injury.

Electrodiagnosis of Peripheral Nerve Injury

The EDX studies are the cornerstone in the diagnosis and management of nerve injuries by providing valuable information as to the location of the lesion, and its severity, pathophysiology, and prognosis (Table C1-3). Intraoperatively, the EDX studies guide the surgeon during the procedure and help assess the status of the regenerating axons within the injured nerve segment. During the recovery stage of peripheral nerve injury that may occur spontaneously or after surgical repair, the EDX studies are also essential in the evaluation of remyelination, regeneration, and reinnervation.

Table C1-3 Role of Electrodiagnostic Studies in Peripheral Nerve Injury

Localize the site of nerve injury
Determine the pathophysiology of the lesion
Estimate the severity of the injury
Determine the prognosis
Assess the progress of remyelination and reinnervation

In contrast to the anatomical classification of nerve injuries, the pathophysiologic responses to peripheral nerve injuries have a limited repertoire: that is, axon loss, demyelination, or a combination of both. The EDX studies evaluate the integrity of the myelin sheath and the axon exclusively, and can only distinguish a neurapraxic injury (myelin injury) from all other degrees of injury that are associated with axonal damage and wallerian degeneration.

Localization of Nerve Lesions Using Nerve Conduction Studies

There are essentially three electrophysiologic consequences to peripheral nerve injury that can be assessed by nerve conduction studies. Two of them, namely focal slowing of conduction and conduction block, are caused by myelin disruption; the third is a manifestation of axonal loss (conduction failure).

Focal Slowing

Focal slowing in peripheral nerve injuries represents a convenient method of localizing lesions. When focal slowing is an isolated finding such as of the ulnar nerve across the elbow, the patient is not symptomatic and has no weakness or sensory loss. In symptomatic peripheral nerve injuries, focal slowing is associated with conduction block due to internodal demyelination, axon loss, or both.

Focal slowing of conduction usually is caused by widening of the nodes of Ranvier (paranodal demyelination) and, sometimes, focal axonal narrowing. It is evident on NCSs by slowing of conduction of a specific nerve segment, while other segments of the same nerve as well as neighboring nerves remain normal. When the large myelinated fibers are slowed to essentially the same extent, focal slowing across the involved nerve segment is synchronized. This is manifested by either a prolongation of distal latencies (in distal lesions) or slowing in conduction velocities (in proximal lesions), while the CMAP amplitude, duration, and area are not affected and do not change when the nerve is stimulated proximal to the lesion. When variable number of the medium or small nerve fibers (average or slower conducting axons) are affected only, desynchronized (differential) slowing of conduction across the nerve segment is evident. In this situation, the CMAP is dispersed on stimulation proximal to the lesion and has prolonged duration, with normal (nondispersed) response on distal stimulation. If this finding is isolated, the distal latency or conduction velocity, which represent the speed of the largest (fastest) axons, are normal. However, in most clinical situations, the large fibers are often involved also, desynchronized slowing is usually accompanied by slowing at the involved segment, resulting in concomitant slowing of distal latency or conduction velocity.

Conduction Block

Normally, the action potential is generated by sufficient temporal and spatial summation of excitatory inputs to motor or sensory axons. The nerve potential travels a myelinated axon in a saltatory fashion, passing hundreds of nodes of Ranvier without failure. The axonal regions at the site of the nodes of Ranvier are rich in Na channels. An abrupt change in Na conductance forms the basis for the generation of nerve action potential and the maintenance of saltatory conduction. Loss of myelin can involve one or more segments of these axons (segmental demyelination). Segmental demyelination can result from in widening of the nodes (paranodal demyelination) or the loss of one or more internodal segments (internodal demyelination). Both forms of demyelination can result in slowing or block of conduction. However, at least in compressive/entrapment neuropathy, focal slowing of conduction is characteristic of paranodal demyelination, whereas conduction block is a manifestation of internodal demyelination.

Before one can understand the electrophysiologic diagnosis of conduction block, the normal conduction studies of nerves, especially in reference to temporal dispersion and phase cancellation, and, ultimately, conduction block, must be discussed.

Three physiologic facts play a pivotal role in the generation of the CMAP which is obtained with surface recording.

1. The CMAP is produced by supramaximal stimulation of peripheral nerve and represents the summation of all individual muscle fiber action potentials directed to the muscle through the stimulated nerve.
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