Key Question No. 1: What is the Temporal Course and Progression of the Polyneuropathy (Acute, Subacute, Chronic; Progressive, Stepwise, Relapsing/Remitting)?

The temporal course and progression can be obtained by the history alone and often confirmed by electrophysiologic studies. Most polyneuropathies are chronic, and their onset cannot be easily determined. Acute polyneuropathies are notably less common ( Box 26–1 ). Among them, Guillain–Barré syndrome (and its most common variant, acute inflammatory demyelinating polyneuropathy [AIDP]) is the most distinctive, with an onset over a few days or a few weeks at most. Similarly, most polyneuropathies are slowly progressive ( Figure 26–2 ). Polyneuropathies that progress in a stepwise fashion are infrequent and are often associated with a mononeuropathy multiplex pattern (discussed later). Likewise, the history of a relapsing/remitting course is distinctly unusual and suggests either an intermittent exposure/intoxication or a variant of chronic inflammatory demyelinating polyneuropathy (CIDP).

Temporal progression of polyneuropathy.

The pattern of the temporal progression is one of the key historical points in determining the etiology of a polyneuropathy (n.b., worsening severity is denoted as going downward). A: Slowly progressive; B: Stepwise progressive; C: Relapsing / remitting.

Key Question No. 2: Which Fiber Types are Involved (Motor, Large Sensory, Small Sensory, Autonomic)?

The next step is to determine which fiber types are involved. This information is obtained primarily from the history and confirmed by physical examination and electrophysiologic tests. Nerve fibers can be categorized either by the modality carried (motor, sensory, autonomic) or by fiber size. All motor fibers are large-diameter, myelinated fibers, whereas all autonomic fibers are small-diameter, mostly unmyelinated fibers. However, sensory fibers may be either large or small in diameter. Large sensory fibers mediate vibration, proprioception, and touch, whereas small sensory fibers convey pain and temperature sensations.

When nerve is diseased, it can react in a limited number of ways. Thus, many peripheral nerve disorders present with similar symptoms despite different etiologies. Symptoms and signs of nerve dysfunction result either from lack of function ( negative symptoms and signs) or from abnormal function or overfunctioning ( positive symptoms and signs). For example, anyone who has “fallen asleep” on his or her arm can remember the initial numbness or lack of feeling (negative symptoms), followed by intense pins-and-needles paresthesias (positive symptoms) as circulation is restored. Characteristic positive or negative sensory symptoms and signs caused by diseased nerves help one recognize which fiber types are involved ( Table 26–1 ).

Determining which fiber types are involved has important diagnostic implications. Most polyneuropathies involve both sensory and motor fibers on electrophysiologic testing, even though, clinically, most distal axonal polyneuropathies exhibit sensory symptoms and findings long before the disease process becomes sufficiently severe to cause actual weakness. Patients with certain hereditary polyneuropathies (e.g., Charcot–Marie–Tooth polyneuropathy) and conditions such as lead poisoning, porphyria, and Guillain–Barré syndrome may exhibit predominantly motor symptoms and signs. On the sensory side, pure sensory neuropathies also are unusual and often suggest a primary process affecting the dorsal root ganglia. These sensory neuronopathies are quite rare and are characteristically seen acutely or subacutely as a paraneoplastic syndrome, postinfectious process, or associated with Sjögren’s syndrome or pyridoxine (B ₆ ) intoxication. Chronic sensory neuronopathies may be seen in the inherited sensory neuropathies and as a component of some inherited neurodegenerative conditions (e.g., Friedreich’s ataxia).

Large and small fibers are affected in most polyneuropathies. Only a few polyneuropathies preferentially affect small fibers ( Box 26–2 ). Manifestations include autonomic dysfunction and a distal sensory deficit, particularly for pinprick, often associated with painful, burning dysesthesias. It is essential to appreciate that routine nerve conduction studies assess only large myelinated fibers. A patient who has a pure small-fiber polyneuropathy, with complete sparing of the large fibers , may have completely normal electrophysiologic studies. Conversely, large-fiber polyneuropathies always show abnormalities on electrophysiologic testing. Predominantly large-fiber polyneuropathies result in clinical sensory deficits (particularly for vibration and touch), weakness, and loss of tendon reflexes, with little or no autonomic and pain/temperature sensation loss.

Key Question No. 3: What is the Pattern of the Polyneuropathy (Distal Dying Back [Distal-To-Proximal Gradient], Short Nerves, Multiple Nerves; Symmetry, Asymmetry)?

The overall pattern of the polyneuropathy is determined largely based on the clinical examination and is supplemented and confirmed by electrophysiologic studies. In most polyneuropathies, there is a distal-to-proximal gradient of symptoms and signs. Distal symptoms and findings occur in most polyneuropathies, in part indicating the frequency with which axonal loss is the underlying pathologic process. Most axonal polyneuropathies exhibit a distal-to-proximal, dying back pattern, reflecting that the chance of damage to a nerve is length dependent ( Figure 26–3 ). Thus, the longest nerves are affected first, resulting in a stocking-glove distribution of symptoms. Patients initially develop numbness or weakness of the toes and feet, which then slowly progresses up the leg. When the process reaches the upper calf, the fingertips become involved as well, because the distance from the lumbosacral spinal cord to the upper calf is the same as that from the cervical spinal cord to the fingertips. Only rarely will polyneuropathies preferentially affect the shorter, more proximal nerves before the distal ones (e.g., in porphyria, proximal diabetic neuropathy, and some cases of inflammatory demyelinating polyneuropathy).

Stocking-glove pattern of polyneuropathy.

Most polyneuropathies, especially axonal polyneuropathies, are length dependent, resulting in a stocking-glove distribution of symptoms and signs. Symptoms first present in the toes and then progress up the leg. When the polyneuropathy reaches the upper calves, the fingertips become involved as well. As the polyneuropathy worsens, symptoms may develop over the anterior chest and abdomen, representing distal degeneration of the thoracic intercostal nerves.

(Reprinted with permission from Schaumburg, H.H., Spencer, P.S., Thomas, P.K., 1983. Disorders of peripheral nerves. FA Davis, Philadelphia.)

After determining whether a distal-to-proximal gradient is present, one should next assess the polyneuropathy for symmetry. Nearly all polyneuropathies are symmetric. The presence of any significant asymmetry is a key finding; it usually excludes a large number of toxic, metabolic, and genetic conditions that cause only a symmetric pattern . Asymmetry implies the possibility of (1) a mononeuropathy multiplex pattern, (2) a superimposed radiculopathy or entrapment neuropathy, or (3) a variant of CIDP. Nerve conduction studies and EMG frequently are useful in sorting out these possibilities.

The pattern of a mononeuropathy multiplex is one of the most important patterns to recognize and differentiate from the length-dependent, dying-back, axonal polyneuropathy. The clinical presentation is distinctive: there is an asymmetric, stepwise progression of individual cranial and/or peripheral neuropathies ( Figure 26–4 ). Over time, a confluent pattern may develop, which may be difficult to distinguish from a generalized polyneuropathy. In most cases, the individual neuropathies are of named nerves (i.e., median, ulnar, peroneal, etc.) as opposed to small nerve twigs. Mononeuropathy multiplex has a limited differential diagnosis ( Box 26–3 ) and most often occurs in the setting of vasculitis and vasculitic neuropathy. As each subsequent nerve is infarcted, pain develops (often severe), followed hours or days later by weakness and numbness in the nerve’s distribution. Although other organ systems are often involved, the initial clinical presentation of systemic vasculitis may involve only the peripheral nervous system. Indeed, there are now well-recognized cases in which vasculitis remains confined to the peripheral nervous system.

Mononeuropathy multiplex pattern of polyneuropathy.

Mononeuropathy multiplex is a distinctive pattern, presenting as an asymmetric, stepwise progression of individual cranial or peripheral neuropathies, usually of named nerves. As time passes, a confluent pattern may develop that often is difficult to distinguish from a generalized polyneuropathy. Mononeuropathy multiplex is characteristically seen in vasculitic polyneuropathy. The pattern shown here is an asymmetric polyneuropathy with involvement of the left ulnar, right median, left distal peroneal, right saphenous, and right peroneal nerves.

(Adapted and reprinted with permission from Schaumburg, H.H., Spencer, P.S., Thomas, P.K., 1983. Disorders of peripheral nerves. FA Davis. Philadelphia.)

Key Question No. 4: What is the Underlying Nerve Pathology (Axonal, Demyelinating, or Mixed)?

Pathologically, injury to nerves consists of two major processes: axonal loss or demyelination. The vast majority of polyneuropathies are primarily axonal. In demyelinating polyneuropathies, the initial injury to the nerves reflects damage to or dysfunction of the Schwann cells and the myelin sheaths. As a consequence of demyelination, conduction is impaired with marked slowing of conduction velocity or frank conduction block. In establishing the differential diagnosis of a peripheral nerve disorder, the presence of demyelination is always a key finding (see later). Demyelination may be demonstrated either by nerve biopsy and pathologic examination or, more easily, by electrophysiologic testing. When nerve conduction studies demonstrate a polyneuropathy to be predominantly demyelinating, the differential diagnosis is readily narrowed to a small group of disorders ( Box 26–4 ).

Key Question No. 5: Is there a Family History of Polyneuropathy?

For any patient with a polyneuropathy, especially when the diagnosis is not clear, particular attention must be paid to family history. There are a large number of inherited polyneuropathies. Although for most of them only symptomatic therapy is available, correct diagnosis is important for genetic counseling and prognosis, and to avoid unnecessary or inappropriate further testing and treatment. Charcot–Marie–Tooth (CMT) neuropathy refers to a group of inherited disorders characterized by a chronic motor and sensory polyneuropathy. CMT accounts for the majority of inherited polyneuropathies and, in many large series, represents a significant proportion of the patients with difficult-to-diagnose polyneuropathies. Four major types of CMT are defined based on their inheritance and physiology: the demyelinating autosomal dominant form is CMT1; the axonal autosomal dominant form is CMT2; the autosomal recessive demyelinating form is CMT4; and the X-linked demyelinating form is CMTX. Within each type, there are several subtypes based on the specific genetic defect. In contrast to CMT, there are a smaller group of inherited polyneuropathies associated with defects of metabolism that have been described. Most are extremely rare and are associated with other systemic abnormalities.

Inherited polyneuropathies may affect certain individuals so minimally or may progress so slowly over an individual’s lifetime that the person never seeks medical attention. Therefore, it often is beneficial to examine family members, both clinically and with nerve conduction studies and EMG, to help determine whether the underlying etiology of the patient’s polyneuropathy is genetic. Several clinical clues, however, suggest the possibility of an inherited polyneuropathy ( Figure 26–5 ):

• •
  

  Foot deformity (pes cavus, hammer toes, high arches)

  
  
• •
  

  History of a long-standing polyneuropathy (many years and often decades)

  
  
• •
  

  History of very slow progression

  
  
• •
  

  Few positive sensory symptoms

  
  
• •
  

  Family history of “polio,” “rheumatism,” “arthritis,” or other disorders that actually might have been inherited polyneuropathy

Pes cavus.

Pes cavus is an orthopedic deformity of the foot, recognized as a foreshortened foot with a high arch and hammer toes. Pes cavus develops during childhood from the combination of intrinsic foot muscle weakness and relative preservation of the long flexors and extensor muscles in the calves. Because most polyneuropathies preferentially affect distal muscles, polyneuropathies that are present during development as a child commonly result in this deformity. As the vast majority of polyneuropathies that are present as a child are genetic, the presence of pes cavus in a patient with a peripheral neuropathy likely indicates that the neuropathy has been present since childhood and is most likely inherited.

Key Question No. 6: Is there a History of Medical Illness or are there Signs Suggesting A Medical Illness Associated with Polyneuropathy?

A careful history and general physical examination are essential in evaluating a patient with polyneuropathy. Several medical conditions are strongly associated with polyneuropathy. Most prominent among them are diabetes and other endocrine disorders, cancer, connective tissue disorders, porphyria, vitamin and other deficiency states, and human immunodeficiency virus (HIV) infection.

Key Question No. 7: Is there any History of Occupational or Toxic Exposure To Agents Associated with Polyneuropathy?

Finally, it is always important to ask about occupational and exposure history. Among drugs, most notable are cancer chemotherapeutic agents, which frequently result in polyneuropathy that is detectable either clinically or electrically. In addition, a large number of prescription drugs, as well as over-the-counter medicines, can cause polyneuropathy. A careful review of all medicines is always important.

Asking about a patient’s occupational and recreational activities occasionally elicits a toxic source for the neuropathy. One should always inquire about the patient’s use of alcohol, which is one of the most frequent causes of toxic polyneuropathy.

Clinical

Clinically, the patient with an axonal polyneuropathy usually presents with a stocking-glove distribution of symptoms and signs, including distal sensory loss and weakness. Ankle reflexes usually are absent, whereas knee and upper extremity reflexes are preserved, unless the polyneuropathy is severe.

In severe cases, the pattern of abnormalities may become more complex. Sensory symptoms and signs may develop not only over the limbs but also over the anterior chest and abdomen (escutcheon sign, which is the shape of a shield), reflecting distal degeneration of the thoracic intercostal nerves, which originate from the back and run around the abdomen and chest. If this pattern is not appreciated, a mistaken impression of a spinal level may result. ( Note : A level will only be found examining the front, not the back.) In the most extreme cases, sensory loss may develop over the top of the head due to degeneration of the distal trigeminal and cervical nerves.

Electrophysiology

Axonal polyneuropathies are associated with a characteristic pattern of nerve conduction results, provided the polyneuropathy has been present long enough for wallerian degeneration to have occurred (i.e., 3–9 days). In general, motor and sensory amplitudes decrease, with normal or only slightly slowed distal latencies, late responses, and conduction velocities. The changes are always more marked in the lower extremities, where the pathology is the greatest.

Likewise, evidence of axonal loss is found on needle EMG examination, more prominent distally than proximally, with the lower extremities more affected than the upper extremities. Of course, EMG findings are dependent on the length of time a polyneuropathy has been present. Denervation typically develops within weeks and reinnervation after weeks to months. Different patterns also develop depending on the tempo of the illness. If the process is relatively active and progressive, a combination of denervation and reinnervated motor unit action potentials (MUAPs) with decreased recruitment will be seen and again will be more prominent distally. In cases where the polyneuropathy is long-standing and only very slowly progressive, reinnervation may completely keep pace with denervation. In such cases, only reinnervated MUAPs with decreased recruitment will be seen distally, with little or no active denervation.

Most polyneuropathies have been present for several months or years before coming to evaluation. Accordingly, when a patient with an axonal polyneuropathy is first seen in the EMG laboratory, a combination of denervation and reinnervation is usually present.

Diabetes

In any discussion of axonal neuropathies, special mention should be made of diabetic neuropathy. Peripheral nervous system manifestations of diabetes are numerous and varied. Isolated mononeuropathies of cranial nerves (e.g., facial palsy), intercostal nerves (known as diabetic thoracoabdominal neuropathy), or peripheral nerves may occur. Several types of polyneuropathy may occur. The most common, a distal sensorimotor polyneuropathy, is a typical axonal polyneuropathy affecting both large and small sensory fibers. On EMG, however, findings of a polyradiculoneuropathy are usually present. Diabetic patients may also present with a pure autonomic polyneuropathy or a small-fiber sensory polyneuropathy, with distal burning and pain. If large sensory and motor fibers are spared, such patients will have completely normal electrodiagnostic studies. Other patients with diabetes will present with more proximal nerve syndromes, either at the root or plexus level, especially in the lower extremity (i.e., proximal diabetic neuropathy, diabetic amyotrophy, etc.). Large-fiber diabetic neuropathies usually demonstrate axonal changes on nerve conduction studies. Although most axonal polyneuropathies, including those associated with diabetes , have some secondary demyelination, the electrophysiologic criteria for primary demyelination are not met. Only in some cases of uremic polyneuropathy, especially when combined with diabetic polyneuropathy, do nerve conduction velocities slow sufficiently to approach or exceed the criteria set for demyelinative slowing.

Guillain–Barré Syndrome (GBS)

Guillain–Barré syndrome is now most properly thought of as a syndrome that comprises several variants, with acute inflammatory demyelinating polyneuropathy (AIDP) being the most common in North America. GBS is an immune-mediated, rapidly progressive, predominantly motor polyneuropathy that often leads to bulbar and respiratory compromise. It is one of the most common of all neuromuscular emergencies. Although the overall prognosis is favorable in more than 80% of patients, the hospital course is frequently long, followed by a prolonged recuperation. Nerve conduction studies and EMG play an important role in the diagnosis of GBS because early recognition is necessary to begin appropriate treatment and avoid potential medical complications.

People of all ages can be affected, although GBS is most common in young adults. An antecedent event, often an upper respiratory infection or gastroenteritis, is found in approximately 60% of patients. Precipitating factors include campylobacter, cytomegalovirus, Epstein–Barr virus, and HIV infection, as well as vaccination, surgery, trauma, and malignancy (especially lymphoma).

Chronic Demyelinating Polyneuropathy

When a patient with a chronic polyneuropathy is found to have evidence of a primary demyelinating process on nerve conduction studies ( Box 26–6 ), the differential diagnosis narrows considerably. However, some of the disorders that might be considered in the differential diagnosis ( Box 26–4 ) are associated with other prominent symptoms outside of the peripheral nervous system, some of which involve the central nervous system or have an onset in early childhood. From a practical point of view, the differential diagnosis of an isolated chronic demyelinating polyneuropathy in an adult without central nervous system or systemic findings likely is limited to either an inherited polyneuropathy (most often CMT type 1A), or CIDP or one of its variants. Nerve conduction studies can often differentiate among these conditions.

Multifocal Motor Neuropathy with Conduction Block

In the early 1990s, attention was brought to patients with pure motor neuropathies, often associated with antiganglioside antibodies (especially anti-GM ₁ ). A monoclonal protein was not present. These patients presented with a pure lower motor neuron syndrome, clinically similar to the progressive muscular atrophy variant of amyotrophic lateral sclerosis (ALS). However, their electrophysiologic studies showed evidence of an acquired segmental demyelinating neuropathy with conduction block along motor nerves, similar to CIDP, although sensory nerves were minimally affected or completely spared. It was unclear whether these patients were a variant of CIDP or represented a unique syndrome. The recognition of this disorder, multifocal motor neuropathy with conduction block (MMNCB), now has important therapeutic and prognostic implications because most of these patients are treated successfully with intravenous immunoglobulin; cyclophosphamide or rituximab therapy have been used successfully in some refractory cases. It often falls to the electromyographer to make the differentiation between ALS, which usually is fatal, and MMNCB, which responds to immunomodulation.

Clinical

Patients with MMNCB present with progressive, asymmetric weakness and wasting, often affecting the distal upper extremity muscles first. Most patients are younger than 50 years, which is younger than patients with typical ALS. Males are more commonly affected than females. In some cases, it may be possible to detect weakness in the distribution of named motor nerves with sparing of other nerves in the same myotome (clinical multifocal motor neuropathy). This pattern is not seen in ALS or its progressive muscular atrophy variant, in which the entire myotome is characteristically affected at the same time. Occasional patients will have prominent weakness but without wasting, a finding usually associated with pure demyelination. Definite upper motor neuron signs are always absent, although retained or inappropriately brisk reflexes for the degree of weakness and wasting may be seen. Bulbar function and sensation are characteristically spared, although mild or transient sensory symptoms may be present.

Many believe that this disorder is a variant of CIDP. However, the asymmetry, upper extremity predominance, relative absence of sensory findings, and typical lack of response to prednisone all suggest the possibility of a unique disorder that differs from the usual presentation of CIDP.

Electrophysiology

Findings on motor nerve conduction studies in MMNCB often are similar to those seen in CIDP. There may be evidence of demyelinative slowing, including markedly prolonged latencies, slowed conduction velocities, and prolonged late responses. The characteristic finding, however, is that of conduction block, temporal dispersion, or both, along the motor nerves .

The precise electrophysiologic definition of conduction block remains controversial (see Chapter 3 ), and much of the interest in defining conduction block is due to this disorder. From results of computer simulation models, a drop in proximal CMAP area of more than 50% always signifies conduction block and cannot be explained by temporal dispersion alone. However, any abrupt drop in CMAP area or amplitude, especially over a short segment, usually signifies conduction block. Of course, conduction block across a known entrapment site (e.g., ulnar nerve at the elbow, peroneal nerve at the fibular neck) cannot be used to diagnose MMNCB or any other acquired demyelinating polyneuropathy.

Because MMNCB is potentially treatable and ALS is usually fatal, an exhaustive search for conduction blocks often is undertaken. Although such testing is worthwhile, it should be done only in patients with predominantly lower motor symptoms and signs. Patients with MMNCB do not have unequivocal upper motor signs (i.e., spasticity, extensor plantar responses, pathologic hyperreflexia) or bulbar dysfunction. Even if strict criteria for conduction block are not met, any markedly slowed conduction velocity or distal latency (excluding entrapment sites and recording from nerves where the recorded muscle is severely atrophic), or markedly prolonged F responses, should seriously put into question the diagnosis of ALS. Electrophysiologic evidence of demyelination does not occur in ALS.

In the search for conduction block, more proximal segments of nerve may be studied (e.g., stimulating axilla, Erb’s point, cervical roots). In exceptional cases, conduction blocks may be demonstrated only proximally. However, conduction blocks in MMNCB typically are present distally in the routine segments of nerve usually studied. One must always remember that with proximal stimulation, technical problems become more marked, especially the problem of ensuring supramaximal stimulation. If supramaximal stimulation is not achieved proximally, a mistaken impression of a conduction block may occur. In addition, the normal effects of temporal dispersion become more marked when longer distances are studied. When patients with ALS are studied proximally, some drop in amplitude and area may occur, but a drop in area of more than 50% is never seen. Last, stimulation of proximal segments also carries the inherent problem of co-stimulation of adjacent nerves. This is especially true for the median and ulnar nerves, where collision studies are needed to exclude the contribution to the CMAP from proximal stimulation of adjacent nerves (see Chapter 30 ).

In MMNCB, sensory studies are usually completely normal (though mild sensory abnormalities have been seen). Indeed, sensory studies usually are normal, even if performed across the same segment of nerve where a motor conduction block is present ( Figure 26–9 ). Of course, any sensory abnormalities should also put into question the diagnosis of ALS, unless there is a known secondary process resulting in a superimposed polyneuropathy.

Motor conduction block in multifocal motor neuropathy with conduction block (MMNCB).

Patients with MMNCB characteristically display conduction block of motor fibers, but often with complete sparing of sensory fibers, even through the same segment of nerve. Shown here are results of median nerve conduction studies in a patient with MMNCB, with the wrist and antecubital fossa stimulated and the abductor pollicis brevis muscle (top) for motor studies and the index finger (bottom) for sensory studies co-recorded. Note the complete block of motor conduction with intact sensory conduction. The amount of amplitude drop in the proximal sensory potential is within the normal range expected for the effects of normal temporal dispersion and phase cancellation for sensory potentials.

Needle EMG findings characteristically show decreased recruitment of MUAPs in weak muscles as a result of proximal conduction blocks. As in CIDP, secondary axonal changes are not unusual; denervating potentials and reinnervated MUAPs are present in most patients.

Nerve Conduction Studies

Nerve conduction studies should begin with motor conduction studies in a lower extremity ( Box 26–7 ). The routine peroneal and tibial motor studies are performed along with their F responses. If motor responses are absent recording the usual distal muscles (i.e., EDB, abductor hallucis brevis), peroneal motor studies can be performed using the tibialis anterior, a more proximal muscle, for recording. Likewise, tibial motor studies can be performed proximally recording the soleus, using the same montage as for the H reflex. ( Note : In this situation, only one stimulation site [i.e., the popliteal fossa] is possible; thus, an amplitude and distal latency can be obtained, but not a conduction velocity.) After the motor studies are completed in a lower extremity, one should obtain a lower extremity sensory response, either the sural or superficial peroneal, and often both. Averaging may be required, especially in polyneuropathy, where the responses may be very small. At least one motor and one sensory nerve conduction study should be performed in the contralateral leg to assess symmetry. In general, amplitude differences of more than 50% comparing side to side are considered abnormal (i.e., a 50% drop from the higher side to the lower; or a 100% increase from the lower side to the higher). Finally, in the lower extremities, the soleus H reflex can be performed. In most polyneuropathies where the ankle reflex is absent, so too is the H reflex, so the study adds little additional information. The H reflex, however, is more helpful in the assessment of very early polyneuropathies. Mild prolongation of the H reflex latency may be one of the first abnormalities seen in mild or early polyneuropathy.

After the lower extremity studies, an upper extremity is conducted. If only the lower extremities are studied, any abnormalities found could be just as consistent with the diagnosis of lumbosacral plexopathies as with that of polyneuropathy. In the upper extremity, median and ulnar motor conduction studies are performed first, with their F responses. Their respective sensory studies are performed next. In axonal polyneuropathy, the median and ulnar sensory responses, although abnormal, usually are better preserved than the sural and superficial peroneal responses in the legs. Of course, one must ensure that these sensory responses are not abnormal secondary to a local entrapment neuropathy. In this regard, measuring the radial sensory response often is helpful, being much less commonly involved in entrapment neuropathy. Comparing the maximal radial sensory amplitude to the maximal sural amplitude can be especially useful (see discussion of the SRAR above). Any ratio <0.21 is supportive of the diagnosis of an axonal polyneuropathy. As with the lower extremities, comparison of one motor and sensory nerve from side to side should be performed to assess symmetry. If there is a clear clinical asymmetry, comparison of more nerves is indicated.

When looking for conduction block and other electrophysiologic evidence of demyelination, it is often worthwhile to study additional nerves. The formal criteria for a demyelinating polyneuropathy are based on finding a number of different abnormalities ( Boxes 26-5 and 26-6 ). If these criteria are only partially met, a more extensive search is warranted to try to secure the electrodiagnosis of a demyelinating neuropathy.

Electromyographic Approach

The EMG strategy in polyneuropathy ( Box 26–8 ) is similar to that of the nerve conduction studies. Distal and proximal muscles of at least one upper and lower extremity must be sampled. In polyneuropathy, there is a characteristic distal-to-proximal gradient of neuropathic changes (lower extremity more affected than upper extremity; legs more affected than thighs; hands more affected than arms). It is very unusual for any polyneuropathy to affect the gluteal muscles or the muscles of the upper arm and shoulder girdle. Important exceptions to this include AIDP, which is a polyradiculoneuropathy, as well as other neuropathies with a proximal predominance, including porphyria and some diabetic neuropathies. As in the nerve conduction studies, it is worthwhile to compare a contralateral muscle in each limb to assess symmetry. In cases in which there is a clinical suggestion of asymmetry, more muscles should be sampled, especially in the distribution of the asymmetry.

Although the intrinsic foot muscles are the most distal muscles, they are best avoided in the EMG evaluation of polyneuropathy. First, examination of these muscles is often painful for most patients to tolerate. Second, activation of these muscles is usually difficult or nearly impossible (especially the tibial-innervated foot muscles), making any assessment of MUAPs difficult. Finally and most importantly, some denervation and reinnervation are commonly found in normal patients, especially in the EDB muscle. Presumably, repetitive trauma over time from shoes, walking, and running injures the distal nerves in the feet. Any EMG abnormality in those muscles must always be interpreted with caution and compared from side to side. In general, the best distal muscles to sample are those in the lower calf, especially the extensor hallucis longus and flexor digitorum longus.

For evaluation of polyneuropathy, the needle EMG is clearly the more sensitive part of the electrophysiologic study. Although the typical polyneuropathy shows distal abnormalities on both nerve conduction studies and EMG, in some mild polyneuropathies the only abnormal findings may be those present on the EMG. The loss of only a few axons may result in fibrillation potentials that are easily seen on the EMG but may cause little appreciable change on the motor and sensory nerve conduction studies. To emphasize this point, take the example of mild neuropathy wherein 10% of the fibers have been lost. A sural sensory amplitude that had been 20 µV drops to 18 µV (normal >6 µV), and a tibial motor amplitude that had been 10 mV drops to 9 mV (normal >4 mV). Thus, the nerve conduction studies still are interpreted as “normal.” However, within the extensor hallucis longus muscle, there may be 200 motor units (i.e., 200 axons). Each motor unit may be composed of 50 muscle fibers. Thus, if 10% of the fibers are lost (20 axons), 20 × 50 = 1000 muscle fibers will be fibrillating and easily appreciated on needle EMG.

Example Cases

In the following polyneuropathy example cases, we use the history, physical examination, and EDX study to answer the seven key questions about the polyneuropathy illustrated in the case. By doing so, the differential diagnosis is narrowed to a much smaller number of disorders, and subsequent evaluation and treatment proceeds in a more directed and logical way.

Case 26–1

History and Physical Examination

A 45-year-old woman was referred for numbness and tingling in the legs. She had been well until 6 months previously, when bilateral numbness began in the toes. Over the next several months, the numbness spread to above the ankles and was associated with pins-and-needles paresthesias. She described her feet as feeling like wooden blocks. More recently, numbness spread to the calves, with some tingling in the fingertips. She noted some difficulty opening jars, buttoning buttons, and turning keys.

On examination, there was mild atrophy of the intrinsic foot muscles bilaterally, especially of both extensor digitorum brevis muscles. Muscle strength testing was normal in all extremities. Ankle reflexes were absent bilaterally. Both knee reflexes were present but hypoactive. The upper extremity reflexes were normal and symmetric. Sensory testing demonstrated decreased vibration at both ankles. Sensation to light touch and pinprick was decreased below the knees bilaterally and in the fingers of both hands. There was no Romberg sign. Gait and coordination were normal. She had no family history of polyneuropathy, and no significant medical illnesses.

CASE 26–1

Nerve Conduction Studies

Nerve Stimulated	Stimulation Site	Recording Site	Amplitude Motor = mV; Sensory = µV			Latency (ms)			Conduction Velocity (m/s)			F Wave latency (ms)
			RT	LT	NL	RT	LT	NL	RT	LT	NL	RT	LT	NL
Median (m)	Wrist	APB	6.2	5.8	≥4	3.5	3.2	≤4.4				31	32	≤31
	Antecubital fossa	APB	6.1	5.8		7.3	7.2		52	50	≥49
Ulnar (m)	Wrist	ADM	7.3		≥6	3.1		≤3.3				32		≤32
	Below elbow	ADM	7.2			6.4			55		≥49
	Above elbow	ADM	7.2			8.3			53		≥49
Median (s)	Wrist	Index finger	13	12	≥20	3.4	3.3	≤3.5	50	51	≥50
Ulnar (s)	Wrist	Little finger	7		≥17	3.1		≤3.1	49		≥50
Tibial (m)	Ankle	AHB	3.2	2.8	≥4	5.7	5.6	≤5.8				57	56	≤56
	Popliteal fossa	AHB	2.5	2.2		12.1	11.7		39	41	≥41
Peroneal (m)	Ankle	EDB	1.1		≥2	6.2		≤6.5				NR		≤56
	Below fibula	EDB	0.9			12.6			39		≥44
	Lateral popliteal fossa	EDB	0.8			15.3			37		≥44
Sural (s)	Calf	Posterior ankle	NR	NR	≥6			≤4.4			≥40

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