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Figure 4.1. Conduction block.

a normal motor response when stimulating the affected nerve distal to the lesion, but either a reduced or absent response is elicited when stimulating proximal to the lesion. (Fig. 4.1) Neuropraxic lesions, (conduction block), may take up to 6 months to resolve, although typically block reverses over 2 weeks to 3 months. The presence of Wallerian degeneration can be especially observed during EMG. In this situation of axon loss, one can see fibrillation potentials of the involved muscles. These patients usually have an incomplete recovery. When a diagnosis of a compressive neuropathy is made, the patient's caretakers can then be given specific instructions for positioning of the patient or other equipment utilized in the care of the patient. Therefore, early recognition of this pattern of weakness is important.

Frequently distal weakness is secondary to a polyneuropathy. Patients commonly develop this neuropathy insidiously. The most common neuropathy is a generalized axonal sensorimotor polyneuropathy. Etiologic causes can be varied: metabolic, nutritional, secondary to a systemic disorder, infectious, or cryptogenic. The most common etiology of a generalized distal axonal sensorimotor polyneuropathy is diabetes mellitus (Melton and Dyck, 1987). The risk of developing a diabetic peripheral neuropathy correlates with the duration of the diabetes, the control of hyperglycemia, and the presence of retinopathy and nephropathy (Dyck et al., 1993). Other causes of axonal polyneuropathy are shown in Table 4.1.

Patients with an axonal polyneuropathy first develop a distal pattern of sensory loss followed by distal weakness. The reflexes, when lost, are in a length-dependent fashion. Electrodiagnostic studies reveal reduced amplitudes of sensory and motor action potentials. Conduction velocities are minimally affected (Oh, 1993). EMG reveals a pattern compatible with denervation mostly affecting the distal muscles. Motor unit potentials are usually large with an increased recruitment and decreased interference pattern (Brown and Bolton, 1987). Spontaneous activity is usually sparse. Identifying the underlying etiology can lead to improvement or at least a stabilization of the polyneuropathy.

Occasionally, patients develop another pattern of weakness that involves both proximal and distal muscles. This pattern of weakness is most consistently seen in a demyelinating polyneuropathy. Demyelinating neuropathies may be divided into acute or chronic presentations. Guillain-Barre syndrome (GBS) or acute inflammatory demyelinating polyradiculoneuropathy (AIDP) has become the most

Table 4.1. Causes of axonal polyneuropathies.

Nutritional deficiencies Thiamine Pyridoxine (B6) Cobalamin (B12) Folate Vitamin E Strachan syndrome

Toxins Alcohol Drugs

Chloramphenicol Chloroquine Dapsone Metronidazole Nitrofurantoin Zalcitabine and other nucleoside analogs Cisplatinum Paclitaxel Misonidazole Thalidomide Heavy metals Hexacarbons Acrylamide Carbon disulfide Ethylene glycol Organophosphates common cause of acute paralysis in Western countries since the virtual elimination of poliomyelitis with vaccination programs (Parry, 1993). GBS is clinically a predominant motor neuropathy affecting the patient in a symmetrical fashion. At the nadir of the illness there is approximately equal weakness of proximal and distal muscles in about 50% of cases (Ropper and Shahani, 1984). Roughly 60-70% of patients with AIDP note some form of acute illness (e.g., a viral syndrome) 1-3 weeks before the onset of neurologic symptoms. In a recent case control study of 154 patients with GBS, serologic evidence of recent infections with Campylobacter jejuni (32%), cytomegalovirus (13%), Epstein-Barr virus (10%), and Mycoplasma pneumoniae (5%) were more frequent than in the control population (Jacobs et al., 1998).

The early and accurate diagnosis of GBS depends primarily on NCs. Carefully performed NCs of multiple nerves, including cranial nerves and proximal segments of spinal nerves, if indicated, are almost always abnormal even when the cerebrospinal fluid protein is normal. The electrophysiologic diagnosis of GBS requires the demonstration of the characteristic changes associated with acute demyelination (Gilliatt, 1966). The most studied aspects in AIDP are motor nerves; distal latencies, compound action potentials, conduction velocities, waveform duration and morphology, and F-waves. The electrophysio-logic hallmarks include prolongation of the distal latencies, slow conduction velocities, temporal dispersion, conduction block, and prolonged F-waves. Another electrodiagnostic finding often seen in GBS is superimposed axonal degeneration (Ensurd and Krivickas, 2001). The degree of axonal loss has been shown to correlate with both acute mortality and long-term disability in patients with GBS (Chio et al., 2003).

Nearly 40% of patients hospitalized with GBS will require inpatient rehabilation (Meythaler et al., 1994). The electrodiagnostic study aids in identifying these patients.

Chronic inflammatory demyelinating polyneuropathy (CIDP) is an acquired demyelinating poly-radiculoneuropathy that shares a similar pathology and presumed autoimmune etiology with AIDP but differs primarily in the time course of presentation and evolution of clinical signs. In typical CIDP, motor and sensory symptoms develop slowly over the course of weeks to months, or even years. The clinical course may be relapsing, with variable periods of remission, or stepwise and chronically progressive. The diagnosis of CIDP is based on a combination of clinical and electrophysiologic findings as well as evaluation of the cerebrospinal fluid. The patient with CIDP has variable weakness in proximal and distal muscles. Sensory impairment is common, and tendon reflexes are either reduced or absent (McCombe et al., 1987). Electrodiagnostic studies usually reveal absent sensory responses. Motor conduction velocities are very slow, usually below 80% of normal values, and distal latencies may be prolonged.

Hydralazine

Aminodarone

Propafenone

FK 506 (Prograf)

Colchicine

Vincristine

Disulfiram

Glutethimide

Simivastin and other statins

Table 4.2. Concurrent diseases with CIDP.

Table 4.3. Drugs that may cause a myopathy.

HIV infection Diabetes mellitus Chronic active hepatitis Inflammatory bowel disease Connective tissue disease Nephrotic syndrome Melanoma

Monoclonal gammopathy Lyme disease Lymphoma

Amiodarone

Anesthetics

Cytarbine

Cyclosporine

Cyclophosphamide

Cholesterol-lowering agent

Colchicine

Corticosteroids

Diazepam

Diuretics

Gemfibrozil

Interferon alpha

Isoniazid

Neuroleptics

Penicillamine

Steroids

Tacrolimus

Vasopressin

Zidovudine

If the pathology is concentrated proximally, the F-wave latencies are markedly abnormal (Nicolas et al., 2002; Scarlato and Comi, 2002). Some patients with CIDP may have a concurrent illness (Table 4.2).

Muscle disease

Some patients may, in the course of their rehabilitation, develop a pattern of proximal weakness. This weakness usually begins in the lower extremities and then progresses to involve the proximal arms. There may or may not be associated muscle pain. In this setting, the most common etiology would be a toxic or iatrogenic myopathy. Certain drugs are known to cause an immunologic reaction directed against muscle (Scarlato and Comi, 2002) (Table 4.3). Electrolyte imbalances have also been associated with vacuolar muscle damage (Saleh and Seidman, 2003). Prompt recognition of this clinical picture is important because toxic or iatrogenic myopathies are potentially reversible, thus reducing their damaging effect.

Inflammatory myopathies are another cause of proximal weakness. The inflammatory myopathies are a diverse group, ranging from focal varieties to widespread skeletal involvement. Etiologic factors are mainly immune-mediated or infectious agents. The most common immune-related varieties in clinical practice are dermatomyositis and polymyositis, each of which have distinctive clinical and histopatho-logical features, and may occur in isolation or associated with a systemic connective tissue disease (Amato and Barohn, 1997). Human immunodeficiency virus

(HIV) causes a myopathy in almost a third of infected patients (Dalakas et al., 1986). Another infectious agent is human T-cell lymphotropic virus 1 (HTLV-1).

Electrodiagnostically, patients with a myopathy should have normal nerve conductions. EMG in these patients demonstrates short duration, small amplitude, polyphasic motor unit potentials with early recruitment (Mastaglia et al., 2003). For additional discussion of myopathies, see Volume II, Chapter 41.

Neuromuscular transmission defect

Infrequent causes of muscle weakness are the diseases of the neuromuscular junction (Volume II, Chapter 40). Neuromuscular transmission defect can be a result an autoimmune disease like Myasthenia gravis (MG) or Eaton- Lambert syndrome (LEMS), or toxins that block neuromuscular transmission. MG occurs with a prevalence of 20 per 100,000 (Phillips, 2003). Like other autoimmune disorders, it is characterized by periods of relapses and remissions (Richman and Agius, 2003). The target of the autoimmune attack in MG is the nicotinic acetylcholine receptor, which is located on the postsynaptic muscle endplate membrane (Tzartos and Lindstrom, 1980). Patients present with weakness of the ocular and or bulbar musculature that over time may generalize to involve proximal limb muscles. An important characteristic of patients with neuromuscular transmission defect is fatigability. Additionally, these patients can present with unexplained respiratory failure.

Repetitive nerve stimulation is a useful electro-physiologic test in the evaluation of patients with suspected neuromuscular junction disorders. In patients with MG, there is a decremental response seen on the compound motor action potential (CMAP) with slow rates of stimulation. In patients with LEMS, fast rates of repetitive stimulation produce a marked increase in the CMAP (Kimura, 1989).

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