28 Atypical Motor Neuron Disorders

There are a heterogeneous group of motor neuron disorders that are rare but nonetheless important to recognize, because they often can mimic the presentation of amyotrophic lateral sclerosis (ALS). These often are referred to as atypical motor neuron disorders. Although many of the atypical motor neuron disorders share some features with ALS, they often can be distinguished by their clinical and electrophysiologic characteristics (Boxes 28–1 and 28–2).

One of the most important atypical motor neuron disorders that can be confused with motor neuron disease is the immune-mediated motor neuropathy multifocal motor neuropathy with conduction block (MMNCB). Strictly speaking, this is a disorder of the motor nerve and as such is discussed in detail in Chapter 26. Patients present with progressive, asymmetric weakness and wasting that often affect the distal upper extremity muscles first. Weakness is in the distribution of named motor nerves, often 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 have weakness without wasting, a finding usually associated with pure demyelination. The disease is slowly progressive, with a male predilection, generally presenting before the fifth decade. Definite upper motor neuron signs are absent, although retained or inappropriately brisk reflexes for the degree of weakness and wasting may be seen. Bulbar function and sensation are characteristically spared. Mild or transient sensory symptoms may be present. The characteristic finding on motor nerve conduction studies is that of conduction block, temporal dispersion, or both, along the motor nerves. Other signs of demyelination also may be seen, including slowed conduction velocities, absent or impersistent F responses, and prolonged distal motor latencies. Sensory conduction studies are typically normal.

Other than multifocal motor neuropathy with conduction block, atypical motor neuron disorders are seen most often in association with certain viral infections or as the result of specific genetic mutations. Rarely, atypical motor neuron disorders are seen as a remote effect of some neoplasms or as a result of electrical injuries or radiation. Because the prognosis in ALS is uniformly poor compared with these atypical motor neuron disorders, it is essential that the correct diagnosis is reached. In addition, some are potentially treatable; in others, genetic counseling is important.

Infectious Motor Neuron Disorders

Paralytic Poliomyelitis and Postpolio Syndrome

Paralytic poliomyelitis was once a common cause of acute lower motor neuron dysfunction. In the United States from 1951 to 1955, an average of more than 15,000 cases occurred per year. Through widespread use of the oral polio vaccine, the incidence of acute poliomyelitis has been drastically reduced. Most cases now are associated with the live attenuated virus in the oral polio vaccine and occur either in vaccine recipients or in individuals who are in contact with vaccine recipients, especially immunocompromised patients. Other cases occur in travelers to areas where poliomyelitis is endemic; in 2011, these countries were Afghanistan, India, Nigeria, and Pakistan. Sporadic outbreaks have also occurred in other underdeveloped countries. In rare, sporadic cases, infection presumably is due to incomplete immunization status. Most sporadic cases are no longer associated with the poliovirus but are the result of coxsackievirus, echovirus, or enterovirus infection.

Patients with acute poliomyelitis present with fever, headache, myalgias, and gastrointestinal disturbance. Weakness, wasting, and depressed reflexes begin to appear during the first or second week of the illness. The distribution of weakness typically is asymmetric, and the lower extremities are most commonly involved. The upper extremities, trunk, diaphragm, and bulbar muscles are occasionally involved. Sensation and autonomic function are spared. Cerebrospinal fluid (CSF) typically shows a lymphocytic pleocytosis, often in the range of 100 to 200 cells per cubic millimeter (rarely, polymorphonuclear leukocytes may be seen early), during the preparalytic phase of the illness. Pleocytosis, while invariably present in the preparalytic phase of the illness, tends to clear with the onset of the weakness. The CSF protein level is commonly elevated within the first several weeks of the illness, whereas CSF glucose is normal. Cultures from CSF usually fail to isolate the virus, although the virus can commonly be isolated from the stool if it is obtained within the first 10 days of the paralysis. In addition, antibody titers from the acute and convalescent phases may allow virus identification.

Weakness associated with poliomyelitis now is seen most often in the electromyography (EMG) laboratory not as an acute process but in patients with postpolio syndrome (PPS). PPS occurs in at least one fourth of previously infected patients, usually 25 to 30 years after the attack of acute poliomyelitis. Patients develop pain, fatigue, and weakness, often most prominent in the muscle groups previously affected by the poliomyelitis. However, muscles that were clinically normal may develop symptoms, reflecting the diffuse underlying nature of the previous poliomyelitis. The etiology of PPS is not completely known, but it is most likely related to the normal aging process (i.e., most individuals lose some motor neurons after age 55 years) superimposed on chronically denervated muscles. Patients with PPS and worsening symptoms often are referred to the EMG laboratory to exclude a new, superimposed process, such as radiculopathy, entrapment neuropathy, myopathy, or motor neuron disease as a source of increased fatigue, pain, and weakness.

West Nile Encephalitis

Over the past several years, there have been an increasing number of reports of a “polio-like” syndrome associated with West Nile encephalitis. The responsible virus, which is a member of the flavivirus family and is composed of a single strand of RNA, was first isolated in 1937 in northern Uganda. In nature, the virus is transmitted between birds by mosquitoes (Figure 28–1). Jays, blackbirds, finches, warblers, sparrows, and crows appear to be most important in maintaining the infection. Most infections in humans occur by way of a mosquito bite, although cases have been reported following transplanted organs and infected blood products. Because the disease is primarily spread to humans by mosquitoes, patients typically are affected in the summer and early fall.

FIGURE 28–1 West Nile virus.

The vector for the West Nile virus is the common mosquito. Although rare, an increasing number of cases of poliomyelitis have been associated with this virus, either alone or in association with encephalitis.

(Courtesy of US Geological Survey.)

Fortunately, most infections with the West Nile virus are asymptomatic, with only one in 150 infections resulting in neurologic involvement. The elderly and the immunocompromised appear to be at highest risk. After an incubation period of several days, a nonspecific flulike illness develops, often with fever, headache, and joint and muscle pain. In some patients there may be additional features suggestive of West Nile infection, including retro-orbital pain, facial congestion, and rash. Definitive diagnosis is made by the presence of immunoglobulin M antibodies in CSF or serum.

In patients with neurologic involvement, a combination of encephalitis, meningitis, and myelitis can occur. Diffuse weakness is common and often thought to be due to the encephalitis. Other patterns of weakness are also seen, among them monoplegia, flaccid quadriplegia, bulbar weakness, and respiratory weakness. In some patients, an acute segmental flaccid paralysis has been described as an initial presentation of West Nile virus, even in the absence of meningitis or encephalitis. Such cases initially were attributed to Guillain–Barré syndrome, although it now is clear that the weakness more likely was due to anterior horn cell disease. In patients in whom electrodiagnostic (EDX) studies have been performed, nerve conduction studies show reduced compound muscle action potential (CMAP) amplitudes with relatively intact sensory conduction studies. No evidence of demyelination is present. Rarely, patients have had abnormal sensory conduction studies, suggesting involvement of the dorsal root ganglia or peripheral sensory nerve as well. Needle EMG shows evidence of axonal loss. The pattern of findings depends on when the study is performed in relationship to the start of the illness.

Thus, in addition to coxsackievirus, echovirus, and enterovirus, West Nile virus can be added to the list of infectious agents that can result in an acute infection of the anterior horn cells. Thus, paralytic poliomyelitis is best regarded as a clinical syndrome that can be caused by a variety of viruses, not simply the poliovirus.

Retrovirus-Associated Motor Neuron Disorders

Human immunodeficiency virus (HIV) is associated with a variety of neuromuscular disorders, including peripheral neuropathies, myopathies, and radiculopathies. Experimental studies in mice have shown that retroviruses can induce a lower motor neuron syndrome in mice and suggest a relationship between retroviruses and the pathogenesis of motor neuron disease. There are rare reports of patients with HIV infection and classic ALS, or a clinical syndrome resembling primary lateral sclerosis, without any other explanation for their symptoms. Other patients have had restricted lower motor neuron signs. Some reports have noted improvement or complete remission of the syndrome in these patients when they are treated with highly active antiretroviral therapy.

Another retrovirus, the human T cell lymphotropic virus-type 1 (HTLV-1), is well known to be associated with spastic paraparesis in endemic areas (i.e., the Caribbean basin, southwest Japan, southeast United States, southern Italy, and sub-Saharan Africa) in a syndrome known as HTLV-1-associated myelopathy or Tropical Spastic Paraparesis (HAM/TSP). Along with spastic paraparesis, patients usually have bladder dysfunction and minor sensory symptoms. A motor neuron syndrome mimicking ALS is also observed in a series of patients with HTLV-1 infection. The presence of spastic paraparesis or even typical ALS symptoms, with minor sensory findings or bladder dysfunction, especially in an endemic area for HTLV-1, should prompt a search for HTLV-1 antibodies.