John D. England, M.D.
Professor and Head
Department of Neurology
Louisiana State University Health Sciences Center
Answer 1: Peripheral neuropathy is a general term that indicates a disorder of the peripheral nervous system. Such a broad definition encompasses the entire spectrum of causes and includes all disorders of peripheral nerve. The cases under discussion are most likely examples of an immune-mediated polyradiculopathy or polyradiculoneuropathy. Such patients usually present with varying combinations of altered sensation, pain, and muscle weakness in the extremities. If loss of proprioception in the lower extremities occurs, patients will have difficulty with balance and walking (sensory ataxia). Symptoms are usually relatively symmetric and most predominant in the lower extremities. If cranial nerves are involved, patients may present with symptoms of cranial neuropathy, especially facial weakness.
Answer 2: The overall prevalence of peripheral neuropathy is approximately 2400 per 100,000 population (2.4%). However, in people older than 55 years, the prevalence rises to approximately 8000 per 100,000 (8%). (J Neurol Neurosurg Psychiatry 1997;62:310-18) The most common cause of peripheral neuropathy is diabetes mellitus, but a host of other conditions such as metabolic disorders, vitamin deficiencies, toxins, drugs, infectious agents, vasculitis, immunological aberrations, malignancies, and inherited diseases can cause peripheral neuropathy. (Lancet 2004;363:2151-61) The cases under discussion from the Mayo Clinic are most likely examples of an immune-mediated inflammatory polyradiculoneuropathy. Although the description of the time of illness progression is variable, most appear to have developed a chronic polyneuropathy over weeks to months (personal communication from Dr. P. James B. Dyck). As such they represent an unusual cluster of a chronic inflammatory polyradiculoneuropathy with demyelinating features. Whether these neuropathies represent a variant of CIDP or whether they represent another dysimmune neuropathy is not known.
Answer 3: The most accurate diagnosis of polyneuropathy is made by the combination of neuropathic symptoms, neuropathic signs, and electrodiagnostic (EMG/NCS) findings. (Neurology 2005;64:199-207) Therefore, most patients should undergo a thorough medical history, a comprehensive neurological examination, and electrodiagnostic studies. Ancillary laboratory studies such as blood tests, CSF examination, neuroimaging studies, and nerve biopsy may be necessary. (Lancet 2004;363:2151-61) For the cases from the Mayo Clinic, electrodiagnostic studies, MRI studies, and CSF analysis showed features compatible with an inflammatory demyelinating polyradiculoneuropathy. (Muscle Nerve 2001;24:311-24) Major involvement of nerve roots is indicated by the minimal sensory nerve conduction abnormalities, the elevated CSF protein, and the thickened/contrast enhancing nerve roots on MRI.
Answer 4: The treatment of peripheral neuropathy depends upon the variety and the etiology (if known). Obviously, if a systemic disorder is found, then the underlying condition must be treated. The neuropathies from the Mayo Clinic most closely resemble CIDP, which is the quintessential example of a chronic dysimmune inflammatory polyneuropathy. Therefore, it would have been reasonable to treat them with intravenous immune globulin, plasma exchange, or systemic steroids. Information is not yet available regarding whether this was done or how the patients responded. Symptomatic treatment for symptoms, especially pain, should be addressed on a case by case basis.
Answer 5: The prognosis for polyneuropathy varies considerably and is directly related to whether the etiology is self-limited or treatable. The immune-mediated inflammatory polyneuropathies often respond to immunomodulatory or immunosuppressive treatments; however, the response may not be complete. Guillain-Barre Syndrome (acute inflammatory demyelinating polyneuropathy) has a relatively better prognosis for recovery than the more chronic varieties of inflammatory demyelinating polyneuropathy such as CIDP.
Answer 6: The description of this cluster of neuropathies conforms most closely to an immune-mediated inflammatory polyradiculoneuropathy. The association with processing of pig head material suggests the possible exposure to an antigen or infectious agent that can precipitate an immune-mediated attack upon peripheral nerve. At this time, the nature of this factor or factors is unknown.
Answer 7: The first step is to confirm that the common exposure is to pig head material and not to some other factor or factors. The next step is to evaluate the patients' cases carefully to ascertain that the condition is the same in all patients. It appears that a large part of these evaluations have already been done, but the details are not yet available. Further steps will be to examine human blood and CSF samples to ascertain whether specific antibodies are present and whether they cross react with antigens in nerve and biological material from swine. An attempt to identify an infectious agent within the swine material and from biological samples (e.g., blood, CSF, and nerve) from the patients should also be done. Obviously, workers at pork processing plants should be warned and immediately protected from pig head material, especially aerosolized brain materials pending further investigation of these outbreaks.
Disclaimer: The opinions expressed in this posting are those of the author only and do not represent the views of the American Academy of Neurology or any of its affiliated subsidiaries.
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