, Ali T. Ghouse2 and Raghav Govindarajan3
(1)
Parkinson’s Clinic of Eastern Toronto and Movement Disorders Centre, Toronto, ON, Canada
(2)
McMaster University Department of Medicine, Hamilton, ON, Canada
(3)
Department of Neurology, University of Missouri, Columbia, MO, USA
The term “peripheral neuropathy” covers a variety of clinical syndromes that affect numerous types of nerve cells and fibers, including motor, sensory, and autonomic fibers. Nearly all peripheral neuropathies affect all these fiber types to a certain degree. However, a single fiber type may be targeted exclusively, and may be affected most, to a greater extent in some disorders than others. There are four cardinal patterns of peripheral neuropathy, including polyneuropathy; a few clinically important polyneuropathies are described below:
- 1.
Polyneuropathy
- 2.
Mononeuropathy
- 3.
Mononeuritis multiplex
- 4.
Autonomic neuropathy
Classification of Polyneuropathies
There are several ways to classify polyneuropathies. This classification depends on the following:
Course or progression (acute, subacute, chronic, progressive, relapsing)
Fiber type (motor, sensory, small fiber, large fiber, autonomic)
Pattern of involvement (symmetrical, multifocal)
Underlying pathology (autoimmune, compression, ischemia)
Part of the nerve cell mainly affected (axon, myelin sheath, cell body)
Hereditary nature (Charcot-Marie-Tooth disease, hereditary neuropathy with a propensity to experience pressure palsy)
Associated illnesses (porphyria, hypervitaminosis, human immunodeficiency virus)
Exposure to toxins (occupational, chemotherapy, poisoning)
Parts of the Nerve Cell Affected
Axonopathy is a disorder that predominantly affects the axons associated with the peripheral nerve fibers. This disorder may be provoked by various metabolic diseases, including diabetes, malnutrition, renal failure, and alcoholism; diseases of the connective tissues; or by effects related to drugs/toxins such as chemotherapy agents. Large-fiber, small-fiber, or both types of axons are affected. The most distal portions of axons are usually the first to degenerate (this is also known as “dying-back neuropathy”), and axonal atrophy is centripetal.
Myelinopathy, also referred to as “demyelinating polyneuropathy,” is a result of a loss of myelin, leading to conduction slowing or blocking of action potentials through the axon of the nerve cell. The most typical cause of this condition is acute inflammatory demyelinating polyneuropathy. Other causes include nerve entrapments, genetic metabolic disorders, and toxins.
Neuronopathy is the result of a derangement of neurons. Motor neuron diseases, toxins, autonomic dysfunction, and sensory neuronopathies may all be a cause of this. Vincristine, a chemotherapy agent, is just one of many neurotoxins that may also cause neuronopathies.
Although polyneuropathies are often suggested by physical examination and history alone, electrophysiological testing is still a large part of the diagnosis, evaluation, and classification of these diseases. A few clinically important polyneuropathies are described below.
Chronic Polyneuropathy
Chronic polyneuropathy is a diagnosis that is very common, having an estimated prevalence of 2.4–8 per 100,000. Polyneuropathy itself has been linked to a variety of causes. However, a cause cannot be established for about 10–15 % of patients, even after a thorough evaluation has been conducted. Those individuals who do not have a cause associated with the neuropathy mostly have axonal neuropathy. Recently, this specific type of neuropathy has been commonly referred to as chronic idiopathic axonal polyneuropathy (CIAP). Patients who are affected by CIAP show, in middle to old age, mild sensory and motor symptoms, along with slow deterioration. However, the occurrence of serious disability is not evident.
Distal Symmetrical Polyneuropathy (Protocol for Evaluation)
A simplified version of a nerve conduction study (NCS) protocol may be used to define the existence of distal symmetrical polyneuropathy:
- 1.
Both sural sensory and peroneal motor NCS are conducted in one lower extremity. If no signs of distal symmetrical polyneuropathy are evident, it is concluded that both studies are normal. If this is the case, then the performance of any further NCS is unnecessary.
- 2.
If the sural sensory or peroneal motor NCS turns out to be abnormal, then it is recommended to conduct further NCS protocols. This includes performing NCS of the ulnar sensory nerves, along with the median sensory and ulnar motor nerves in an upper extremity. NCS may also be conducted for a contralateral sural sensory and one tibial motor nerve. However, proceed with caution when interpreting and understanding median and ulnar studies, since there is a possibility of superadded compression neuropathy.
- 3.
If a response is not present for any of the given nerves studied (sensory or motor), then NCS of the contralateral nerve should be completed.
- 4.
If there is no presence of a peroneal motor response, then NCS of an ipsilateral tibial motor nerve should be completed.
Demyelinating Neuropathies
There are several demyelinating diseases associated with the peripheral nervous system. These include:
Guillain-Barré syndrome and chronic inflammatory demyelinating polyneuropathy
Anti-MAG (myelin-associated glycoprotein) peripheral neuropathy
Charcot-Marie-Tooth Disease
Copper deficiency
Guillain-Barre Syndrome (GBS): Key Subtypes
Acute Inflammatory Demyelinating Polyradiculoneuropathy (AIDP)
AIDP is an autoimmune disorder that is mediated by antibodies. It is elicited by the presence of antecedent viral or bacterial infections. Demyelination is evident via the analysis of electrophysiological findings. More specifically, inflammatory demyelination may also be associated with the loss of axonal nerves. Nevertheless, remyelination can occur once the immune reaction has been brought to a halt.
Acute Motor Axonal Neuropathy (AMAN)
AMAN is a pure motor axonopathy. Approximately 67% of patients who are affected are shown to be seropositive for campylobacteriosis. The electrophysiological studies conducted are normal in sensory nerves, but show reduced or even absent conduction in motor nerves. However, the recovery process for those affected by AMAN is usually very quick. Of note, a large proportion of affected individuals are pediatric patients.
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