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PERIPHERAL NEUROPATHIES

INTRODUCTION

The nervous system is often artificially organized into two parts, a central nervous system (CNS) and a peripheral nervous system (PNS). The CNS consists primarily of the brain and spinal cord. The PNS consists mainly of the peripheral nerves that transmit impulses outside the CNS.  

The peripheral neuropathies are an assorted group of disorders that affect these peripheral nerves (PNS). Diseases of the peripheral nerves are a common presenting problem both in general practice and neurology. Their causes are diverse and includes infection, heredity, trauma and entrapment, cold, toxins, certain nutrients and drugs, radiation, severe illness, gland and kidney diseases, some cancers, immune system abnormalities and inflammation. Examples of diseases associated with peripheral neuropathies include diabetes mellitus and thyroid disease.

 Despite the large number of causes, the ways in which peripheral nerves respond to injury are relatively limited so that there is a relatively small number neuropathic syndromes (the collections of symptoms and signs that neuropathies cause).

It is notable, that many of the inherited peripheral neuropathies (outside link) can now be attributed to specific gene mutations and biochemical abnormalities. It is now apparent that many of these diseases, which were previously considered a single entity, are a genetically heterogeneous group. This recent understanding has been brought about by application of gene mapping and linkage techniques.

ANATOMIC CONSIDERATIONS

Understanding the principle components of the peripheral nervous system, allows understanding of how nerves respond when things go wrong. 

An individual nerve is often made up of different types of nerve fibers that transmit different types of signals. Automatic (termed ‘autonomic’) activities and painful or temperature sensation are principally transmitted by small uninsulated (unmyelinated) or small insulated (myelinated) fibers respectively. This contrasts with nerve fibers going to muscle that are typically large well insulated and fast conducting, as are fibers supplying receptors for light touch and vibration sensation. Deep tendon reflexes also depend upon larger myelinated fibers.

Because of this distribution of fibers, diseases principally destroying the nerve fiber insulation (demyelination) will predominately affect muscle power, light touch and vibration sensation and reduce reflexes. Diseases affecting principally small fibers may spare muscle and reflex function, but affect autonomic control and pain sensation.

Symptoms however are often mixed, as some diseases may affect both the large and small fibers giving rise to a full sensory, muscle (termed ‘motor’) and autonomic disorder. In addition, some diseases cause complete death of the nerve cells of a specific fiber type (e.g.ALS). 

SYMPTOMS OF PERIPHERAL NERVOUS SYSTEM DISEASE

There are eight principle manifestations of peripheral nervous system disorders. These are listed below:

•  Impairment of muscle function.

• Impairment of deep tendon reflexes.
• Twitching, cramps, and spasms of muscles.
• Loss of sensation.
• Altered sensation such as tingling or painful burning (termed paresthesias and dysesthesias)
• In coordination due to loss of sense of position of the limbs (termed ’Sensory ataxia’).
• Deformity, loss of muscle mass and hair loss
• Autonomic dysfunction (e.g. impotence, abnormal sweating)

Various combinations are possible. The overall pattern of symptoms varies depending upon the types and location of the nerves that are affected, the degree to which they are damaged, and how fast the disease progresses. In some disorders only an individual nerve may be affected but include all the various fiber types within it, causing both abnormalities in sensation and muscle activity in the specific area of the body supplied by that nerve (termed a ‘mononeuropathy’, e.g. Carpal Tunnel Syndrome). More diffuse diseases may affect one or more fiber types in all of the nerves (termed ‘polyneuropathies’, e.g. Charcot-Marie-Tooth disease). In these latter cases the longest nerves, those that travel from the lower back to the feet, are often the most vulnerable and are affected first. This causes foot numbness or weakness in the small muscles of the feet early on.  

What can I do about it?

Treatment of peripheral neuropathies varies as widely as their causes. Knowing what to do depends largely on the underlying process causing the neuropathy. Finding this out usually involves consultation with your doctor. Blood tests or electrical testing of the nerves (Electromyography, EMG) may be required. A consultation with a neurologist is often sought if the cause of the peripheral neuropathy is not apparent. Even in highly specialized centers for studying peripheral nerve diseases, however, causes of chronic polyneuropathy may not be found in up to a quarter of cases. None the less, symptom alleviation is often possible.

Dr. R. O'Brien

This data is provided for informational purposes only. It does not substitute for individualized advice from a qualified physician. Although attempts have been made to ensure the material is accurate and up to date it is provided in an 'as is' state. Neither the author nor Neurology BC assumes any liability for errors or omissions or any problems that might arise due to them. Always consult your physician or qualified health professional before acting on information that concerns your health.