The brain and spinal cord make up the central nervous system. The nerves that radiate out from the central nervous system to muscles (motor nerves) and sense receptors (sensory nerves) make up the peripheral nervous system. A third type of peripheral nerve are autonomic nerves and these contol body temperature, heart beat, breathing and bowel, bladder, sexual function etc.

An illness of the nervous system is called a neuropathy. An illness of the peripheral nervous system is called a peripheral neuropathy. An illness of many peripheral nerves is called a polyneuropathy.

Perpheral nerves are made up of bunches of single celled neurons - rather like the wiring loom of a car. The long body of a neuron is called an axon. This conducts a nerve impulse rather like a wire conducts electricity. Some, but not all, axons are surrounded by a fatty insulating sheath called myelin. Myelin greatly increases the efficiency of a transmitted nerve impulse. An axon normally surrounded by myelin (this is termed myelinated) that becomes demyelinated for some reason is very inefficient and will not work normally. Sometimes the nerve axons themselves become damaged.

An illness that causes nerves to become demyelinated is called a demyelinating neuropathy. An illness that causes direct axonal damage to nerves is called an axonal neuropathy.

An illness that damages motor nerves is called a motor neuropathy. An illness that damages sensory nerves is called a sensory neuropathy. An illness that damages the autonomic nerves is called an autonomic neuropathy. A neuropathy may damage just one type of nerve (eg motor nerves), two types of nerve or three types of nerve.

Some illnesses are caused by a dysfunction of the immune system. This is often termed autoimmune disesase and there are many forms that attack different parts of the body. Those that attack the peripheral nerves are sometimes called dysimmune neuropathies.

Illnesses can be short (acute) or long-lasting (chronic). In dysimmune neuropathies, acute is defined when the nadir (worse point) is comes within 28 days. Chronic is defined when the nadir is comes after 56 days. Unusually, if the nadir comes between 28 and 56 days, the illness is termed sub-acute.

GBS is short for Guillain-Barré syndrome. A clinical syndrome is a collection of symptoms. GBS therefore describes an illness of patients who exhibit the symptoms described by Guillain and Barré in 1916. It has been shown that the symptoms might be caused by either demyelination or by a direct attack on nerve axons. GBS is an acute illness. Patients are only affected for a short time though recovery can be short or protracted.

CIDP is short for chronic inflammatory demyelinating poly(radiculo)neuropathy. 'Radiculo' is often omitted, especially in North America. The symptoms are similar to GBS but follow a chronic course. CIDP does not refer to a description of the symptoms but describes the mechanism that causes them. Inflammatory means the condition is caused by inflamation or irritation. Demyelinating poly(radiculo)neuropathy means that the inflammation demyelinates many neurons (and their roots).

This spectrum of peripheral neuropathy is very wide and multidirectional:

The most common of the acute dysimmune neuropathies is GBS. It can be either demyelinating or cause axonal damage. Variants exist that restrict the area of attack.

The most common of the chronic dysimmune neuropathies is CIDP. By definition it is demyelinating though there is a rare related axonal condition. Variants of CIDP exist and related conditions affect just part of the body.

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