The cause of the disease is unknown, but about two-thirds of cases are preceded by a respiratory or gastrointestinal infection. A significant minority of cases follow infection by two herpes viruses, cytomegalovirus or Epstein-Barr virus, the cause of infectious mononucleosis. Some cases have been triggered by immunizations against infections such as influenza.
Myelin, the fatty protective sheath around the nerves, is inflamed and patchily destroyed, disrupting messages from the nerves to the muscles, which become weak and non-functional. The first symptom is usually a tingling sensation and numbness in the feet, legs and arms. Over the next few days there is progressive loss of sensation and muscle power affecting the whole body. Symptoms may continue to worsen for two to three weeks. In the worst cases, the muscles that control speech, eye movements and breathing may be paralysed (one-third of patients develop breathing problems and one-fifth require artificial ventilation). Most experts believe the syndrome represents an autoimmune attack in which antibodies in the blood mistakenly attach themselves to normal tissue and destroy it. However, the usual treatments for autoimmune disorders - steroids and anti-inflammatory medications -- do not help and may make things worse. Plasma exchange, involving removal and replacement of the liquid part of the blood that contains the antibodies with a slat and protein solution, or pooled gamma globulin, which contains antibodies from blood donors, give the best results.