The ongoing Zika virus outbreak in South America has brought media and research attention to several rare neurological disorders.
Early reports suggested links between Zika infection and microcephaly (abnormal smallness of the head) in newborn infants. These were quickly followed by reports of increased rates of auto-immune disorder Guillain-Barre syndrome in Zika-infected adults. The most recent reports propose a connection between Zika infection and acute disseminated encephalomyelitis (ADEM).
These three rare conditions all cause damage to the nervous system, which includes the brain, spinal cord and nerves. Our nervous system functions to send signals throughout the body to co-ordinate movement, sense our environment and regulate body function.
What is acute disseminated encephalomyelitis (ADEM)?
ADEM is a rare autoimmune disease that causes lesions in the brain and spinal cord. Disease is usually triggered by a previous infection or vaccination, although why this occurs is not well understood. For example, one in 1,000 people infected with measles goes on to develop ADEM. Rates of disease have decreased in developed countries, due to reduced rates of infection.
Immune cells normally protect our body against disease, by killing invading viruses and bacteria. In ADEM, these immune cells attack our nerves within the brain and spinal cord and cause damage. Damage destroys the insulating coating on our nerve cells, called myelin, and interferes with signalling in the nervous system.
Early symptoms include fever, low energy, headache nausea and vomiting. Within several days, symptoms escalate and can range from low energy to coma, with weakness along one side of the body or in the legs (hemiparesis/paraparesis). Symptoms can also include loss of control of body movements (ataxia) and other movement disorders. Anti-inflammatory drugs are usually used to try to reduce damage.
Symptoms can start to improve quickly (within days) and people usually fully recover within six months. Most people experience no long-term symptoms. However, some individuals do “relapse”, meaning they go on to experience another round of symptoms.
If patients continue to having recurring symptoms, they may be diagnosed with multiple sclerosis. Multiple sclerosis causes similar damage and symptoms to ADEM and is a life-long disease. It is currently unclear whether ADEM leads to multiple sclerosis in some people or whether ADEM is simply confused for the first episode of multiple sclerosis.