Motor neurone disease affects about 2 in every 100,000 people each year in the UK and about 5,000 people are living with the disease at any given time.
In the United States, about 5,600 individuals are diagnosed every year The condition is slightly more common among men than women.
People who develop motor neurone disease usually start to experience symptoms in their early 60s, although people in their 90s or still in their teenage years can also develop the condition.
The majority of cases of motor neurone disease are sporadic and there is no family history of the condition. However, in about 5% of cases, a close family relative also has the condition or a similar disease called frontotemporal dementia. In familial cases, a number of defective genes that are thought to contribute to the disease have been identified.
Several environmental factors have also been implicated in causing motor neurone disease but none have yet been proved to be associated with the condition. One example is American football, as three individuals who play the game have developed the condition. In addition, 50 association footballers in Italy have motor neurone disease.
For the majority of countries, the prevalence of motor neurone disease is similar. Among some areas of the world, however, an increased prevalence of the condition has been known, although this appears to be tailing off in recent decades.
One area of increased incidence is the Kii peninsula of Japan where the cycad seed is used as a medicine. The ingestion or topical application of this seed has been linked to the development of motor neurone disease. Chamorro inhabitants from the island of Guam in the Pacific Ocean also develop a special form of motor neurone disease called Guamanian ALS-PD-dementia complex or lytico bodig. It has been suggested that this condition develops in these regions due to the use of cycad nut flour in the diet or the inclusion of bats in the diet that have eaten cycad nuts.
Reviewed by Sally Robertson, BSc