ALS generally appears between the ages of 40 and 70, but can also affect adults older or younger and, in rare cases, adolescents.
Compared to other neurodegenerative diseases, the number of victims at a given moment in a given population (prevalence) is relatively low. It affects around 5000 people in France. Its incidence, that is to say the number of people who develop ALS each year, is around 1500 new cases each year in France.
Less than 10% of ALS cases are hereditary. At the moment, only a few genes which are deemed familial have been identified. Among them, the gene ALS1, situated on chromosome 21, represents the most frequent deregulation linked to ALS. This genetic dysfunction can be transmitted from generation to generation and might be at the origin of the disease’s breakout. Researchers have shown that this mutation directly affects not only motor neurons, but also their neighboring cells. Regardless, the gene ALS1 is only responsible for about 15% of all the genetic forms of the disease. The gene ALS2 is responsible for a form of ALS which affects youths and is linked to chromosome 2. The gene ALS3 on chromosome 18 is responsible for a form of ALS affecting adults. The gene ALS4 on chromosome 9 corresponds to a rare form of familial ALS which strikes boys during the course of adolescence and women during their 30’s. The gene ALS5 on chromosome 5 affects adults principally. The genes ALS6 and ALS7, discovered in 2003, are linked to chromosomes 16 and 20 respectively; researchers are working to determine their exact placement.
This shows that an important variety of genetic mutations could be at the origin de ALS. Recent experiments point to a genetic deficiency in a protein, dynein, which could be at the origin of a mechanism of ALS development and other motor neuron diseases.
In 90 % of cases, the disease appears nevertheless without reason and is not hereditary. This frequent form is called sporadic: these cases of ALS appear rarely and more or less isolated and show no hereditary factor. Even if the causes of these sporadic forms are unknown, it is likely that simultaneous anomalies of multiple genes favor predisposition of the disease.
Numerous laboratories thus focus their efforts on the research of other probable genetic mutations involved in ALS. Identification of these mutations should in time allow an understanding of these complex sporadic forms, which are far more frequent and present exactly the same symptoms. These studies could also open the way to new therapies.
Lifestyle and environment
Even if none of them has been incriminated yet, exposure to certain environmental factors could increase an individual’s susceptibility to developing ALS. This include along others:
- Military service
- Considerable physical activity
- Repeated trauma
- Certain agrochemical products
Therefore, in connection with this final factor, a slight rise in the number of ALS cases has been detected in south-west Ontario. The water supply originates in an area of high industrial activity. Such factors as food, alcohol, exercise, nicotine, life in a distant region, exposure to certain work toxins, physical trauma and a preexisting connection to the disease have all been studied. However, to this day, epidemiological research has been unable to connect smoking as a factor which is considered ‘’probable,’’ but ‘’not yet confirmed.’’ (Shaw and Strong, 2003)
Furthermore, risk elevation has been observed among patients with recent mechanical trauma, particularly with electrocution. Certain viruses could also cause certain cases of ALS.
Although no direct environmental cause has been deducted, epidemiological research conducted throughout the world shows consistently that environmental factors can play a role in the breakout of the illness. They could possibly work in connection with motor neuron dysfunction by rendering them susceptible to cellular mechanisms.
Other elements could, on the other hand, have a protective element. Several studies on the impact of diet on the occurrence of ALS suggest that a diet rich in vitamin E and unsaturated fatty acids could reduce the risk of breakout, but no formal link has been established.