ALS can be grouped in different ways. Most commonly used are two classifications based on the underlying cause of the disease, either familial or sporadic. Sometimes ALS is also classified by the clinical onset or progression of the disease.
About 10 percent of cases are classified as hereditary or Familial ALS (fALS). Although clinically indistinguishable from Sporadic ALS, Familial ALS or fALS is inherited through an autosomal dominant gene mutation. When we are born, we get one copy of DNA from each parent, but normally only one of these genes expresses itself. The expressed gene is considered the more dominant gene. An autosomal dominant gene is the gene that will always be expressed in a carrying individual. There are several different genes that have been associated with fALS. The first discovered gene is SOD1, which comprises 12 percent of fALS cases. Because there are so many different genes, most cases are not tested for fALS unless there is known history of the disease in the patient's family.
Sporadic ALS (sALS) makes up the other 90 percent of ALS cases in the U.S. Men are one and a half times more likely to be diagnosed with sALS than women. The cause of sALS is not well understood, but research has connected it with various different sources. Around 10 percent of sALS cases have a known genetic mutation. Other correlations are found in environmental factors, such as active duty involvement in U.S. military service and smoking. Other factors such as stress, poor sleep, trauma, and strenuous physical exertion may also be involved in the development of sALS.