Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder associated with a loss of motor neurons. These neurons carry messages between the brain and the voluntary muscles, and their loss causes worsening motor control, marked by symptoms that include difficulty in eating and breathing, and muscle wasting (atrophy).

ALS can be grouped into different forms. Most commonly, this is based on the underlying cause of the disease, or whether it is sporadic or familial. ALS can also be classified by the clinical onset or progression of the disease.

Classification by underlying cause

Sporadic ALS

Sporadic ALS is the most common form of ALS, accounting for around 90 to 95 percent of all cases in the U.S. This form is diagnosed in patients with no known family members affected by the disease.

There may be genetic factors that can slightly increase the risk of developing sporadic ALS, but they do not directly cause the disease itself.

Familial ALS

Familial, or inherited, ALS runs in families and accounts for around 5 to 10 percent of cases. It can be caused by one of several mutations that are passed from the parents to their children in an autosomal dominant manner. This means that if a parent has familial ALS, there is a 50 percent risk that this will be passed to a child.

Several mutations have been identified and linked to familial ALS, and there may be others still unknown.

The most common defect, associated with 25 to 40 percent of familial ALS cases, is in a gene called C9ORF72. Changes to this gene are also associated with frontotemporal dementia (FTD). People with a mutation in this gene may develop both diseases, referred to as ALS-FTD.

Another familial form of ALS is the ALS-parkinsonism-dementia complex 1 (ALS-PDC) or Lytico-bodig disease, where patients experience both symptoms of ALS, dementia, and Parkinson’s disease. This form is most commonly found in areas of Japan and in Guam.

Other known genes associated with familial ALS include superoxide dismutase 1 (SOD1), TARDBP, FUS, and ubiquilin 2 (UBQLN2).

Classification by clinical onset

Spinal ALS

Around two-thirds of ALS patients have what is called the spinal onset form of the disease. Initial symptoms of this form include muscle weakness or wasting in the arms and legs, and involuntary muscle contractions resulting in twitches. Patients with spinal ALS tend to progress to paralysis or death within three to five years.

This form is also sometimes referred to as classical Charcot ALS.

Bulbar ALS

In the bulbar form of ALS, the muscles involved in speaking, swallowing, and breathing are generally the first to be affected. Initial symptoms include slurry speech and difficulty swallowing. However, muscle weakness can quickly progress to the arms and legs, making it difficult to distinguish between spinal and bulbar ALS.

Bulbar ALS is slightly more common in women, and people over age 70. With bulbar ALS, patients may be paralyzed within one to two years.

Other forms of ALS

Another disease form is called Guamanian ALS and is especially common in people of Guam and the Trust Territories of the Pacific. The exact cause of this form is not known.

Two other rare forms of ALS are known to exist. One is juvenile ALS, an extremely rare form that, as its name implies, affects children, adolescents, and young adults under age 25; the other is primary lateral sclerosis, a very slowly progressing type of ALS.

