Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative condition that affects nerve cells in the brain and spinal cord. These motor neurons are responsible for controlling voluntary muscle movement throughout the body. As the disease advances, these nerve cells gradually deteriorate, leading to muscle weakness, atrophy, and eventually paralysis.
General Prevalence of ALS
ALS is generally considered a rare neurological disorder, with its occurrence varying across different populations. The incidence rate, which refers to new diagnoses, typically ranges from 1.5 to 3 new cases per 100,000 people each year in the United States and Europe. The prevalence, indicating the total number of people living with the condition at a given time, is estimated to be between 5.2 and 9.1 cases per 100,000 people in the U.S.
The average age for an ALS diagnosis is around 55 years, with most individuals developing symptoms between the ages of 40 and 70. The likelihood of developing ALS increases with age, peaking in those aged 60 to 79.
ALS in Younger Adults
While ALS is more commonly diagnosed in older individuals, cases do occur in younger adult populations, including those in their 20s and 30s. These instances are often referred to as “young-onset ALS” or “early-onset ALS,” typically defined as symptoms appearing before the age of 45 or 50.
Data indicates that only about 13% of all ALS patients develop symptoms before the age of 40. The age group of 18 to 39 years shows incidence rates of approximately 0.1 per 100,000 people. The prevalence for this younger demographic is also low, ranging from 0.5 to 1.2 cases per 100,000 individuals. Young-onset ALS often progresses at a slower rate than cases diagnosed in older individuals, contributing to longer survival times for some patients.
Factors Influencing Age of Onset
The age at which ALS symptoms first appear can be influenced by various factors, including whether the disease is sporadic or familial. Most ALS cases, about 90% to 95%, are classified as sporadic, meaning they occur without a known family history of the disease or a clear genetic cause. Sporadic ALS generally has a later age of onset, typically in the late 50s to early 60s.
In contrast, familial ALS accounts for a smaller proportion, about 5% to 10% of all cases, and is hereditary. Individuals with familial ALS often experience an earlier age of onset, frequently in their late 40s or early 50s, which is about five years younger than the average for sporadic cases. This earlier onset in familial forms is directly linked to specific gene mutations inherited from parents.
Mutations in genes such as C9orf72 and SOD1 are among the most common genetic causes identified in familial cases. Research indicates that carrying multiple rare mutations in genes associated with ALS can also contribute to a significantly earlier age of disease onset.