Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig’s disease, is a progressive neurodegenerative condition that attacks motor neurons in the brain and spinal cord. This loss leads to progressive muscle weakness, speech difficulties, and eventually affects breathing. The risk of a child developing ALS when a parent is diagnosed is not uniform for every family. Assessing personal risk requires understanding the distinction between the two main classifications of the disease.
Sporadic Versus Familial ALS
ALS is categorized into two primary forms based on family history: sporadic and familial. Determining which category an affected parent’s case falls into is the first step in estimating a child’s risk.
Sporadic ALS (SALS) is the most common form, accounting for 90% to 95% of all diagnoses. SALS occurs randomly in individuals who have no known family history. While the exact cause remains unknown, it involves a complex interaction between genetic predispositions and environmental factors. For the majority of people with SALS, the risk of passing the disease to their children is near the general population risk.
Familial ALS (FALS) represents the remaining 5% to 10% of cases. FALS is defined by having two or more family members diagnosed with the disease. In FALS, a specific genetic mutation is passed down, making the condition hereditary. This clear inheritance pattern significantly elevates the genetic risk for first-degree relatives.
Determining Your Specific Inheritance Risk
The specific risk depends entirely on whether the mother’s case is sporadic or familial. If the mother’s ALS is classified as sporadic, the risk of her child developing the disease is low. While about 10% of SALS cases involve a genetic mutation, the risk of transmission remains negligible for the vast majority.
If the mother’s diagnosis is FALS, the inheritance risk is substantially different and follows a predictable pattern. Familial ALS is most often inherited in an autosomal dominant manner. This means the gene mutation is on a non-sex chromosome, and only one copy of the mutated gene is necessary to cause the disorder.
This autosomal dominant pattern results in a 50% chance for the affected parent to pass the gene mutation to each child. The most common genes associated with FALS include C9orf72, SOD1, and TARDBP. The specific gene involved can influence the disease characteristics; for instance, C9orf72 mutations are the most frequent genetic cause globally and are also linked to frontotemporal dementia.
The only way to confirm a child’s specific statistical risk is to determine the genetic status of the affected parent. If the parent tests positive for a known ALS-related mutation, their child faces that 50% risk of inheriting the mutation. However, inheriting a mutation does not guarantee the disease will develop, a concept known as reduced penetrance.
Genetic Counseling and Testing Options
For individuals concerned about elevated risk due to a family history of ALS, engaging with a genetic counselor is a prudent first step. A genetic counselor is trained to interpret complex family histories and assess the likelihood of FALS. They construct a detailed family tree, or pedigree, which helps clarify the inheritance pattern and informs the risk assessment.
Counselors explain the options for genetic testing, which fall into two main categories. Diagnostic testing is performed on the affected parent to identify a specific gene mutation associated with their FALS. If a mutation is successfully identified, it paves the way for predictive testing in asymptomatic family members.
Predictive testing is for an unaffected adult to determine if they inherited the specific family mutation. This testing is often a profound personal decision with significant emotional and ethical implications. A genetic counselor will carefully discuss the potential impact of a positive result on career, family planning, and mental health before any testing is initiated. The support and guidance provided by a counselor are designed to help individuals make an informed choice that aligns with their personal values and circumstances.