Although the genetic transmission mechanisms common to attention deficit hyperactivity disorder (ADHD) and Alzheimer’s disease are far from proven, a team of researchers has just pointed to a link between the two diseases by studying a cohort of more than two million people linked to their biological parents.
Attention deficit hyperactivity disorder is a behavioral disorder in childhood that can persist into adulthood. It affects 5% of children worldwide and 2.5% of adults. It is characterized by disproportionate symptoms of inattention, impulsivity, and motor agitation (hyperactivity). Alzheimer’s disease is a neurodegenerative disease characterized by cognitive impairment and loss of independence leading to dementia. It is associated with aging and can appear from the age of 60 (prevalence worldwide is between 5 and 7% in people over 60 years of age).
To what extent are ADHD and Alzheimer’s disease hereditary?
Attention deficit hyperactivity disorder (ADHD) is a complex neurological disorder in the development of which many factors are involved. It is a hereditary disease and several genes seem to be involved. On the other hand, inheritance of ADHD remains rare, with less than 1% of cases being associated with a genetic cause.
The possible mechanisms of the relationship between the two disorders have not been established, mainly because the diagnosis of ADHD is difficult, although awareness and knowledge of the disorder have increased in recent years.
Common heredity has been hypothesized, although no common genetic variants have been found. However, a 2015 study published in the American Journal of Medical Genetics suggests that nucleotide polymorphisms may play a role in the molecular basis of ADHD in adults. Furthermore, these variations are thought to be located in or near genes involved in neurodegenerative disease processes.
The relationship between attention deficit hyperactivity disorder and Alzheimer’s disease
A multigenerational cohort study has been conducted in Sweden and published in The Journal of the Alzheimer’s Association. The aim was to find out whether older relatives of people with ADHD had been diagnosed with dementia. Using several national registries – which report births, treatment data, and deaths – more than 2 million people born between 1980 and 2001 were followed, of whom more than 3% were diagnosed with ADHD. These individuals were then linked to their biological relatives (parents, grandparents, aunts, and uncles) to trace possible familial links between ADHD and Alzheimer’s disease.
The authors showed an association between ADHD and Alzheimer’s disease across generations. Relatives of people with ADHD have an increased risk of developing Alzheimer’s disease compared to relatives of people without ADHD. This association decreases with decreasing genetic relatedness (second-degree relatives).
Although no causal relationship was demonstrated in this study, the authors suggest mechanisms to explain these associations. This study hypothesizes genetic variants that have not yet been identified, environmental risk factors, or the impact of ADHD on physical health that may promote dementia. The researchers also mention their interest in exploring these different pathways in future research.