Head injury, autism and dementia: A brief conjecture
This is outside my usual wheelhouse, but it came up in my reading and- even if I am ridiculously wrong- I felt it was at least worth circulating the idea. If I am right, it could do us a lot of good to know. This is not medical advice, it is a conjecture, nothing more.
Autism and dementia
There is evidence of increased rates of dementia in people with autism, with the 5 year prevalence of dementia 4x higher in autistic people without intellectual disability than in the general population without intellectual disability:
Participants were 30–64 year-old adults who were Medicaid beneficiaries and had either a diagnosis of ASD only (n = 12,648), a diagnosis of ASD with co-occurring intellectual disability (ID) (n = 26,168), a diagnosis of ID without ASD (n = 406,570), or no ASD nor ID diagnoses (n = 798,828). The 5-year prevalence of dementia was 4.04% among adults with ASD only, and 5.22% for those with ASD and co-occurring ID. This prevalence was higher compared to the prevalence of dementia in individuals with no ASD and no ID (0.97%), but lower compared to individuals with ID only (7.10%).
Why?
Genes
Now there are many possible explanations for this link, and several probably have some truth. Dr B. Blair Braden, in commenting on this issue remarked:
“Studies also show a higher incidence of the APOE4 gene that predisposes people for Alzheimer's disease in later life and earlier shrinking of the hippocampus, the primary memory structure in the brain.”
I am not denying a possible role for genes, but I want to focus on a possible simpler link.
Cognitive reserve
Another factor that the authors of that study are undoubtedly aware of is the issue of cognitive reserve. It is widely held- on the basis of substantial evidence- that individuals with greater intelligence are less likely to develop dementia, and when they do develop dementia, develop it more slowly. This can be explained by a pretty simple analogy- if you cutoff food to a person with more body fat they’ll last longer than if you cutoff food to a skinnier person- so too with dementia. Some intelligent people who maybe have the underlying pathology of dementia are so shielded from the effects due to their extra intelligence that they do not show symptoms before they die of other causes.
Autistic people are widely seen as exceptionally intelligent, and some of us are, but the average IQ of a person with autism is lower than the average IQ of a person without autism. Even if you cut out autistic people with intellectual disabilities, on average the remaining autistics may have a lower cognitive reserve than the neurotypical comparison sample. This is one possible explanation, however it’s not what I’ll be focusing on.
Head injury
The explanation I’m most interested in is not one I was able to find anywhere in the small literature on the topic. It’s the explanation I’ve written this blog post to advance. I think the effect might be driven by head injury.
It is long and well-established that a history of head injury is a powerful risk factor for Alzheimer’s:
https://n.neurology.org/content/35/2/264.short
https://www.tandfonline.com/doi/abs/10.3109/02699059509008194
It appears that people with autism may be greatly more injury prone than people without autism, e.g.
https://ajph.aphapublications.org/doi/abs/10.2105/AJPH.2017.303696
https://onlinelibrary.wiley.com/doi/abs/10.1002/aur.2086
There is some evidence that suggested this injury/autism link may be wholly mediated by attention deficits- that is, the effect of autism on injury risk may be 100% mediated through the connection between autism and ADHD/ADD but even if that’s true, it doesn’t make the association go away.
So my conjecture is simple. I think any connection between autism and dementia may be powerfully mediated by head injury. I could easily be wrong, but a hypothesis like this is very easily checked- run a mediation analysis to see whether the relationship between autism and dementia is heavily statistically mediated by head injuries, TBI’s etc. Maybe one of my readers even has the right data to do it.
There are two important practical implications:
We need to protect children and adults with autism against head injury. This is already known and observed, but if this connection is real, it underlines the point.
As treatments are developed for the aftereffects of head injury (and work is going on in this regard- e.g. for the treatment of former athletes who may be at risk of CTE)- autistic people should be kept in mind as potential beneficiaries.