It is well known that there is a link between autism and epilepsy. I’ve heard numbers as high as 20% as to the fraction of people with autism who also have epilepsy, but this number appears to be going down with time.

A recent paper has just become available online: Epilepsy in Autismnext term is Associated with Intellectual Disability and Gender: Evidence from a Meta-Analysis.

As the title says, this is a meta-analysis. They used results from a number of previous papers (including one unpublished by one of the authors) to get larger numbers and come to conclusions. As such, I am not going to go into details about the methods, except to say that the studies used go back as far as 1963. That is a huge time during which the understanding of autism has changed quite a bit. I don’t mention that as a limitation, just an observation.

We reviewed all data available from published reports (1963–2006) on previous autism and epilepsy and conducted a meta-analysis of 10 and 14 studies, respectively, to assess the relative risk (RR) of epilepsy in previous autism term according to ID and gender. The pooled groups included 2112 (627 with IQ ≥ 70, 1485 with IQ < 70) and 1530 (1191 male, 339 female) patients, respectively

As you can see, they got a large group by combining studies. This is especially important, I believe, in gathering good statistics for females.

The results are, to me, difficult to read. There is a large increase in the relative risk of ID (intellectual disability) with autism+epilepsy. Also, there is a large relative risk for being female and epilepsy for people with autism.

There was a strong discrepancy in relative risk (RR) according to IQ, with more autistic patients with ID having epilepsy (RR = .555; 95% confidence interval [CI]: .42-.73; p < .001). The pooled prevalence of epilepsy was 21.5% in autistic subjects with ID versus 8% in autistic subjects without ID. There was a strong discrepancy in RR according to sex, favoring comorbidity of epilepsy in autistic girls (RR = .549; 95% CI: .45-.66; p < .001). The male:female ratio of autism comorbid with epilepsy was close to 2:1 whereas the male:female ratio of autism without epilepsy was 3.5:1.

To me, that is enough of a statement. However, the conclusions state that because of this, autism with epilepsy is a subgroup of autism.

The results of this meta-analysis indicate that risk for epilepsy in previous termautismnext term is a function of ID severity and distinguishes previous autism term associated with epilepsy as a subgroup of previous autism term by its male-female ratio.

I include a figure from the paper here, which summarizes the data on IQ and percentage with epilepsy.

Percentage of people with autism who have epilepsy by IQ

Again, to me this is difficult to read. I wouldn’t wish epilepsy on people (and I’m not here!), but it just seems so unfair that those with the greater cognitive challenges should face such a high incidence of epilepsy as well. There has been, as noted above, a trend seen in some data (e.g. the CDDS datasets) showing a decrease in time of the fraction of people who have autism also having epilepsy. This could be due to an increase in time of the fraction of people with autism and less or no ID.

Amiet C, Gourfinkel-An I, Bouzamondo A, Tordjman S, Baulac M, Lechat P, Mottron L, Cohen D. (2008). Epilepsy in autism is associated with intellectual disability and gender: evidence from a meta-analysis. Biological Psychiatry, 54 (7), 577-582