# The impact of comorbidity of intellectual disability on estimates of autism prevalence among children enrolled in US special education

## TL;DR

This study shows that the rise in autism prevalence among children in US special education may be partly due to reclassifying cases of intellectual disability as autism.

## Contribution

The study highlights how comorbid intellectual disability may influence autism diagnosis and prevalence estimates.

## Key findings

- There was a 331% increase in autism prevalence from 2000 to 2010 in special education data.
- The decrease in ID prevalence accounted for 64.2% of the increase in autism prevalence.
- Older children were more likely to be recategorized from ID to autism than younger children.

## Abstract

While recent studies suggest a converging role for genetic factors towards risk for nosologically distinct disorders including autism, intellectual disability (ID), and epilepsy, current estimates of autism prevalence fail to take into account the impact of comorbidity of these neurodevelopmental disorders on autism diagnosis. We aimed to assess the effect of potential comorbidity of ID on the diagnosis and prevalence of autism by analyzing 11 years of special education enrollment data.

Population study of autism using the United States special education enrollment data from years 2000-2010.

US special education.

We analyzed 11 years (2000 to 2010) of longitudinal data on approximately 6.2 million children per year from special education enrollment.

We found a 331% increase in the prevalence of autism from 2000 to 2010 within special education, potentially due to a diagnostic recategorization from frequently comorbid features like ID. In fact, the decrease in ID prevalence equaled an average of 64.2% of the increase of autism prevalence for children aged 3-18 years. The proportion of ID cases potentially undergoing recategorization to autism was higher (p=0.007) among older children (75%) than younger children (48%). Some US states showed significant negative correlations between the prevalence of autism compared to that of ID while others did not, suggesting differences in state-specific health policy to be a major factor in categorizing autism.

Our results suggest that current ascertainment practices are based on a single facet of autism-specific clinical features and do not consider associated comorbidities that may confound diagnosis. Longitudinal studies with detailed phenotyping and deep molecular genetic analyses are necessary to completely understand the cause of this complex disorder. Future studies of autism prevalence should also take these factors into account.

We present a large-scale population study of autism prevalence using longitudinal data from 2000-2010 on approximately 6.2 million children enrolled in US special education.

We provide one possible but compelling explanation for increase in autism prevalence and show that current ascertainment of autism is based on single facet of clinical features without considering other comorbid features such as intellectual disability

We are not able to dissect the exact frequency of comorbid features over time as US special education allows for enrollment under only one diagnostic category and does not document comorbidity information.

This study examines how comorbidity of related phenotypes such as ID can impact estimates of autism prevalence and does not assess other factors reported to impact autism prevalence.