A systematic review of two outcomes in autism spectrum disorder –
epilepsy and mortality
SUE WOOLFENDEN1,2| | VANESSA SARKOZY1,2| | GRETA RIDLEY2| | MICHAEL COORY3,4| |
1 University of New South Wales, Randwick, Sydney, NSW. 2 Sydney Children's Hospital Network, Randwick, NSW. 3 University of Melbourne, Melbourne, Victoria. 4 Murdoch
Children's Research Institute, Parkville, Victoria. 5 Royal Children's Hospital Melbourne, Melbourne, Victoria, Australia.
Correspondence to Dr Sue Woolfenden at Sydney Children's Hospitals Network (Randwick), High St, Randwick, 2031 NSW, Australia. E-mail: email@example.com
Accepted for publication 23 October 2011.
Published online 21st February 2012.
ASDAutism spectrum disorder
SMRStandardized mortality ratio
AIM It has been reported that rates of epilepsy and mortality are higher among the population
with autism spectrum disorder (ASD) than in the general population. The aim of this systematic
review is to provide comprehensive evidence for clinicians, carers, and people with ASD regarding
METHOD Studies were eligible for inclusion if the main focus of the study involved observation
over a period of 12 months or more of an initially defined population (with appropriate diagnostic
label). Studies were also required to have at least 30 participants in order to differentiate case ser-
ies from cohort studies. The Cochrane Database of Systematic Reviews, the Database of Reviews
of Effectiveness, MEDLINE, PsycINFO, EMBASE, and CINAHL were searched. The date of the last
search was September 2010. The risk of bias of included studies was assessed and a meta-analysis
RESULTS Twenty-one studies were identified, 16 measuring the percentage of participants with
epilepsy and five measuring mortality using a standardized mortality ratio. The pooled estimate
for the percentage of participants with epilepsy was 1.8% (95% CI 0.4–9.4%) in studies in which the
majority did not have an intellectual disability and the mean age was <12 years at follow-up, and
23.7% (95% CI 17.5–30.5%) in studies in which the majority did have an intellectual disability and
the mean age at follow-up was more than 12 years. The pooled estimate for the standardized
mortality ratio was 2.8 (95% CI 1.8–4.2).
INTERPRETATION The prevalence of epilepsy is higher among the population with ASD than in
the general population. People with ASD have a higher risk of mortality than the general
population. This has important health promotion implications.
Autism spectrum disorder (ASD) affects between 60 and 70
children per 10 000.1The core features of ASD are severe and
pervasive deficits in social communication and interactions
and restricted, repetitive patterns of behaviour, interests, and
activities.2Males are affected about four times more frequently
than females. Although there is currently no known cause, evi-
dence suggests that the cause is highly genetic with multifacto-
rial risk factors that interact, leading to changes in brain
The International League Against Epilepsy defines epilepsy
as ‘a chronic neurologic condition characterized by recurrent
spontaneous epileptic seizures’.3The lifetime prevalence of
epilepsy in the general population has been reported to range
from 14 to 92 per 10 000 people and the incidence from 32 to
66 per 100 000 person-years.4The prevalence of epilepsy has
been reported to be higher among individuals with ASD, with
the highest prevalence evident in adolescence and young
adulthood.5The incidence and prevalence of epilepsy reported
in studies vary and are dependent on factors such as coexistent
intellectual disability, family history, severe language delay,
underlying genetic conditions⁄syndromes, the age and sex of
the participants in the study, and the severity of autistic fea-
tures.5A recent meta-analysis of 23 studies found a pooled
prevalence of epilepsy of 21.5% (2150⁄10 000) among partici-
pants with autism and intellectual disability compared with
8% (800⁄10 000) among participants with autism but without
intellectual disability.6Being female also increased the risk of
having epilepsy. This meta-analysis predominantly consisted
of cross-sectional studies and included only studies that
reported epilepsy as a function of IQ or sex.
Increased mortality among people with ASD has been
reported in follow-up studies of clinic and population cohorts.
Those with comorbid epilepsy or intellectual disability have
have reported mortality as an outcome in follow-up of a clinic
Clinicians need robust evidence to support the advice that
they provide to families and children with autism about the
ª The Authors. Developmental Medicine & Child Neurology ª 2012 Mac Keith Press
DEVELOPMENTAL MEDICINE & CHILD NEUROLOGYSYSTEMATIC REVIEW
future risks caused by their condition. Epilepsy and mortality
are important outcomes for children affected by ASD and
their families. The purpose of this systematic review is to iden-
tify studies that investigate these outcomes, evaluate their
methodological quality, and describe the occurrence of
epilepsy and mortality of people with ASD.
Types of participants
Studies were included if participants were children who had
received a diagnosis of pervasive developmental disorder, per-
vasive developmental disorder not otherwise specified, atypical
autism, unspecified pervasive developmental disorder, Asper-
ger disorder⁄syndrome, autism, autistic disorder, or childhood
autism. The diagnosis must have been made using a standard-
ized diagnostic instrument or by using established diagnostic
criteria, using an accepted classification system at the time of
DSM III-IV-IV TR9,10or ICD 9–10.11,12A dual diagnosis
(e.g. Asperger disorder and attention-deficit–hyperactivity
disorder, or autism and fragile X) did not prevent inclusion.
Types of studies
Studies were eligible for inclusion if the main focus of the
study involved observation over a period of 12 months or
more of an initially defined population (with appropriate diag-
nostic label). To differentiate case series from cohort studies,
we required included studies to have at least 30 participants.
Types of outcome measures
There were two types of outcome measures: epilepsy and mor-
tality. Epilepsy required a clear history of non-febrile clinical
seizures or ‘epilepsy’ to have been reported in the study during
the period of follow-up. Electroencephalographic changes
alone without clinical seizures were not sufficient.
Mortality as a standardized mortality ratio (SMR) was mea-
sured from a population-based register. The SMR is used to
compare the mortality risk of a study population with that of a
standard population. It is the ratio of the observed to expected
Search strategy for identification of studies
The Cochrane Database of Systematic Reviews, the Database
of Reviews of Effectiveness, MEDLINE, PsycInNFO, EM-
BASE, and CINAHL were searched. The last search date was
September 2010. The methodological search filter ‘prognosis,
sensitive’ for the MEDLINE database, devised by Wilczynski
and Haynes,13was adapted for the databases that did not
include this search filter at the time of the searches. Content
search terms that limited the search to autism and ASD were
also used.1Conference proceedings and dissertation abstracts
were also searched, reference lists of articles identified through
the search strategy were reviewed, and known experts in the
field were contacted. The search strategy was examined by the
search coordinator of the Cochrane Developmental, Psycho-
social and Learning Problems.
Review of studies
The titles and abstracts of all references identified were
screened and non-relevant studies were excluded. The initial
screening of titles and abstracts was performed by two of the
reviewers. A second ‘strict’ screening of titles and abstracts was
performed by three reviewers, with two reviewers assessing
every title and abstract. Potentially relevant articles were then
retrieved for a final screening based on full text assessment by
at least two of three reviewers. Disagreement in all cases was
resolved by consensus, and articles that did not fulfil inclusion
criteria were discarded.
Clinical information that would influence the applicability
and interpretation of findings and is necessary to allow the
assessment of the homogeneity of the studies included in
this review, such as autism diagnostic groups, presence or
absence of intellectual disability in the majority of partici-
pants, age of inception cohort, and duration of follow-up,
The risk of bias was assessed by examining the sample
selected, recruitment method, completeness of follow-up, tim-
ing of diagnosis, and blinding. This was modified from current
literature that addresses the assessment of quality in prognos-
tic systematic reviews.14Studies were assigned as being low
risk if the sample came from a population base, the follow-up
period was prospective, the follow-up was more than 80% of
the sample, the cohort was diagnosed with ASD as stated in
the inclusion criteria at baseline or before recruitment to
study, and blinding of outcomes was adequate.2Studies could
receive a tick, cross, or question mark for each of these criteria.
A study that recruited from a clinic that served a designated
population was given a ‘population’ rating, otherwise a ‘clinic-
based’ rating was applied. Analysis for confounders was not
assessed as we were not investigating predictors of outcomes
and in only one study was there analysis of IQ in relation to
the outcome of epilepsy. The variability in these predictor
measures and data presented meant that the authors did not
feel that a meaningful comparison was possible.
If the information required to make an assessment was not
available in studies published after 2000, the authors were
emailed to ask for further information.
DATA MANAGEMENT AND STATISTICAL ANALYSIS
Data extraction was done by at least two reviewers indepen-
dently. We have presented the information in a way in which
1For full search strategy please email corresponding author.
2For full quality criteria please email corresponding author.
What this paper adds
In this systematic review, the percentage of study participants with ASD who
had epilepsy at follow-up ranged from 1.8% to 23.7% depending on the mean
age of the participants and whether the majority of the study group had an
The expected number of deaths is two to three times higher in populations
with autism spectrum disorder than in the general population.
This systematic review illustrates the need for ongoing health promotion and
surveillance for people with ASD.
variations in similar outcomes can be examined, taking into
account length of follow-up, age at ascertainment, and other
clinically important differences, such as diagnostic group or
presence⁄absence of intellectual disability in the majority of
participants (defined as more than 70% of participants having
an IQ or equivalent measure more than 2SD below the norm)
when that information was available.
To determine pooled estimates of the percentage of par-
ticipants with epilepsy at the end of the follow-up period,
the variances of the raw percentages were stabilized using a
Freeman–Tukey arcsine square root transformation.15Back-
transformation used the harmonic mean of the denomina-
tors.16The I2statistic was calculated as a measure of the
percentage of the overall variation in the pooled estimates
of outcome that was attributable to between-study heteroge-
neity.17We anticipated large heterogeneity between studies
considering the potential variations in baseline characteristics
such as differences in diagnostic groups, intellectual disabil-
ity, and the average age of participants at the end of the fol-
low-up period. The DerSimonian–Laird random effects
model was used to pool the transformed percentages.18This
method regards the studies that were included in the model
as a sample from theoretical potential studies. It implicitly
incorporates uncertainty due to heterogeneity into the confi-
dence intervals (i.e. produces wider confidence intervals than
those produced with fixed-effects methods). Pooled esti-
mates of SMR were also obtained from random effects
models (after first stabilizing the variances with log transfor-
The literature search for this review was completed at two
time points, with the final search completed at the end of Sep-
tember 2010. The combined search yielded 13 293 titles.
Full-text reviews were conducted on 117 promising papers
and 21 studies met the inclusion criteria.
Characteristics of included studies
Sixteen studies measured the percentage of participants with
epilepsy at the end of follow-up as an outcome.8,19–33Seven
studies were of participants classified as having autistic disor-
der (AD) only and the remainder of the studies included
participants from the broader autism spectrum (i.e. autistic
disorder, and/or pervasive developmental disorder not other-
wise specified and⁄or Asperger syndrome). In 11 studies, the
majority of participants had an intellectual disability. Mean
age at baseline ranged from 2 years 7 month to 16 years
11 months. Study duration ranged from 2 to 24 years. In three
studies, the mean age at follow-up was under 12 years. In
terms of risk of bias, two studies had a low risk of bias for all
measures, three studies had one measure that was at high risk
of bias (a retrospective study design or lack of blinding), and
the remainder11had two or more measures that were at a high
risk of bias and⁄or were unclear (Table I).
Pooled estimate of percentage of participants with epilepsy
Figure S1 (supporting information published online) shows
the pooled estimates of the percentage of participants with
epilepsy for the duration of follow-up and the presence or
absence of intellectual disability in the majority of the cohort.
In the one study in which the majority of participants did not
have an intellectual disability and the age at follow-up was
under 12 years,211.8% (95% confidence interval [CI] 0.4–
9.4%) had epilepsy at follow-up. For the four studies in which
the majority of participants did not have an intellectual disabil-
ity and the age at follow-up was 12 years or more,19,24,25,28the
pooled estimate of the percentage of participants having
epilepsy at follow-up was 8.9% (95% CI 3.7–15.7%). Of these
papers, two gave separate data for sex,21,25with 2% to 5.2% of
females in the sample having epilepsy, in contrast to 0% to
3.9% of males.
Table I: Characteristics of the included studies on the outcome of epilepsy in ASD
Risk of bias
Mean age at
(SD or range)
ASD, autism spectrum disorder; AD, autistic disorder.
Developmental Medicine & Child Neurology 2012, 54: 306–312
For the two studies in which the majority of participants did
have an intellectual disability and the age at follow-up was
under 12 years,30,31the pooled estimate of the percentage of
participants having epilepsy at follow-up was 6.1% (95% CI
3.8–9.0%). For the nine studies in which the majority of
participants did have an intellectual disability and the age at
follow-up was 12 years or more,8,20,22,23,26,27,29,32,33the pooled
estimate of the percentage of participants having epilepsy at
follow-up was 23.7% (95% CI 17.5–30.5%) with high hetero-
geneity. Of these papers, three gave separate data for sex,8,22,27
with 3% to 10% of females in the sample having epilepsy, in
contrast to 13% to 29% of males (Fig. S1; Table II).
Characteristics of included studies
Five population-based studies measured the SMR. Two of
these studies were from the same population-based register in
the USA, the California Developmental Disability System, at
different time periods, the first being 1983 to 199725and the
second being 1998 to 2002,34so there would have been some
overlap of participants between these two studies. The other
three studies were from population-based registers in Sweden
(2) or Denmark (1).35–37The two Swedish studies measured
mortality as an outcome for the same group of 341 partici-
pants from the same population-based register but at different
time periods.36,37The mean age of participants at baseline
ranged from 8 years 6 months to 11 years. The mean duration
of follow-up ranged from 14 to 36 years. All of these studies
had a low risk of bias for all measures. They had between 120
and 13 111 participants. There appeared to be no trend in
SMR over time (Table III).
Risk factors identified for increased risk of mortality in the
participant studies were moderate to profound intellectual dis-
ability, having epilepsy, and female sex. In the study by Shav-
elle et al.,25the SMR for participants with no or mild
intellectual disability was 1.4, compared with 3.1 for those
with moderate, severe, or profound intellectual disability. Gill-
berg et al.35found a significant increase in the proportion of
deaths among the group of children with severe intellectual
disability and any medical disorder (with or without epilepsy).
However, no difference in mortality risk between those with
or without an intellectual disability was noted by Mouridsen
et al.36Shavelle et al.25also reported that seizures were associ-
ated with an SMR of 36.9, and other medical conditions were
also associated with an increased SMR. Mouridsen et al.36
reported an SMR of 35.0 associated with epilepsy. All studies
found an increased risk of death associated with female sex. As
in other chronic conditions, the relative disparity (as measured
by the SMR) was highest in the youngest age groups.34,36
However, as for other chronic diseases, and for the population
generally, the absolute risk of death for people with autism will
increase as they age into middle adulthood and beyond.
Causes of death (Table IV) were categorized as physical,
accidental, and suicide. Epilepsy accounted for only 7% to
30% of the deaths; a wide range of other conditions were also
found to cause death, including circulatory, malignancy, and
respiratory conditions, and external causes including drown-
ing, motor vehicle accidents, and suffocation played a role. It
Table II: Pooled estimates of epilepsy percentage and heterogeneity statistics for subgroup analyses
Pooled estimate of
epilepsy, % (95% CI)I2(%)Qa(df)p-value
<70% with IQ <70; mean age <12y
<70% with IQ <70; mean age +12y
>70% with IQ <70; mean age <12y
>70% with IQ <70; mean age <12y
aCochran’s Q statistic; df, degrees of freedom; NA, not applicable.
Table III: Characteristics of the studies on the outcome of mortality in ASD
Denmark 1999 9.5 (2–17)
201011 (4–18) ASD
Denmark 2008 9.5 (2–17)ASD5⁄5Yes35.51960–20062613.51.9 (1.3–2.8)
2001 8.5 (2–15)
2.6 (2.1–3.2)– 78
USA– 8.5 (2–15) ASD5⁄5n191983–2002 280114.22.5 (2.2–2.8)
SMR, standardized mortality ratio; CI, confidence interval; ASD, autism spectrum disorder.
is unclear if accidental suffocation was synonymous with sud-
den unexplained death in epilepsy.
Pooled estimate of standardized mortality ratio
It was not sensible to statistically pool all five of the mortality
studies because of overlapping populations (Table III). We
therefore combined the three most recent studies with mutu-
ally exclusive populations: those by Pickett et al.,34Gillberg
et al.,35and Mouridsen et al.36The resultant pooled SMR was
2.8 (95% CI 1.8–4.2) (Fig. S2, supporting information pub-
lished online). Among males, the overall SMR was 2.1 (95%
CI 1.7–2.7) with minimal heterogeneity (I2=0.0%), and among
females the overall SMR was 7.2 (95% CI 3.0–17.2) with high
heterogeneity (I2=77.3%). For females, the Gillberg et al.35
study reported an extremely large SMR of 20.7 (95% CI
7.6–45.0), but the SMRs obtained by Mouridsen et al.36(4.0;
95% CI 1.7–7.9) and Pickett et al.34(5.2; 95% CI 3.0–8.4)36
were also higher than the SMR for males.
Combining the three studies of Pickettet al.34Gillberg
et al.,35and Mouridsen et al.36produced an overall SMR of
2.8 (95% CI 1.8–4.2) with high heterogeneity (Fig. S2).
When a child is diagnosed with an ASD, his or her parents
want and need clear and accurate information regarding possi-
ble long-term outcomes associated with this complex neuro-
developmental condition. This systematic review of the
literature examined the outcomes of epilepsy and mortality to
provide the best currently available evidence on these serious
outcomes. It has also provided future directions for research in
terms of what is needed in future studies to help fill our
To provide high-quality evidence regarding the outcomes
of ASD, we applied well-described methodological approaches
that have been used in other systematic reviews of prognosis
and outcome studies.14First, information should be collected
prospectively on a sample of children who are diagnosed
according to best practice at the start of the study. Of the stud-
ies that met the inclusion criteria for this review, less than
one-third were retrospective in follow-up design, with all chil-
dren meeting DSM or ICD diagnostic criteria at the begin-
ning of the study. In addition, in 80% of the studies there was
adequate follow-up, with over 80% of the original sample
traced. All the identified mortality studies used population-
based samples; however, the majority of the epilepsy studies
used a clinic-based sample, which has a potential impact on
the applicability of the study results for practitioners if they
are working in different clinical environments. In over half of
the studies there was either no blinding or it was unclear from
the paper whether there was blinding of outcome assessors,
although one can argue that this a less important source of bias
when objective outcomes such as epilepsy or mortality are
This systematic review found that the overall percentage of
participants with epilepsy at follow-up ranged between 1.8%
in participants aged under 12 years, the majority of whom did
not have an intellectual disability, and 23.7% of participants
Table IV: Findings of the three most recent studies on causes of death in the population with ASD
Cause of death
Epilepsy or neurological
External (accidental ordeliberate) (%)
SUDEP 3⁄9 (30)
Cerebral infection 1⁄9 (11)
Accident 1⁄9 (11)
Unknown 1⁄9 (11)
Cardiac 6⁄26 (23)
Epileptic attack 4⁄26 (15)
Meningitis 1⁄26 (4)
Pneumonia 4⁄26 (15)
Accidental overdose 1⁄26 (4)
Drowning 1⁄26 (4)
Jump 1⁄26 (4)
Deliberate overdose 1⁄26 (4)
Malignancy 2⁄26 (8)
Acute appendicitis 1⁄26 (4)
Urethral bleed 1⁄26 (4)
Unknown 1⁄26 (4)
Seizures 15⁄202 (7)
Diseases of the nervous
and sense organs 10⁄202 (5)
Respiratory 13⁄202 (6)
Suffocation 8 (4)
Drowning 11⁄202 (5)
Other external causes 30⁄202,
mostly motor vehicle accidents (15)
Cancer 21⁄202 (10)
Digestive 13⁄202 (6)
All other causes not
previously listed 43⁄202 (22)
SUDEP, sudden unexpected death in epilepsy.
Developmental Medicine & Child Neurology 2012, 54: 306–312
aged over 12 years, of whom the majority did have an intellec-
tual disability. These are significantly greater percentages than
those reported in the literature for the general population, but
are similar to those found for intellectual disability.38,39Our
findings were also consistent with a previous systematic review
that showed that the pooled prevalence of epilepsy was 21.5%
among participants with autism and an intellectual disability
compared with 8% among participants with autism without
intellectual disability.6Of interest, our systematic review
included only one study20that overlapped with the review by
Amiet et al.6and identified an additional 15 studies. One could
conjecture that the increased rates of epilepsy seen in people
with ASD is a function of their comorbid intellectual disabil-
ity; however, it is interesting to note that both systematic
reviews found a higher prevalence among those without intel-
lectual disability as well as in comparison with the general
population. Amiet et al.6in their systematic review argue that
this may be attributable to the heterogeneous nature of ASD
and that differing neurobiological and genetic processes in the
pathogenesis of ASD result in subgroups with a greater or
lesser risk of epilepsy, independent of comorbid intellectual
disability. As our systematic review did not investigate predic-
tors, we were unable to investigate these possible relationships
This systematic review found that people with ASD have
SMRs ranging from 1.9 to 5.6. Overall, the SMR when all
studies were combined was 2.8 (95% CI 1.8–4.2). This means
that the expected number of deaths among the population
with ASD is approximately two to three times higher than that
among peers of the same age and sex in the general popula-
tion. In our systematic review, we found that the SMR to be
higher for females than for males. Our findings are similar to
the all-cause mortality rates among the population with intel-
lectual disability alone, which is reported to be up to three
times higher than among the general population, with mortal-
ity being particularly high among young adults, women, and
people with Down syndrome.40Epilepsy as a condition in its
own right has also been found to increase mortality rates, par-
ticularly when there is comorbid intellectual disability and⁄or
recurrent seizures.41,42However, the causes of death usually
reflect patterns of morbidity in the general population.42This
was the case in our systematic review, in which, although epi-
lepsy was responsible for 7% to 30% of deaths, it is worth not-
ing that the causes of death were heterogeneous and reflected
the wide range of causes found in the general community,
which emphasizes the importance of general health promotion
strategies for people with ASD around maintaining health and
This systematic review was limited by the available studies,
which measured only the patterns of prevalence of the out-
comes in clinical subgroups categorized by, for example, sex,
age, and intellectual disability (in which case primary studies
could be grouped) rather than predictors of outcomes. Any
differences between these subgroups cannot be analysed in any
further depth in terms of the relationship between possible
predictors and the outcomes studied. This was in part because
the relationship between potential predictors and outcome
was not adequately reported or available in the primary studies
and partly because there was great variability in how potential
predictors were measured in the studies included. Analysis of
confounders was not assessed as we did not investigate predic-
tors of outcomes.
The challenge for clinicians, parents⁄carers, and individuals
with ASD is how to sensibly use this evidence that people with
ASD are at an increased risk of having epilepsy or dying rela-
tive to comparison individuals without ASD. We would argue
that this information alerts us to the need for health promo-
tion and regular health surveillance of individuals with ASD,
especially as children and adolescents with ASD transition into
adulthood. In particular, we would advocate that clinicians be
alert for any new signs or symptoms that could indicate the
emergence of a physical or mental health problem, and
support those with autism and their carers to attend regular
review of their well-being. Increased vigilance with injury
prevention and encouragement of other health-promoting
activities such as smoking cessation and an active lifestyle is
also required. This information is important to parents, clini-
cians, and those developing services to cater for the needs of
children with ASD across their lifespan. However, risk of bias
among the studies published to date and the relative lack of
information about outcomes in clinically important subgroups
of children mean that we are a long way from offering families
high-quality information about the risk of adverse or favour-
able outcomes for their child.
For researchers, we recommend that both epilepsy onset
and resolution be reported. In addition, epilepsy occurrence
should be reported in association with duration of follow-up
for each child, and survival curves could be used for this. This
would provide more accurate information about the peak age
of diagnosis of epilepsy and likelihood of resolution.
We would like to thank the Early Years Research Group in Sydney
South West for their support of Dr Ridley. This review was funded
partly through the Financial Markets Foundation for Children.
Supporting information may be found in the online version of this
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