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Research Article                                                                                                                                        Open Access

## Injury Patterns among Individuals Diagnosed with Infantile Autism during Childhood: A Case-Control Study

Svend Erik Mouridsen 1* , Bente Rich, Torben Isager 2

1 Child and Adolescent Psychiatry Center, Bispebjerg University Hospital, Copenhagen, Denmark Child and Adolescent Psychiatry Center, Glostrup University Hospital, Glostrup, Denmark

*Corresponding author: svend.erik.birkebaek.mouridsen@regionh.dk

## Abstract

Background: To date, injury risk among people with infantile autism (IA) has been a relatively poorly researched issue.

Objective: The purpose of our study was to compare the prevalence and types of injuries in a clinical sample of 118 patients diagnosed with IA during childhood with those of 336 age- and sex-matched controls from the general population.

Method: All participants were screened through the nationwide Danish National Hospital Register. The average amount of time that the participants were observed was 30.3 years (range, 27.3 to 30.4 years), and the mean patient age at the end of the observation period was 42.7 years (range, 27.3 to 57.3 years).

Results: Among the 118 patients with IA, a total of 52 (44.1%) were registered in the Danish National Hospital Register with at least one injury diagnosis. In the comparison group, 226 of 336 individuals (67.3%) had at least one such diagnosis. The difference is statistically significant ( P < .0001; odds ratio, 0.4; 95% confidence interval, 0.3 to 0.6), and the nature of the injuries also seems to differ. Gender, intellectual level, and concurrent epilepsy were not predictive of injury risk.

Conclusions: Our results lend support to the notion that injuries that require medical attention are not uncommon among an  adult  population  of  people  diagnosed  with  IA  during  childhood,  but  they  are  less  common  than  the  rate  found  in  a comparison group from the general population. It is proposed that a diagnosis of IA is related to the likelihood of institutional care, which may have a protective effect with respect to acute hospital use.

Keywords:

infantile autism; injury; patterns of injury

## Introduction

Infantile autism (IA) belongs to a group of neurodevelopmental disorders that are classified  as pervasive developmental disorders or autism spectrum disorders (ASDs), of which IA forms the main prototype. IA is analogous to childhood autism as defined in the International Statistical Classification of Diseases and Related Health Problems, 10th Revision (ICD10) (1) and  autistic disorder  as defined  in the Diagnostic  and  Statistical  Manual  of  Mental  Disorders, Fourth Edition (2). The diagnosis is based on clinical criteria; people with IA have deficits in communication  skills,  impaired  social  interaction, and restricted interests. IA commonly co-occurs with intellectual disability (3), epilepsy (4), and attentiondeficit/hyperactivity disorder (5). Accumulating evidence indicates that people with these conditions are at higher risk for injury or accidents than people without  these  conditions  (6-8).  In  addition,  the literature  consistently  reports  higher  rates  of  injury among males as compared with females (9).

To maintain the health and quality of life of people with ASD, it is important to minimize their risk of acquiring  new  disabilities.  Understanding  the  risktaking behaviors and injury patterns of people with ASD can help to guide decision making related to long-term planning for individuals with ASD, and it can also have implications for health policy decisions. To  date, the body  of systematic research that addresses injury patterns in people with ASD is small. Two studies  carried  out  in  the  United  States  have assessed the relationship between ASDs and injuries. In their study, Lee and colleagues (10) used parentreported injury data for the most recent 12 months from 82 children with ASD between the ages of 3 and 5 years. The ASD diagnoses of the children were also parent-reported and not validated by clinicians. The studied children all participated in the National Survey  of  Children's  Health.  Results  showed  that participants with ASD were about two times more likely  to  experience accidents that required medical attention as  compared  with  unaffected  controls (24.2% vs. 11.9%). Males were 1.30 times more likely to have had an injury as compared with females. In another study, McDermott  and  colleagues (11) compared differences in the frequencies and types of injuries of 1610 children and adolescents with ASD between the ages of 1 and 18 years old and 91,571 typically developing peers. The authors used administrative  data  from  Medicaid,  which  insured both study groups. The observation period was the 12 months of 2003. The results indicated that the risk of receiving emergency department or hospital treatment for any injury was elevated in individuals with ASD. Participants with ASD had higher rates of head,  face,  and  neck  injuries  and  lower  rates  of sprains and strains. The researchers found this to be in  agreement  with  the behavioral  profile  of  people with ASD:  they  are less likely to compete  in organized sports or to participate in group play, both of which may precipitate sprains and strains.

In the same vein, research suggests that elopement behavior among children with ASD increases their risk of injury or death and places a major burden on families and care professionals. Anderson and colleagues (12) observed that 48% of children with ASD had attempted to elope from a safe environment; this rate was nearly four times higher than that of their unaffected siblings. In particular, participants  with  more  significant  ASD  symptom severity  were  more  likely  to  engage  in  risk-taking behaviors and to sustain frequent injuries. Of those who had run away, 24% were in danger of drowning and  65%  were  in  danger  of  traffic  injury,  which suggests  that  addressing  elopement  behavior  is  an important intervention for many children with ASD. Cavalari  and  Romanczyk  (13)  reported  that  the likelihood of sustaining more frequent injuries increases  with  a  child's  engagement  in  more  risktaking behaviors.

The purpose of the present study is to expand on the  study  of  injury  treatment  among  children  and adolescents with ASD by Lee and colleagues (10) and McDermott and colleagues (11). We have the ability to  study  injury  patterns  in  a  sample  that  includes adults who were originally diagnosed with IA during childhood. On the basis of the outlined literature, we hypothesized that participants with diagnoses of IA would display higher rates of injury as compared with non-autistic community controls who were individually matched for gender, chronological age, place of birth, and  social class.  It  was  further hypothesized  that  male  gender  or  a  concurrent diagnosis of intellectual disability or epilepsy would be associated with an elevated risk for injury.

## Participants and Methods

Case group

The case group was recruited from the population of patients consecutively attending the Departments of Child Psychiatry in the university hospitals of Copenhagen (the  capital  of  Denmark)  and  Aarhus (the  second  largest  city  in  Denmark)  from  1960 through  1984  inclusive.  They  were  selected  at  that time to participate in a study of parental age (14). The clinics provided services to the entire population of Denmark. All patients who were given a diagnosis of childhood  psychosis-the  ICD-8  term  for  ASDs, including borderline autistic condition-were rediagnosed  by  the  first  two  authors  in  1985  in accordance  with  the  ICD-9  criteria  (15).  The  core symptomatology  of  IA  according  to  the  ICD-9  is essentially similar to that later delineated for childhood  autism  in  the  ICD-10  (1).  The  revised diagnosis was based on a review of the patient's child psychiatric  records.  As  a  result  of  this  rediagnosis, 118 individuals  (85  males  and  33 females)  with  IA were identified. These 118 patients make up the case group for the current study. At their index admission during  childhood,  the  mean  age  of  the  case  group was 4.9 years (median, 4.0 years; range, 2 to 15 years). All  but  one  medical  record  included  information about the intelligence quotient (IQ): 53 (45%) individuals had an IQ of less than 50; 30 (25%) had an IQ of between 50 and 69; and 34 (29%) had an IQ of more than 69.

## Comparison group

A comparison group was retrieved from the Danish Civil Registration System. This register was established on April 2, 1968, and all persons alive and living in Denmark were enrolled at that time. Since then, all persons with permanent residence in Denmark have been added to this register, including every live-born baby and every new inhabitant (16). All  persons  in  the  register  are  assigned  a  unique identification number called a CPR number, which is then used in all national registers to enable accurate linkage among them. In the current study, wherever possible, each child with IA was matched with three control children by sex, time of birth (the controls were born the same day or the day before or after the patient), place of birth (region), and social group. In 6 cases, it was not possible to select three matched controls, because the patients in question were born abroad; this led to the loss of 18 controls. Consequently,  the  comparison  group  included  336 participants (246 males and 90 females).

## Hospital register

All  participants  were  screened  through  the  Danish National  Hospital  Register  (DNHR)  (17)  with  the use  of  their  CPR  numbers.  The  register  holds information  about  all  individuals  discharged  from Danish  medical  hospitals  since  January  1,  1977; information  about  visits  to  outpatient  clinics  and emergency  departments  has  been  included  in  the register  since  January  1,  1995.  In  Denmark,  all hospital  and  outpatient  treatment  is  offered  to  all Danish  citizens  regardless  of  their  socioeconomic status,  because  all  treatment  is  paid  for  via  the  tax system. No matter where in the country diagnoses and  treatments  are  given,  all  patient  contacts  are registered in the DNHR.

Members of both groups were followed from their birthdays or January 1, 1977 (whichever came later), until May 15, 2007 (the closing date of the observation period). The average observation period was  30.3  years  (standard  deviation  [SD],  0.4  years; range, 27.3 to 30.4 years), and the mean age at the end of the observation period was 42.7 years (SD, 7.7 years; range, 27.3 to 57.3 years). The mean age at the start  of  the  register-based  observation  period  for injuries  was  12.4  years.  When  calculating  the  mean age at the end of the observation period, we assumed that  an  individual  had  been  alive  throughout  the observation period.

For individuals  registered  in  the  DNHR,  we ascertained  the  place  of  admission,  the  dates  of admission and discharge, the primary diagnosis, and eventually  one  or  more  secondary  diagnoses.  The register provides diagnoses according to the ICD-8 (18) and, after January 1, 1994, the ICD-10 (1).

The  conditions  inquired  about  for  the  current study included injuries, poisoning, and certain other consequences of external causes and epilepsy. Claims with ICD-8 diagnoses codes 800.xx through 999.xx or  ICD-10  diagnoses  codes  S00.x  through  T98.x were used to define these injuries.

We  counted  the  total  number  of  injury-related diagnoses for each participant. The total number of injury diagnoses during the observation period was used  to  classify  each  participant  into  one  of  three groups  by  the  number  of  DNHR  diagnoses:  one diagnosis,  two  to  four  diagnoses,  or  five  or  more diagnoses. In addition, each type of bodily region was counted independently so that a count was given for a specific type (e.g., fracture) or location (e.g., ankle and foot) of injury if the ICD-8 or ICD-10 codes for that type or location of injury were registered during an  episode  of  care.  For  example,  a  person  who experienced  both  an  arm  and  a  foot  injury  on  the same day would be counted in the outcome groups for  both  arm  and  foot  injuries.  Participants  with epilepsy were  identified  with a diagnostic  code related to epilepsy: ICD-8 codes 345.xx or ICD-10

codes G40.x and G41.x. To simplify data presentation,  all  diagnoses  are  listed  as  ICD-10 diagnoses.

## Statistical Analysis

Data handling and statistical analysis were performed with  the  use  of  the  STATISTIX  PC  software packages  (19).  Comparisons  between  groups  were based  on  the  two-sample t -test  for  continuously scaled data and Fisher's exact test for dichotomous (present, absent) data. A probability value of less than 0.01  per  a  two-tailed  test  was  used  to  indicate significant differences between the groups.

## Ethics

The Danish Data Inspectorate approved the study protocol.

## Results

Prevalence of injury

Table  1  summarizes  the  prevalences  and  types  of injuries in the case and comparison groups.

At the end of the observation period, a total of 52 people with IA (35 males and 17 females) were found in  the  DNHR  to  have  diagnoses  of  any  injury variants  (ICD-10  codes  S00  through  T98).  The corresponding  figure  from  the  comparison  group was  226  (169  males  and  57  females).  The  overall prevalence of any injury among patients with IA was 52 out of 118 (44.1%). This is statistically significantly lower than the injury prevalence found in the comparison group, which was 226 out of 336 (67.3%; P <  .0001;  odds  ratio  [OR],  0.4;  95% confidence  interval  [CI],  0.3  to  0.6).  However,  the nature of the injuries seen in these two groups were somewhat different. When males and females were analyzed  separately,  the  difference  was  still  highly significant for males (35 out of 85 [41.2%] vs. 169 out of 246 [68.7%]; P < .0001; OR, 0.3; 95% CI, 0.2 to 0.5) but not for females (17 out of 33 [51.2%] vs. 57 out of 90 (63.3%); P = .30; OR, 0.6; 95% CI, 0.3 to 1.4).

## Number of injury diagnoses

A total of 250 injury diagnoses had been recorded in the  DNHR  among  the  118  individuals  with  IA (mean, 2.1; SD, 5.1; range, 0 to 36). In the comparison group, there were 1101 diagnoses (mean, 3.3; SD, 5.3; range, 0 to 46; two-sample t -test, t = 2.07;  degrees  of  freedom  =  452; P =  .04  [equal variance]). In total, 66 of 118 individuals in the case group  (55.9%)  and  110  of  336  individuals  in  the comparison group (32.7%) had no injury diagnoses recorded  in  the  DNHR  during  the  observation period  ( P <  .0001;  OR,  2.7;  95%  CI,  1.7  to  4.2). Fifteen of 118  individuals with IA  (12.7%)  as compared with 60 of 336 individuals in the comparison group (17.9%) had one injury diagnosis in the register ( P = .25; OR, 0.7; 95% CI, 0.4 to 1.2). The corresponding numbers with respect to two to four  injury  diagnoses  were  23  of  118  individuals (19.5%) and 76 of 336 individuals (22.6%), respectively ( P = .51; OR, 0.8; 95% CI, 0.5 to 1.4). For five or more injury diagnoses, the numbers were 14 of 118  individuals (11.9%) and  90 of 336

individuals (26.8%), respectively ( P = .0008; OR, 0.4; 95% CI, 02 to 0.7). Most participants in the study had a low number of injury diagnoses (i.e., one to four) in the  DNHR. However, participants in the comparison group represented a statistically significant larger proportion of the individuals who had received five or more injury diagnoses.

TABLE 1. Frequencies  and  types  of  injuries  (International  Statistical  Classification  of  Diseases  and  Related  Health Problems, 10th Revision codes in parentheses) in a sample of 118 individuals diagnosed with infantile autism during childhood and 336 matched controls from the general population*

|                                                                                           | Individuals Infantile (n = 118)   | With Autism   | Controls (n = 336)   | P Values   |
|-------------------------------------------------------------------------------------------|-----------------------------------|---------------|----------------------|------------|
| Any injury, poisoning, or certain other consequences of external causes (S00-T98) (%) †   | 52 (44.1)                         |               | 226 (67.3)           | <0.0001    |
| Injuries to the head (S00-S09)                                                            | 25 (21.2)                         |               | 74 (22.0)            | 0.90       |
| Injuries to the neck (S10-S19)                                                            | 4 (3.4)                           |               | 15 (4.5)             | 0.79       |
| Injuries to the thorax (S20-S29)                                                          | 0 (0.0)                           |               | 21 (6.2)             | 0.009      |
| Injuries to the abdomen, lower back, lumbar spine, pelvis and external genitals (S30-S39) | 0 (0.0)                           |               | 13 (3.9)             | 0.05       |
| Injuries to the shoulder and upper arm (S40-S49)                                          | 9 (7.6)                           |               | 27 (8.0)             | 1.00       |
| Injuries to the elbow and forearm (S50-S59)                                               | 8 (6.8)                           |               | 27 (8.0)             | 0.70       |
| Injuries to the wrist and hand (S60-S69)                                                  | 21 (17.8)                         |               | 111 (33.0)           | 0.002      |
| Injuries to hip and thigh (S70-S79)                                                       | 3 (2.5)                           |               | 13 (3.9)             | 0.60       |
| Injuries to the knee and lower leg (S80-S89)                                              | 9 (7.6)                           |               | 71 (20.1)            | 0.001      |
| Injuries to the ankle and foot (S90-S99)                                                  | 11 (9.3)                          |               | 64 (19.0)            | 0.02       |
| Unspecified multiple injuries (T00-T07)                                                   | 2 (1.7)                           |               | 9 (2.7)              | 0.74       |
| Injury of unspecified body region (T08-T14)                                               | 4 (3.4)                           |               | 10 (3.0)             | 1.00       |
| Effects of foreign body entering throughnatural orifice (T15- T19)                        | 6 (5.1)                           |               | 28 (8.3)             | 0.31       |
| Burns and corrosions (T20-T32)                                                            | 3 (2.5)                           |               | 10 (3.0)             | 1.00       |
| Poisoning by drugs, medicaments, and biological substances (T36-T50)                      | 2 (1.7)                           |               | 6 (1.8)              | 1.00       |
| Toxic effects of substances chiefly nonmedical as to source (T51-T65)                     | 5 (4.2)                           |               | 13 (3.9)             | 1.00       |
| Other and unspecified effects of external causes (T66-T78)                                | 2 (1.7)                           |               | 4 (1.2)              | 1.00       |
| Certain early complications of trauma (T79)                                               | 0 (0.0)                           |               | 1 (0.3)              | 1.00       |
| Complications of surgical and medical care, not elsewhere classified (T80-T88)            | 1 (0.8)                           |               | 16 (4.8)             | 0.09       |
| Sequelae of injuries, of poisoning and of other consequences of external causes (T90-98)  | 0 (0.0)                           |               | 17 (5.1)             | 0.02       |

* Different injury types may co-occur in the same individual

†The diagnostic categories shown are those where at least one case or control were represented

## Types of injuries

The injury types listed in Table 1 are not mutually exclusive;  some  participants  had  been  recorded  as having more than one type of injury at the end of the observation period. Most injury types occurred with nearly  the  same  prevalence  in  the  two  groups. However, participants with IA had statistically significantly lower rates for injuries to the thorax ( P = .009), the wrist and hand ( P = .002), and the knee and  lower  leg  ( P =  .001).  In  addition,  there  was  a trend  with  respect  to  a  lower  rate  of  injury  of  the ankle and foot ( P = 0.02) among those with IA.

## Injuries and intelligence

Stratification of the 117 cases with known IQ values (the IQ of one case was unknown) into two groups with IQs of 69 or less (n = 83) or of 70 or more (n = 34) demonstrated no statistically significant difference between the two groups with respect to risk of injury (40 of 83 [48.2%] vs. 12 of 34 [35.3%]; P = .23; OR, 1.7; 95% CI, 0.8 to 3.9). However, there was a tendency for those individuals with IQs of 69 or less to present with injuries more frequently than those participants with IQs of 70 or more.

## Injuries and gender

Among  the  85  males  with  IA,  35  (41.2%)  had received an injury diagnosis. For the 33 females with IA, 17 (51.2%) had received such a diagnosis. The difference  was  not  statistically  significant  ( P =  .41; OR, 0.7; 95% CI, 0.2 to 1.5).

In  the  comparison  group,  169  males  and  57 females had injury diagnoses (169 of 246 [68.7%] vs. 57 of 90 [63.3%]; P = .35; OR, 1.3; 95% CI, 0.8 to 2.1).  Thus, gender was not a statistically significant risk factor with respect to injury diagnosis in either group.

## Epilepsy and risk of injury

Of the 118 individuals with IA, 29 (24.6%) received at  least  one  epilepsy  diagnosis;  5  individuals  in  the comparison group (1.5%) received such a diagnosis. Fourteen  of  the  29  individuals  with  both  IA  and epilepsy (48.3%) were also found to have an injury diagnosis.  In  the  complementary  group  of  the  89 individuals with IA but with no known epilepsy, 38 (42.7%) also had an injury diagnosis ( P = .67; OR, 1.3; 95% CI, 0.5 to 2.9).

In  the  comparison  group,  4  of  the  5  individuals with epilepsy diagnoses (80.0%) were also found to have  no  injury  diagnoses  in  the  DNHR.  In  the complementary  group  of  331  individuals  with  no epilepsy  diagnoses,  221(66.8%)  were  found  in  the DNHR to have injury diagnoses ( P = .67; OR, 2.0; 95% CI, 0.2 to 18.0). Thus, epilepsy did not reach statistical significance or increase odds as a risk factor in either the case group or the comparison group.

## Discussion

To our knowledge, this is the first study to investigate injury patterns in a population of adults with IA. Our study expands on two previous studies (10,11) that observed  that  children  and  adolescents  with  ASD were  two  times  more  likely  to  have  experienced accidents that required medical attention as compared with controls without ASD. Contrary to our hypothesis and the above-mentioned results, we observed that a diagnosis of IA was associated with a lower rate of injury as compared with the rate seen among people of similar age and gender without IA from  the  general  population.  Of  our  118  patients with IA, 44.1% had DNHR records of a diagnosis of any injury; the corresponding rate in the comparison group was 67.3% ( P < .0001). Furthermore, individuals with IA also had a lower mean number of injury diagnoses as recorded in the DNHR (2.1 vs. 3.3 diagnoses; P = .04) than those in the comparison group from the general population. Most participants in both groups were registered in the DNHR with a low number of injury diagnoses (i.e., one to four). However,  participants  in  the  comparison  group represented a significantly  larger  proportion  of  the individuals  that  had  received  five  or  more  injury diagnoses  (11.9%  vs.  26.8%).  Taken  as  a  whole, injuries  that  required  medical  attention  were  not uncommon in an adult population of people who had been diagnosed with IA during childhood, but they were  less  common  than  the  numbers  seen  in  a comparison  group  from  the  general  population. However,  the  nature  of  the  injuries  in  these  two groups  were  somewhat  different.  Injuries  to  the thorax, wrist and hand, knee and lower leg, and ankle and foot were statistically significantly lower among participants from the IA case group.

What can be the explanation for the unexpected low prevalence of injuries and the reduced risks of certain  types  of  injuries  among  the  case  group  of individuals with IA? The reasons are not straightforward.  Variability  in  sampling,  diagnostic criteria, and methodology make it difficult to compare the studies that have been published so far. It  is  of  particular  note  that,  in  Denmark,  hospital treatment is offered to all citizens regardless of their socioeconomic status, because treatment is paid for via  the  tax  system.  Financial  factors  are  thus  less likely to influence pathways to care in this study as compared with studies from many other nations. The Danish setting therefore offers a unique opportunity to  address  the  issue  of  equality  of  treatment  for people with and without mental disorders.

We studied 118 carefully diagnosed people with IA. The two previously published studies (10, 11) included individuals with ASD diagnoses ascertained from  parent-reports  or  Medicaid  records.  We  also studied  a  much  older  age  group  than  did  Lee  and colleagues  (10)  or  McDermott  and  colleagues  (11), who only included participants who were 18 years old or younger. With this in mind, it is important to note that  the  risks  people  face  change  with  age  and  life stage. In other words, the risks that a toddler faces are not the same as the risks that a grandmother faces (20).

In  Denmark, there is a long tradition  of  treating people with IA. This treatment, which is also offered to  people  with  other  developmental  disorders,  is provided through the government social care system, without  charge  to  the  individual  and  without  the availability  of  private  treatment  settings.  From  a previously published study, we have been informed that  many  of  our  participants  with  IA  have  been living  in  long-term  care  settings  for  autistic  people for many years (21). However, the exact number of participants with IA who were living in institutional settings at the end of the observation period is not known, and the duration of their placement in such settings is also unclear. The institutional environment may have a protective effect. A contributory explanation  could  be  that  many  of  these  people's health needs are dealt with within the institution or that there at least may be a greater threshold required before acute hospital care is deemed necessary (22). Consequently, this could bias our observations about lower injury rates. There is research from Denmark that  shows  that  patient  contact  with  hospitals  as either inpatients or outpatients due to injury, poisoning, and certain other consequences of external causes (ICD-10 codes S00 through T98) is different  before  and  after  the  diagnosis  of  ASD. Atladottir and colleagues (23) observed that children with ASD had a significantly higher risk of having hospital contact before the time of the  ASD diagnosis as compared with children of the same age without ASD, but they also had a significantly lower risk  of  hospital  contact  after  the  time  of  the  ASD diagnosis. These results  suggest  that  the  diagnostic process and the implementation of therapeutic and educational interventions may result in safer environments for this population. Another explanation  may  be  that  people  with  IA  are  less inclined to travel around in society and therefore they are  at  lower  risk  for  accidents  and  injuries. For example,  people  with  IA  and  other  disabilities  are unlikely to drive and  may  be  less  involved  in accidents related to work and sport, so they may be at  reduced  risk  for  certain  types  of  injuries,  as suggested  by  our  study.  In  the  2003  US  National Survey of Children's Health, children with ASD were found  to  be  less  likely  to  participate  in  sports  or extracurricular  activities  than  children  with  other developmental  disabilities  (24).  In  line  with  this, Mangerud and colleagues (25) observed that adolescents  with  ASD  had  low  levels  of  physical activity,  and,  of  all  the  assessed  diagnostic  groups (i.e., adolescents with  mood disorders, anxiety disorders, eating disorders, hyperkinetic disorders, or other disorders), the lowest rate of participation in team sports. The significantly reduced prevalence of injuries related to the wrist and hand, the knee and lower leg, and the ankle and foot observed in our case group  may  give  support  to these observations. However, we have no data related to sports and other physical activities, so caution is advised  in the interpretation of these data.

## Risk factors for injury

To reduce injury-related disability, it is important to identify the factors that contribute to an individual's predisposition to it. IA is an example of a disorder that commonly co-occurs with other disorders, such as  intellectual  disability  (3)  and  epilepsy  (4).  These comorbidities  can  potentially  increase  the  risk  of injury among people with ASD (7,8). However, it is uncertain whether an increased risk of injury-if it exists-is related to the IA itself or to the concurrent conditions.  In  our  study,  injury  risk  was  elevated (OR, 1.7) among the patients with IA who also had an intellectual disability (IQ < 70), but the difference did not  reach statistical significance ( P =  .23). Epilepsy occurs in approximately 25% of people with IA,  although  rates  from  5%  to  40%  have  been reported (4), and there is widespread use of anticonvulsant  medications  among  those  who  are affected. Epilepsy may predispose patients to injury as a result of falls and other accidents. In addition, anticonvulsant  treatment  may  interfere  with  bone metabolism, affect bone quality, and increase the risk of  fracture  (8).  However,  epilepsy  was  not  a  risk factor in our study in either the case or the comparison group. Nevertheless, sample sizes were small,  which  raises  the  possibility  that  small  or moderate effect sizes may have been missed.

The literature consistently reports higher rates of injury  among  boys  as  compared  with  girls  (9,10). Contrary  to  our  hypothesis,  the  expected  gender difference in the injury rates was only observed in the comparison group (OR, 1.3), and it was not statistically significant ( P = .43). Among people in the IA case group, more females than males had received a  diagnosis  of  injury  (51.2%  vs.  41.2%; P =  .41). Although the numbers are small, this observation is of particular interest, because several mortality studies have reported that the risk of death is greater among females with ASD than males with ASD (26).

## Methodological considerations

When  considering  the  results  of  our  study,  the strengths  and  weaknesses  of  the  data  must  be recognized.  Our  results  are  based  on  register  data that  cover  an  entire  national  population,  but  the participants  were  not  assessed  face-to-face  by  the authors. It is a major advantage of this study that data were registered in a setting of universal health care, which  virtually  eliminates  the  risk  of  recall  and selection  bias.  In  addition,  the  use  of  register  data provides complete follow-up information about all cases,  without  the  attrition  that  may  limit  personal follow-up  studies. A number of limitations  of  this study should also be considered. Our study focused on  more  severe  injuries,  which  would  be  captured through hospital treatment and emergency department  visits. It  may  be  argued  that  inpatient admissions and, since 1995, outpatient data (including emergency department) are relatively crude measures of the injuries that occurred among the  member  of  the  two  study  groups.  Relatively minor injuries for which no hospital treatment was sought may have escaped our attention and consequently biased our findings toward lower prevalence values for cases as well as controls. It is also important to emphasize that our results mainly apply to injuries among preteens, teens, and young adults: the participants were an average of 12.4 years old when they entered the observation period. Lastly, we acknowledge the important limitation of our data not including information about the patient's activity (e.g., sports participation) when the injury occurred or the location in which the accident took place.

The issues related to the identification of participants with epilepsy have been published elsewhere  (27).  The  validity  of  the  medical  data  is unknown. The DNHR collects data for administrative purposes and not for research; therefore, the quality of the data may differ among hospitals. However, a validation study had estimated the quality of the diagnostic codes and the accident codes to be about 85%, and this has been confirmed (28). In  another study that evaluated the quality of epilepsy diagnoses,  the results  showed  that  the positive predictive value of an epilepsy diagnosis in the DNHR was 81% (29).

In sum, as a consequence of their core symptoms or their accompanying disorders (e.g., epilepsy, learning disability), it would seem that people with IA may be at  increased  risk  for  injury.  Alternatively,  many  of these patients live in institutional environments, and their reluctance to interact with society at large may have a protective effect. Although a few studies have indicated that injury risk is increased among children and  adolescents  with  ASDs,  the  results  from  the current study lend no support to the hypothesis that injury risk is elevated in a population mainly composed  of  adults diagnosed with IA  during childhood.  However,  these  results  may  only  be applicable  in  Danish society,  where  there  is  a  long tradition of treating people with IA in institutional settings.

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