Abstract

Dysphagia (feeding and swallowing disorder) is associated with serious health complications and psychosocial sequelae. This review summarizes international research relating to the prevalence of dysphagia in people with intellectual disability. Studies published from 1990 to July 2016 were identified using Medline, Cinahl, PsycINFO, Web of Science, email requests, and cross-citations. Twenty studies were identified. Dysphagia in people with intellectual disability appears to be associated with more severe levels of intellectual disability, comorbid cerebral palsy, and motor impairments. However, further research with representative samples of people with intellectual disability using adequate methods of assessment are required in order to provide more precise prevalence estimates and clarify factors that may be associated with dysphagia in this population.

Dysphagia (feeding and swallowing disorder) is associated with many health complications. Aspiration pneumonia is considered by many to be the main complication of dysphagia and is of significant concern due to its link with subsequent morbidity and mortality (Heslop et al., 2014; Hollins, Attard, von Fraunhofer, McGuigan, & Sedgwick, 1998; Martino et al., 2005). Other health complications include choking and airway blockage (National Patient Safety Agency, 2007; Samuels & Chadwick, 2006) and compromised nutritional status and dehydration (Kennedy, McCombie, Dawes, McConnell, & Dunnigan, 1997; MacDonald, McConnell, Stephen, & Dunnigan, 1989). In addition, urinary tract infections, headaches, constipation, oesophagitis, and reduced ability to combat infections have all been associated with dysphagia (Cichero, 2006; Eustace, Gui, & Iacucci, 2015; Robbins et al., 2008).

In addition to health complications, the psychosocial impacts of dysphagia include loss of opportunity for communication during meals, increased stigma when eating in community settings, and loss of dignity associated with being supported to eat and drink (Chadwick, Jolliffe, Goldbart, & Burton, 2006; Miller, Noble, Jones, & Burn, 2006). Further, reduced choice may occur alongside a managed eating and drinking regime, with an associated loss of enjoyment of meals and drinks due to dietary modification (e.g., thickening drinks, mashed food, and food exclusions) reducing individual quality of life and well-being.

The association between dysphagia, its complications, and mortality appears pronounced in people with intellectual disability (ID). Respiratory disease, particularly bronchopneumonia, is a leading cause of death in people with ID, particularly in those with profound intellectual and multiple disabilities (PIMD), accounting for significantly more deaths than in the local general population (Hollins et al., 1998; Patja, Mölsä, & Iivanainen, 2001). Preventable lung inflammation caused by solids or liquids and foreign bodies has also been associated with mortality in people with ID (Glover & Ayub, 2010). The importance of dysphagia in relation to the well-being of people with ID led the United Kingdom National Patient Safety Agency (NPSA) to identify swallowing difficulties as one of five priority areas in relation to safety risks for people with ID using healthcare services (National Patient Safety Agency, 2004), and there has been a call for research investigating mealtime safety incidents involving people with intellectual and developmental disabilities (Hemsley, Balandin, Sheppard, Georgiou, & Hill, 2015).

In addition, the issue of dysphagia in people with ID may be complicated by medical co-morbidities, psychiatric, communicative, cognitive, and behavioral issues. For example, there is a link between the side effects of neuroleptic medications and dysphagia (Dziewas et al., 2007), and people with ID are more likely than others to be prescribed these (e.g., antipsychotic medication; Glover et al., 2015). Further, specific syndromes associated with ID can result in both anatomical and neurological precursors for dysphagia, including Down syndrome (O'Neill & Richter, 2013; Smith, Teo, & Simpson, 2014), Rubinstein-Taybi syndrome (Shashi & Fryburg, 1995), and Rett syndrome (Abraham, Taragin, & Djukic, 2015).

The issues outlined above indicate the importance of considering dysphagia specifically in relation to people who have ID. Internationally, there appears to be a lack of research investigating the prevalence of dysphagia in people with ID (Chadwick & Jolliffe, 2009; Leslie, Crawford, & Wilkinson, 2009). A systematic review of 189 studies on dysphagia found that two-thirds of the literature focussed on adults over 50 years of age (Roden & Altman, 2013), with only two studies (Calis et al., 2008; Henderson et al., 2009) focusing on people with ID. Further, although there have been systematic reviews conducted in relation to dysphagia and people with cerebral palsy (e.g., Benfer, Weir, & Boyd, 2012; Hirata & Santos, 2012), it cannot be assumed that people with cerebral palsy have ID (Enkelaar, Ketelaar, & Gorter, 2008). No reviews exist specifically relating to people with ID and dysphagia.

The prevalence of dysphagia varies depending on the diagnostic instrument used, concomitant medical disorders, and the population studied (Wilkins, Gillies, Thomas, & Wagner, 2007). For the general population, literature reviews have reported prevalence rates ranging from 2.3–16% (Kertscher, Speyer, Fong, Georgiou, & Smith, 2015) and 1.7–11.3% (Roden & Altman, 2013), with prevalence being higher in older people (Kertscher et al., 2015; Roden & Altman, 2013). Although ID is a risk factor for dysphagia, with increased likelihood of dysphagia occurring with increasing severity of cognitive impairment (Chadwick & Jolliffe, 2009; Parkes, Hill, Platt, & Donnelly, 2010), it has been noted that the prevalence of dysphagia in populations with ID remains unknown (Leslie et al., 2009). Accurate estimates of dysphagia prevalence are important, allowing services and policy makers to be better informed when planning associated health and social care resources and raising awareness of this health issue in people with ID to prevent it from being overlooked. We conducted a systematic review of studies that include information on the prevalence of dysphagia in groups of people with ID published from 1990 to July 2016. The purpose of this article is to present the findings from our review, and to address the question of what is known about the prevalence of dysphagia in groups of people with ID.

Method

Electronic literature database searches were conducted in Medline, Cinahl, and PsycINFO (all on EBSCO) and Web of Science (SCI-EXPANDED, SSCI, and A&HCI) in June 2015 and subsequently updated in July 2016. Searches combined terms for dysphagia and ID with the Boolean operator “and.” An example of database-specific search terms (Medline) is given in the Appendix. Searches included broad terms relating dysphagia and people with ID to create a pool of studies, with studies on topics other than prevalence being retained for separate review. Specific criteria relating to prevalence studies were applied to this pool of studies. The reference lists of studies meeting the inclusion criteria were searched. In addition, in June 2015, a request for information on research relevant to dysphagia and people with ID was sent to members of the International Association for the Scientific Study of Intellectual and Developmental Disabilities (IASSIDD) Health Special Interest Research Group and the Intellectual Disability UK Research mailing list, with the request subsequently being published in the TAC Bulletin in October 2015 (www.teamaroundthechild.com).

Study Selection

Inclusion criteria

Studies were required to meet all of the following criteria: peer reviewed; published between 1990 and 2016; English language full text; primary research; samples of people with ID, samples where at least 50% of the sample (e.g., people with cerebral palsy) are explicitly noted to have ID, or mixed samples where results are disaggregated for people with ID; and findings that include exact figures on the number of people with dysphagia or indicators of dysphagia such as particular items on the screening tool of eating problems (STEP).

Exclusion criteria

Studies published before 1990 were excluded as they predate major changes in service provision for people with ID (Emerson, 2004). Additionally, the following exclusion criteria were applied: not peer reviewed or peer review status unclear; reviews, letters, commentaries, editorials, or meeting or conference abstracts; only included information relating to specific syndromes associated with ID (e.g., Rett syndrome), with the exception of Down syndrome, which is the most common genetic cause of ID (Sherman, Allen, Bean, & Freeman, 2007); conditions where ID cannot be assumed (e.g., cerebral palsy, autistic spectrum disorder) unless the proportion of the sample with ID is explicitly reported to be above 50% or results are disaggregated for people with ID; study relates solely to infants (less than 1 year of age).

Initially, titles and abstracts were used (by the first author) to exclude studies that were obviously not within scope. Those retained for further screening were those for which relevance could not be assessed without accessing full text, or those that were chosen as being potentially within scope. These studies were screened by the first and second author and discussed until consensus was reached on whether or not they met the specific inclusion criteria with regards to dysphagia prevalence.

Data were extracted from the full text of included articles by the first author and entered into an Excel database. This included bibliographic details, the country within which the study took place, details of the focus of the study, sample size and characteristics, study design and data sources, measures employed, main results, and issues raised in the discussion.

Quality Assessment

A method for evaluating aspects of quality considered important to obtaining unbiased estimates of the prevalence of dysphagia in people with ID was developed based on a subset of items from the checklist provided by Munn, Moola, Lisy, Riitano, and Tufanaru (2015; e.g., Was the sample frame appropriate to address the target population?; Were valid methods used for the identification of the condition?). These items were expanded. In addition, an item was developed to indicate the extent to which each study considered prevalence in relation to subgroups of the overall sample.

The following scores were allocated for the item “Was the sample frame appropriate to address the target population?”: 4 = study includes a population-based sample of people with ID; 3 = study includes a sample of people with ID with limited generalizability (e.g., only includes certain types of accommodation, or a specific age band); 2 = study includes a representative sample of people with selected characteristics (e.g., limited to a specific severity of ID, or to people with Down syndrome); 1 = study includes a sample of people with selected characteristics (as per score 2) but with limited generalizability (e.g., sample chosen from only one type of setting); and 0 = study has a poorly defined sample.

The following scores were allocated for the item “Were valid methods used for the identification of the condition?”: 5 = dysphagia identified via instrumental assessment, e.g., videofluoroscopic swallowing study (VFSS) specifically carried out for the study; 4 = dysphagia identified via clinical examination specifically carried out for the study including use of dysphagia screening tools by professionals; 3 = dysphagia identified based on information extracted from medical records or other health-related documentation (which may or may not include results of methods specified for scores of 4 and 5); 2 = dysphagia identified via interview with an informant (including STEP); 1 = questionnaire self-completion by an informant; and 0 = method of identification unclear or not stated. If multiple methods were used, the highest level was entered as the score. Finally, for the item “Prevalence figures presented for subgroup(s),” scores could range from 0 to 6 based on the extent to which additional prevalence figures were available. One point was allocated for each of the following criteria: severity of dysphagia; age; gender; level of ID; specific conditions (e.g., cerebral palsy); and other.

Quality assessment scores are presented in the third column of Table 1. All relevant studies were included in the review regardless of methodological quality. Ninety-five percent (95%) confidence intervals for prevalence rates were calculated using the Wilson Score Method (Eayres, 2008) in Microsoft Excel using the spreadsheet available at http://www.apho.org.uk/resource/view.aspx?RID=48617. It was not possible to compare results between studies directly due to variation in the methods used and, therefore, a meta-analysis was not conducted.

Table 1

Summary of Studies Including Figures Relating to the Prevalence of Dysphagia in People With Intellectual Disability

Summary of Studies Including Figures Relating to the Prevalence of Dysphagia in People With Intellectual Disability
Summary of Studies Including Figures Relating to the Prevalence of Dysphagia in People With Intellectual Disability

Results

The process of identifying studies for inclusion is summarized in Figure 1. Electronic database searches identified 799 references, with 561 remaining after deletion of 238 duplicates. Following screening based on titles and abstracts, 441 references were excluded and a pool of 120 remained for further screening. After examination of full text and the addition of studies cited within these and from other sources, 20 studies met the criteria for inclusion with regards to dysphagia prevalence. These are summarized in Table 1, which gives the definition of dysphagia as used in the study, with some studies presenting figures for multiple items associated with dysphagia rather than a single figure for dysphagia overall. Only one of the 20 studies had a sample that was not entirely comprised of people with ID, with 89% of the sample explicitly noted to have ID (Waterman, Koltai, Downey, & Cacace, 1992).

Figure 1

Flow chart of study identification.

Figure 1

Flow chart of study identification.

Geographical Spread and Quality Scores

Table 1 shows that all studies were undertaken in high-income countries. In terms of geographical spread, eight studies were from the United States; four from the Netherlands; three from England; one from Belgium and the Netherlands jointly; and one each from Ireland, Israel, Japan, and Spain. With regards to quality scores, two studies attained the maximum score of 4 for sample frame (Ball et al., 2012; Chadwick and Jolliffe, 2009), with 10 of the 20 studies being based on samples with selected characteristics (e.g., Down syndrome, PIMD) with limits to generalizability (e.g., only including those living in particular types of settings). The most common method of identifying dysphagia was via information from records (12 studies), with one study using instrumental assessment and four conducting clinical evaluation. Only eight studies provided results for subgroups.

Study Design

Studies were almost entirely cross-sectional and, based on retrospective review of medical records (or other routinely collected information), questions were completed either by self-report or interview, or by clinical examination. One study conducted clinical assessment as part of prospective evaluation of esophageal motor dysfunction (Zárate, Mearin, Hidalgo, & Malagelada, 2001) and one study collected data as part of longitudinal study on recurrent lower respiratory tract infections (Calis et al., 2008).

Prevalence of Dysphagia or Indicators of Dysphagia

The figures presented in Table 1 (also presented visually as a forest plot in Figure 2) range from 1% of adults with ID specifically noted not to have cerebral palsy (Henderson et al., 2009) to 99% of children with severe generalized cerebral palsy and ID (Calis et al., 2008). Variation in the characteristics of study samples, and methods used for the definition and ascertainment of dysphagia, prohibited the application of meta-analysis procedures.

Figure 2

Forest plot of estimates relating to the prevalence of dysphagia in people with intellectual disability.

Figure 2

Forest plot of estimates relating to the prevalence of dysphagia in people with intellectual disability.

Overall, only two studies employed samples that could be considered representative of the general population of adults with ID (Ball et al., 2012; Chadwick & Jolliffe, 2009). The first of these gives a crude estimate of 11.5% based on detailed investigation of a sample of those identified via speech and language therapists, social care providers, and members of the community ID team as requiring mealtime support (Ball et al., 2012). The second gives an estimate of 8.1% based on referrals to a speech and language therapy service where dysphagia was subsequently confirmed via clinical assessment involving videofluoroscopy in 52% of cases (Chadwick & Jolliffe, 2009). The oral stage of swallow was affected for 94.1% of those with dysphagia, the pharyngeal stage for 51.5%, the oesophageal stage for 25.7%, and more than one stage for 58.4%. In both studies, estimates were reliant on dysphagia having been recognized initially by caregivers or professionals, and subsequently confirmed via further investigations.

Higher estimates have been reported in nonpopulation-based studies where estimates are not reliant on initial recognition of dysphagia by caregivers or professionals. In one study, 484 out of 929 adults with ID (52.1%) aged 50 or more were reported to have dysphagia based on mealtime observation by trained speech therapists using the Dysphagia Disorders Survey (Hermans & Evenhuis, 2014). In a second study, 290 of 416 adults and children with ID (69.7%) were reported to have mild to profound dysphagia based on clinical dysphagia evaluation (Sheppard, Hochman, & Baer, 2014).

There is some indication that there may be under-recognition of dysphagia by staff and family carers. In a study of children with severe generalized cerebral palsy and ID, parents' opinions on the presence of dysphagia did not correlate with the actual presence of clinical features of dysphagia, with parents tending to underestimate the severity of dysphagia (Calis et al., 2008). In one study using both oral motor evaluations during feeding and radiological studies, it was noted that, although patients had generally presented to a feeding disorders clinic with a prior diagnosis of rumination or had unexplained regurgitation, the study identified many abnormalities of deglutition and gastroesophageal function that had not been diagnosed or treated previously (Rogers, Stratton, Victor, Kennedy, & Andres, 1992).

Factors Associated With the Presence of Dysphagia

There are a number of factors associated with the presence of dysphagia that contribute to the variation in prevalence rates reported in Table 1. A critical factor is the severity of ID. Namely, dysphagia appears to be more common in those with more severe ID. Problems on the eating skills category of the STEP (which includes independence) increased with severity of ID. In one study, 11 out of 25 children (44%) with mild levels of ID were noted to have a problem on the eating skills category of the STEP, with the figure increasing to 24 out of 32 (75%) of those with moderate levels of ID, and 33 out of 34 (97.1%) of those with severe or profound levels of ID (Gal, Hardal-Nasser, & Engel-Yeger, 2011). In this study it was also reported that dysphagia (defined as oral-motor disorders such as abnormalities in sucking, chewing, and lingual movement, and swallowing discoordination) was observed in half the children who had severe levels of ID (the exact figure was not presented). In a U.S. study using the STEP, 14 out of 23 items (60.9%) were significantly more frequent in those with profound levels of ID compared to those with mild to severe levels of ID including: does not demonstrate ability to chew (13.2% of those with profound levels of ID vs. 3.4% of those with mild to severe levels of ID); chokes on food (3.5% vs. 2.3%); does not demonstrate ability to swallow (3.8% vs. 1.1%); swallows without chewing sufficiently (5.7% vs. 3.4%); special positioning for feeding (26.1% vs. 3.4%); and requires special equipment for feeding (43.4% vs. 11.4%; Matson, Fodstad, & Boisjoli, 2008). Although based on small numbers of children with cerebral palsy, one study found increased dysphagia presence with increasing level of ID: none of the six children without ID (0%) had dysphagia, compared to two of 16 children with mild or moderate levels of ID (12.5%), six of 15 children with severe levels of ID (40%), and seven out of 16 children with profound levels of ID (44%; Waterman et al., 1992).

In studies of people with profound ID and multiple disabilities, swallowing difficulties have been reported in up to 49.0% of participants (Petry, Maes, & Vlaskamp, 2009), with 47.5% reported to have dysphagia or a feeding tube due to dysphagia (van Timmeren, van der Putten, van Schrojenstein Lantman-de Valk, van der Schans, & Waninge, 2016) and 29.5% reported to have dysphagia in a further study (Gittins & Rose, 2008). Although one study reports a lower rate of 20% for dysphagia in nonambulatory people with profound levels of ID, the definition of dysphagia is unclear, with it also being noted that 23 (44%) were at risk for aspiration pneumonia due to significant oral motor dysfunction (Kozma & Mason, 2003).

Cerebral palsy is also clearly associated with dysphagia. Calis et al. (2008) found that, of a sample of 166 children with generalized cerebral palsy and ID, only two (1%) did not have dysphagia, with 8% having mild dysphagia, 76% having moderate to severe dysphagia, and 15% having profound dysphagia. In one study of those with ID aged 40 or more living in small group homes, dysphagia was reported in 12 of 1196 (1%) of those who did not have cerebral palsy, compared to 11 of 177 (6%) of those who did have cerebral palsy (Henderson et al., 2009). Even when cerebral palsy was adjusted for Severity of Functional Impairment Index (SFII) and ID level, adults with cerebral palsy were still eight times more likely to have dysphagia than those without. Calis et al. (2008) also reported an association between motor impairment (as measured by the Gross Motor Function Classification System; GMFCS) and dysphagia severity in children with generalized cerebral palsy and ID, with 24 of 139 children (17.3%) at GMFCS level V having profound dysphagia compared to none of 27 children (0%) at GMFCS level IV (Calis et al., 2008).

One Japanese study of adults with ID and severe motor disabilities with Lennox-Gastaut syndrome suggests an association between seizure outcome and dysphagia (Ogawa et al., 2001). For those with unfavorable seizure outcome (with a favorable outcome defined as a reduction of total seizure frequency to below half of the initial frequency), 14 of 21 individuals (66.7%) had progression of dysphagia compared to 1 of 17 individuals (5.9%) with a favorable outcome (Ogawa et al., 2001). The authors suggest that dysphagia was caused by the persistent epileptic activity itself.

The professional literature provides very scant findings regarding the association of age with dysphagia. For psychiatric inpatients with ID, dysphagia was reported more often in patients in age groups 46-60 and 60 or more than in those aged 16–25 or 26–45 (Exact Permutation Test, p < 0.0001; % figures for age groups not given; Charlot et al., 2011).

Discussion

This review found extremely wide variation in the figures reported for the prevalence of dysphagia in people with ID. This can be attributed to differences in the characteristics of the samples included, the definition of dysphagia used, and the method of dysphagia ascertainment. This variation makes comparison between studies difficult. From the international evidence reviewed, two estimates from population-based samples of people with ID from areas of England are available and indicate that around 8.1% to 11.5% of adults known to formal ID services have dysphagia (Ball et al., 2012; Chadwick & Jolliffe, 2009). However, both estimates are reliant on dysphagia having been initially recognized by caregivers or professionals prior to further investigation. Moreover, there is some indication that there may be under-recognition of dysphagia (Calis et al., 2008; Rogers et al., 1992) and, therefore, the actual prevalence may be higher. Indeed, for less representative samples, rates of over 50% have been reported based on mealtime observation by trained speech therapists (Hermans & Evenhuis, 2014) and clinical dysphagia evaluation (Sheppard, Hochman, & Baer, 2014).

The primary aim of many of the included studies was not focused on identifying the prevalence dysphagia in people with ID, with some studies collecting information on a wide range of health issues in people with ID (e.g., McBrien & Macken, 2009). As such, procedures for identifying dysphagia, understandably, did not generally attain high-quality scores, although the quality of studies in relation to their primary aim may have been high. Chadwick and Jolliffe (2009) acknowledge that the prevalence estimate they give is likely to be an underestimate as it is based on those referred to speech and language therapists with a confirmed diagnosis of dysphagia and, thus, may only include those with more severe dysphagia. Accurate prevalence estimates are important to those planning services and screening studies of representative samples of people with ID using adequate methods of assessment (e.g., videofluoroscopy) alongside clinical assessment is required (Chadwick & Jolliffe, 2009).

Studies suggest that increasing level of severity of ID (Gal et al., 2011; Matson et al., 2008; Parkes et al., 2010; Waterman et al., 1992) and cerebral palsy (Calis et al., 2008; Henderson et al., 2009; Perez et al., 2015) are associated with dysphagia. Motor impairment has also been associated with dysphagia (Calis et al., 2008). This may contribute to the high prevalence of dysphagia found in people with profound levels of ID and multiple disabilities (Petry, Maes, & Vlaskamp, 2009; van der Heide, van der Putten, van den Berg, Taxis, & Vlaskamp, 2009), and severe or profound levels of ID and motor disabilities (van Timmeren et al., 2016). There is little evidence regarding an association between age and dysphagia in people with ID, but, given that dysphagia has been found to increase with age in the general population (Kertscher et al., 2015; Roden & Altman, 2013), it is reasonable to assume that this will be the case for people with ID (even taking into account possible differential mortality with those with severe dysphagia dying younger) and the association is likely to be compounded by co-morbidities occurring in people with ID, such as dementia in people with Down syndrome. Larger scale studies employing multivariate analytic methods would help clarify the factors associated with dysphagia in people with ID.

Limitations

There are a number of limitations to this review. First, although studies were identified from a range of countries, the review is restricted to English language publications and this may have contributed to the lack of any identified studies from low and middle income countries. Second, all data was extracted by one reviewer and extraction of data by two reviewers independently would have reduced the possibility of errors. Third, the quality scores employed were developed specifically for this review and there is no information on the reliability or validity (for example, in relation to the accuracy of methods for identifying dysphagia) of the scores. Fourth, studies on relatively rare specific syndromes associated with ID were excluded and future review should consider the unique issues that may arise in these syndromes in relation to dysphagia (c.f., Abraham et al., 2015; Shashi & Fryburg, 1995). Finally, no studies have included the “hidden majority” of adults with ID who are not known to formal ID services (Emerson, 2011).

Conclusion

Dysphagia is common in people with ID and may be under-recognized. There is a need to more accurately discern the prevalence of dysphagia in people with ID, especially those in low and middle income countries. Improved recognition and management of dysphagia may reduce the occurrence of associated health conditions and reduce hospital admissions and premature death. Those providing services and supports to people with ID need access to resources that provide comprehensive information concerning dysphagia. A recent report by Marriot and Turner (2016) would be an excellent starting point for those seeking additional information. They not only provide essential information, but also offer numerous examples of best practices in relation to reasonable adjustments, as well as other practical ideas to improve dysphagia care for people with ID (Marriott & Turner, 2016).

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Appendix

Example of Database Specific Search (Medline)

( (MH “Deglutition Disorders+”) OR TI dysphagi* OR AB dysphagi* OR TI swallow* OR AB swallow* OR TI deglutition OR AB deglutition ) AND ( (TI ( learning N1 (disab* or difficult* or handicap*) ) OR TI ( mental* N1 (retard* or disab* or deficien* or handicap*) ) OR TI ( intellectual* N1 (disab* or impair* or handicap*) ) OR TI development* N1 disab* OR TI ( multipl* N1 (handicap* or disab*) ) OR TI “Down* syndrome” OR (MH “Developmental Disabilities”) OR (MH “Intellectual Disability+”) OR (MH “mentally disabled persons”)) OR (AB ( learning N1 (disab* or difficult* or handicap*) ) OR AB ( mental* N1 (retard* or disab* or deficien* or handicap*) ) OR AB ( intellectual* N1 (disab* or impair* or handicap*) ) OR AB development* N1 disab* OR AB ( multipl* N1 (handicap* or disab*) ) OR AB “Down* syndrome”) )

Limits: English language, human, published from 1990.

Author notes

This work was supported by Public Health England. However, the findings and views reported in this article are those of the authors and should not be attributed to Public Health England.