Introduction

Mounting evidence suggests that attention-deficit/hyperactivity disorder (ADHD) is more common in people with ID than in the general population (Fox and Wade, 1998; O’Brien, 2000; Baker et al., 2010; Neece et al., 2013a). However, diagnostic assessment and management in this population can present significant challenges, and clinicians report feeling less confident in diagnosing ADHD in people with ID compared with those with ADHD alone (Buckley et al., 2006). Within people with ID, ADHD is an additional source of disability which compounds the ID: those with ADHD comorbid to ID show significantly greater impairments in adaptive behavior than those with ID alone (Carmeli et al., 2007), and ADHD diagnoses and symptoms in this population are associated with as high a psychosocial impact as for those without ID (Simonoff et al., 2007), as well as high rates of other psychiatric comorbidities (Neece et al., 2013a).

What is ADHD?

ADHD is a neurodevelopmental disorder with onset in childhood, which is characterized by a persistent pattern of inattention and/or hyperactivity and impulsivity (American Psychiatric Association, 2013). The Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) criteria emphasize the requirements of: symptom onset before the age of 12 years; at least six months duration of symptoms; evidence for symptoms across several settings; significant functional impact; and lack of explanation of symptoms by the presence of another mental disorder (American Psychiatric Association, 2013). Of significance is the requirement for symptoms to be considered with reference to the developmental level of the individual. A lower threshold of inattentive or hyperactive/impulsive symptoms is required for older adolescents and adults (five of nine symptoms in either or both domains) compared to children (six of nine symptoms in either or both domains). However, thresholding of these criteria have not been undertaken for people with an ID. Subtypes are labeled as “combined” or as “predominantly inattentive,” or “predominantly hyperactive/impulsive” presentations of the disorder (American Psychiatric Association, 2013). The International Classification of Diseases and Related Health Problems, Tenth Revision (ICD-10) uses the term “Hyperkinetic disorder” to refer to a similar disorder, but specifies symptoms must have been apparent no later than seven years of age (World Health Organization, 1992).

Specific adaptations of ADHD criteria have been published for people with ID. The Diagnostic Criteria for Psychiatric Disorders for Use with Adults with Learning Disabilities/Mental Retardation (DC-LD; Royal College of Psychiatrists, 2001) is applicable only to adults, whereas the Diagnostic Manual – Intellectual Disability (DM-ID; Fletcher et al., 2007) applies to children, adolescents, and adults with ID. These systems simplify the criteria for ADHD; however, neither the DM-ID nor the DC-LD specify adapted thresholds for symptoms of ADHD in each domain for those with ID, nor do they contextualize the evaluation of such symptoms in the context of ID, beyond stating the need to consider whether symptoms are accounted for by level of ID. External validation and field testing of these criteria for different levels of ID is required.

Epidemiology

Although ADHD appears to be more common in people with ID than in the general population, findings from studies are inconsistent. Prevalence figures in people with ID range from 8.7% to over 40% for children and adolescents (e.g., Strømme and Diseth, 2000; Neece et al., 2013a), and from less than 2% to 55% for adults (e.g., Cooper et al., 2007). Potential reasons for discrepant findings include different sampling strategies resulting in differing age ranges and levels of ID represented in each study, differing assessment methods (e.g., clinician diagnosis vs. criteria-based structured interview schedules, and different approaches to measuring impairment) along with the different diagnostic criteria employed and whether inattentive subtypes are included. Taken together, in keeping with a range of other mental disorders, ADHD is probably over-represented around threefold in people with an ID compared to the general population (Baker et al., 2010; Neece et al., 2011, 2013a).

The relationship between the prevalence of ADHD and ADHD symptoms and level of ID is unclear. For example, O’Brien’s (2000) epidemiological study of young adults with ID found increasing rates of ADHD in those with more severe ID, while other studies (e.g., Pearson and Aman, 1994) do not support this observation. These discrepant findings highlight a potential difficulty in identifying ADHD in those with more severe ID (Fox and Wade 1998; Antshel et al., 2006b).

Vulnerability to ADHD is conferred by a number of means, including many biological, environmental, and social risk factors, which give rise to both ADHD and ID. Further, a number of syndromes associated with ID are also associated with an ADHD phenotype. These include tuberous sclerosis complex (De Vries et al., 2007), Williams syndrome (Leyfer et al., 2006), Fragile X syndrome (Hagerman, 2002), Angelman syndrome (Barry et al., 2005), and velocardiofacial syndrome (Antshel et al., 2006a). The added value of diagnosis of ADHD in the setting of a syndrome with a characteristic behavioral phenotype is debatable, but may assist in the prioritization of intervention if clinically meaningful symptoms of ADHD are present.

The impact of ADHD

The functional, social, psychological, and economic impact of ADHD in the general population is well documented (Jensen et al., 2001,). Studies of people with ID have suggested that those with comorbid ADHD experience a “double deficit,” which manifests as further impairment of functional skills (Carmeli et al., 2007) and cognitive functions (Di Nuovo and Buono, 2007; Rose et al., 2009). As with the general population, the combination of ADHD and ID in adolescence has significant impact in the educational setting, with ADHD symptoms being predictive of later suspension from school (Handen et al., 1997). In adulthood, the combination of ADHD and ID is associated with increased aggressive and self-injurious behaviors (Cooper et al., 2009a, 2009b), which in turn is likely to be associated with poorer outcomes.

ADHD in the general population is associated with high rates of psychiatric comorbidity, including disorders such as oppositional defiant disorder (ODD), conduct disorder, specific learning disorders, tic disorders, anxiety disorders, affective disorders, and substance-use disorders (e.g., Mayes et al., 2000; Jensen et al., 2001; Biederman et al., 2010). Less is known regarding psychiatric comorbidities in children with ID and ADHD, but emerging evidence suggests very high rates of other mental health problems in this group, particularly ODD (Neece et al., 2013a,). Patterns of additional psychiatric comorbidity in people with both ADHD and ID also appear high in the clinical context, and underscore the complex neuro-behavioral and psychological vulnerabilities of people with both ADHD and ID.

Diagnostic validity in the context of ID

Historically, the question of whether an ADHD diagnosis is valid in the presence of ID has been debated (Burack et al., 2001), and indeed, there is some evidence of a discontinuity between ADHD in people with and without ID, such as differing gender distributions (e.g., Hastings et al., 2005; Neece et al., 2013b). However, validity for the diagnosis is bolstered by several lines of evidence. First, studies which support higher prevalence of ADHD in people with ID (e.g., O’Brien, 2000; Strømme and Diseth, 2000) are consistent with the well-documented, significant negative association between IQ and ADHD symptoms in the general population (Frazier et al., 2004; Antshel et al. 2006b). Second, rates of ADHD symptoms are elevated in people with ID, even after controlling for mental age (e.g., Pearson and Aman, 1994; Hastings et al., 2005), suggesting that lowered IQ alone cannot explain ADHD symptoms. Third, there are consistencies in findings in people with ADHD with and without ID, with respect to the nature of symptoms, their developmental course, the risk factor profiles, and comorbidity profiles (e.g., Antshel et al., 2006b; Neece et al., 2013a, 2013b).

Assessment of ADHD

A multimodal approach is required that follows best practice guidelines for the general population (National Institute for Health and Clinical Excellence, 2008; Royal Australian College of Physicians, 2009), with modifications for the person with ID. Detailed evaluation of current symptoms, an evaluation of their impact across a variety of settings, and the collection of information from multiple different sources is required. The issue of whether to evaluate the individual’s symptoms relative to expectations based on chronological age, developmental age, or level of ID has previously been debated (Pearson and Aman, 1994; Seager and O’Brien, 2003). From a clinical perspective, comparing to peers of a similar age and level of ID is the most sensible approach (Fletcher et al., 2007). Observational data may assist in determining the severity, frequency, and impact of symptoms, relative to expectation based on level of ID. Detailed evaluation for possible causes or contributors to symptoms, including screening for relevant medical factors (e.g., seizures, sleep disorders, and thyroid disorders) and psychiatric factors is essential.

A thorough developmental assessment and psychiatric history will allow the contextualization of the symptoms with regard to the person’s level of ID, the identification of other relevant psychiatric comorbidities, and if necessary, a review of the etiology of the ID itself. An evaluation of intellectual and adaptive functions may be required if this has not been performed or previous results are unavailable or very old. In the context of mild ID, a psychoeducational assessment may be required if the history suggests the presence of an additional learning disorder. Allied health assessments can assist in determining comorbid speech and language, motor coordination, and sensory problems. Assessing the environmental context to identify possible drivers and moderators of symptoms is important and should take into account the physical environment, adequacy of supports, and dynamics and mental health within the family or cohabitation setting.

Detailed neuropsychological assessment is not routinely recommended but may be of value where there is diagnostic uncertainty in individuals with mild ID, and where more detailed understanding of cognitive strengths and weaknesses is required in planning educational goals. Neurophysiological tests such as the measurement of event-related potentials (ERP) or quantitative analysis of electroencephalogram (EEG) are not recommended in people with ID. Structural brain imaging such as magnetic resonance imaging (MRI) of the brain is only recommended if ADHD symptoms are considered to be secondary to a condition associated with underlying structural brain changes. Functional brain imaging such as functional MRI (fMRI) is not recommended in the diagnostic assessment.

For adults with ID who have come for assessment of ADHD symptoms for the first time, a detailed retrospective evaluation for ADHD symptoms in childhood must be performed. Whilst retrospective self-reports of symptoms may be of value, independent verification, preferably from a parent, is important. Other sources of childhood information, such as previous psychometric assessments, school reports, and information from residential and respite placements, may also be of use.

Symptom rating scales are commonplace in the assessment of ADHD in people without ID. With few exceptions, the validity of these scales in people with ID is either uncertain or questionable: Miller et al. (2004a, 2004b) examined the psychometric properties of popular rating scales assessing symptoms of ADHD in a small sample of children with ID and found very low correlations between parent rating scales. Further, the applicability of such scales in more severe levels of disability appears limited (e.g., Deb et al., 2008). Behavioral rating scales validated for use with people with ID may be more useful (Miller et al., 2004a). The Developmental Behavior Checklist Hyperactivity Index (Einfeld and Tonge, 2002) has been found to have construct validity and discriminant function when used with young people with ID (Einfeld and Tonge, 2002), and to discriminate between children and adolescents with autism spectrum disorders alone and ADHD plus autism spectrum disorders (Gargaro et al., 2014). However, replication with larger samples is required.

Management of ADHD

Current clinical guidelines (National Institute for Health and Clinical Excellence, 2008; Royal Australian College of Physicians, 2009; Canadian ADHD Resource Alliance, 2011) provide comprehensive management recommendations that should be used as a framework for the approach to ADHD management in people with ID. These guidelines outline an integrated approach to care that encompasses a broad range of social, personal, educational, and occupational needs. Key principles in the management of ADHD in people with ID include: the adoption of a person-centered approach; promotion of independence whilst acknowledging the age and capacity of the individual; undertaking multidisciplinary assessment and management that engages family and carers; the setting of clear goals for management and monitoring the effectiveness of intervention; and ensuring clinical practice is based on the best available evidence. Significant adaptation of the clinical approach is often required for the assessment and management of ADHD in people with ID. Adaptations may include: preparation and adjustments of consultation processes and length; adaptation of communication; engaging the person and where appropriate the guardian/s in decision-making; and allowing for extended models of working with families and carers.

As with ADHD in the general population, an individual management plan that takes into account specific needs and individual and carer preferences should be developed. However, for individuals with ID and ADHD, consideration of a more complex array of factors is necessary, including: (i) where multiple psychiatric or developmental comorbidities exist, prioritizing interventions, and seeking to minimize impact of ADHD treatments on co-occurring conditions; (ii) careful evaluation of the potential adverse consequences of ADHD pharmacotherapy in the context of individuals with complex medical comorbidities and reduced ability to spontaneously communicate side effects; (iii) determining priorities for intervention in a person-centered manner, including consultation with guardians and family carers as appropriate; (iv) where multiple psychosocial disadvantage exists, carefully prioritizing intervention strategies that are most likely to benefit the mental health and well-being of the individual, thus, avoiding overwhelming the individual or their support networks; and (v) the coordination of clinical care across a potentially more complicated array of settings including educational, vocational, respite, and place of residence.

Education about ADHD for the affected individual, their family, and carers is a core aspect of sound management. For young children with moderately severe ADHD without ID, first-line treatment is parent training/education programs, either as a group or on an individual basis (National Institute for Health and Clinical Excellence, 2008). Although specific evidence for the effectiveness of parent training in children with ID and ADHD is lacking (Reilly and Holland, 2011), parent training can be effective for reducing behavioral difficulties in children with developmental disabilities (Hudson et al., 2003; Tellegen and Sanders, 2013), suggesting it may also be of value in children with ID and ADHD. Cognitive-behavioral therapies, social skills training, and family therapy should be employed where indicated, though direct evidence for their effectiveness in people with ID and ADHD is lacking.

As with the general population, pharmacotherapy in people with ID and ADHD should be limited to those individuals with severe ADHD symptoms and impairments, or where those with moderate symptoms and impairments either refuse non-pharmacological treatments or their symptoms fail to respond to such interventions (Royal Australian College of Physicians, 2009). Stimulants such as methylphenidate and amphetamine salts are the mainstay of drug treatment, but non-stimulant treatments such as atomoxetine or other treatments may also be appropriate (Rowles and Findling, 2010). A small number of short-term randomized controlled trials suggest the effectiveness of methylphenidate at low doses (Aman et al., 2003), higher doses (Pearson et al., 2003), and optimal dosing (Simonoff et al., 2013) in children and adolescents with ADHD. Other types of studies have also added some support for the effectiveness of both methylphenidate (e.g., Handen et al., 1999) and amphetamines (Jou et al., 2004) in ADHD with ID. However, it remains unclear whether stimulants are as effective in people with ID compared to people without ID (e.g., Aman et al., 2003), and whether they are effective in reducing symptoms across all settings (e.g., Pearson et al., 2003). Significant side effects, including sleep disturbance, appetite changes, and weight loss, have been noted in some stimulant studies (Pearson et al., 2003; Simonoff et al., 2013), but not in others (Aman et al., 2003).

In children with ADHD without ID, risperidone treatment is recommended only in the presence of significant conduct disorder with associated aggression (Royal Australian College of Physicians, 2009). No randomized controlled trials on the effectiveness of risperidone have been conducted in people with ADHD and ID. A direct comparison between methylphenidate and risperidone found a more pronounced effect in the latter for reducing ADHD symptoms in children and adolescents with moderate ID (Correia Filho et al., 2005). However, risperidone was associated with significant side effects, including weight gain and somnolence. Risperidone is not, therefore, generally recommended in people with ADHD and ID unless significant comorbidity with severe aggressive behaviors exists.