Intellectual disability (ID) is the impairment of general mental abilities, which affects an individual’s functioning in everyday life. According to DSM-5, ID has an impact on 3 broad domains in a person’s life: conceptual (eg, language and memory), social (eg, empathy, social judgment), and practical (eg, personal care, money management).1 Individuals with ID have a higher risk of psychiatric disorders than individuals with intelligence in the normal range: prevalence is as high as 40.9% based on clinical diagnosis and 15.7% based on DSM-IV-TR.2 When specific ID diagnostic criteria are used (diagnostic criteria for psychiatric disorders for use with adults with learning disabilities [DC-LD]), the most common comorbid psychiatric disorders are problem behavior (18.7%), affective disorder (5.7%), autism spectrum disorder (4.4%), psychotic disorder (3.8%), and anxiety disorder (3.1%).2
Problem/challenging behavior is heterogeneous and its assessment and management are beyond the scope of this article. The focus here is on the comorbid presentation of affective and anxiety disorders, psychotic disorder, and autism spectrum disorder.
Specific challenges in the field of ID can make the diagnosis of comorbid psychiatric disorders difficult. Individuals with ID can experience communication difficulties that vary from problems expressing psychological experiences to being unable to produce speech. As may be expected, this results in an under-reporting of psychiatric symptoms.3 Clinicians must be aware of this and place greater emphasis on observ-ing the individual and on obtaining collateral history from family and care staff.
There can be a tendency to attribute any behavioral/psychological disturbance to an individual’s ID (known as diagnostic overshadowing).4 This significantly increases the risk of missed diagnoses. Individuals with ID often have the same form of psychopathology (eg, auditory hallucinations) as those with intelligence in the normal range. However, the content of this psychopathology can differ significantly (see Case Vignettes for examples). As a result, the presentation of a comorbid psychiatric disorder in this group is often atypical. Be alert to this when conducting assessments.
Affective and anxiety disorders
Affective and anxiety disorders occur in individuals with ID at a rate of around 5.7% and 3.1% respectively.2 In addition to the classic symptoms seen with these disorders, their atypical presentation can include features such as aggression and self-injurious behavior.
In a study on depression symptoms in individuals with ID, Esbensen and Benson5 found that those with MDD and ID had a more negative attributional style and lower self-esteem. We have found that because of the often challenging nature of diagnosing depression in individuals with ID, clinicians frequently find themselves relying on the biological symptoms of depression for diagnosis: sleep and appetite disturbance, poor concentration and memory. Mayville and colleagues6 found that persons with depression and ID had a lower food intake than those with ID who were not depressed.
A review of rapid cycling bipolar affective disorder found that hypersomnia occurred in 70% of sleep-disturbed depressed patients with ID.7 There is little literature on the presentation of mania in individuals with ID. In their review of rapid cycling bipolar affective disorder in persons with ID, Vanstraelen and Tyrer7 found that mood states were generally described in terms of observable behavior rather than affective state. They found that signs of manic episodes typically included insomnia, increased activity, pressured speech, and agitation.
Because of the lack of randomized controlled trials for the management of anxiety, depression, and bipolar affective disorder in persons with ID, recommended pharmacological options do not differ significantly from those for individuals with intelligence in the normal range. It is worth emphasizing that efforts should be made to identify and manage potential precipitating and perpetuating factors (eg, change in environment, disrupted sleep).
Harry is 35 years old, with moderate ID. He presents with a long-standing history of anxiety symptoms that, when particularly severe, include excessive talking, asking the same questions repeatedly, and tremulousness. On several occasions and seemingly without warning, Harry hit other service users and members of staff. Following one of these altercations, he began referring to others as “naughty” and telling them they would be punished.
An adapted form of CBT is used to explore and address some of Harry’s anxieties. He is also treated with an SSRI.
David is 22 years old, with moderate ID and bipolar affective disorder; he has no speech. He presents with a 1-week history of significantly increased activity including destruction of property and no need of sleep. There had been a recent change in David’s day center, and he had struggled to settle in there. He has no family history of any psychiatric disorder.
In the acute phase, David is treated with sodium valproate and a benzodiazepine; later, risperidone is added. His sleep is also managed pharmacologically. Following sufficient clinical improvement, ID services manages David’s phased return to the day center, with additional support from care staff.
Dr Kendall is a WCAT Fellow, MRC Centre of Neuropsychiatric Genetics and Genom-ics, Cardiff University School of Medicine, UK. Professor Owen is Director of the MRC Centre for Neuropsychiatric Genetics and Genomics, Director of the Institute of Psychological Medicine and Clinical Neurosciences, and Emeritus Director of the Neuroscience and Mental Health Research Institute, Cardiff University School of Medicine. The authors report no conflicts of interest concerning the subject matter of this article.
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