Intellectual Disability and Psychiatric Comorbidity: Challenges and Clinical Issues

CLINICAL

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.

Challenges

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).

CASE VIGNETTE

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.

CASE VIGNETTE

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.

Psychotic disorder

The prevalence of psychotic disorder (including schizoaffective disorder) in individuals with ID is about 3.8%.² This figure is in keeping with results from previous studies.⁸ As with the conditions discussed above, individuals with ID in whom a psychotic disorder develops may present differently from someone without ID. Bouras and colleagues⁹ compared psychopathology in 53 individuals with schizophrenia-spectrum psychoses and ID with that in 53 individuals with schizophrenia-spectrum psychoses but no ID. Those with psychosis and ID had more negative symptoms and functional disability than those without ID.

It can be very difficult to describe certain psychotic phenomena such as somatic passivity when one’s intelligence is in the normal range, so it is perhaps unsurprising that it is even more of a struggle for persons with communication difficulties. In keeping with this, Bouras and colleagues described a higher rate of observable psychopathology in those with psychosis and ID over and above the rate of reported psychopathology in this group. Pharmacological treatment of psychosis in those with ID does not differ significantly from psychosis in those without ID.

CASE VIGNETTE

Sarah, a 20-year-old with mild ID, presented with a 1-month history of delusions that her best friend was trying to kill her and her family. Third person auditory hallucinations told Sarah to kill her friend. She lacked motivation and often did very little with her day. There was a history of schizophrenia in Sarah’s mother and ID in her father.

Sarah was treated with olanzapine and experienced a good response with full resolution of her hallucinations and delusions over the next few months. Her negative symptoms persisted. A careful multidisciplinary approach facilitated her discharge from the hospital.

Autism spectrum disorder

Cooper and colleagues² found the prevalence of autism spectrum disorder to be 4.4% in persons with ID. It has been found that increased severity of ID is positively correlated with the incidence of autism.¹⁰ In fact, it can sometimes be difficult to distinguish autism from severe ID. Matson and colleagues¹¹ compared the symptoms of 57 adults with autism and severe ID with the symptoms of 57 matched controls with ID. They found a significant difference in the range of symptoms experienced by each group, which was sufficient to predict the presence or absence of autism with high accuracy. The features characteristic of autism in these individuals were deficits in social interaction and repetitive behaviors.

Individuals with autism spectrum disorder and ID may require additional support from the multidisciplinary team with regard to managing day-to-day living (eg, occupational therapy for sensory stimulation, speech therapy input for communication). Currently there are no medications to treat autism. When problems such as challenging behaviors arise, efforts must be made to identify and manage precipitating and perpetuating factors and comorbid psychiatric disorders. When symptom control is required (eg, for aggression) and the above issues have been addressed, an antipsychotic medication is often used. However, this should be a short-term option only, and efforts must be made to ensure that this is not simply an attempt to oversedate the patient.

Comorbid physical disease

Individuals with ID have more physical disorders per person than persons in the general population. In a population study by Beange and colleagues,¹² 95% of individuals had a medical condition other than ID. Genetic syndromes are often associated with particular physical illnesses (eg, heart defects, hypothyroidism, cataracts, external auditory canal stenosis in Down syndrome). Many persons with ID do not have a distinct genetic syndrome but still have a high rate of physical comorbidity.

A general practice–based study of individuals with ID found the prevalence of epilepsy to be 18%.¹³ Persons with ID often suffer from multiple seizure types, and the rate of resistance to treatment is high, which necessitates multiple antiepileptic medications.¹⁴

Compared with the general population, persons with ID are at increased risk for being obese, with its accompanying risks of diabetes mellitus and other problems. They also have an increased risk of being underweight.¹⁵ In addition, persons with ID are at increased risk for cardiovascular disease, which is a significant cause of death in this group.¹⁶

In view of medication use for comorbid conditions, clinicians should be aware of the potential for drug-drug interactions-particularly in those patients who are receiving antiepileptic medication. Altered pharmacokinetics resulting from abnormal weight may necessitate adapted dosing regimens. Finally, the adverse effects of psychopharmacological treatments may worsen comorbid physical disorders, such as obesity and low seizure threshold.

Conclusion

The risk for a psychiatric disorder is greater in persons with ID than in those with intelligence in the normal range. Communication deficits present a challenge to diagnosis, and clinicians must be able to adapt to ensure they ascertain the relevant information that will allow them to make a diagnosis. They should also be alert to the dangers of diagnostic overshadowing.

For patients with ID, once a diagnosis of a psychiatric disorder is made, pharmacological treatment does not differ significantly from that in the general population, but care may be required to avoid medication interactions and underdosing or overdosing, as well as the potential to worsen comorbid conditions. Finally, individuals with a psychiatric disorder and ID may require additional multidisciplinary support.

Disclosures:

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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