Although it is possible to make a diagnosis of major depression and dysthymic disorder in people with ID, the boundary between major depression and subclinical depression is more difficult to define in those with severe and profound ID; it is easier to establish whether they suffer from depressive symptoms than whether the disorder is major depression or dysthymia.

In developed countries, the prevalence of ID from all causes is 1.5% to 2%.3 In adults with ID, about 8% suffer from a depressive disorder.5 There are no reliable studies on the prevalence of depressive illness in children with ID. However, prevalence of psychiatric symptoms overall has been estimated as 30% to 40%, three to four times the rate in children in general.3

The United Kingdom’s Royal College of Psychiatrists has reviewed and modified the ICD-10 diagnostic criteria to produce The Diagnostic Criteria for Psychiatric Disorders for Use with Adults with Learning Disabilities/Mental Retardation (DC-LC).6 The National Association for the Dually Diagnosed in the United States has produced the Diagnostic Manual-Intellectual Disability (DM-ID), which includes evidence-based modifications to the DSM-IV criteria for use in persons with ID, including children.1 These landmark publications provide guidance regarding the diagnosis of depressive disorders in young people with ID and highlight the limitations surrounding diagnosis in this population.

Symptoms of depression in youth with ID include depressed mood, observed sad or miserable facial expressions, crying and irritability, loss of interest in usual activities or usual stereotypical interests, and appetite and sleep disturbances.7 Self-injury behaviors may become more common or pronounced during a depressive episode.7 Reduced food intake or food refusal is reported to be a marker of depression in adults with severe and profound ID.8 Slowed movements, impaired self-care, particularly if progressing onto catatonia, are suggestive of a psychotic depression. Also, negative automatic thoughts and hopelessness are reported as symptoms of depression in adults with mild ID.9 Depression is associated also with a decline in daily living skills, social withdrawal, and adversely affects the quality of life of young persons with ID and their family.

In addition to the depressive symptoms, suicidal thoughts and suicide attempts are reported in adult patients with both mild and severe ID.10 Suicidal thinking or deliberate self-harm should alert clinicians that patients may be suffering from a depressive disorder. Developmentally disabled adolescents are at increased risk of depression and suicide, and they commonly experience other risk factors associated with suicide, including difficulties coping with change, increased anxiety, being bullied, social isolation, and lack of understanding from the social support network.11 As a young person with ID develops greater self-awareness during adolescence, the risk of self-harm may increase.

Clinicians need to gather information from others who know the young person well, including parents, caregivers, teachers, and respite workers, in addition to the direct examination of the young person. Daily mood charts or diaries completed by parents or caregivers recording information such as weight, appetite, self-harm episodes, and sleep patterns can also assist in diagnosis. Clinical evaluation should include standardized assessment tools. Several clinician, parent, caregiver, teacher, and self-rated instruments are available.12, 13, 14, 15, 16, 17 Table 22.1 summarizes some of those specifically developed for use in ID. Other scales devised for general use, such as Child Behavior Checklist, have also been used in patients with mild ID15 (see Chapter 3).

Most self-report questionnaires for the measurement of mood in youth assume a level of self-awareness, social comprehension, and understanding of abstract ideas, such as moods and feelings, which is not present in most youth with PDD and moderate and severe ID. Adolescents with mild ID are able consistently to report on their mood in questionnaires used in the general population such as the Beck Depression Inventory.9 As occurs in children and adolescents without developmental disabilities, there is often poor agreement between parent reports and reports by youth with mild ID.16

The literature refers to atypical manifestations of depressive symptoms in patients with ID when using modified criteria (e.g., reduced self-care, apparent loss of capacity to self-care), or when substituting symptoms (e.g., aggression or tantrums) for DSM-IV criteria.18 It has been argued that other
behaviors should be substituted for the traditional depressive symptoms in people with severe or profound cognitive impairment who have diminished capacity to report depressive symptoms. Davis et al.17 described substituting DSM-IV criteria with symptoms based on observed behavior. In adults with ID, observer-reported tearfulness has been substituted for depressed mood; observer-reported marked reduction in social participation has been substituted for diminished interest or pleasure; significantly elevated challenging behaviors, aggression, and self-harm have been substituted for feelings of worthlessness or guilt.17 However, this approach has been criticized for lacking a rationale on which the substitution is based.18 In severe or profound ID, other authors have argued for substituting challenging behaviors (such as self-injury, aggression, and screaming) for depressive symptoms, allowing clinicians who have detected other depressive symptoms to make a diagnosis of depression.19 The validity of these “depressive equivalents” has not been established.20

The DC-LD is a consensus document that combines modified depressive symptoms (e.g., social withdrawal, reduction in the quantity of speech/communication, reduction in self care) with standard DSM-IV criteria (loss of interest or pleasure).6 DC-LD also allows for depressive episodes to be classified into categories of recurrent, in remission, with psychotic symptoms, and bipolar depression.21

Both adults and children with mild cognitive impairment are more likely to show depressive symptoms and to be diagnosed with major depression than those with severe ID. 22,23 The course of psychiatric symptoms also varies in youth depending on the level of cognitive impairment. For example, an 11-year follow-up study showed that overall psychiatric symptoms decreased over time, but youth with mild ID showed a larger reduction than those with severe or profound ID.24 It is unclear, however, how this relates to depressive illness, not specifically considered in the study, which increases in frequency during adolescence.

Where there has been deterioration in overall functioning, diagnoses other than depression should also be considered, including anxiety, bipolar disorder, psychosis, and medical and neurologic conditions. Persons with very low IQ are often unable to express their complaints; they may look sad or tired as a result of a physical illness. Because anxiety is highly comorbid with depression, if the young person is reacting to environmental stressors—such as changes in routine, an illness (e.g., urinary tract infection, constipation), or unfamiliar caregivers—associated emotional or behavioral problems may be caused by anxiety rather than depression. Delusions related to worthlessness and guilt in the context of low mood are suggestive of a psychotic depression in young people with mild ID who are able to communicate these thoughts.

Known causes of ID, such as fragile X, Prader-Willi, Down, and Williams syndromes, are associated with specific behavioral phenotypes or clusters of symptoms.25 Behavioral phenotypes are temperamental characteristics or psychiatric symptoms that are frequently seen in association with a genetic condition but are not a core diagnostic feature of this condition.26 For instance, in Williams syndrome there are often marked levels of anxiety, significant hyperactivity, hyperacusis, and uninhibited social behavior.26 Rates of psychopathology are relatively low in children with Down syndrome. Although they show more problems than children in the general population, these are predominantly externalizing (e.g., stubbornness, oppositionality, inattention).27 Depression is rare in children with Down syndrome but increases in adults and may be mistaken for early dementia.

IQ needs to be taken into account when considering psychosocial factors. For example, the capacity to evaluate and compare oneself with others is unlikely to exist in persons of very low IQ. Children and adolescents with mild or moderate ID are more prone to experience low self-esteem because they have greater difficulty negotiating the tasks of growing up. An important role for the clinician is helping parents and teachers to better understand the young person’s competencies and emotional maturity and how this might not match their chronological age.