Intellectual/developmental disabilities (herein referred to as IDD) represent a group of developmental disorders characterized by 1) deficits in measured intelligence and 2) adaptive deficits in self-care, social, self-help, communication, educational, and occupational functioning and 3) an onset prior to age 18 (1, 2). Clinically, this tripartite diagnostic approach casts a wide net and creates a situation in which mental health professionals must deal with a large number of developmental disorders with multiple etiologies. This paper is constructed to assist clinicians in working through some of this complexity and provide some assistance in caring for these patients. Unfortunately we will be unable to discuss all of these issues in great depth but hope to generate sufficient interest to motivate the reader to learn more.
Current lifetime prevalence data suggest that between 20%–70% of individuals with IDD experience significant behavioral problems or meet current diagnostic criteria for a mental disorder (1, 3–5). These high prevalence rates are linked to the severity of intellectual disability, global impairments in adaptability and resilience, coexisting neurological/medical conditions, histories of major psychosocial and familial dysfunction, and abuse and neglect (5–7). In addition, life transitions such as physiological changes associated with puberty, the end of educational programs, transitioning to out-of-home living arrangements, declining vigor of aging parents and the physiological effects of aging create major challenges for individuals with IDD. Each of these transitions require major psychological and social adaptation and frequently trigger the onset of both challenging behaviors and the onset of many mental disorders. Unfortunately our clinical understanding of age related changes in brain function, dementias, and other neuropsychiatric conditions are still in their infancy. Much remains to be explored (2, 7).
Each life transition also influences the timing of referrals for mental health services. Most mental health referrals result from increases in troublesome or challenging behaviors (e.g. self-injury or aggression). Because the severity of these behaviors garners immediate attention, the clinician may overlook or not recognize associated primary psychiatric disorders unless a high index of suspicion for their presence is maintained. The reverse can also be true: clinicians can assume that a psychiatric disorder is present even though there is insufficient evidence to make an appropriate diagnosis. For example hallucinations prompt the diagnosis of schizophrenia regardless of contradictory findings or other core features of this severe mental illness.
Why might such misdiagnoses occur? Many patients with IDD are frequently unable to adequately describe mood or cognitive states because of limited expressive language, difficulty describing such internal states, or are subject to regression and cognitive/behavioral disorganization in response to environmental stressors. In addition, some are unable to provide useful information or fully understand the process of the psychiatric examination. As a result the clinician must rely on secondary sources or direct observation, inference and extrapolation of complex behaviors to fit the diagnostic criteria of psychiatric disorders. In addition, a failure to consider the contribution of medical or neurological illness or medication side effects on challenging behaviors can lead to misdiagnosis of serious organic mental disorders (e.g. delirium) as primary mental illness. Such misdiagnoses can also lead to further complications secondary to inappropriate treatment strategies.
To accommodate these complex features of neurodevelopmental disorders we will expand the usual definition of diagnostic overshadowing (misattributing symptoms of mental illness to IDD) to include behavioral changes associated with neurological and medical disorders. Thus the misdiagnosis of medical/neurological disorders as primary psychiatric conditions and misdiagnosis of primary psychiatr