es. 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 psychiatric conditions as medical/neurological disorders are added diagnostic overshadowing. Resolving these issues requires a high index of suspicion and a reasonable working knowledge of neuropsychiatry. Such awareness can improve our understanding of the complex nature of differential diagnosis and help us empathize with the problems faced by colleagues working in the field of dual diagnosis (IDD and MH) (2).
A second set of challenges may be as complicated as those outlined above. Understanding and treating individuals with dual diagnosis also forces the clinician to integrate multiple clinical perspectives. For example:
Behavioral therapy and learning theory: This system is based on target behaviors rather than syndromes (e.g. aggression in response to changes in key environmental factors or escape from task demands) (8). The challenge to mental health professionals is to determine when appropriate target symptoms are identified with specific psychiatric syndromes, e.g. dysphoric mood in major depressive disorder.
Nomenclature issues: Clinicians need to adapt our current nomenclature to better fit adult patients with IDD. The Diagnostic Manual-Intellectual Disability (DM-ID) and the Diagnostic Criteria-Learning Disorders respectively represent attempts to adapt the DSM-IV-TR and ICD-9 uniquely to these differences in nosology.
Adapting to multidisciplinary teams. Best practices in the field of dual diagnosis include mental health professionals as part of a team that includes behavioral psychologists, occupational therapists, physical therapists, pediatric neurology, medicine, psychiatry, educators, family members, and direct support professionals and others. The goal is to integrate different points of view and treatment models into a coherent treatment plan (5–7).
A third challenge arises from an information explosion in the clinical neurosciences of IDD and mental illness. At a practical level we need to translate new information into clinical practice. At a theoretical level we need to update the biopsychosocial m