CLINICAL HANDBOOK OF SCHIZOPHRENIA

42. Intellectual Disability and Other Neuropsychiatric Populations 439vulnerability, or both, interact with stressful life events of a social, psychological, or possiblyphysiological nature to produce pathology. Three percent is an often-quoted figurefor persons with ID who also have schizophrenia (Turner, 1989), but this number is impreciseand might be low. Some clinical and research interest has focused on whetherthere might be specific genetic abnormalities that could both cause disabling syndromesand increase risk of schizophrenia and other psychiatric disorders. For example, there appearsto be increased risk of developing psychiatric disorders, especially schizophrenia,for persons with velocardiofacial syndrome (VCFS) (Karayiorgou et al., 1995; Murphy,Jones, & Owens, 1999). VCFS is associated with small interstitial deletions on chromosome22q11. Murphy and colleagues (1999) reported that 12 of 50 persons with VCFSmet DSM-IV criteria for schizophrenia.Another developmental disorder associated with risk of psychosis is Prader–Willisyndrome (Vogels et al., 2004). Characteristic features are neonatal hypotonia and developmentaldelay, hypogonadism, cognitive abnormalities, and dysmorphia of the face,body, and extremities. Hyperphagia and obesity begin later in childhood. Behavioralproblems also begin in childhood, and psychosis emerges in young adulthood in approximately5–10% of patients (Cassidy, 1997). The onset of psychotic symptoms in personswith Prader–Willi syndrome is typically before age 21 and often follows a period ofpsychosocial stress. Delusions, hallucinations, anxiety and agitation, bizarre behavior, affectivedisturbance, and mental anguish are among the described symptoms (Vogels et al.,2004).Persons with schizophrenia and those with ID might also share other nongenetic riskfactors, for example, obstetrical complications (O’Dwyer, 1997) or acquired brain diseaseof other types.Brain injury in childhood from trauma also causes cognitive disability. Traumaticbrain injury has also been implicated as a synergistic risk factor for development ofschizophrenia-like psychosis in individuals whose family pedigrees carry elevated risk forschizophrenia (Malaspina et al., 2001). Nutritional deficiencies, lead poisoning, andsociocultural factors, such as severe neglect or abuse, are additional causes of brain dysfunctionin children. ID occurs 1.5 times more frequently in males than in females. Currently,biological causes can be determined for only about one-fourth of individuals.Most cases are of idiopathic etiology.UNDERSTANDING PSYCHIATRIC ILLNESS IN PERSONS WITH IDID and psychiatric illness are not mutually exclusive. Instead, the opposite is true. In1990, the American Psychiatric Association Committee on Psychiatric Services for Personswith Mental Retardation and Developmental Disabilities estimated that 40–70% ofindividuals with MR have psychiatric disorders that meet established diagnostic criteria.More recent studies suggest that the prevalence of psychiatric disorders in the IDpopulation is three to five times higher than that in the non-ID population (Linna et al.,1999). Attention deficit disorder, affective disorders, and pervasive developmental disorderare most commonly seen. The presentation of these disorders, however, can be alteredin an individual with ID. Our experience is that depression is underdiagnosed, and thatatypical features, especially in more severely impaired groups, often confound the presentation.Schizophrenia and schizophrenia-like syndromes occur in 2–5% of individuals withID, but these disorders may account for a much higher percentage of persons with IDwho are admitted to psychiatric hospitals. Signs and symptoms of schizophrenia may be