Mental retardation is a condition characterized by limitations in intellectual and adaptive functioning. Impairments in cognitive and verbal skills may undermine the ability of a patient to clearly express the symptoms of his or her illness. Thus, when a psychiatric disorder coexists, the limited capacity of the individual to describe his or her internal state may lead to an erroneous diagnostic hypothesis.

A clinical case was followed and reviewed to illustrate the difficulties concerning the diagnosis of mental illness in patients with lower intelligence.

A 31 year old male was admitted in our acute psychiatric department with predominant auditory hallucinations. Prior to his admission, based on the assumption that he suffered from paranoid type schizophrenia, the patient had been treated by his private physician with heavy doses of antipsychotic medication, showing no clinical response. During his psychodiagnostic evaluation he underwent intelligence and personality testing, the former indicating mild mental retardation and the latter revealing no signs of psychotic symptoms. Careful clinical interviewing proved that his auditory hallucinations were in fact obsessive thoughts which he could not understand and describe as such due to his limited cognitive skills. Upon these, further investigation revealed typical obsessive compulsive behaviours which led us to a diagnosis of OCD. Following a proper adjustment in his medication and the initiation of psychotherapy, the patient showed marked improvement in his symptomatology.

Deficits in receptive and expressive language skills in people with mental retardation may obscure the diagnostic indicators of psychiatric disorders. Clinicians should be vigilant to adequatelly explore this area in their standard evaluation and modify the diagnostic assessment accordingly.