Cognitive impairment in dual diagnosis inpatients with schizophrenia and alcohol use disorder

Abstract

Cognitive impairment has been found independently among individuals with schizophrenia and individuals with alcohol use disorders. Less is known about the nature and severity of cognitive impairment in patients with a dual diagnosis, though the co-occurrence of these disorders may further exacerbate cognitive impairment. The study investigates the possible additive effect of alcohol use disorder and schizophrenia on cognitive impairment among patients diagnosed with schizophrenia. Participants were inpatients with schizophrenia (n = 30), inpatients with a dual diagnosis of schizophrenia and alcohol use disorder (n = 30), and matched controls (n = 30): all completed a comprehensive neuropsychological battery. Both patient groups were significantly impaired, relative to controls, across the battery. Dual diagnosis patients were significantly more impaired than schizophrenia patients on delayed verbal memory, and executive functioning, primarily set-shifting, working memory, and planning, and had higher psychiatric morbidity scores. The findings provide support for an additive effect of the two disorders on cognitive impairment. These cognitive deficits may affect capacity to engage in treatment, increase risk of relapse, and adversely affect treatment outcomes. An understanding of the cognitive profile of people with dual diagnosis may help to tailor treatment delivery to meet their specific needs, enhance cognitive strengths, accommodate deficits and improve treatment outcomes.

Introduction

Cognitive impairment is a central and enduring characteristic of schizophrenia (Hoff and Kremen, 2003, Bilder et al., 2000) and is common in individuals dependent on alcohol (Bates et al., 2002, Grant, 1987). Relatively little is known of the possible ‘additive effect’ of heavy drinking on cognitive impairment in patients with a dual diagnosis, (i.e. if the co-occurrence of the disorders exacerbates cognitive deficits associated with schizophrenia alone). Alcohol is the most frequently abused substance among individuals with schizophrenia (Cantor-Graae et al., 2001, Fowler et al., 1998, Miles et al., 2003). Between a quarter and one-third of schizophrenia patients report current (past year) alcohol use disorder (Menezes et al., 1996, Cantwell et al., 1999, Weaver et al., 2001), with higher rates among institutionalised patients (Isherwood and Brooke, 2001, Wheatley, 1998).

Previous studies have reported mixed findings on the additive effect of schizophrenia and alcohol use disorder on cognitive impairment. On a face recognition and set-shifting task, Nixon et al. (1996) failed to find differences between patients with a dual diagnosis compared to those with only schizophrenia or alcohol dependence. In contrast, schizophrenia patients with a history of alcoholism were found to show greater (subtle but consistently identifiable) cognitive impairment on the Cognitive–Perceptual factor of the Neurologic Evaluation Scale (Allen et al., 2000). Using a comprehensive battery of neuropsychological tests, greater cognitive impairment was found among dual diagnosis patients compared to patients with only schizophrenia or alcoholism (Allen et al., 1999). Other US studies have found evidence of an additive effect among older patients with dual diagnosis (Bowie et al., 2005, Mohamed et al., 2006). Our earlier work on a UK sample (Manning et al., 2007) was suggestive of greater global cognitive impairment among dual diagnosis outpatients, though no specific deficits were observed, possibly due to the insensitivity of the screening tool used (Mini Mental State Examination).

Contradictory findings from the limited number of studies in this area may arise from differences in sampling (e.g. inpatient versus outpatient, chronic versus acute schizophrenia, alcohol use versus alcohol dependence), methodology (cognitive domain examined, sensitivity of neuropsychological test and performance parameter selected) and other limitations such as small samples reducing statistical power to detect group differences and failing to match on confounding factors such as premorbid functioning.

The present paper reports a further investigation of this issue using a comprehensive neuropsychological battery administered to inpatients with a diagnosis of schizophrenia, half of whom had alcohol use disorders. The purpose of the study was to delineate the profile of cognitive deficits in dual diagnosis patients and to determine the possible additive effect of alcoholism and schizophrenia on memory and executive functioning impairment, using matched controls.