It has been suggested that in order to sustain the lifestyle of substance abuse, addicted schizophrenia patients would have less negative symptoms, better social skills, and less cognitive impairments. Mounting evidence supports the first two assumptions, but data lack regarding cognition in dual diagnosis schizophrenia. Seventy-six schizophrenia outpatients (DSM-IV) were divided into two groups: with (n = 44) and without (n = 32) a substance use disorder. Motor speed and visuo-spatial explicit memory were investigated using CANTAB. As expected, dual diagnosis patients showed a better cognitive performance. Our results suggest either that substance abuse relieves the cognitive deficits of schizophrenia or that the patients with less cognitive deficits are more prone to substance abuse.
Epidemiological studies show that the lifetime prevalence of substance use disorders (SUD) is close to 50% among schizophrenia patients. Psychoactive substances (alcohol, cannabis, and cocaine) exert a negative impact on the course of the pathology. Compared to abstinent patients, addicted schizophrenia patients relapse more frequently, they are more depressed and suicidal, they engage more often in criminal activities, and they are more frequently homeless and unemployed (Mueser, Drake, & Wallach, 1998).
It has been suggested that in order to find, acquire, and sustain alcohol and/or drug use, schizophrenia patients would have better social skills, less negative symptoms, and better cognitive functioning than abstinent patients (Joyal, Hallé, Lapierre, & Hodgins, 2003). Supporting this assumption, it has been shown that schizophrenia patients addicted to cannabis or cocaine have less severe negative symptoms (Bersani et al., 2002 and Serper et al., 1995). When compared to abstinent ones, dual diagnosis patients also appeared to have a better pre-morbid adjustment (Arndt, Tyrrell, Flaum, & Andreasen, 1992). They also seemed to have better social functioning (Côté, Lesage, Chawky, & Loyer, 1997). Further, it has been shown that the DSM-IV deficit syndrome of schizophrenia is related to less substance abuse (Kirkpatrick et al., 1996).
Regarding cognition of dual diagnosis schizophrenia, robust evidence is lacking. Recently, Carey, Carey, and Simons (2003) have shown that dual diagnosis patients suffer from less global cognitive impairments. Joyal et al. (2003) obtained similar results but their sample size was small (total n = 30). The current study was undertaken in order to further strengthen the preliminary evidence supporting the hypothesis of a better cognitive functioning in dual diagnosis patients. To find, acquire, and sustain alcohol and/or drug use, we hypothesised that dual diagnosis patients would have less deficits in explicit memory. A fronto-temporal function, explicit memory is significantly impaired among schizophrenia patients, and it is an important predictor of their social and occupational functioning ( Hoff & Kremen, 2003).
