Comparing cognitive functions of smoking and non-smoking patients with schizophrenia

Objectives: Patients suffering from schizophrenia have a high prevalence of cognitive function impairments even though they are in remission. Schizophrenia is associated with cognitive deficits including verbal learning and memory, attention, problem solving, visual learning and memory, executive function and working memory. Available data suggests that dysregulation of the neuronal nicotinic acetylcholine receptor (nAChR) system contributes to the pathophysiology of schizophrenia and nAChR stimulation has a positive effect on cognitive functions in schizophrenia. It is known that the frequency of smoking in schizophrenic patients is higher than in the normal population and the cessation of smoking is more difficult in these individuals. One of the major reasons for this situation lies in the beneficial neuropsychological effects of nicotine provided to patients with schizophrenia. This study aimed to compare cognitive functions of smokers and non-smoking patients with schizophrenia by using neurocognitive tests.

Methods: The study sample consisted of 58 smokers (mean age: 35.32±8.26; education level: 9.36±3.12 years) and 40 non-smokers (mean age: 35.17±7.35; education level: 9.00±2.99 years) patients diagnosed with schizophrenia (SCZ) who were in a follow-up program at Sakarya University Education and Research Hospital. All participants were made to perform Trail Making Test (TMT), Stroop Color Word Test (SCWT), Rey Auditory Verbal Learning Test (VLT) and the Wechsler Memory Scale-Visual Production Subscale (WMS-V) to measure their neurocognitive functions.

Results: There were no statistically significant differences between groups regarding mean age, gender, average education period, age of illness onset and duration of illness. There was no significant difference between SCZ smokers and non-smokers in TMT-A time, but SCZ smokers significantly performed TMT-B in a shorter time. There were no significant differences between SCZ smokers and non-smokers in response times of Stroop 1 (reading the color words colored in different ink) and Stroop 2 (saying the names of colors written in different ink), the number of word corrections and word errors associated with Stroop 1 and Stroop interference. Significant group differences were found just only for the number of color corrections and color errors associated with Stroop 2 where SCZ non-smokers were observed to have more difficulties. The differences in VLT and WMS-V between two groups were significant. SCZ smokers were observed to perform higher than SCZ non-smokers in highest learning scores, total learning scores and long-term memory scores of VLT sub-tests and WMSvisual memory scores.

Conclusions: In general, this study revealed that the differences in impairments of cognitive skills between the two groups were significant in favor of SCZ smokers. There is evidence to suggest that SCZ smokers performed better than the non-smokers group, particularly in the tests measuring attention, verbal and visual memory, and working memory. These findings, consistent with previous studies, supported the ‘self-medication hypothesis’ for smoking in schizophrenia which assumes patients’ trying unconsciously to improve cognitive deficits of schizophrenia by nicotine administration.