One of my current projects, recently published, assesses the effects of cigarette-smoking behavior on general and social cognition in a bipolar cohort.
Individuals diagnosed with mental illness demonstrate a higher likelihood of substance use, such as cigarette smoking, compared to individuals without a psychiatric diagnosis. A number of theories have been developed to explain this curious finding, particularly in relation to schizophrenia. For example, nicotine is a stimulant, and as such, may enhance focus and attention in individuals with mental illness like schizophrenia. Also, people with mental illness report feeling subjectively better and temporary relief from their psychiatric symptoms, suggesting nicotine use as a form of self-medication. Finally, biological studies demonstrate overlap in both developing mental illness and addictive behavior, suggesting shared genetic and neuroanatomical substrates underlying both disorders.
Interestingly, nicotine use in schizophrenia has shown to improve aspects of cognition, like attention, processing speed, and working memory. While most work assessing the beneficial effects of nicotine on cognition have been conducted in psychotic disorders like schizophrenia, very few, if any, have been conducted in bipolar disorder. While primarily considered an affective disorder characterized by mood lability (i.e., switching between manic/hypomanic and depressive symptomatology), bipolar disorder also shares schizophrenic-like symptoms, particularly positive symptoms of psychosis (i.e., delusions and hallucinations). However, only approximately 65% of bipolar patients present with psychotic-like features. Therefore, given common biological determinants found in both schizophrenia and bipolar disorder, we hypothesized to see a similar relationship between smoking behavior and cognition in bipolar patients who smoke, namely: Bipolar smokers with psychotic history would demonstrate better performance on aspects of cognition compared to bipolar non-smokers with psychotic history.
Participants included 105 bipolar patients, with a roughly equal number of smokers and non-smokers, who were questioned on pre-existing cigarette-smoking behavior and administered a battery of cognitive measures. A number of factors that may additionally affect cognition were also accounted for, like: affective symptoms, other substance use, premorbid IQ, comorbidity of other psychiatric disorders (particularly anxiety disorders), total number of psychiatric medications taken at time of testing, and number of cigarettes smoked daily. Generally, results ran counter to our expectations: bipolar smokers with psychosis history performed worse on measures of general and social cognition compared to bipolar non-smokers with psychosis history. However, in bipolar patients with no evidence of psychosis, smokers outperformed non-smokers on these same cognitive tasks. As a psychostimulant, nicotine has been demonstrated to improve aspects of cognition, which may explain similar results in our non-psychotic group of bipolar patients. However, bipolar smokers with psychosis history may present as a significantly more impaired group of patients, given that smoking has been demonstrated to be an additional risk factor for poorer cognition and general overall functioning (i.e., illness severity).
While the current study did present with some limitations (i.e., no experimental administration of nicotine, no objective assessment of nicotine levels, etc.), results suggest cognitive performance in response to nicotine use is partly dependent on psychosis history in patients with bipolar disorder, which may point to differing neurobiological substrates between these subtypes. Further experimental manipulation is necessary to disentangle the effects of smoking/nicotine and psychosis history on cognition in bipolar disorder.