Patients with bipolar disorder can be separated into three groups based on their cognitive performance, a study suggests.
Previous research indicating that the cognitive ability of bipolar disorder patients is midway between that of mentally healthy controls and patients with schizophrenia does not account for this heterogeneity among bipolar patients, say Katherine Burdick (Icahn School of Medicine at Mount Sinai, New York, USA) and colleagues.
They identified three distinct cognitive groups among 136 bipolar disorder patients who completed the MATRICS Consensus Cognitive Battery (MCCB).
One group, comprising 28.7% of the patients, performed comparably to 148 mentally healthy controls for all MCCB cognitive domains and in fact had significantly better performance for social cognition.
This is “in line with the anecdotal evidence of a high-performing, socially savvy” bipolar disorder subgroup, say Burdick et al in Psychological Medicine.
A second group, of 31.6% of the patients, had selective cognitive impairment. Their performance on the MCCB domains ranged from normal relative to the controls, to one standard deviation below. Performance was unaffected for visual learning and most impaired for processing speed, attention and social cognition.
The team notes that patients in this group had a cognitive profile similar to that usually reported for bipolar disorder patients as a whole.
The third group, which included 39.7% of the patients, had marked cognitive impairment across all domains, with scores between two and three standard deviations below the controls' scores.
Indeed, the deficits in this group were very similar to those found in 185 patients with schizophrenia that the researchers had previously tested. The bipolar disorder patients outperformed the schizophrenia patients on the social cognition domain, but all other domains, and the global score, were comparable.
Psychosis history did not differ among the three bipolar cognitive groups, and neither did age, gender, race, current manic or depressive symptoms or history of substance abuse.
Pre-morbid IQ differed, being significantly lower in the group with marked global cognitive impairment. Also, the total number of previous mood episodes was higher in the group with selective impairment – significantly so relative to the unimpaired group, but not to the group with global impairment.
“By subgrouping [bipolar disorder] patients based on neurocognitive profiles we can reduce the heterogeneity of the phenotype to allow for a more targeted assessment of clinical and biological predictors of cognitive impairment”, conclude the researchers.
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