Cognitive subgroups in first episode bipolar I disorder: Relation to clinical and brain volumetric variables

Distinct cognitive subgroups are seen in patients with long duration bipolar I disorder (BDI), possibly reflective of underlying pathophysiological differences. It is unknown whether such cognitive heterogeneity is present at illness onset. We applied latent class analysis (LCA) to cognitive test scores in first episode BDI patients. Exploratory analysis elucidated whether impaired subgroups were characterized by 'early neurodevelopmental' (low premorbid IQ and intracranial volume) versus 'later neurodevelopmental' (decline from premorbid to current IQ, changes in relative grey (GM)/white (WM) matter volumes) pathology.

Recently recovered first manic episode BDI patients (n = 91) and healthy controls (HC, n = 63) comprised the study sample. LCA identified subgroups based on processing speed, verbal memory, non-verbal memory, executive functioning, attention and working memory scores. Subgroups were compared amongst each other and HC on premorbid/current IQ, intracranial (ICV), total brain and regional volumes.

Three cognitive subgroups emerged: (i) globally impaired (GI, n = 31), scoring 0.5-1 SD below demographically corrected norms across domains, (ii) selectively impaired (SI, n = 47), with predominant processing speed deficits and (iii) high performing (HP, n = 13), with above-average cognitive performance. GI patients showed a 'later neurodevelopmental' pattern, with normal ICV, significant decline from premorbid to current IQ, higher total GM and lower total WM (with respect to total brain volume) versus SI and HC (p = 0.003). GI patients had higher left frontal pole GM versus HC (p < 0.05, FWE corrected).

A globally impaired patient subgroup is identifiable in first episode BDI, possibly characterized by unique neurodevelopmental pathologic processes proximal to illness onset.