Executive deficits and regional brain metabolism in Alzheimer's disease

Executive deficits are common in patients with Alzheimer's disease (AD), contribute prominently to clinical disability, and may be associated with frontal lobe pathology. This study examined regional brain hypometabolism associated with executive dysfunction in patients with AD.

Forty-one patients with probable AD underwent [(18)F] fluorodeoxyglucose positron emission tomography (FDG-PET) imaging at rest. Neuropsychological measures of executive control included the Conceptualization (Conc) and Initiation/Perseveration (I/P) subscales of the Mattis Dementia Rating Scale (DRS), the Wechsler Adult Intelligence Scale (WAIS) Similarities subtest, the Tower test, and the Ruff Figural Fluency test (Ruff). Voxel-based analyses were conducted using statistical parametric mapping (SPM2) to measure the correlation between regional cerebral metabolism and executive measures. Correlations independent of global cognitive impairment were identified by including Mini-Mental State Examination (MMSE) score as a covariate in the model.

Executive deficits, as measured by poor performances on the DRS I/P and Conc subscales, were associated with hypometabolism in the bilateral mid-dorsolateral frontal region. Activity in posterior cortical regions also contributed uniquely to some aspects of executive functioning, as lower resting metabolism in parietal or temporal cortex was correlated with poor performance on four of the five executive measures. After controlling for global cognitive score, there were significant extra-frontal correlations with hypometabolism in insula, occipital lobe, and temporal cortex.

Some but not all executive deficits in AD are associated with neural activity in the dorsolateral frontal cortex. Activities in distributed neural systems that include parietal and temporal cortex also contribute to some executive abilities. The pathophysiology of executive dysfunction is complex and includes abnormalities not limited to a single region.