Assessing Motor Cortex Excitability and Its Relationship with Cortical Atrophy in Alzheimer's Dementia.

Background: Cortical excitability has been proposed as a novel neurophysiological marker of neurodegeneration in Alzheimer's dementia (AD). However, the link between cortical excitability and structural changes in AD is not well understood.

Objective: To assess the relationship between cortical excitability and motor cortex thickness in AD.

Methods: In 62 participants with AD (38 females, mean ± SD age = 74.6 ± 8.0) and 47 healthy control (HC) individuals (26 females, mean ± SD age = 71.0 ± 7.9), transcranial magnetic stimulation resting motor threshold (rMT) was determined, and T1-weighted MRI scans were obtained. Skull-to-cortex distance was obtained manually for each participant using MNI coordinates of the motor cortex (x = -40, y = -20, z = 52).

Results: The mean skull-to-cortex distances did not differ significantly between participants with AD (22.9 ± 4.3 mm) and HC (21.7 ± 4.3 mm). Participants with AD had lower motor cortex thickness than healthy individuals ((92) = -4.4, = <0.001) and lower rMT (i.e., higher excitability) than HC ((107) = -2.0, = 0.045). In the combined sample, rMT was correlated positively with motor cortex thickness ( = 0.2, = 92, = 0.036); however, this association did not remain significant after controlling for age, sex and diagnosis.

Conclusions: Patients with AD have decreased cortical thickness in the motor cortex and higher motor cortex excitability. This suggests that cortical excitability may be a marker of neurodegeneration in AD.