Short latency afferent inhibition differs among the subtypes of mild cognitive impairment

Mild cognitive impairment (MCI) is considered a transitional stage between normal aging and a diagnosis of clinically probable Alzheimer disease (AD). The role of the cholinergic system in MCI is not clearly defined and needs to be further investigated. A transcranial magnetic stimulation (TMS) protocol, the short latency afferent inhibition (SAI), may give direct information about the function of some cholinergic pathways in the human motor cortex. We aimed to evaluate in the present study the relationship of SAI to the specific clinical subtypes of MCI. SAI was examined in 20 patients with amnestic MCI (10 SD, 10 MD), twenty patients with nonamnestic MCI (10 SD, 10 MD) and ten control subjects. Motor threshold, central motor conduction time, intracortical inhibition and facilitation to paired-TMS were also evaluated. Mean SAI was significantly reduced in amnestic MCI-MD patients when compared with the controls, while it was not significantly different in amnestic MCI-SD patients and in nonamnestic patients. SAI was increased after administration of a single dose of donepezil in a subgroup of four amnestic MCI-MD patients. The other TMS parameters did not differ significantly between the four MCI groups and the control group. We demonstrated that this putative marker of central cholinergic activity differs among MCI subtypes. The amnestic-MD type of MCI might be a phenotype of incipient AD. However, this hypothesis would be better addressed in a longitudinal study of individual patients. TMS studies may be useful in identifying MCI individuals in whom cholinergic degeneration is occurred and therefore at increased risk of conversion to AD.