Noninvasive in vivo assessment of cholinergic cortical circuits in AD using transcranial magnetic stimulation

Background: A recently devised test of motor cortex excitability (short latency afferent inhibition) was shown to be sensitive to the blockade of muscarinic acetylcholine receptors in healthy subjects. The authors used this test to assess cholinergic transmission in the motor cortex of patients with AD. Methods: The authors evaluated short latency afferent inhibition in 15 patients with AD and compared the data with those of 12 age-matched healthy controls. Results: Afferent inhibition was reduced in the patients (mean responses ± SD reduced to 85.7% ± 15.8% of the test size) compared with controls (mean responses ± SD reduced to 45.3% ± 16.2% of the test size; p < 0.001, unpaired t-test). Administration of a single oral dose of rivastigmine improved afferent inhibition in a subgroup of six patients. Conclusions: The findings suggest that this method can be used as a noninvasive test of cholinergic pathways in AD. Future studies are required to evaluate whether short latency afferent inhibition measurements have any consistent clinical correlates.