Vagus Somatosensory Evoked Potentials and Near-infrared Spectroscopy in the Early Diagnosis of Dementia

Plain-language summary

In its long preclinical course, AD shows a spreading pattern of specific pathology in a uniform sequence of predictable steps including brainstem nuclei in early stages. Many of these nuclei which are early involved in AD take equally part in the afferences of the Xth cranial nerve, the Vagus nerve. A method for the functional assessment of Vagus-related nuclei in the lower brainstem is the technique of somatosensory evoked potentials of the Vagus nerve (VSEP). This method targets the accessibility of early functional changes by evoked potentials on one hand and the early affection of specific brainstem nuclei comprising Vagus afferences in the course of AD on the other hand. The method of VSEP takes advantage of the transcutaneous stimulation of the auricular branch of the Vagus nerve (ABVN) which is presumed to be the only sensory part of this nerve innervating parts of the outer meatus acoustics at the tragus. This cutaneous branch was shown to gain access to Vagus afferences via brainstem regions which are affected in the course of AD. VSEP latencies in AD were shown to be significantly longer as compared to healthy controls. Yet, if VSEP really are suited for the early detection of AD is still not known. Functional near-infrared spectroscopy (fNIRS) measures changes in cerebral oxygenation by means of near-infrared light using wavelengths of 650-850 nm. The principle of fNIRS is based on the principle that regional neuronal activation of the brain leads to an increase in metabolism and oxygenation of brain tissue in that region which is accompanied by an elevated regional cerebral blood flow. In AD, there is a growing body of literature reporting deviant fNIRS activation patterns for a variety of tasks. For example, it was shown that the fNIRS activation pattern in frontal and parietal cortex areas in subjects with AD performing the line orientation paradigm is clearly different from healthy controls. Yet, if fNIRS is suited as a means of early detection of AD is not known. Therefore we aimed at testing the predictive value of VSEP and fNIRS in the early detection of AD. The hypothesis to be tested within this study states that subjects developing AD or MCI within an observation period of 6 years depict longer VSEP latencies, a different fNIRS oxygenation pattern and a lower performance in neuropsychologic rating below the level of dementia at baseline than those who remain cognitively stable.

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