PRIMARY AUDITORY PATHWAY AND RETICULAR ACTIVATING SYSTEM DYSFUNCTION IN ALZHEIMERS-DISEASE

Patients with Alzheimer's disease (AD) have pathologic involvement of several important components of the primary auditory pathway, includin g the inferior colliculus, medial geniculate body, primary auditory co rtex, and secondary auditory cortex. The main components of the brains tem auditory evoked response (BAER) and middle latency response (MLR) reflect the function of portions of the primary auditory pathway, incl uding those affected pathologically in AD, The amplitude of the P1 com ponent of the MLR reflects the degree of neuronal activity of midbrain portions of the ascending reticular activating system (ARAS) with cor tical cholinergic projections. To determine whether there is dysfuncti on of the primary auditory pathway and ARAS in AD, we compared simulta neous BAER and MLR component latency and amplitude measurements in pat ients with mild-moderate AD (n = 35) and age-matched healthy elderly c ontrols (n = 34). There were significant latency delays in brainstem t ransmission time (BAER I-V interpeak latency; p < 0.05) and in primary auditory cortex evoked potential generation (MLR Pa latency; p < 0.05 ) in the AD group compared with controls. In addition, there was a sig nificant reduction in the P1 component amplitude of the MLR in the AD group (p < 0.01). These results indicate dysfunction of the primary au ditory pathway and ARAS in patients with mild-moderate AD and support the hypothesis that impairment of auditory function and of arousal are intrinsic features of AD.