Central auditory dysfunction may precede the onset of clinical dementia in people with probable Alzheimer's disease.

OBJECTIVES: To document the prognostic significance of a central auditory speech-processing deficit for the subsequent onset of probable Alzheimer's disease.
DESIGN: Prospective cohort study.
SETTING: Framingham Heart Study. PARTICIPANTS: Seven hundred forty dementia-free volunteers from the Framingham Heart Study cohort with symmetric hearing thresholds at biennial examination 15 (1983-1985). MEASUREMENTS: The diagnosis of probable Alzheimer's disease was made prospectively using the National Institute of Neurological and Communicative Disorders and Stroke-Alzheimer's Disease Related Disorder Association criteria. A central auditory speech-processing deficit was defined as a score of 50% of less correct on the Synthetic Sentence Identification with Ipsilateral Competing Message test in at least one ear with normal word recognition ability in both ears. Cox proportional hazards regression assessed the relationship between a central auditory speech-processing deficit and the age at diagnosis of probable Alzheimer's disease.
RESULTS: Forty subjects (5.4%) received a diagnosis of probable Alzheimer's disease during an average of 8.4 years (range 3-12) of follow-up; seven (17.5%) of these had a central auditory speech-processing deficit. The presence of a central auditory speech-processing deficit had an age-adjusted risk ratio for probable Alzheimer's disease of 10.8 (95% CI = 4.6-25.2), and the estimated risk ratio adjusted for age, gender, education level, apolipoprotein allele E4 presence, and hearing level was 23.3 (95% CI =6.6-82.7). A central auditory speech-processing deficit had a positive predictive value for subsequent probable Alzheimer's disease of 47% but the sensitivity was only 17.5%.
CONCLUSION: Central auditory speech-processing deficits may be an early manifestation of probable Alzheimer's disease and may precede the onset of dementia diagnosis by many years.