[Olfactory disorders in Alzheimer's disease and in Parkinson's disease. Review of the literature].

Olfactory disorders in Alzheimer's disease and Parkinson's disease have been the topic of a large body of work over the last decades. Work devoted to olfactory disorders in Alzheimer's disease includes over 300 papers providing clinical and fundamental data. Anatomy studies in Alzheimer's disease have demonstrated a specific concentration of lesions in peripheral and central olfactory structures (senile plaques, neurofibrillary degeneration) as well as lesions in layers II and III of the entorhinal cortex. These neuropathological findings led to the development of the hypothesis that olfactory disorders in Alzheimer's disease would result from a toxic process. Observed olfactory deficits involve both identification and recognition of odors and detection thresholds. Nevertheless, patients with Alzheimer's disease rarely consult for sensorial deficits as the other signs of the disease predominate. Neuropathology data on the olfactory system are much more sparse in Parkinson's disease. Lewy bodies suggestive of Parkinson's disease have been observed in the olfactory bulb and pathways, but, unlike Alzheimer's disease, the olfactory disorders appear to be stable, changing little over time, as opposed to the evolution of neurological symptoms and cognition impairment. Clinicians should be aware that olfactory disorders are an integral part of Alzheimer's disease and Parkinson's disease. Screening for sensorial impairment however is a secondary objective in the context of these neurodegenerative diseases.