Comparison of olfactory and gustatory disorders in Alzheimer's disease

Patients with Alzheimer's disease (AD) develop olfactory and gustatory disorders. However, the order of failure and relevance of the pathophysiology are unclear. We compared olfactory identification and whole mouth gustation in patients with AD to those with mild cognitive impairment (MCI) and to healthy controls (HC) and assessed correlations with pathophysiology. Patients with AD (n = 40), MCI (n = 34), and HC (n = 40) were recruited. We performed the Odor Stick Identification Test for Japanese (OSIT-J), gustatory test by the intraoral dropping method using taste solutions, Mini-Mental State Examination (MMSE), Alzheimer's Disease Assessment Scale-cognitive subscale Japanese version (ADAS-J cog), Touch Panel-type Dementia Assessment Scale (TDAS), and measurement of amyloid β (Aβ) 42 and phosphorylated tau (p-tau) 181 levels in cerebrospinal fluid (CSF). Patients with AD and MCI had lower OSIT-J scores than did the HC. The OSIT-J score was correlated with the MMSE, ADAS-J cog, TDAS, and Aβ42 results. There were no significant differences in the gustatory test scores among the three groups. The gustatory test score was only correlated with the MMSE, ADAS-J cog, and TDAS results. Olfactory function decreased in AD and MCI patients and was associated with CSF biomarker levels and cognitive disorders. The results suggest that olfactory function is impaired in early stage of AD. Gustatory function was not correlated with CSF biomarkers, which suggests that it may not be impaired in early stage of AD.