Abstract
Purpose: The olfactory system is particularly vulnerable in an ageing brain, both anatomically and functionally, and these brain changes are more pronounced among individuals with trisomy 21. Furthermore, the age of the system starts to deteriorate, and the mechanism involved is unclear in an individual with trisomy 21. Therefore, the present review aims to summarise the available information related to this topic and to suggest questions still unanswered which can be a subject of further research. Methods: A systematic literature search of trisomy 21 and olfactory dysfunction was conducted using PubMed/MEDLINE and Scopus electronic database following PRISMA guidelines. References and citations were checked in the Google Scholar database. Reports were extracted for information on demographics and psychophysical evaluation. Then, the reports were systematically reviewed based on the effects of ageing on the three olfactory domains: threshold, discrimination, and identification. Results: Participants with trisomy 21 show an early onset of olfactory impairment, and the age effect of the olfactory deficit is fully expressed at age > 30 years old. The three olfactory domains, threshold, discrimination, and identification, are suggested to be impaired in trisomy 21 participants with age > 30 years old. Conclusions: Olfactory dysfunction in an individual with trisomy 21 commences at a relatively young age and affects the three olfactory domains. A challenge for the future is to quantitatively establish the olfactory function of an individual with trisomy 21 at all ages with more detailed measurements to further understand the pathophysiology of this brain deterioration.
Highlights
Trisomy 21, or Down syndrome (DS), is a genetic disorder affected by the manifestation of all or part of the third copy of chromosome 21 and is the most common congenital chromosome disorder in humans [1]
The present review aims to address the hypothesis that adults with trisomy 21 show greater deficits in olfactory function than younger adults with trisomy 21
The present review aims to summarise at what age this olfactory dysfunction starts to develop in trisomy 21 and gather available information related to this topic, and may suggest questions still unanswered, which can be a subject of further research
Summary
Trisomy 21, or Down syndrome (DS), is a genetic disorder affected by the manifestation of all or part of the third copy of chromosome 21 (trisomy 21; +21) and is the most common congenital chromosome disorder in humans [1]. Individuals with trisomy 21 manifest intellectual and physical delay, and dysmorphic facial features and/or congenital heart malformations [4,5], susceptibility to leukaemia [6], and infections. The most consistent differences, involve the brain [7] These differences are still incompletely understood, causing a developmental abnormality that results in lifelong cognition differences [7]. Individuals with trisomy 21 are associated with a group of clinical manifestations of ‘accelerated ageing’ [8,9] in which individuals with trisomy 21 will age faster than the general population.
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