Santiago Ezquerro García. Alumno del Master en Biotecnología y Bioingeniería (2015-2016)
LOSS OF OLFACTION DUE TO AGING IN HUMANS AND ITS QUANTIFICATION
The loss or dysfunction of olfaction can significantly impact the individual´s appetite, nutrition and physical/mental wellbeing. The importance of this sense is evident not only in nutritional aspects, but also in many aspects of everyday life. In this sense, Stevens et al (1985) estimated that 45% of the elderly are not capable of detecting diluted petrol gas in the levels dictated in the norms of safety, being only 10% in young adults. This is due to functional and pathological changes that appear in the olfactory system during aging. In particular, Alzheimer´s and Parkinson Disease give rise to an important loss of olfaction.
There are several functional tests to determine loss of olfaction. The most relevant are:
- PSYCHO-PHYSICAL TEST: Are the most commonly used and include tests of identification. These tests allow the detection of olfaction loss related to aging.
- ELECTROPHYSIOLOGICAL TESTS: Is the most useful procedure, performed by measuring brain electrical activity induced by odours and detected using electrodes placed on the scalp. Reflects changes in large populations of neurons when activated by smells.
- PSYCHO-PHYSIOLOGICAL TESTS: Measure the response of the nervous system due to olfactory stimulation combining the previous methods.
CAUSES OF OLFACTORY LOSS RELATED TO AGING
Loss of olfaction is due to alterations in the nasal structure, olfactory epithelium, olfactory bulb and superior brain structures. These alterations have been associated in certain degrees to olfactory dysfunction. Also, several genes have been shown to contribute, in a slight manner, to the decrease in identification of smells related to aging. The main causes are:
A) Changes in non-olfactory elements of the nose
Curiously, aging is related to a significant decrease in asthma and a series of nasopharyngeal anomalies, such as adenoid hypertrophy, inflammation, cystic degeneration or secretion of thick mucus. Recently, it has been shown that sleep apnoea, which is related to restrictions of nasal air flow (snoring), increases its prevalence with age and has an adverse effect in olfactory function. Also, a non-neuronal process that greatly compromises olfactory function is the decrease in size and number holes of the ethmoid bones or cribriform plate. This corresponds to a bone located in the nose that contains numerous pores for olfactory nerves. The occlusion or decrease of the size of the pores can cause the loss of axons of the neurons that connect with the olfactory epithelium.
B) Changes in olfactory neuroepithelium
Histological studies of human olfactory epithelium have demonstrated that changes related to age, including decrease in receptor numbers, thinning of the epithelium and alterations of the pattern and distribution of olfactory cells, embedded in the respiratory epithelium. Basically, it has been observed that the olfactory epithelium is substitute by the respiratory epithelium. There are several reasons for the decrease in olfactory cell receptors and other elements of this epithelium:
- The neurogenic processes are affected by aging.
- The age-related decrease in size and number of cribriform plate holes can cause necrosis of the olfactory cells.
- Immunological and enzymatic defence mechanisms are affected, which are necessary to maintain epithelium integrity.
- Age-related loss of specificity response in individual cellular receptors.
- The exposure to air-transported environmental agents, including pollution, cigarette smoke, virus, bacteria and other xenobiotics; damage the olfactory epithelium that increases with age.
C) Changes in the olfactory bulb
Similar to the olfactory epithelium, the size of the olfactory bulb decreases with age in humans and animals. This decrease seems to be mainly due to damage of the olfactory neuroepithelium due to nasal infections, chronic rhinitis, lack of air flow and exposure to xenobiotics.
D) Changes in central regions of the brain implicated in olfaction
Aging is accompanied by loss of brain mass. This is due to the rapid volume loss of hippocampal, amygdala, piriform cortex, anterior olfactory nucleus and frontal poles of the brain. It is important to note that certain neurochemical changes can occur in specific regions due to aging, such as limbic structures. Image studies suggest that neurotransmitter receptors significantly decrease in the brains of the elderly.