Author: Eloy Gutierrez Sendra (Master´s Student)
Reference: Lafreniere D and Mann N. Anosmia: Loss of smell in the elderly. Otolaryngol Clin N Am 2009, 42: 123-131
Loss of olfaction and taste give rise to changes in an individual´s appetite and food preferences, affecting one´s quality of life and nutritional state. The sensitivity to smells decreases with age, and occasionally the individual is not aware of this loss. These changes in olfaction are more pronounced in Alzheimer´s and Parkinson disease, since the deterioration of the olfactory process is further affected by the loss of central processing of information.
It has been observed that smells reach the olfactory epithelium through the nose and retro-nasal pathway. The mitral cells of the olfactory epithelium detect the smell by the binding of the smells to their corresponding receptors. The mitral cells then send the information from the epithelial cilia to the olfactory bulb, and here it transforms the chemical signals to electric responses and are sent directly to the limbic system and neocortex, where the information is processed. The olfactory receptor´s function depends on the composition of the mucosal layer. In this manner, the composition of the nasal secretions can change in the presence of inflammation, toxic exposure or with certain diseases, such as Alzheimer´s and Parkinson´s disease. Therefore, damaged olfactory epithelium puts in risk smell processing at the sensorial or perceptive level. However, it has been observed that the olfactory epithelium can replace lost olfactory neurons, known as olfactory neurogenesis. A reduced mucosal-cilia movement, decreased enzymatic activity or low hydration can damage the olfactory epithelium.
It has been shown that the number of olfactory neurons decreases with age while olfactory mucosa irregularities decreases, due to apoptotic processes that cause traumatic anosmia. The study published by Loo et al discovered that anterior olfactory cells suffer more changes than medial and posterior ones. Furthermore, after the environmental exposure there is a strong decrease in immature neurons compared to mature neurons. Other factors such as smoking, viral infections, and disease, among others, degenerate the peripheral neurons, developing into aberrant synapses and eventually contributing to the loss of mitral cells in the olfactory bulb.
The major neurodegenerative disorders that accompany olfactory alterations include:
– Alzheimer´s disease: Pathological lesions have been observed in the brain tissue due to the production of neurofibrillary plaques composed from a protein associated with abnormally phosphorylated fibrillary microtubules (TAU). There is a loss of smell identification and detection that aggravates as the disease progresses. The heterozygote and homozygote individuals with the genotype E4 apolipoprotein present a higher risk to develop Alzheimer´s disease and major cognitive degeneration.
-Parkinson´s Disease: In this disease, one of the characteristic symptoms correspond to olfactory dysfunction that is usually detected early on. Also, there is a decrease in smell processing and identification. The neurons that are affected by the disease present what is known as Lewy bodies, which are distributed throughout the whole cerebral cortex and deep zone of the mid-brain or brainstem.
-Frontotemporal dementia: The frontal lobes are affected, which causes a deficiency in smell processing.
-Semantic dementia: The temporal lobes are affected, which causes deficiencies in olfactory memory.
Furthermore, there is a loss of olfactory taste that corresponds with the aging process and the conditions of the mouth, which affects retronasal olfaction. Certain objects such as prosthetic dentures affects olfactory sensitivity as well as the mechanical propagation of smells to the nasal pit. Lastly, there are a large population of individuals that lose olfaction after surgery, traumatism or viral infection. In these cases, its recovery is difficult to treat.