50 % of the population between 65 and 80 years of age report a reduced ability to smell. In patients over 80 years of age it reaches 75%. One study illustrated that people over 50 years of age had more difficulty in detecting pleasant smells than unpleasant smells.
Chronic alcohol and tobacco use
Enolism alters the smell The problem improves with cessation of alcohol consumption due to involvement of certain brain areas.
Smokers of more than 20 cigarettes a day are more likely to have smell and taste problems.
Neurodegenerative disorders
Olfactory problems affect 90% of patients with a diagnosis of Alzheimer's diseaseof the disease of ParkinsonThe most common symptoms of dementia are Lewy body dementia and frontotemporal dementia. Individuals with these pathologies do not usually report problems with smell or taste directly; the disorder is initially suspected indirectly by changes in appetite and changes in the patient's weight (weight loss).
Alzheimer's disease
In this pathology, the severity of the loss of smell correlates with the degree of dementia. Anatomically speaking, amyloid plaques and neurofibrillary tangles are observed in the medial temporal lobe (in the entorhinal and piriform cortex, but also in the olfactory tract and bulb and in the anterior olfactory nucleus).
Olfactory tests can distinguish Alzheimer's disease (AD) from vascular dementia, pseudo-dementia or depression (the latter three pathologies do not necessarily involve impairment of this sense).
In the early stages of Alzheimer's disease, olfactory disturbances can already be found.
Parkinson's disease (PD) , Parkinson's disease dementia and Lewy body dementia
Patients with Parkinson's disease where the clinical features of tremor dominate have the sense of smell less altered than the rigid type. Usually the loss of smell does not progress over time (unlike in AD). Abnormal accumulation of amyloid precursor fragments and alpha-synuclein is observed. Improper folding makes these deposits highly insoluble (these complexes are located in the olfactory bulb, anterior olfactory nucleus and medial temporal lobe).
Not all patients with PD have problems with the smell.
Frontotemporal dementia
In 70 % of cases, the loss of smell is asymptomatic, the patient is not aware of the problem. In this disease the dysfunction is localised in the orbitofrontal cortex and temporal lobes.
Multiple sclerosis
The prevalence of olfactory dysfunction is from 40%. The lesion is located in the inferior part of the frontal lobe and in the temporal lobes (some studies included the olfactory tract and the olfactory bulb. The dysfunction would be related to flare-ups and disease progression.
Epilepsy
Olfactory hallucinations may be the manifestation of an epileptic seizure, and are usually short-lived (one minute or less) as opposed to those occurring in stroke (lasting from five minutes to hours).
COVID-19
The loss of the smell and of the Taste after COVID-19 infection affects 20-85% of patients. Much research is still ongoing to understand the pathophysiological basis and to find adapted treatments.
Interaction of the coronavirus with ACE-2 receptors expressed on sustentacular cells (supporting cells) and taste buds results in direct damage to the olfactory and gustatory systems. The virus also invades olfactory neurons and produces local inflammation. Therefore, COVID-19 patients with loss of smell or taste may benefit from neuroprotective (statins, minocycline, intranasal vitamin A, insulin, omega-3 and melatonin), anti-inflammatory (corticosteroids, melatonin, omega-3 etc.) or depolarising (phosphodiesterase inhibitors such as pentoxifylline, caffeine and theophylline) agents (2).
Important ideas
Loss of smell may go unnoticed but is no less important. Weight loss or decreased appetite sometimes reflect such a problem which will have to be correctly diagnosed. Once this problem has been confirmed the investigation will have to continue as it may only be the tip of the iceberg (neurodegenerative disease o ageing accelerated...).
On the other hand, the sense of smell can be worked, and there are home training kits (e.g. Arkopharma Olfae). In the diagnosis and neurologists and otolaryngologists may be involved in the treatment of olfactory disorders.
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