Post-viral effects of COVID-19 in the olfactory system and their implications.

BACKGROUND: The mechanisms by which any upper respiratory virus, including SARS-CoV-2, impairs chemosensory function are not known. COVID-19 is frequently associated with olfactory dysfunction after viral infection, which provides a research opportunity to evaluate the natural course of this neurological finding. Clinical trials and prospective and histological studies of new-onset post-viral olfactory dysfunction have been limited by small sample sizes and a paucity of advanced neuroimaging data and neuropathological samples. Although data from neuropathological specimens are now available, neuroimaging of the olfactory system during the acute phase of infection is still rare due to infection control concerns and critical illness and represents a substantial gap in knowledge. RECENT DEVELOPMENTS: The active replication of SARS-CoV-2 within the brain parenchyma (ie, in neurons and glia) has not been proven. Nevertheless, post-viral olfactory dysfunction can be viewed as a focal neurological deficit in patients with COVID-19. Evidence is also sparse for a direct causal relation between SARS-CoV-2 infection and abnormal brain findings at autopsy, and for trans-synaptic spread of the virus from the olfactory epithelium to the olfactory bulb. Taken together, clinical, radiological, histological, ultrastructural, and molecular data implicate inflammation, with or without infection, in either the olfactory epithelium, the olfactory bulb, or both. This inflammation leads to persistent olfactory deficits in a subset of people who have recovered from COVID-19. Neuroimaging has revealed localised inflammation in intracranial olfactory structures. To date, histopathological, ultrastructural, and molecular evidence does not suggest that SARS-CoV-2 is an obligate neuropathogen. WHERE NEXT?: The prevalence of CNS and olfactory bulb pathosis in patients with COVID-19 is not known. We postulate that, in people who have recovered from COVID-19, a chronic, recrudescent, or permanent olfactory deficit could be prognostic for an increased likelihood of neurological sequelae or neurodegenerative disorders in the long term. An inflammatory stimulus from the nasal olfactory epithelium to the olfactory bulbs and connected brain regions might accelerate pathological processes and symptomatic progression of neurodegenerative disease. Persistent olfactory impairment with or without perceptual distortions (ie, parosmias or phantosmias) after SARS-CoV-2 infection could, therefore, serve as a marker to identify people with an increased long-term risk of neurological disease.

Olfactory function in chronic renal failure.

BACKGROUND: Patients with chronic renal failure (CRF) show a high prevalence of poor nutritional state so that dietary treatment becomes a significant part of the therapeutic regimen. Because smell plays an important role in nutrition, this study aimed to investigate olfactory function in CRF patients. METHODS: A total of 64 CRF patients were investigated. Forty-nine of them were treated with hemodialysis, 15 CRF patients were not dialysis dependent. For comparison we examined 15 healthy subjects. Olfactory function was assessed for odor discrimination, odor identification, and butanol odor thresholds. RESULTS: Olfactory loss was found in 56% of the patients, with 3 functional anosmics and 33 hyposmics. CRF had specific effects on individual tests of olfactory function. Elevated odor thresholds werefound in 11% of patients, 38% of patients had reduced odor discrimination, and 48% of patients exhibited deficits in odor identification. Results of psychological tests (Mini-Mental State Examination and Trail-Making Test) correlated with results from odor identification (p < 0.01) and discrimination (p < 0.01) but not with odor thresholds. CONCLUSIONS: The ability to discriminate and identify odors was found severely impaired whereas odor thresholds were similar to what is seen in the general population. Consequently, CRF patients should be counseled with regard to the possibility of reduced chemosensory functions.