COVID-19 and olfactory dysfunction: a looming wave of dementia?

Olfactory dysfunction is a hallmark symptom of COVID-19 disease resulting from the SARS-CoV-2 virus. The cause of the sudden and usually temporary anosmia that most people suffer from COVID-19 is likely entirely peripheral-inflammation and other damage caused by the virus in the sensory epithelium inside the upper recesses of the nasal cavity can damage or prevent chemicals from properly activating the olfactory sensory neurons. However, persistent olfactory dysfunction from COVID-19, in the form of hyposmia and parosmia (decreased or altered smell) may affect as many as 15 million people worldwide. This epidemic of olfactory dysfunction is thus a continuing public health concern. Mounting evidence suggests that the SARS-CoV-2 virus itself or inflammation from the immune response in the nasal sensory epithelium may invade the olfactory bulb, likely via non-neuronal transmission. COVID-19-related long-term olfactory dysfunction and early damage to olfactory and limbic brain regions suggest a pattern of degeneration similar to that seen in early stages of Alzheimer's disease, Parkinson's disease, and Lewy body dementia. Thus, long-term olfactory dysfunction coupled with cognitive and emotional disturbance from COVID-19 may be the first signs of delayed onset dementia from neurodegeneration. Few treatments are known to be effective to prevent further degeneration, but the first line of defense against degeneration may be olfactory and environmental enrichment. There is a pressing need for more research on treatments for olfactory dysfunction and longitudinal studies including cognitive and olfactory function from patients who have recovered from even mild COVID-19.NEW & NOTEWORTHY More than 15 million people worldwide experience persistent COVID-19 olfactory dysfunction, possibly caused by olfactory bulb damage. SARS-CoV-2 can cause inflammation and viral invasion of the olfactory bulb, initiating a cascade of degeneration similar to Alzheimer's disease and Lewy body disease. People who have had even mild cases of COVID-19 show signs of degeneration in cortical areas connected with the olfactory system. These data suggest a wave of post-COVID dementia in the coming decades.