Long-Term Smell Loss in COVID-19 Tied to Damage in the Brain's Olfactory Bulb

Pauline Anderson

April 12, 2022

Editor's note: Find the latest COVID-19 news and guidance in Medscape's Coronavirus Resource Center.

Patients with COVID-19, especially those with an altered sense of smell, have significantly more axon and microvasculopathy damage in the brain's olfactory tissue versus non-COVID patients. These new findings from a post-mortem study may explain long-term loss of smell in some patients with the virus.

"The striking axonal pathology in some cases indicates that olfactory dysfunction in COVID-19 may be severe and permanent," the investigators led by Cheng-Ying Ho, MD, PhD, associate professor, Department of Pathology, Johns Hopkins University School of Medicine, Baltimore, Maryland, write.

"The results show the damage caused by COVID can extend beyond the nasal cavity and involve the brain," Ho told Medscape Medical News.

The study was published online April 11 in JAMA Neurology.

A More Thorough Investigation

Patients infected with SARS-CoV-2, which causes COVID-19, present with a wide range of symptoms. In addition to respiratory illnesses, they may exhibit various non-respiratory manifestations of COVID-19.

One of the most prevalent of these is olfactory dysfunction. Research shows such dysfunction, including anosmia (loss of smell), hyposmia (reduced sense of smell), and parosmia (smells that are distorted or unpleasant), affects 30% to 60% of patients with COVID-19, said Ho.

However, these statistics come from research before the advent of the Omicron variant, which evidence suggests causes less smell loss in patients with COVID, she said.

Previous studies in this area mainly focused on the lining of the nasal cavity. "We wanted to go a step beyond to see how the olfactory bulb was affected by COVID infection," said Ho.

The study included 23 deceased patients with confirmed COVID-19 ranging in age from 28 to 93 years at death (median 62 years, 60.9% men). It also included 14 controls who tested negative for COVID-19, ranging in age from 20 to 77 years (median 53.5 years, 50% men).

Researchers collected post-mortem tissue from the brain, lung, and other organs and reviewed pertinent clinical information.

Most patients with COVID died of COVID pneumonia or related complications, although some died from a different cause. Some had an active COVID infection and others were "post-infection, meaning they were in the recovery stage," said Ho.

Six patients with COVID-19 and eight controls had significant brain pathology.

Compared with controls, those with COVID-19 showed significantly worse olfactory axonal damage. The mean axon pathology score (range 1-3 with 3 the worst) was 1.921 in patients with COVID-19 and 1.198 in controls (95% CI, 0.444 - 1.002; P < .001).

The mean axon density in the lateral olfactory tract was significantly less in patients with COVID-19 than in controls (P = .002), indicating a 23% loss of olfactory axons in the COVID group.

Comparing COVID patients with and without reported loss of smell, researchers found those with an altered sense of smell had significantly more severe olfactory axon pathology.

Vascular Damage

Patients with COVID also had worse vascular damage. The mean microvasculopathy score (range, 1-3) was 1.907 in patients with COVID-19 and 1.405 in controls (95% CI, 0.259 - 0.745; P < .001).

There was no evidence of the virus in the olfactory tissue of most patients, suggesting the olfactory pathology was likely caused by vascular damage, said Ho.

What's unique about SARS-CoV-2 is that although it's a respiratory virus, it's capable of infecting endothelial cells lining vessels.

"Other respiratory viruses only attack the airways and won't attack vessels, but vascular damage has been seen in the heart and lung in COVID patients, and our study showed the same findings in the olfactory bulb," Ho explained.

The researchers divided patients with COVID by infection severity: mild, moderate, severe, and critical. Interestingly, those with the most severe olfactory pathology were the ones with milder infections, said Ho.

She noted other studies have reported patients with mild infection are more likely to lose the sense of smell than those with severe infection, but she's skeptical about this finding.

"Patients with severe COVID are usually hospitalized and intubated, so it's hard to get them to tell you whether they've lost smell or not; they have other more important issues to deal with like respiratory failure," said Ho.

Advanced age is associated with neuropathologic changes, such as tau deposits, so the researchers conducted an analysis factoring in age-related brain changes. They found a COVID-19 diagnosis remained associated with increased axonal pathology, reduced axonal density, and increased vascular pathology.

"This means that the COVID patients had more severe olfactory pathology not just because they had more tau pathology," Ho added.

New Guidance for Patients

Commenting for Medscape Medical News, Davangere P. Devanand, MD, professor of psychiatry and neurology and director of geriatric psychiatry, Columbia University Irving Medical Center, New York City, said the findings indicate the damage from COVID in the olfactory pathway may not be reversible as was previously thought.

"This has been suggested before as a possibility, but the autopsy findings in this case series indicate clearly that there may be permanent damage," he said.

The results highlight the need to monitor patients with COVID for a smell deficit, said Devanand. 

"Assuring patients of a full recovery in smell and taste may not be sound advice, although recovery does occur in many patients," he added.

He praised the study design, especially the blinding of raters, but noted a number of weaknesses, including the small sample size and the age and gender discrepancies between the groups.

Another possible limitation was inclusion of patients with Alzheimer's and Lewy body pathology, said Devanand.

"These patients typically already have pathology in the olfactory pathways, which means we don't know if it was COVID or the underlying brain pathology contributing to smell difficulties in these patients," he said.

He noted that, unlike deceased COVID cases in the study, patients who survive COVID may not experience axonal and microvascular injury in olfactory neurons and pathways and their sense of smell may make a full return.

Devanand said he would have liked more detailed information on the clinical history and course of study participants and whether these factors affected the pathology findings.

The study was supported by grants from the National Institutes of Health.

Ho and Devanand have reported no relevant financial disclosures.

JAMA Neurol. Published Online April 11, 2022. Full text

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