Plasma Injections May Ease Post-covid Loss of Smell

woman smelling pink flower

In a recent trial, platelet-rich plasma injections helped more than half of the patients with COVID-19-related persistent smell loss.

Zara Patel suspected this early in the pandemic when people infected with COVID-19 reported losing their sense of smell. Patel, an otolaryngology professor at Stanford Medicine, has spent years researching loss of smell as a symptom of viral infections.

“Many viruses can cause smell loss, so it wasn’t surprising to us as rhinologists when we found out that COVID-19 causes loss of smell and taste. It was almost expected,”

she said. Patel was also aware that the condition could last for a long time and that there were few effective treatments available.

Platelet-rich Plasma

About 15% of those who experienced smell loss from COVID-19 continued to have issues six months later, according to a 2022 survey by Patel and colleagues. In the US, that amounts to about 9 million people, and the number is rising.

Due to the importance of smell in how we perceive food, many people who report a loss of smell also report a loss of taste.

Using injections of platelet-rich plasma derived from a patient’s own blood, Patel’s team has now tested a new treatment for long-term COVID-19-related loss of smell. In a study involving 26 participants, those who received the treatment were 12.5 times more likely to experience improvement than those who received placebo injections.

Plasma is the liquid portion of blood, and platelet-rich plasma is a concentrated form of plasma from which blood cells and other blood components have been removed. It is rich in platelets and, more importantly, growth factors, which are known to aid in tissue regeneration.

When injected into joints, platelet-rich plasma is said to treat mild arthritis, reduce wrinkles on the face, and even regrow hair when injected into the scalp.

A Neurological Issue

Patel was skeptical of such a panacea, but she was intrigued by a study demonstrating that platelet-rich plasma injections were just as effective as surgery in treating carpal tunnel syndrome, which is caused by nerve compression and injury in the wrist.

She was aware that COVID-19-related olfactory dysfunction was also a neurological issue, as the virus’s long-term effects prevent the regeneration of nerves in the deep nasal cavity. These nerves are connected to the brain and regenerate every three to four months on average.

“It’s a nerve damage and nerve regeneration issue that we’re dealing with,”

she explained.

SARS-CoV-2 Virus Nerve Damage

The SARS-CoV-2 virus does not directly target nerve cells; rather, it infects sustentacular cells, which contain the ACE-2 receptor used by the virus to infect cells. These cells play a role in proper nerve regeneration; therefore, persistent inflammation and damage to these cells may result in permanent function loss.

When the pandemic struck, Patel had already completed a small pilot study demonstrating the safety of platelet-rich plasma injections in the nasal cavity, so she shifted her plans for a larger trial to focus specifically on COVID-19-associated smell loss.

All participants had confirmed prior COVID-19 infections and ongoing odor loss that persisted for six to twelve months. Additionally, they had to have previously tried other therapies like steroid rinses and olfactory training.

“I wanted to make sure that whatever intervention I was going to study was not just in place of or equivalent to the treatments we’re already doing, but a benefit above and beyond,”

Patel said.

Sniffin’ Sticks

Every two weeks for six weeks, half of the participants received platelet-rich plasma injections into the tissue deep inside their nasal cavities, while the other half received injections of saline as a placebo. This was a randomized, controlled trial, and both participants and researchers were in the dark about who received what.

The Sniffin’ Sticks test, a common olfactory test, was used by the researchers to gauge participants’ ability to smell. In order to receive a possible score of 48, test takers must be able to recognize a variety of odors, including some that are pleasant (flowers) and some that are unpleasant (rotten eggs).

The platelet-rich plasma group scored on average 6.25 points higher than they did prior to treatment, which was 3.67 points higher than the placebo group, when the participants were reassessed three months after their initial injection.

They improved most in smell discrimination, the capacity to distinguish between various odors. At three months, 57.1% of those receiving platelet-rich plasma had demonstrated a clinically significant improvement, as opposed to 8.3% of those receiving placebo.

Placebo Effect

It’s interesting to note that both groups reported similar improvements when the participants rated their own sense of smell. According to Patel, earlier research has shown that subjective and objective improvements don’t always coincide.

According to Patel, the improvements in the placebo group may have been partially brought on by a placebo effect, but they may also indicate that some spontaneous recovery may still occur even six months later.

COVID-19 has drawn attention to post-viral smell loss, as well as perhaps a greater appreciation for the role smell plays in our daily lives, noted Patel. Though this study did not assess taste loss, the recovery of smell is likely to aid in the recovery of taste.

“People tell me all the time that they never realized how important their sense of smell and taste was to them and their quality of life until they lost it. People say, ‘My life has gone gray,'”

she said. Patients not participating in the trial can now receive platelet-rich plasma injections from Patel.

“Our olfactory systems can be resilient. But the sooner you perform some sort of definitive intervention, probably the better chance you have of improvement,”

Patel concluded.

  1. Yan, CH, Jang, SS, Lin, H-FC, et al. Use of platelet-rich plasma for COVID-19–related olfactory loss: a randomized controlled trial. Int Forum Allergy Rhinol. 2022; 1- 9.


Last Updated on January 30, 2023