Early in the pandemic, when people with COVID-19 began reporting that they lost their sense of smell, Zara Patel, MD, figured as much. A professor of otolaryngology at Stanford Medicine, Patel has, for years, studied 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."

Zara Patel, MD, Professor of Otolaryngology, Stanford Medicine

Patel also knew that the condition could last a while and that few effective treatments were available.

According to a 2022 survey by Patel and colleagues, about 15% of people who experienced smell loss from COVID-19 continued to have problems six months later. That's roughly 9 million people in the U.S., and the number is growing. Many who report loss of smell also report loss of taste because smell is such a major component of how we experience food.

Now Patel's team has tested a new treatment for long-term, COVID-19-related smell loss using injections of platelet-rich plasma derived from a patient's own blood. In a trial of 26 participants, those who received the treatment were 12.5 times more likely to improve than patients who received placebo injections. The study was published Dec. 12 in the International Forum of Allergy and Rhinology.

Platelet-rich plasma is a concentrated form of plasma, the liquid portion of blood, with blood cells and other blood components removed. It's rich in platelets and, most importantly, growth factors — compounds known to help regenerate tissue. Platelet-rich plasma has been purported to treat mild arthritis when injected into joints, reduce wrinkles when used on the face, and even regrow hair when injected into the scalp.

Patel was skeptical of such a cure-all but was intrigued by a study showing that platelet-rich plasma injections were as effective as surgery in treating carpal tunnel syndrome, which is caused by compressing and injuring a nerve in the wrist. She knew that COVID-19-related smell loss also was a neurological problem, in which long-term effects of the virus prevent nerves deep in the nasal cavity from regenerating correctly. These nerves connect to the brain and normally regenerate every three to four months.

"It's a nerve damage and nerve regeneration issue that we're dealing with," she said.

Stealth attack

The SARS-CoV-2 virus doesn't target nerve cells directly; it attacks supporting cells known as sustentacular cells, which have the ACE-2 receptor the virus uses to infect cells. These cells play a role in correct nerve regeneration, so persistent inflammation and damage to these cells may lead to long-term loss of function.