Injection might help long COVID patients smell normally again

woman leans over a steaming pan on a stovetop and wafts its scent towards her face
After a bout of COVID-19, some people develop "parosmia," or an altered sense of smell, which can make pleasant scents smell awful. (Image credit: Hinterhaus Productions via Getty Images)

A nerve-block injection may help people with long COVID restore their sense of smell, new research suggests.

The injection, called a stellate ganglion block, is given into a bundle of nerves in the neck. The new study, presented Nov. 20 at the annual meeting of the Radiological Society of North America, along with other preliminary reports, hints that the injection could help long COVID patients with parosmia, or an altered sense of smell. This condition can make pleasant scents seem revolting and can have a major impact on a person's quality of life.

However, other researchers raised concerns about the study's design limitations.

"They lost a third of their patients to follow-up, which is terrible," and means they're working with very limited data, said Dr. Leigh Sowerby, an assistant professor of otolaryngology, or head and neck surgery, at Western University in Ontario, Canada who was not involved with the research.

Related: Brain fog in long COVID may be linked to blood clots

The stellate ganglia are bundles of nerves on either side of the voice box. They contain "sympathetic" nerves that go to the head, neck, arms and upper chest. The sympathetic nervous system controls the body's "fight-or-flight" response and is part of the autonomic nervous system that deals with body functions we don't consciously control, such as heart rate.

Stellate ganglion blocks block the activity of those neck nerves, often to treat chronic pain, headaches or certain vascular diseases.

In the new study, 54 patients received a stellate ganglion block to potentially treat parosmia after COVID-19, but only 37 provided follow-up information. Participants also received a steroid injection to decrease any inflammation in nerves that could impact smell.

Of the 37 patients, 22 reported some improvement one week after treatment and 18 reported notably increased improvement one month afterward. Three months out, the 22 patients reported that their symptoms improved 49%, on average.

Twenty-six participants returned for a second injection at least six weeks after their first. A second injection improved the symptoms of those who responded to the first, but it wasn't helpful for patients who had no effect from the first.

It's not clear why this treatment might be effective for parosmia, said Dr. Adam Zoga, an author of the new research who administered the injections and a radiologist at Jefferson Health in Philadelphia.

Some think the distortion to a person's sense of smell might be due to an overactive sympathetic nervous system, sent into overdrive during the COVID-19 infection, Sowerby said. The theory is that the injection "resets" this part of the nervous system, he said.

There have been previous case reports, as well as survey-based and observational studies, that suggest stellate ganglion blocks might help treat parosmia. But these studies, like the new research, do not include a control group that received a placebo. Without a control group, it's hard to know if the nerve block is truly effective, Sowerby said. Sowerby and a group of other researchers plan to conduct this type of study.

Sowerby said the study's most significant limitation was the number of participants that didn't provide follow-up data. Details of those patients' symptoms or why they didn't follow up could make a substantial difference in the results. Another limitation is that patients reported their symptom improvements, rather than the improvements being objectively measured.

"There are some olfactory [smell] testing tools out there," said Kai Zhao, an associate professor of otolaryngology at the Ohio State University who was not involved in the research. "I'm hoping some future studies can use objective testing."

While more research is needed on stellate ganglion blocks for parosmia, Zoga noted that the procedure is usually fairly low risk and seems to work well for some people, while few other treatments do.

Sowerby agreed — even if the improvements are really a placebo effect, the treatment has the potential to help people. "It's still ultimately about healing your patient," he said.

This article is for informational purposes only and is not meant to offer medical advice. 

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Rebecca Sohn
Live Science Contributor

Rebecca Sohn is a freelance science writer. She writes about a variety of science, health and environmental topics, and is particularly interested in how science impacts people's lives. She has been an intern at CalMatters and STAT, as well as a science fellow at Mashable. Rebecca, a native of the Boston area, studied English literature and minored in music at Skidmore College in Upstate New York and later studied science journalism at New York University.