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When they were examining the brain scans of COVID-19 patients hospitalized for particularly severe infections, French doctors found something unusual: inflamed growths called nodules directly behind their eyes.

The University of Paris scientists studied 129 patient MRIs and found nodules behind the eyes of nine of them, eight of whom had nodules behind both eyes, Live Science reports. The discovery, while still surrounded by unanswered questions, serves as a warning that some coronavirus symptoms might be flying under the radar as doctors focus on the more urgent, life-threatening problems caused by COVID-19.

"The eye problems we found can be potentially very serious because they occur in the... macular region, which is the region responsible for giving us clear vision and the ability to see fine detail," study lead author Dr. Augustin Lecler told Live Science. "If persisting, it might potentially lead to severe vision loss or even blindness."

For now, scientists aren't sure why these nodules popped up. Their research, published Tuesday in the journal Radiology, gathered MRIs from from patients who had previously been hospitalized. That means they weren't able to definitively establish a causal link between severe COVID-19 and the nodules, nor were they able to nail down why the growths formed, how long they lasted, or whether they actually led to noticeable vision loss.

"It is critical to remember that eye problems can go unrecognized in the ICU, and clinicians need to be vigilant in first identifying if there is an orbital [eye] problem to protect the patient's vision," Northwell Health Zucker Hofstra School of Medicine neuroradiology chief Dr. Claudia Kirsch said in an accompanying Radiology editorial.

On one hand, the eye contains the same ACE2 receptors that the coronavirus gloms onto when attacking a cell, so they could be the result of a direct infection. But the growths also could stem from an inflammatory or immune response to the coronavirus elsewhere in the body.

But the team is working on answering those questions, Live Science reports, by launching a follow-up study with the original and new coronavirus patients. And this time, crucially, their medical assessments will include eye exams.