Harvard Medical School researchers at Massachusetts Eye and Ear have, for the first time, induced a sense of smell in humans by using electrodes in the nose to stimulate nerves in the olfactory bulb, a structure in the brain where information received from the nose is processed and sent to deeper regions of the brain.
Reporting online in International Forum of Allergy & Rhinology, the research team describes their results, which provide a proof of concept for efforts to develop implant technology to return the sense of smell to those who have lost it.
“Our work shows that smell restoration technology is an idea worth studying further,” said corresponding author Eric Holbrook, HMS associate professor of otolaryngology and chief of rhinology at Mass. Eye and Ear.
“The development of cochlear implants, for example, didn’t really accelerate until someone placed an electrode in the cochlea of a patient and found that the patient heard a frequency of some type,” Holbrook said.
Smell loss, or anosmia, has an estimated prevalence of five percent in the general population. While some cases of anosmia may be treated by caring for an underlying cause such as nasal obstruction (i.e., due to swelling, polyps or sinus disease), other cases involving damage to the sensory nerves of the nose (e.g., head injury, viruses and aging) are much more complex. There are currently no proven therapies for these cases.
Sense of smell not only contributes to enjoyment of life but also to daily safety and well-being. We rely on our sense of smell to make us aware of smoke in order to detect fire, to alert us to natural gas leaks and to help us avoid eating rotten food.
It is estimated that more than 50 percent of the population over age 65 has experienced some loss of smell, so getting proper nutrition can be difficult because the sensation of flavor is closely tied to the sense of smell—and as flavor diminishes, appetite decreases in this population.
A cochlear implant for the nose
Motivated by work conducted by research colleaguesat Virginia Commonwealth University School of Medicine, Mass. Eye and Ear physicians wanted to address the question of whether electrical stimulation of the olfactory bulb could induce the sense of smell in human subjects.
Their findings demonstrate this feasibility. In their report, the researchers describe endoscopic procedures to position electrodes in the sinus cavities of five patients with an intact ability to smell. Three patients described sensations of smell (including reports of onion, antiseptic, sour and fruity aromas) as a result of the stimulation.
This breakthrough in human patients opens the door for a “cochlear implant for the nose” to be developed—though the study authors caution that the concept of an olfactory stimulator is more challenging than existing technologies may allow.
The most successful neuroprosthetic device in the world, cochlear implants have been on the market for more than three decades to electrically stimulate the auditory nerve to restore hearing in people with profound hearing loss.
“There’s currently so little that we can do for these patients, and we hope to eventually be able to re-establish smell in people who don’t have a sense of smell,” Holbrook said. “Now we know that electrical impulses to the olfactory bulb can provide a sense of smell—and that’s encouraging.”
Additional authors include co-first author Sidharth Puram, Washington University School of Medicine in St. Louis; Reiner See, HMS instructor in neurology at Massachusetts General Hospital; Aaron Tripp, Massachusetts General Hospital; and Dinesh Nair, HMS instructor in neurology at Brigham and Women’s Hospital.