Smell dysfunction significantly impacts quality of life and safety, with limited effective treatments. This open-label study evaluates the preliminary efficacy of combining non-invasive trigeminal nerve stimulation (TNS) with standard smell training (ST) to improve olfactory function. Participants will complete 8 weeks of at-home treatment and attend three in-person visits for assessment. Improvements in smell, mood, sleep, and quality of life will be measured.

Smell dysfunction is a serious health issue associated with everyday dangers like the inability to correctly identify nearby hazards (e.g. burning, gas leaks, or spoiled food), psychosocial problems including social withdrawal and reduced quality and enjoyment of life, a myriad of psychiatric conditions, and a generally increased risk of mortality. Further escalating the gravity of olfactory dysfunction is mounting evidence of olfactory disturbance as prodromal to neurodegenerative disorder.

Smell/olfactory training (ST) has emerged as the first-line, and only (current) treatment for smell loss (SL). Classic ST requires twice daily practice of sniffing odorants over the course of 3 months to regenerate olfactory neurons, engage smell-related cognitive functions, and retrain the brain to smell. ST is promising as a stand-alone treatment for SL. However, limitations of ST include the burden of many months of daily practice that leads to sub-optimal compliance, dropout and modest effect sizes. Methods to accelerate and enhance the benefits of ST are needed.

The olfactory and intranasal trigeminal circuits are structurally and functionally coupled brain systems, both essential to smell function. In fact, there is widespread central overlap and convergence of these systems that is believed to underlie the strong mutual, inhibitory influence between the odor-processing functions of the olfactory pathway and the pungency-processing functions of the trigeminal pathway. Given the convergence and functional influence the circuits have on one another, it is likely that the trigeminal system can be leveraged to activate or enhance olfaction. As such, a method that directly engages the intranasal trigeminal system may be an effective strategy to improve olfactory function.

Trigeminal nerve stimulation (TNS) is a non-invasive, pain-free, method of neuromodulation that delivers low levels of electrical stimulation to the trigeminal circuit, having potential to enhance smell function through activation of the highly connected olfactory-intranasal trigeminal systems. The study investigator's previously published work demonstrated TNS-enhanced psychophysical detection of odorants in healthy adults. The TNS device is FDA 510(k)-cleared for the at-home treatment of pain and attention deficit hyperactivity disorder (ADHD); however, its use in this study is to enhance olfactory function.

For this study, the study investigators will conduct an at-home, open label trial of TNS-enhanced ST in adults. The primary objectives are to determine initial efficacy of TNS-enhanced ST at treating smell loss. Objectives will be achieved through the following aims:

Aim 1: Determine the effects of TNS-enhanced ST on olfactory deficits. The study investigators hypothesize that 8 weeks of TNS-enhanced ST will show significant improvement in psychophysical and subjective olfactory function at 8 weeks compared to baseline.

Aim 2: Determine if TNS-enhanced ST improve other symptoms associated with smell loss. The study investigators hypothesize that 8 weeks of TNS-enhanced ST will show significant improvement in mood, sleep and quality of life at 8 weeks compared to baseline.

Inclusion Criteria:

• Age 18-80
• Self-reported or medically documented smell loss
• Treatment-seeking for their smell loss
• Naïve to TNS
• Able to comprehend English and provide informed consent

Exclusion Criteria:

• Medical Records not available to confirm eligibility
• Documented/Suspected head injury (e.g. sport, accident, blast exposure)
• Neurological disorder (e.g. epilepsy, neurodegenerative disorder)
• Serious mental illness (e.g. schizophrenia, bipolar disorder)
• Suicidal ideation within the last month
• Current (≤6 months) and/or heavy cigarette smokers (i.e. ≥ 10 pack-years)
• Oral/nasal steroids or other intranasal medications
• Immunomodulatory medications
• Pregnant or trying to become pregnant