Stellate Ganglion Block for the Treatment of COVID-19-Induced Parosmia: Double-Blinded, Placebo-Controlled Randomized Clinical Trial

One of the hallmark symptoms of infection with SARS-CoV-2 is olfactory dysfunction (OD). While the majority of patients recover from COVID dysosmia, up to 15%-25% have long-term hyposmia and it is estimated that up to 1.2 million people in the United States will experience chronic OD from the COVID-19 pandemic. A unique feature of COVID-19-associated OD is the high rate of persistent parosmia. In one study of 222 patients with COVID-19-associated olfactory dysfunction by Lerner et al, 148 (67%) of these patients experienced parosmia at some point, and estimates of persistent parosmia 6 months after COVID-19 infection range from 25% to 57%.Patients with OD have decreased quality-of-life and have described their lives as if "living in a box." These patients have concerns for environmental safety, decreased enjoyment of their food, depression, anxiety, and even a higher risk of mortality. Unpublished work by our group has demonstrated a relationship between parosmia and increased risk of screening positive for anxiety, depression, and suicidality.

The COVID-19 pandemic has highlighted the importance of the sense of olfaction, but no standard of care treatment for post-viral OD exists. The most commonly used treatment for post-viral OD is olfactory training; however, a large proportion of patients do not receive benefit and continue to have persistent symptoms. A multitude of other therapies have been tried in randomized clinical trials with minimal success, including theophylline, vitamin A, sodium citrate, and intranasal insulin. As a result, there is a critical need for the development of a novel intervention to address the large number of patients with OD due to the COVID-19 pandemic.

The stellate ganglion block (SGB) is proposed to inhibit the sympathetic neural connections within the head, neck, and upper extremity, improve regional blood flow, reduce adrenal hormone concentration, and even reestablish circadian rhythms through modulation of melatonin. The SGB has been used successfully in a multitude of disorders, including post-traumatic stress disorder, migraine, and complex regional pain syndrome. A meta-analysis of 12 clinical trials found that SGB was superior to placebo in reducing pain scores among patients with various sympathetic hyperactivity-associated disorders. Many "long COVID" symptoms, those that persist after recovery from acute COVID-19 infection, are hypothesized to be, at least in part, a result of sympathetic hyperactivity resulting in positive feedback loops. Therefore, the stellate ganglion block (SGB) is hypothesized to reset the balance of the autonomic nervous system and provide relief for long COVID symptoms, including OD.

The SGB was first proposed to treat OD by Lee et al in 2003, where 38 post-viral OD participants were treated with SGB and 13 participants remained untreated as controls. Subjective olfactory function improved in 27 (71%) of the treated participants compared to zero (0%) of the controls. Olfactory perception was improved significantly in the SGB group assessed both by the butanol threshold test and odor identification test. Additional studies by Moon et al noted improvement in OD of various etiologies after repeated SGBs. However, each of these studies were limited by their use of unvalidated outcome measures in a heterogeneous OD population in the pre-COVID era, limiting their external validity to the present day.

A multitude of anecdotal news reports and published case series point to a possible beneficial effect of the SGB on both chronic COVID-19-induced OD and various other long COVID symptoms. Numerous pain management clinics across the country are offering the SGB for long COVID with thousands of dollars of out-of-pocket costs to patients without adequate evidence to justify its use. One case series of 195 parosmic patients noted up to a 75% response rate after SGB.