Anodal transcranial direct current stimulation (A-tDCS) significantly improved smell performance in patients with anosmia following COVID-19 infection compared with sham treatment, according to findings published in the Journal of Neurology, Neurosurgery, and Psychiatry.
A characteristic symptom of COVID-19 infection is loss of smell, which may last from 15 to 180 days and contribute to mood disorders, decreased identification of environmental hazards, and reduced pleasure in eating.
Researchers conducted a double-blind, sham-controlled study to determine the impact of A-tDCS combined with olfactory training in 7 patients post-COVID-19 infection with olfactory dysfunction. Exclusion criteria included individuals with severe mood disorder, rhinologic diseases, epilepsy, and a sensitive scalp. Patients were not allowed to take medications to alleviate olfactory symptoms.
Prior to treatment, the researchers obtained baseline olfactory function measurements using a visual analog scale (VAS) in which patients self-rated their loss of smell on a scale of 0 to 10 with 0 indicating complete loss of smell and the Sniffin’ Sticks test. The test was scored out of 16 total points (12-16, normosmia; 9-11, hyposomia; and 8 or below, anosmia). All patients scored below 8, indicating anosmia at baseline.
Patients received sham stimulation (S-tDCS) first to avoid potential carry-over of treatment effects by applying actual treatment first. Actual treatment consisted of 20 minutes of A-tDCS at an intensity of 1.5 mA over 5 consecutive days for 2 weeks with anodal placement over the left prefrontal cortex (since tDCS cannot access the orbitofrontal cortex) and cathode placement over the contralateral shoulder. Concurrent olfactory training while the patients underwent neural stimulation consisted of the patients sniffing 10 different odors (rose, eucalyptus, lemon, star anise, rosemary, strawberry, coconut, vanilla, pine tree, bergamot) for 10 seconds each in random order, identifying the odor, and rating odor intensity.
Patients repeated the smell assessment tests following S-tDCS, A-tDCS, and 3 months following the end of treatment. Between actual treatment and the 3-month follow-up, patients demonstrated significant improvements in both subjective VAS and objective Sniffin’ Stick measurements with average scores either doubling or tripling compared with scores at baseline and following S-tDCS. After S-tDCS, neither the mean VAS or Stiffin’ Stick scores improved significantly (P =.0625 and P =.2500, respectively), whereas they improved after actual treatment and at 3-month follow-up (all P =.0156).
Six of the 7 patients achieved normosmia. The 1 patient who did not achieve normosmia still demonstrated significant improvements after A-tDCS. Treatment effects lasted at the 3-month follow-up after the treatment ended.
“Although placebo effects may play a nontrivial role in any trial, our data indicate that [olfactory training] coupled with S-tDCS did not achieve any benefit,” the researchers stated. “However, the significant and long-lasting improvement of olfactory function observed in the current sample suggests that [A-tDCS] treatment could alleviate chronic post-COVID-19 hypo/anosmia.” The researchers speculated that A-tDCS potentially achieved its effects through altering neuroplasticity, thus changing olfactory performance.
The small sample size limited conclusions obtained from this study.
Vestito L, Mori L, Trompetto C, et al. Impact of tDCS on persistent COVID-19 olfactory dysfunction: a double-blind sham-controlled study. J Neurol Neurosurg Psychiatry. Published online May 23, 2022. doi:10.1136/jnnp-2022-329162