BACKGROUND: Persistent post-COVID olfactory dysfunction continues to be studied due to the controversy in its pathophysiology and neuroimaging.
METHODS: The patients had confirmed mild COVID-19 infection with olfactory dysfunction of more than one month of evolution and they were compared to controls with normal olfaction, assessed using the Sniffin\' Sticks Olfactory Test and underwent brain, magnetic resonance imaging (MRI) of the olfactory bulb and olfactory function.
RESULTS: A total of 8 patients and 2 controls participated. The average age of the patients was 34.5 years (SD 8.5), and that of the controls was 28.5 (SD 2.1). The average score in the patients\' olfactory test was 7.9 points (SD 2.2). In brain and olfactory bulb MRI tests, no morphological differences were found. When evaluated by functional MRI, none of the patients activated the entorhinal area in comparison to the controls, who did show activation at this level. Activation of secondary olfactory areas in cases and controls were as follows: orbitofrontal (25% vs 100%), basal ganglia (25% vs 50%) and insula (38% vs 0%) respectively.
CONCLUSIONS: There were no observed morphological changes in the brain MRI. Unlike the controls, none of the patients activated the entorhinal cortex in the olfactory functional MRI.

Post-COVID olfactory dysfunction continues to be studied due to the controversy of the mechanisms involved. The aim was to investigate the olfactory dysfunctions in association with other post-COVID symptoms.
Observational, descriptive and single-center study. The patients had confirmed mild COVID-19 and subjective olfactory dysfunction of more than a month of evolution, which was assessed by Sniffin\' Sticks Olfactory Test.
A total of 86 patients participated. The mean age was 37.2 years (SD 9.82). 70.9% reported parosmia and 46.5% symptoms of brain fog. A pathological test result was obtained in 72.1% of the participants. The most failed pen was 11 (apple) in 76.7%. Anosmia of pen 15 (anise) was reported more frequently in 24.4% and cacosmia of pen 9 (garlic) in 27.9%. We observed a significant association between patients who reported parosmias and brain fog (RR 2.18; p=0.018), also between parosmia and phantosmia (RR 6.042; p<0.001).
There is some pathological selectivity for certain test pens, a higher prevalence of cognitive symptoms and many patients with combined parosmia and brain fog.