Table 1.
Data extracted from included studies.
| Authors, year | n, gender | Mean age ± SD, range | Olfactory assessment methods | Cognition assessment methods | Olfactory results | Cognition results timepoint | Relationship between olfaction and cognition |
|---|---|---|---|---|---|---|---|
| Alemanno et al. (2021) | 87, 25 female | 67.23 ± 12.89 | Survey | MMSE, MoCA | 18/87 had anosmia | Acute respiratory intervention:intubation: 74.2% had MoCA deficit, higher than Venturi mask (p = 0.005); 12.9% had MMSE deficit, higher than Venturi mask (p = 0.024) BIPAP: 94.4% had MoCA deficit, 55.6% had MMSE deficit Venturi mask: 77.8% had MoCA deficit, 48.3% had MMSE deficit no O2: 77.8% had MoCA deficit, 44.4% had MMSE deficit |
No significant differences in cognitive functions between anosmics and non-anosmics |
| Almeria et al. (2020)* | 35, 19 female | 47.6 ± 8.9, 24–60 | Retrospective chart review | Digit span (backwards) | 20/35 had anosmia | 45.7 ± 7.5, 30.0–57.5 | Anosmics had lower scores (t = 2.259, p = 0.031) |
| Azcue et al. (2022) | 73, 51 female | 44.36 ± 9.47, 18–85 | BSIT | MoCA, SPCT, SDMT, HVLT-R, BVMT-R, TMT, Benton JLO | 53 normal (9.96 ± 0.99), 3 relatively abnormal (7.67 ± 0.58), 15 abnormal (6.47 ± 0.99) | MoCA 25.09 ± 3.06; SPCT-3 16.75 ± 5.26; SDMT 47.23 ± 10.93; HVLT-R trial 1 5.31 ± 1.55, total 22.57 ± 5.92, trial 4 7.90 ± 2.83, DI 9.53 ± 2.47; BVMT-R trial 1 5.63 ± 3.54, trial 1–3 22.38 ± 7.37, trial 4 8.31 ± 2.70, DI 5.64 ± 1.04; TMT-A 38.41 ± 14.50; Benton JLO 24.52 ± 4.98 | BSIT showed significant positive correlation with MoCA, SPCT-3, SDMT, HVLT-R trial 1–3, BVMT-R discrimination index, and Benton JLO and significant negative correlation with TMT-A. Participants with abnormal BSIT had significantly worse general cognition, attention, verbal memory, visual memory, visuospatial perception, and abstraction capacity. |
| Cacciatore et al. (2022) | 83, 20 females | 66.9, 95% CI: 64.2–69.7 | Survey | MoCA | 15/83 had hyposmia/hypogeusia | Mean 24.1, range 23.4–24.8 | No significant correlation between cognition and hyposmia/hypogeusia |
| Caspersen et al. (2022) | 774, 449 female | 25–65+ | Survey | Survey | COVID-19 dx 11–12 months ago: 28 had altered smell or taste COVID-19 dx 1–6 months ago: 128 had altered smell or taste |
COVID-19 dx 11–12 months ago: 30 had poor memory, 20 had brain fog COVID-19 dx 1–6 months ago: 81 had poor memory, 84 had brain fog |
No significant correlation between altered smell or taste and poor memory or brain fog |
| Cecchetti et al. (2022) | 49, 13 females | 60.8 ± 12.6 | Survey | Phonemic fluency, SDMT, RAVLT immediate recall | 22/49 had dysgeusia/hyposmia during acute COVID-19 | baseline: phonemic fluency 27.9 ± 10.2, SDMT 35.1 ± 1.9, RAVLT 29.3 ± 9.4 follow up: phonemic fluency 31.9 ± 11.4, SDMT 41.8 ± 1.3, RAVLT 35.8 ± 11.2 |
Those with dysgeusia/hyposmia had less RAVLT (immediate recall memory) improvement; no significant difference in improvement on phonemic fluency or SDMT |
| Chen et al. (2022) | 200, 129 females | 44.6 ± 15.46, 19–82 | Survey, UPSIT | MoCA, NIH Toolbox | Survey: 109/200 had smell changes UPSIT: 53/164 had normosmia, 62/164 had mild hyposmia, 32/164 had moderate hyposmia, 13/164 had severe hyposmia, 4/164 had anosmia |
102/191 had normal MoCA, 89/191 had cognitive impairment; for NIH-TB language, 138/196 had >25% and 58 had ≤25%; for NIH-TB working memory, 134/196 had >25% and 62/196 had ≤25% |
Weak correlation between UPSIT and MoCA (r = 0.30, p = 0.0002); weak correlation between UPSIT and NIH-TB language (r = 0.36, p < 0.0001) |
| Damiano et al. (2023) | 701, 334 females | 55.3 ± 14.6, 95% CI: 54.3–56.3 | Survey | MCS, MMSE, TMT, DSST, Neuropsychological Battery CERAD | 9% had parosmia, 18% had moderate and severe olfactory deficits | MCS 5.2 ± 4.16, MMSE orientation 8.27 ± 3.25, TMT-A 65.5 ± 48.0, verbal fluency 15.57 ± 5.43, DSST 32.3 ± 19.3, Boston naming 13.15 ± 2.27, word list 15.35 ± 4.7, construction praxis 8.26 ± 2.55, word list recall 4.86 ± 2.25, word list recognition 7.88 ± 2.77 | Parosmia significantly associated with MCS (p = 0.001) and Boston naming (p = 0.017); moderate & severe olfactory deficit associated with TMT-A (p = 0.008), digit-symbol (p = 0.009), word list memory task (p = 0.041) |
| Delgado-Alonso et al. (2022) | 50, 37 females | 51.06 ± 11.65 | BSIT | Digit span (backwards), ROCF, Stroop A, inhibition test, determination test, divided attention, selective attention, FGT | 9.00 ± 2.33 | Frequency of impairment 2x more than expected for digit span, ROCF (memory at 30 min); frequency of impairment at least 3x more than expected for Stroop A; inhibition test 7.74 ± 3.91, determination test 198.31 ± 48.63, divided attention 561.37 ± 216.40, selective attention 429.66 ± 124.86, FGT Delayed Free Recognition I 5.70 ± 2.99 | BSIT showed moderate correlations with digit span (backwards) (R = 0.505), ROCF (memory at 30 min) (R = 0.383), Stroop A (R = 0.387), inhibition test (R = -0.374), determination test (R = 0.36), divided attention (R = 0.335), selective attention (R = -0.318), and FGT (Delayed Free Recognition I) (R = 0.347) |
| Desai et al. (2022) | 49, 36 female | 18–76 | Survey, UPSIT | CNS Vital Signs validated cognitive remote testing website, neurocognitive index, composite memory, verbal memory, visual memory, psychomotor speed, reaction time, complex attention, cognitive flexibility, processing speed, executive function, simple attention, motor speed | Survey: active COVID-19: 13% had anosmia and 50% had hyposmia recovered: 4% had anosmia and 67% had hyposmia UPSIT: active COVID-19: 37.5% had anosmia, 18.75% had hyposmia, 43.75% had normosmia recovered: 33.33% had anosmia, 46.67%, had hyposmia, 20% had normosmia |
Active vs. recovered cognitive flexibility 48.9 vs. 34.8, complex attention 46.6 vs. 49.0, composite memory 42.7 vs. 43.8 executive fxn 52.4 vs. 34.7, motor speed 49.1 vs. 43.2, neurocognitive index 47.1 vs. 35.8, processing speed 57.5 vs. 42.0; rxn time 49.0 vs. 31.0, simple attn. 46.4 vs. 49.8, verbal memory 43.1 vs. 45.9, visual memory 45.3 vs. 45.8 |
No correlation between self-reported smell loss and cognitive function; nonsignificant inverse association between UPSIT score and processing speed in recovered; no correlations with cognitive percentiles and UPSIT total scores |
| Di Stadio et al. (2022) | 152, 102 females | 41.2 ± 11, 18–65 | Sniffin’ Sticks identification | MMSE, survey | 50/152 had anosmia, 25/152 had hyposmia, 10/152 had parosmia/cacosmia, 58/152 had combination of hyposmia and parosmia | MMSE wnl; 23.7% reported mental clouding | Patients with mental clouding had higher risk of suffering from anosmia (OR 19, p = 0.05), hyposmia + parosmia (OR 33, p = 0.01), hyposmia alone (OR 15, p = 0.07), and moderate risk of suffering from parosmia (OR 3, p = 0.5) compared to patients with no neurological symptoms |
| Ferrucci et al. (2022) | Time1: 76, 20 females; Time2: 53, 15 females | 56.24 ± 12.08, 18–75 | Survey | BRB-NT, SRT, SPART, SDMT, PASAT, WLG | Time1: 44.6% had hyposmia, 42.1% had hyposmia and dysgeusia Time2: 9.4% had hyposmia |
SPART-D = 5.66 ± 2.07, SRT-LTS = 35.64 ± 13.77, SRT-CLTR = 27.75 ± 13.06, SRT-D = 6.92 ± 2.66, SPART = 17.75 + 5.01, SDMT = 38.81 ± 9.88, PASAT-3 = 41.66 ± 11.98, PASAT2 = 30.81 ± 9.36, WLG = 24.75 ± 4.69 |
SPART-D (delayed visuospatial memory recall) score worse in those who reported hyposmia |
| Fiorentino et al. (2022)* | 84, 55 females | 42.8 ± 13.6, 19–59 | Sniffin’ Sticks, TODA | PPTT, generic naming test from Grémots battery: Evaluation du langage dans les pathologies neurodégénératives | Sniffin’ Sticks: age 19–39: T 4.76 ± 4.04, D 9.51 ± 3.84, I 9.40 ± 3.92, TDI 23.68 ± 9.68 age 40–59: T 4.25 ± 3.26, D 9.55 ± 3.86, I 10.15 ± 3.52, TDI 23.95 ± 8.61 TODA age 19–39: threshold 1.66 ± 0.97, identification 3.97 ± 1.76 age 40–59: threshold 1.41 ± 0.87, identification 4.19 ± 1.63 |
Age 19–39: PPTT 47.31 ± 2.63; generative naming strict 34 ± 2, broad 34 ± 1, time 63.93 ± 17.51 age 40–59: PPTT 49.20; generative naming strict 34 ± 1, broad 35 ± 1, time 59.46 ± 16.34 |
For PPTT and TODA T, small significant correlation between semantic memory and odor threshold detection |
| Jennings et al. (2022) | 108, 76 females | 46.3 ± 10.3, 25–78 | Survey | Survey | 21/108 had dysosmia | 71 had brain fog, 37 did not have brain fog | 25.4% of participants with brain fog reported dysosmia, 8.1% of participants without brain fog reported dysosmia; in cluster analysis, dysosmia was more prevalent in the brain fog group in a two-cluster model |
| Kopishinskaia et al. (2021) | 187, 152 females | 35, 21–87 | Survey | Survey | All patients had parosmia/phantosmia | 40/187 had brain fog | Brain fog was significantly higher in patients with parosmia/phantosmia compared to controls |
| Llana et al. (2022)* | 42, 38 females | 31–51 | Survey | MTT, PAL, MoCA, DSST | 17/42 had anosmia, 25/42 did not have anosmia | MTT ERI-d2: anosmics −9.01 ± 55.78, non-anosmics −49.81 ± 44.19; MoCA: anosmics 25.81 ± 2.42, non-anosmics 26.39 ± 2.64, controls 28.07 ± 1.58; DSS-M: anosmics 64.50 ± 16.08, controls 84.59 ± 10.64; DSS-IL anosmics 10.19 ± 4.79, controls 13.41 ± 3.71; DSS-R anosmics 6.81 ± 1.47, controls 7.83 ± 1.04 | For MTT, ERI-d2 index was Higher in anosmics than non-anosmics with COVID-19; anosmics had lower scores on MoCA and DSST than controls without COVID-19 |
| Tavares-Júnior et al. (2022) | 141, 89 females | 48 ± 14, 16–90 | Survey | ACE-R, MMSE, CDR | Normal cognition: 19 had anosmia, 29 did not Cognitive impairment:1 had anosmia, 24 did not Subjective cognitive decline: 23 had anosmia, 45 did not |
48 had normal cognition, 25 had cognitive impairment, 68 had subjective cognitive decline | Cognitive impairment group had a lower frequency of anosmia than the normal and subjective cognitive decline groups |
*These studies contain only participants between ages 18 and 60.
ACE-R, Addenbrooke’s cognitive examination-revised; BRB-NT, brief repeatable battery of neuropsychological tests; BVMT-R, brief visuospatial memory test-revised; CDR, clinical dementia rating; DSS-IL, digit symbol substation incidental learning; DSS-M, digit symbol substation correctly matched; DSS-R, digit symbol substation incidental learning registered; DSST, digit symbol substitution test; FGT, figural memory test; HVLT-R, Hopkins verbal learning test-revised; JLO, judgment line orientation test; MCS, memory complaint scale; MMSE, mini-mental state examination; MoCA, Montreal cognitive assessment; MTT, mirror tracing test; MTT ERI-d2, mirror tracing test consolidation of learning; PAL, paired-associated learning; PASAT, paced serial additions test; PPTT, pyramids and palm trees test; RAVLT, Rey auditory verbal learning test; ROCF, Rey-Osterrieth complex figure; SDMT, symbol digit modality test; SPART, spatial recall test; SPCT, Salthouse perceptual comparison test; SRT, serial recall test; TMT, trail making test; WLG, word list generation.