Patients with smell loss with subjective flavor perception might be experiencing unconscious memory recall from previously experienced cross‐modal sensory interactions

Retronasal olfactory function in patients with smell loss but subjectively normal flavor perception. David Tianxiang Liu et al. The Laryngoscope, 130:1629–1633, 2020. https://doi.org/10.1002/lary.28258

Abstract

Objectives: The human sense of smell constitutes the main part of flavor perception. Typically, patients with loss of olfactory function complain of diminished perception during eating and drinking. However, some patients with smell loss still report normal enjoyment of foods. The aim of the present study was to compare orthonasal and retronasal olfactory function in patients with non‐sinonasal smell loss and subjectively normal flavor perception.

Methods: Nineteen patients (mean age [range] 52.0 [8–83 years]) with self‐reported olfactory impairment but subjective normal flavor perception were included. Olfactory performance was assessed using the Sniffin’ Sticks (TDI) for orthonasal and the Candy Smell Test (CST) for retronasal function. Visual analogue scales were used for self‐assessment of odor (SOP), taste (STP), and flavor perception (SFP), ranging from 0 (no perception) to 10 (excellent perception).

Results: Mean (SD) SFP was 8.0 (1.8). Mean (SD) orthonasal TDI‐score of all patients was 14.4 (5.3, range 6–25.3) with 11 patients classified as anosmic and eight as hyposmic. Mean/SD retronasal CST‐score was 8.8 (2.7, range 3–13) within the range of anosmia/hyposmia. No correlation was found between SFP and the CST (P = .62).

Conclusion: The present results showed that despite claiming normal flavor perception, our patients were ortho‐ and retronasally dysosmic using standard tests for olfactory function. Although other explanations could be possible, we suggest that this subjective flavor perception might be due to unconscious memory recall from previously experienced cross‐modal sensory interactions.

DISCUSSION

An estimated 25% of all people over 50 years of age experience olfactory impairment.5, 6, 21, 22 A survey from Vennemann et al. on the prevalence of olfactory dysfunctions in the general population showed impairments in almost 18% of the general population with 3.6% classified as functionally anosmic.6 The olfactory system plays the leading role in human multisensory flavor perception,23, 24 therefore it is expected that a loss of olfactory function leads to altered perception of flavors, which is also confirmed in larger series of patients.25 However, some patients report smell loss but simultaneously state normal to excellent flavor perception. Published26 and unpublished data of our group demonstrate a relatively low percentage of patients with severe olfactory dysfunction but normal subjective flavor perception at the same time (between 3.7% for VAS = 10 and 28% for VAS ≥4).

As the major finding of our study, retronasal olfactory performance as measured by an established retronasal smell test did not confirm normal flavor perception in the investigated subjects. All patients yielded scores within the range of hyposmia/anosmia with ortho‐ and retronasal tests, demonstrating striking discrepancies between subjective and measured flavor identification abilities. In contrast, orthonasal smell test results correlated significantly with self‐assessed olfactory abilities. Our findings are in accordance with current scientific publications stating a moderate but significant correlation between self‐assessment of smell perception and measured olfactory acuity in patients with olfactory dysfunction and confirm a trend that self‐assessment of olfactory function becomes more accurate with decreasing performance.7, 14, 27 However, it has to be kept in mind, that on an individual patient's level, olfactory performance can only be assessed by means of validated smell tests.28

Regarding gustatory function, the question could arise of whether gustatory function in patients with smell loss and subjectively normal flavor perception is increased, compared to patients with smell loss and concordant loss of flavor perception. This was not found to be the case in our patients, as the majority of achieved TST scores projected in the medium to lower percentile range of normogeusia compared with normative data,16 which is also in accordance with a previously published study showing no significant influence of smell loss on gustatory function.29 As previously described, normosmic patients tend to rely on their odor imagery abilities for self‐assessment of olfactory function7 although this ability seems to decrease with the duration of olfactory loss.30, 31 A tendency of these patients to rely more on gustatory, textural, auditory (during mastication), and visual information of foods could be a reason for the lack of correlation between self‐assessment and test results of retronasal olfactory function.32 Our findings show that relying exclusively on subjective reports on flavor perception in patients with olfactory dysfunction can be misleading and additional testing of retronasal olfactory function can provide more information for the management regarding hazardous events (eg, ingestion of spoiled food).33

Why does the loss of retronasal olfactory function go unnoticed in some patients? Although we cannot give answers to this question based on our results, some thoughts might be relevant for further research. In our patients subjectively normal flavor perception during food intake was not mediated by intact retronasal olfactory function. In another investigation retronasal olfactory event‐related potentials could be recorded from some patients with unimpaired flavor perception which were ortho‐ and retronasally tested to be dysosmic by means of psychophysical tests.14 However, this might not be clinically relevant, since olfactory event‐related potentials can also be present in patients with functional anosmia, for whom residual olfactory function is not useful in everyday life.17, 18, 34

Part of the contribution of retronasal smell stimuli to overall flavor perception seems to be mediated by memory recall. Therefore unconscious memory recall of “flavor templates” from previously experienced cross‐modal sensory interactions (eg, somatosensory–olfactory interactions) may be an explanation for normal flavor perception in orthonasally anosmic patients with noncongenital causes.30, 35, 36 All three patients in our study with congenital smell loss yielded scores within the range of anosmia in ortho‐ and retronasal tests presuming “flavor” is an individual concept, consisting of interaction of all other sensory modalities (for example vision, taste, sound, and somatosensory) independently from olfactory perception. Long‐term olfactory recognition memory, which plays a vital role in food preference and food habits, happens unconsciously and incidentally through repeated presentation of individual components together, as is the case with food and beverages.37-39 A further mentionable point is that the development of our multisensory flavor perception probably already starts in the mother's womb39 and continues into adulthood. The frequent presentation and co‐occurrence of olfactory stimuli with other sensory stimuli, eg, of gustatory and olfactory quality, consequently allow qualities of one sensory system to evoke qualities in another.40 Further studies using functional imaging methods, for example, are needed for more clarity regarding different brain activities with variability of self‐assessment of different sensory modalities.

Finally, as shown in a recent publication, olfactory changes are not as strongly perceived as visual changes. While olfactory changes were only detected with an accuracy of 61%, visual changes were detected with an accuracy of over 97%. Only 24% of the participants were able to detect olfactory changes reliably above chance. Notably, these subjects also rated their personal interest in olfaction and its use in daily life as most important.41 Regarding our subgroup of patients with smell loss and no subjective change in flavor perception, it might be speculated that these patients rely more on visual, gustatory, and trigeminal cues during eating and drinking leading to an unawareness of a decreased retronasal odor identification ability.