Phantosmia May Predict Long-Term Measurable Olfactory Dysfunction After COVID-19

Olfactory Epithelium Infection by SARS-CoV-2: Possible Neuroinflammatory Consequences of COVID-19

BACKGROUND

The loss of smell is a typical diagnostic symptom of coronavirus disease 2019 (COVID-19). This sensorial deprivation may be expressed as quantitative (anosmia or hyposmia) or qualitative (dysosmia) alterations as a consequence of anatomical disturbances of the nasal epithelium structure. The olfactory system sends direct neuronal connections to brain structures that are involved in emotional processing, including deep temporal nuclei. This anatomical and functional feature may be related to the occurrence of emotional disorders among COVID-19 patients.

SUMMARY

We identify a possible sequence of events, from typical olfactory dysfunction that is associated with COVID-19 and caused by olfactory epithelium damage to disturbances in the quality of life and emotional state of infected patients that is attributable to possible neuroinflammatory processes. Sensorial deprivation causes deleterious actions on mood, negatively affecting quality of life. Olfactory dysfunction that is associated with COVID-19 occurs concurrently with psychological distress, symptoms of anxiety, and depressive disorders and impinges on self-perceived quality of life.

KEY MESSAGES

Changes in mood are certainly associated with multiple factors, including the environment and isolation, but the observation that the virus may penetrate the central nervous system through the olfactory bulb and the connection between the olfactory system and prefrontal and orbitofrontal cortices and the amygdala-hippocampus do not allow one to discard neural factors that are involved in the pathophysiology of emotional symptoms in post-COVID-19 patients. Behavioral symptoms of COVID-19 involve local olfactory actions and the participation of central neuronal systems.

Clinical factors influencing olfactory performance in patients with persistent COVID-19 smell loss longer than 1 year

Objectives

Factors affecting persistence of COVID-19-related olfactory dysfunction (OD) remain partially unknown. We aim to evaluate the clinical factors which could influence olfactory performance in patients with persistent COVID-19-related smell loss.

Methods

A retrospective analysis of 100 patients with persistent COVID-19-related OD was performed between October 2020 and December 2022 at a single-center long-COVID smell clinic. All subjects underwent smell assessment using Sniffin' Sticks (S'S) extended test, nasal endoscopy, nasal airflow evaluation (peak nasal inspiratory flow [PNIF]), allergy test (skin prick test [SPT]) for common aeroallergens, MRI of the head and patient-reported outcome measures (PROMs-VAS, SF-36, Short QOD-NS, SNOT-22). Based on S'S score, subjects were divided into normosmics (TDI ≥ 30.75) and dysosmics (TDI < 30.75).

Results

The median age was 42 years and the median length of patient-reported OD was 1.4 years. 20 patients (20.0%) were normosmic at the time of S'S assessment. Dysosmic patients were found to have significantly lower scores at the SF-36 health domains for energy/fatigue (p = .0004) and emotional wellbeing (p = .04) when compared to normosmics. A moderate correlation (r = .45-.59) between S'S scores and some PROMs was also demonstrated. At the multivariate analysis higher PNIF scores positively influenced odor threshold (p = .001) while positivity to SPT negatively influenced odor identification (p = .04).

Conclusions

Impairment of nasal airflow and sensitivity to aeroallergens can negatively affect olfactory performance in COVID-19-related OD. Long-COVID smell loss deeply affects QoL although recovery of olfaction can bring it back to a normal range.

Level of Evidence

IV.

The assessment of qualitative olfactory dysfunction in COVID-19 patients: a systematic review of tools and their content validity

Background

There is a lack of overview of the tools used to assess qualitative olfactory dysfunction, including parosmia and phantosmia, following COVID-19 illness. This could have an impact on the diagnosis and treatment offered to patients. Additionally, the formulations of symptoms are inconsistent and often unclear, and consensus around the wording of questions and responses is needed.

Aim of study

The aim of this systematic review is to provide an overview of tools used to assess qualitative olfactory dysfunction after COVID-19, in addition to addressing the content validity (i.e., item and response formulations) of these tools.

Methods

MEDLINE, Web of Science, and EMBASE were searched 5th of August 2022 and updated on the 25th of April 2023 to identify studies that assess qualitative olfactory dysfunction in COVID-19 patients. Primary outcomes were the tool used (i.e., questionnaire or objective test) and item and response formulations. Secondary outcomes included psychometric properties, study design, and demographic variables.

Results

The assessment of qualitative olfactory dysfunction is characterized by heterogeneity, inconsistency, and lack of validated tools to determine the presence and degree of symptoms. Several tools with overlapping and distinct features were identified in this review, of which some were thorough and detailed, while others were merely assessing the presence of symptoms as a binary measure. Item and response formulations are also inconsistent and often used interchangeably, which may lead to confusion, incorrect diagnoses, and inappropriate methods for solving the problem.

Conclusions

There is an unmet need for a reliable and validated tool for assessing qualitative olfactory dysfunction, preferably one that also captures quantitative olfactory issues (i.e., loss of smell), to ensure time-effective and specific assessment of the ability to smell. A consensus around the formulation of items and response options is also important to increase the understanding of the problem, both for clinicians, researchers, and the patient, and ultimately to provide the appropriate diagnosis and treatment.

Registration and protocol

The URL is https://www.crd.york.ac.uk/PROSPERO/display_record.php?RecordID=351621. A preregistered protocol was submitted and accepted (12.09.22) in the International prospective register of systematic reviews (PROSPERO) with the registration number CRD42022351621.

Olfactory Nomenclature: An Orchestrated Effort to Clarify Terms and Definitions of Dysosmia, Anosmia, Hyposmia, Normosmia, Hyperosmia, Olfactory Intolerance, Parosmia, and Phantosmia/Olfactory Hallucination

BACKGROUND

Definitions are essential for effective communication and discourse, particularly in science. They allow the shared understanding of a thought or idea, generalization of knowledge, and comparison across scientific investigation. The current terms describing olfactory dysfunction are vague and overlapping.

SUMMARY

As a group of clinical olfactory researchers, we propose the standardization of the terms "dysosmia," "anosmia," "hyposmia," "normosmia," "hyperosmia," "olfactory intolerance," "parosmia," and "phantosmia" (or "olfactory hallucination") in olfaction-related communication, with specific definitions in this text.

KEY MESSAGES

The words included in this paper were determined as those which are most frequently used in the context of olfactory function and dysfunction, in both clinical and research settings. Despite widespread use in publications, however, there still exists some disagreement in the literature regarding the definitions of terms related to olfaction. Multiple overlapping and imprecise terms that are currently in use are confusing and hinder clarity and universal understanding of these concepts. There is a pressing need to have a unified agreement on the definitions of these olfactory terms by researchers working in the field of chemosensory sciences. With the increased interest in olfaction, precise use of these terms will improve the ability to integrate and advance knowledge in this field.