Assessment of studies on oral manifestations and COVID-19: a meta-research study

Aim: This study aimed to evaluate how different study designs influences the presence and characteristics of diagnosis of oral manifestations in patients with COVID-19. Methods: This study is a meta-search conducted with studies evaluating the relationship between patients with oral manifestations and COVID-19. A search was conducted in three databases (PubMed, Web of Science, and Scopus), using terms specific to COVID-19 and oral lesions, including only primary studies. Study selection was undertaken by two reviewers independently. A descriptive analysis was performed. Results: The results included 55 studies in the data synthesis analysis. Most of the studies were case reports and case series. Some of the studies did not relate a biological test to confirm COVID-19. The ulcer lesion and that clinical examination by specialists were the most reported lesion in case reports and case series studies, while taste alterations and patient self-report were more reported in observational studies. Conclusions: This review concluded that the oral lesions in patients with COVID-19 differ according to the study design. The lack of high-quality scientific evidence in the dental literature on COVID-19 makes the findings inconsistent by the lack of sample selection criteria, diagnostic criteria and classification of oral lesions.


Introduction
World Health Organization (WHO) declared a global pandemic for COVID-19 in early 2020.The rapid spread of the disease caused by the SARS-CoV-2 virus brought the need to produce quick answers on the topic, generating a substantial number of articles.In dentistry, it was no different.In the first year of the pandemic, more than one thousand five hundred articles were published in the PubMed database about "COVID-19 and Dentistry" 1 .Despite the urgency of knowledge, this mass production also generated concern in the scientific community about the quality and accuracy of these studies 2 .
The oral mucosa is one of the entry routes for SARS-CoV 2, arising from human salivary gland receptors, believing that oral symptoms such as loss of smell, taste, and xerostomia, may be precursor symptoms of COVID-19, even before the typical clinical symptoms 3 .Thus, the clinical picture pattern of this disease may include oral signs and symptoms 4 .A living systematic review, which aims to be updated every six months, reported in its first publication in 2020 that the most prevalent symptom was taste alteration 5 .In their last update, published in 2021, the authors analyzed an increase in the publication of studies on the topic and the most prevalent oral symptom, up to that point, was xerostomia, followed by taste alterations 6 .However, afterward, a cohort study of 154 patients positive for viral disease reported that the most prevalent oral manifestation was oral ulcers 7 , disagreeing with the results of previous systematic reviews 5,6 .
Despite there are systematic reviews on this topic, most of the primary studies published diverge methodologically, assessed the prevalence of oral lesions as secondary outcomes 8 , presenting a lack of a plausible understanding of the establishment of the causal relationship between oral conditions and the virus 6 , and the diagnostic methods are not standardized which can jeopardize the understanding of this subject.Therefore, considering that previous literature presents different study designs involving COVID-19 and the oral manifestations with different results, this study aimed to evaluate how different study designs influences the presence and characteristics of diagnosis of oral manifestations in patients with COVID-19.

Methodology
This a meta-search study conducted with studies evaluating the relationship between patients with oral manifestations and COVID-19.The protocol for this study was previously registered with OSF Registries (https://doi.org/10.17605/OSF.IO/MS2ZF).

Eligibility criteria
We included studies published in scientific journals with adult subjects, with no maximum age restriction, who tested positive for COVID-19 through a confirmatory diagnostic test, which may be the results of the SARS-CoV-2 polymerase chain reaction (PCR) or other, and who reported some type of oral lesion, regardless of how this lesion was diagnosed.Original studies of any design such clinical trials, observational studies and case-reports were included.However, review articles, letters to the editor, and pilot studies were excluded.Due to financial constraints, only studies in the English language were included.Review articles, letters to the editor, and pilot studies were excluded.

Search strategy
Searches were conducted in the electronic databases PubMed, Scopus, and Web of Science from January 1, 2020 (corresponding to the first reported cases of the disease worldwide) to November 1, 2022.The search strategy was designed for PubMed, based on Mesh terms and keywords, and after, adapted for the Scopus and Web of Science databases (full search strategies could be found in supplementary table 1).The references cited in the included articles were also reviewed to identify other relevant studies.

Selection process
After the search strategy, the studies were imported into the Rayyan QCRI program (https://rayyan.qcri.org) to perform duplicate article removal.A pilot test was conducted to test agreement in study selection between the reviewers involved in this phase.After, two reviewers independently reviewed the titles and abstracts of all studies.Articles meeting the eligibility criteria and those with insufficient data in the title and abstract were selected for full-text reading.Subsequently, the full-text reading was performed independently by the same two reviewers considering the same criteria once more.Discrepancies in selecting titles, abstracts, and full-text articles were resolved through discussion with a third reviewer until a consensus was reached.

Data extraction
For data collection, a standardized form was created using Excel software.Two reviewers analyzed the selected studies to test the data extraction and ensure consistency in the analysis and collection of information.Subsequently, one reviewer extracted the data from the included studies, and then the second reviewer checked the consistency of the data.
The following data were extracted: year of publication, first author name, the continent of study conduct, study design (randomized clinical trials, cross-sectional, cohort, case-control studies, case report or case series), diagnostic test for COVID-19 (PCR or other), presence and clinical presentation (signs and symptoms) of the oral lesion(s), time of manifestation of oral lesion symptomatology, and how the diagnosis of the oral lesion(s) was made (clinical examination, laboratory examination, teledentistry, or self-report).Case report was considered only when a single case was reported in the study, and case series were considered studies that reported two or more cases.
According to the eligibility criteria of this review, studies that had patients diagnosed with COVID-19 through a confirmatory test would be included.In contrast, some studies did not show this confirmation through a laboratory test, but rather the confirmation was through patient self-report.We chose not to exclude these studies because we believe it would be important to discuss the lack of standardization in confirming the disease.

Data analysis
Data were analyzed using Stata 14.0 software.Descriptive analysis was performed to assess the relationship between the oral manifestations and COVID-19 in the different study designs.In addition, a comparison analysis was performed between the oral manifestations of the lesions that appeared most in the studies (ulcer, taste alteration, xerostomia, candidiasis, periodontal disease, and herpes simplex), diagnosis of oral manifestations (clinical examination, laboratory examination, or self-report/tele odontology), and the study design.Taste alteration was considered anosmia, dysgeusia, or ageusia.Periodontal disease was the report of gingival or periodontal disease.Results are presented as frequency (n) and percentage (%).

Results
Figure 1 shows a flow chart summarizing the article selection process.The search initially yielded 2064 potentially relevant articles (PubMed: n = 1151; Scopus: n = 559; Web of Science: n = 354), after removing duplicates, 1486 articles were included based on title and abstract, and among these, 95 articles were included for reading the full text.Finally, 55 studies were included in the data synthesis analysis.Supplementary Table 2  Regarding the most prevalent lesion according to each type of study (Table 2), it was observed that ulcer lesion was the most reported in case report studies.In contrast, taste alteration was most present for analytical observational studies, followed by ulcer and xerostomia.When considering the type of examination, most studies that performed clinical and laboratory examinations were case reports and case series.

Discussion
This was the first article to verify the influence of study design in the report of oral manifestations of COVID-19 related articles.We found that the reported oral manifestations are different depending on the study design, as well as there is a divergence and non-standardization of the diagnostic method performed in these studies.Moreover, there is no standardization in reporting the data evaluated, such as oral lesions.
Related to the oral lesions identified, ulcer was the most reported lesion in case reports and case series studies, agreeing with previous literature 22,63 .Furthermore, these studies presented a diagnosis of the oral manifestations through clinical examination.A possible explanation for this finding is that these lesions were more noticeable due to the direct damage to the oral tissue and mucosa, causing pain and discomfort, often requiring specialized care by patients 8 .In addition, some publications show that the edema 47 and ulcers were found in patients who required oxygen or intubation 7 and it is unlikely that these oral lesions were a direct manifestation of the virus or a marker of the progression of COVID-19 7 .In this sense, the relationship between ulcers and COVID-19 should be interpreted with caution, as they may be biased by the study design and lack of theoretical causal plausibility.
In agreement with previous literature, the oral condition that appeared the most in the group of analytical observational studies was taste alteration 13,14,26,51 , diagnosed by questionnaires and patient self-report 5,64,65 .There is still no consensus on the hypothesis and mechanism of action between COVID-19 and taste alteration; however, the data collection method of these studies can be discussed.Applying data collection by questionnaires and online format may make it easier to interpret oral symptoms, such as loss of taste, and have hindered the visual diagnosis of other oral lesions, thus making it challenging to interpret oral signs and symptoms during or after COVID-19 65 .Moreover, one of the first reports of symptomatology in COVID-19 patients was taste alteration.This report may have had a nocebo effect on patients, being a consequence of previous reports from other patients about such a symptomatologic possibility and may have then caused interference when answering the questionnaire 66 .
After analyzing the studies in this research, some oral lesions were reported in a smaller number, such as herpes simplex 10,67 , oral lichen planus 68 , mucormycosis 41,44 , and osteonecrosis 58 , among others 43 .These manifestations may be considered opportunistic lesions with great potential to affect patients, often resulting from actions for the treatment of COVID-19 and the deterioration of systemic health 9 .Other authors also report that several factors can affect the oral mucosa and trigger numerous lesions, including lack of oral hygiene during the disease, stress, and secondary infections 68 .In addition, these manifestations are some of the most common factors which can cause immune dysregulation in the oral cavity 68 and be present in patients with COVID-19.
Most of the studies analyzed were case reports and case series, which critically agree with other reviews that mapped the level of evidence of published studies on COVID-19 8 .These studies have the lowest level of scientific evidence when considering the different pyramids of evidence 69 because they have several methodological biases, no comparison group, and low reproducibility.The fact that no clinical trials were found shows the scarcity of studies with high methodological levels within the theme and when discussing the level of evidence.However, because this is a pandemic period, with restrictive measures and no knowledge about the disease scenario, the large number of case report studies and case series becomes plausible.Moreover, cohort studies are a reliable design to determine the occurrence of the disease, identify risk factors, and even determine a causal relationship.In the present review, these studies were most of the analytical observational studies found.Being aware that the hierarchical scale of evidence is not linear 70 and when well conducted, a cohort study can be very reliable 71 .
An important aspect to consider in this study is that most of the studies performed the diagnosis of lesions through clinical examination by health professionals, but the use of criteria for diagnosis of lesions or training and calibration of examiners was not reported.The diagnosis of oral diseases has a high degree of subjectivity and depends of professional qualification, so the World Health Organization (WHO) recommends and reports that it is crucial to use the technical guide to calibrate oral disease examiners 72 .Furthermore, taking into consideration that the diagnostic method performed was basically visual/tactile in clinical examinations, the possibility of reproducibility and sensitivity become weak, and some diseases may also not be recognized by examiners, causing a lack of diagnoses or underestimated lesions in each population 73 .
Braz J Oral Sci.2024;23:e243678 With the impact of the pandemic, a minimal understanding of the behavior of the virus was necessary and, in the face of the restrictive measures imposed to contain the advance of the disease, one of the ways to produce and conduct new research was through teledentistry and online surveys with self-report 74,75 .In the group of analytical observational studies that were analyzed, self-report and teledentistry were methods commonly used as diagnosis.However, according to previous literature, online surveys do not guarantee consistent validity of results because there are limitations such as sample characterization and data collection method used, which may influence the research results 74 .In this sense, different results between analytical observational studies and case reports/series may be related to the conduct of the study design, sample selection bias, and the use of non-validated methods, which may cause response bias or affect specific groups of people assessed 75 .
There are some limitations of this study that need to be emphasized.First, COVID-19 is still a new disease, and its manifestations, characteristics, and sequelae are still challenging for researchers.Next, the lack of standardization in the questionnaires used, and the classification and nature of the lesions may have reflected on the results of this research since it made it difficult to collect the data in a standardized manner.Moreover, the necessity of rapid responses about covid and its consequences to reduce its harms and damages may be contributed for the researchers neglect some methodological aspects or reduce its quality criteria.However, the revisors of this study were trained to avoid errors or lack of data from the studies found.Third, the language restriction for selecting articles was only in English, which may have provided the non-inclusion of studies written in other languages since the pandemic started in China.However, English is considered the universal language in science, and studies conducted in Asia were found in greater numbers in this review.
Future research on this topic is encouraged to better plan the design of their studies, including designed studies with careful sample selection, training, and calibration for data collection for the examination of oral manifestations, and standardizing the diagnostic methods of the evaluated diseases.
This study concluded that the oral lesions present in patients with COVID-19 are different according to the study design and that these differences may be related to the diagnostic method used and the way of data reporting.

Figure 1 .
Figure 1.Flowchart of the selection process.
Most of the studies were case reports (n=21, 38.2%) and case series (n=13, 23.6%), and among the analytical observational studies, 10 studies were cohort and cross-sectional studies (18.2%), and 1 study was case control (1.8%).The PCR diagnostic test was the most used, present in 76.4% (n=42) of the articles.About half of the studies (43.6%, n=24), the oral manifestations were diagnosed by clinical examination (physician or dentist diagnosis).The oral manifestation most frequent was ulcer (60.0%, n=33), followed by taste alteration (n=22, 40.0%), and candidiasis described in 15 studies (27.3%) and xerostomia in 14 studies (25.5%).The described onset time of oral manifestations in the analyzed studies ranged from less than 21 days (before diagnosis of COVID-19) to 150 days after diagnosis of COVID-19.

Table 1 .
Descriptive analysis of the analyzed articles (n=55).
ContinueBraz J Oral Sci.2024;23:e243678 *PCR: polymerase chain reaction **Taste alteration: anosmia.dysgeusia or ageusia Supplementary table 1. Search strategy PubMed (( "oral manifestations"[MeSH Terms] 19 OR sars-cov OR 2019-ncov OR sars-cov 2 OR coronavirus OR covid OR sars)) TS= (oral manifestations OR oral manifestation OR oral lesion OR oral lesions OR oral mucosal lesion OR oral mucosal lesions OR mucocutaneous lesions OR mucocutaneous viral lesions OR oral alteration OR oral alterations OR oral pathology OR oral pathologies OR oral complication OR oral complications OR oral change OR oral changes OR mouth diseases OR mouth disease OR oral disease OR oral diseases) AND TS=(2019 nCoV OR 2019ncoV OR 2019 novel coronavirus OR COVID 19 OR covid19 OR new coronavirus OR novel coronavirus OR SARS CoV-2 OR covid-19 OR sars-cov OR 2019-ncov OR coronavirus OR covid OR sars OR sars-cov-2) Braz J Oral Sci.2024;23:e243678

Table 2 .
Comparison analysis between the oral manifestations of lesions, their diagnoses, and the design of the studies (n=55).