The oral cavity is particularly susceptible to viral infections because of its conformation, particularly its soft tissue and salivary glands. Several viruses, including herpes simplex virus (HSV) and human papillomavirus (HPV), are associated with oral disease-causing primary lesions. Furthermore, oral mucosa can be affected by the secondary pathological processes of a bacterial or fungal nature due to viral immunosuppression, such as the human immunodeficiency virus (HIV). Consequently, the oral cavity could be considered a “biological barometer” of the viral immunosuppression advancement. Moreover, an implication of certain viral agents has been seen in dysplastic and neoplastic transformations of squamous epithelium (i.e., HPV) (1, 2). General and specialist dentists play a crucial role in evaluating, diagnosing, and managing such lesions, particularly considering the impacts of oral diseases on overall health and quality of life.

A new coronavirus, known as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), responsible for coronavirus disease 2019 (COVID-19), was identified in Wuhan, Hubei Province, China, at the end of 2019. It then spread worldwide, becoming a global pandemic (3).

The main transmission route is via large respiratory droplets, even though the virus has also been identified in the stool and urine of affected individuals. It presents great variability in the severity of clinical manifestations, such as dry cough, shortness of breath, and fever (4, 5), passing from a mild flu-like illness to severe respiratory syndrome. Mortality rates vary according to region and change as the number of affected individuals is updated (6). Angiotensin-converting enzyme 2 (ACE2) receptor is considered the main functional receptor through which SARS-CoV-2 infects cells. The wide expression of ACE2 receptors in different anatomical sites, including the respiratory and gastrointestinal tracts, could explain the variability of reported clinical manifestations (7).

Interestingly, dermatological manifestations have also been observed in some patients affected by COVID-19. The most common skin lesions in these subjects include erythematous rash, urticaria, and vesicle formation, especially localized on the trunk, which seems to be the anatomic region most involved (5, 8). Oral lesions, such as unspecific ulceration, desquamative gingivitis, petechiae, and coinfections, such as candidiasis (9–13), have also been reported. Moreover, Xu et al. (14) reported a high level of expression of ACE2 receptors in epithelial cells of the oral mucosa, particularly tongue epithelial cells. These results suggest that oral mucosa could be a target of SARS-CoV-2 infection. Nevertheless, it is still unclear whether these manifestations are a specific clinical pattern derived from direct SARS-CoV-2 infection or a consequence of systemic involvement because of the possibility of coinfections, a compromised immune system, and adverse reactions to medical treatment (13, 15, 16). Because the prevalence of clinical oral manifestations is still unknown, the range of COVID-19 manifestations in the oral mucosa is of broad and current interest.

Therefore, after a brief excursus into the main viral agents associated with oral mucosa lesions, this review aims to summarize the updated literature on oral lesions in patients with COVID-19 and emphasize their clinical implications.

An electronic search was conducted in PubMed, Scopus, and Web of Science for literature updated to November 20, 2020. A combination of the following keywords was used: “oral mucosal lesions” OR “oral lesions” AND “COVID-19” OR “SARS-CoV-2” OR “novel coronavirus disease.” The full-text articles of all potential studies were evaluated, and the references cited by the relevant studies were manually searched for further studies. Given the lack of available data, all types of studies reporting oral mucosal lesions in patients with laboratory-confirmed COVID-19 were included; only literature reviews were excluded. However, we decided to include only those with laboratory-confirmed COVID-19 when evaluating the reported cases; suspected COVID-19 were excluded. Other exclusion criteria were articles for which the full text was not accessible or not available in English. Duplicate articles were removed, and a first screening was performed by reading only the titles and abstracts of the studies.

From the 86 studies retrieved, only 17 satisfied the inclusion criteria, of which 11 were letters to the editor, 3 were case reports, 2 were case series, and 1 was a short communication. Table 2 provides a detailed description of the cases included. Given that the studies were published between April and November 2020, the results were listed in the table in alphabetical order of the first author's surname. Excluding the only study in which the gender and age of the participants were not reported (58), 33 cases were female and 24 cases were male. The mean age of reported cases was 42.92 ± 18.05.

The documented manifestations of oral mucosa were quite heterogeneous, varying in the kind of lesion and the location. The most frequent findings were ulcerations (9, 50, 60–62), sometimes associated with necrotic areas (50, 51), aphthous-like lesions (51, 55, 56, 58), and petechiae (51, 52, 54). Maculae (53, 60), blisters (10, 57), lingual papillitis or depapillation (58), and erythema or red plaques (52, 63) were also among the described oral lesions. Besides, a case of dark brown hyperpigmentation was documented by Corchuelo and Ulloa (63).

Tongue (9, 51, 54, 55, 58–62), lips (10, 50, 51, 57, 59, 60, 63) and palate (52, 54, 60) were the most frequently described anatomical locations.

An increasing number of atypical clinical presentations have been reported during SARS-CoV-2 infection, including dermatological and oral manifestations (16, 64–67). The pathogenesis of skin damage during COVID-19 is not well known, but some hypotheses have been formulated. For example, the presence of viral particles in cutaneous blood vessels could induce lymphocytic vasculitis through cytokine production, i.e., interleukin-1 (IL-1), interferon gamma (IFN-γ), and tumor necrosis factor alfa (TNF-α) by CD4+ T helper lymphocytes and the migration of eosinophils, CD8+ cytotoxic T cells, B cells, and natural killer (NK) cells (68, 69). Another possible explanation of cutaneous disturbances correlated to SARS-CoV-2 is the formation and accumulation of microthromboses, which could reduce the blood flow to the cutaneous microvasculature (70), and the presence of deoxygenated blood in venous plexi could further contribute to these cutaneous lesions. Moreover, the deposition of complement components C5b-9 and C4d in pauci-inflammatory thrombogenic vasculopathy and their co-localization with COVID-19 spike glycoproteins were shown by Magro et al. (71). It is reasonable to hypothesize that skin involvement is due to a combination of these mechanisms rather than a single one (5). Taste disorders were the most common oral symptom in patients with COVID-19, probably due to a local inflammatory response resulting from rhinitis triggers, which can hamper taste buds' normal function (15, 72). Additionally, oral mucosa involvement was described during SARS-CoV-2 infection. Since the first description of oral lesions in SARS-CoV-2 positive patients, reported by Martín Carreras-Presas et al. (10), several more recent studies have also reported oral mucosa lesions in COVID-19, such as ulcers (50, 60–62), aphthae (51, 55), and maculae (53, 60). The clinical significance of oral mucosa involvement during SARS-CoV-2 infection remains controversial.

As previously reported (14), the high expression of ACE2 on oral epithelial cells, especially on the tongue, suggests that the oral cavity might be an anatomical site particularly susceptible to SARS-CoV-2 infection. Consequently, as suggested by Brandão et al. (51), the interaction between SARS-CoV-2 and ACE2 might disrupt the oral keratinocytes' function, resulting in painful oral ulcers. Furthermore, oral mucosa lesions during COVID-19 could be justified by the variable inflammatory reaction, which can induce vascular inflammation, as observed for cutaneous manifestations (5, 73). The most recent publications on oral mucosa lesions in patients affected by COVID-19 support an association with organic damage and/or complications for thrombocytopenia, anticoagulant therapy, disseminated intravascular coagulation, and systemic inflammation (60, 62, 63). According to Cruz Tapia et al. (54), clinical manifestations and histological findings suggest the possibility that the oral cavity presents the primary or secondary alterations of vascular-hematologic damage associated with COVID-19. Nevertheless, as reported by Martín Carreras-Presas et al. (10) and Hedou et al. (74), ulcers or vesiculobullous lesions can occur as in other viral infections. It is largely documented that high levels of fatigue and stress can increase the risk of the reactivation of HSV (75).

Moreover, oral damage could also be a manifestation of an immunosuppression state and microbiome dysbiosis caused by a viral infection (76). According to Bezerra et al. (50), it is reasonable to think that COVID-19 systemic immune deregulation may cause a more prolonged immune imbalance, which could predispose these late, secondary oral lesions. In addition, as stated by de Sousa et al. (13), most patients developed oral mucosal injury during the hospitalization period, which supports the hypothesis of coinfections, immunity impairment, and adverse reactions to COVID-19 treatment medications.

Interestingly, Martín Carreras-Presas et al. (10) suggested that oral lesions, such as ulcers, could be an inaugural symptom of COVID-19. According to Amorim Dos Santos et al. (15), in mild cases, oral mucosal lesions occurred before or at the same time as the initial respiratory symptoms; however, in those who required medication and hospitalization, the lesions developed approximately 7–24 days after the onset of symptoms. Limited to the cases reported, the time to onset was variable, ranging from 4 to 90 days; however, the time on onset was unavailable in several reported studies, confirming the necessity to verify this data in larger patients' cohorts.

Regarding the age of patients presenting with oral injuries, Brandão et al. (51) reported two distinct oral lesions patterns. One was represented by aphthous-like ulcers in young patients with mild cases of COVID-19, and the other resembled HSV-1 necrotic ulcers in more severe cases of immunosuppressed older patients. Again, the lack of data available from a large sample of participants makes further investigations necessary to support these hypotheses.

Another interesting fact emerging from the reported cases is how online professional consultation using photography (telemedicine) could be a very useful, additional tool to support clinicians in the early diagnosis of oral lesions, especially when direct observation is not possible (57, 63). Indeed, an early clinical diagnosis and the development of sensitive diagnostic tools are essential for a correct management of the disease (77, 78).

There are some limitations to underline in this study. First, almost all the reports were published as letters to the editor, thus imposing editorial limitations that reduce reporting comprehensibility (12). Moreover, the oral manifestations' real incidence could have been underestimated due to exposure and contamination risk while conducting photographic imaging (15) and the limited data available on oral lesion injuries in asymptomatic patients. Another important aspect to consider is the limited availability of microscopic and histological data of oral mucosa lesions in COVID-19. The only accessible data referred to histological characterizations performed by Soares et al. (60) and Ansari et al. (49), which confirmed the presence of an inflammatory infiltrate, suggesting that the patients' lesions could be associated with COVID-19 disease. It is auspicious that the characterization of the oral lesions of COVID-19 infected patients should include incisional biopsies, followed by direct viral testing for SARS-CoV-2, as suggested by Brandão et al. (51).

Further research is necessary to determine the diagnostic and pathological significance of oral manifestations during COVID-19. Indeed, oral mucosa involvement during viral infection may assume differing clinical significance: it could represent either the first sign of viral disease or coexist as a co-symptom or represent a unique sign of the viral infection (26). In this context, the importance of the clinical oral examination of patients with confirmed or suspicious COVID-19 infection should be emphasized, given the need for support, pain control, and quality of life. In addition, dental operators are potentially exposed to a high degree of contamination with SARS-CoV-2 because of dental procedures that produce aerosols (79, 80). Consequently, an accurate inspection of the oral cavity, always with the mandatory dispositive protection (79), could be crucial in the dental setting to perform a more accurate triage of patients and improve safety operator, avoiding underestimation and misdiagnoses of oral signs and symptoms.

The new SARS-CoV-2 responsible for the global COVID-19 pandemic has become a sanitary emergency of primary importance. Although the typical symptoms include fever, shortness of breath, and a dry cough, cutaneous manifestations have also been reported, including some oral lesions. An association between oral diseases and SARS-CoV-2 infection is still unclear and currently poorly investigated. The appearance of such lesions could be related to the direct or indirect action of SARS-CoV-2 over the oral mucosa cells, coinfections, immunity impairment, and adverse drug reactions. Nevertheless, oral manifestations of this disease seem to be underreported, especially due to lockdown periods and the lack of mandatory dispositive protection. Consequently, based on these outcomes, we can conclude that (1) further studies are necessary to establish the diagnostic and pathological significance of oral manifestations during COVID-19; (2) the oral examination in patients with COVID-19 should not be unattended but rather promote a specialist multidisciplinary approach, including especially dental practitioners; (3) early recognition of oral lesions associated to COVID-19 could be crucial in the dental setting to perform a more accurate triage of patients and improve operator safety, avoiding underestimation and misdiagnosis of oral manifestations.

GL, ML, and EP contributed to all steps of manuscript preparation. RD and SF participated in editing and critical revision of article. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.