Introduction

While respiratory and gastrointestinal complications were the main manifestations of coronavirus disease 2019 (COVID-19), many more manifestations became apparent as the pandemic progressed [1]. Lingual involvement has been extensively reported in COVID-19 patients. The most primary features are chemosensory disorders, such as smell and taste problems [2]. These complications frequently happen during many viral infections, such as influenza [3].

Tongue Features in COVID-19

Chemo-sensory disorders can be defined as diseases related to the sense of taste and smell. Taste disturbances can be categorized as quantitative and qualitative disorders. Hypogeusia, a diminished sense of taste, and ageusia, or the absence of taste, are examples of quantitative taste problems, while dysgeusia is a qualitative change in taste perception [4]. Temporary taste changes may be the sole manifestation of COVID-19 [5]. Dry mouth or xerostomia, which is reported in several patients due to hyposalivation or decreased saliva production [6], is associated with taste disturbances [7] and is manifested by an increased need for drinking fluids in patients with COVID-19 [8]. Some of these patients may even experience tongue burns or spicy, salty, sour, sweet taste, or even dysgeusia [9]. The tongue’s blisters, scattered reddish macules on the tongue [10], ulcers, and the tongue’s painful inflammation have also been reported [11].

Moreover, COVID-19 has been attributed to a Kawasaki-like disease with glossitis, also called red strawberry tongue [12]. Furthermore, the tongue can also be involved as a component of angioedema in the context of COVID-19 [13]. A new feature of COVID-19 has been reported in a patient presenting with Melkersson-Rosenthal syndrome (MRS), a triad of orofacial edema, facial paralysis, and fissured tongue [14].

Different medications used to treat COVID-19, such as remdesivir, favipiravir, ribavirin, and lopinavir/ritonavir, can also cause xerostomia and taste disorders [15, 16]. Also, chloroquine has been reported to be linked to transient oral pigmentations [17]. Moreover, azithromycin played a role in tongue discoloration and oral candidiasis [18]. Furthermore, intensive care unit (ICU) admitted patients who have been intubated routinely develop tongue coatings and candidiasis [19].

The tongue features of COVID-19 have been investigated in several studies, particularly in traditional Chinese medicine. Changes in tongue characteristics are correlated to the progression and severity of the disease [20]. The shape, color, and coating of the tongue, and color and thickness of the fur, are among the characteristics studied previously. The most common subjective characteristics of lingual involvement were red or light red tongue, yellow coating, and greasy coating. However, the pale tongue, purple tongue, white coating, grayish-black coating, rough tongue, tender tongue, puffy tongue, spots, prickles tongue, fissured tongue, and tooth-marked tongue were also reported [21]. Interestingly patients with milder COVID-19 had thinner tongues with lighter colors, while in patients with more severe disease, thicker coatings, more tender and purplish tongues were observed [22]. Figure 1 illustrates the extrapulmonary manifestations of COVID-19, and a patient with COVID-19 who manifested with erythematous tongue with deep grooves is presented.

Fig. 1
figure 1

a A summary of the extrapulmonary manifestations of COVID-19, b Erythemeatous tongue with deep grooves in a patient with COVID-19

Full size image

Black hairy tongue (BHT) or lingua villosa nigra is quite commonly demonstrated in the COVID-19 setting. It is a painless and benign disorder due to the lack of desquamation and increased proliferation and hypertrophy of the tongue’s filiform papillae, leading to a black, sometimes brown, yellow, or green discoloration of the tongue, halitosis, and a metallic taste [23]. Several underlying conditions have been suggested to lead to this disorder, which include antibiotics, such as bismuth [24], amoxicillin [25], tetracycline [26], linezolid [27], and psychotropic agents, including olanzapine, phenothiazines, and tricyclic antidepressants [28]. Dehydration decreased saliva production, trigeminal neuralgia, poor oral hygiene, smoking, alcoholism, and infections were also observed to trigger this condition [29]. BHT may be asymptomatic or cause nausea, halitosis, or dysgeusia [30].

Pathophysiology of Tongue Involvement

In addition to the respiratory and gastrointestinal tract, the angiotensin-converting enzyme 2 (ACE2), which is a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) receptor in the human body, is also expressed in the epithelium of the oral cavity, including the epithelial cells of the tongue [31]. ACE2 receptors are more densely located on the tongue’s dorsum, where the epithelium is significantly altered during COVID-19 and predisposes to overgrowth of anaerobic bacteria, leading to halitosis [32]. Any infectious or inflammatory conditions can lead to hyposalivation due to disturbed salivary secretion, and infection with SARS-CoV-2 can also initiate this process [33].

Dysgeusia and xerostomia may be due to nasal congestion and rhinorrhea caused by oral respiration in viral infections of the upper respiratory [34]. However, in the context of SARS-CoV-2 infection, they are attributed to an olfactory malfunction or neurological impairment [35]. Taste damage results in dysgeusia and ageusia mainly due to an underlying olfactory disturbance, but sometimes primary taste impairment may occur in the context of COVIDjav-19 [36]. The distribution of oral microbiota equilibrium caused by systemic drugs used in COVID-19 may also have a role in oral manifestations and, in particular, halitosis [37]. The pathophysiology of tongue color changes can be microcirculation damage, decreased arterial pressure of oxygen, and increased platelet aggregation [22].

Moreover, the yellow discoloration of the tongue can be attributed to fever and infection, which is correlated with lung damage severity [38]. Furthermore, the tongue’s greasy coatings may indicate either damage to the free oxygen radicals scavenger system or dysbiosis, which latter is a decrease in the levels of Lactobacillus and Bifidobacterium [39]. Besides, the tongue’s tenderness reflects organ failure within the context of a systemic disease, such as COVID-19 [40]. It should be noted that all oral lesions associated with COVID-19 are related to immunosuppression and physical stress [17].

Management of COVID-19 Patients with Tongue Involvements

Most of the lingual problems and taste disturbances subside spontaneously [41]. Nevertheless, some may need specialized treatment, like immunoglobulin and corticosteroids for the Kawasaki-like disease [42] or antihistamines and corticosteroids for angioedema [13]. BHT management increases hydration, induces salivation, discontinues tobacco or alcohol use, and other predisposing factors [43]. If ineffective, gentle tongue cleaning with a brush, topical retinoids or salicylic acid, keratolytic agents, or surgical excision may be curative [44].

Conclusion

The current COVID-19 pandemic is unique because of the diverse multi-organ manifestations it has presented. Taste disturbances and tongue changes are among the various characteristics of SARS-CoV-2 infection. Therefore, because oral complaints are relatively common in COVID-19 patients, an intraoral examination should be conducted in all suspected cases of SARS-CoV-2 infection.