SARS-CoV-2 Endothelial Infection Causes COVID-19 Chilblains

Histopathological, Immunohistochemical and Ultrastructural Study of Seven Paediatric Cases

I. Colmenero; C. Santonja; M. Alonso-Riaño; L. Noguera-Morel; A. Hernández-Martín; D. Andina; T. Wiesner; J.L. Rodríguez-Peralto; L. Requena; A. Torrelo


The British Journal of Dermatology. 2020;183(4):729-737. 

In This Article


Lymphocytic vascular damage was the hallmark feature in biopsies from our seven patients with COVID-19-related chilblains. The intensity of the vasculitic damage ranged from minor findings, such as endothelial cell tumefaction and mild lymphocytic infiltration of the walls, to more severe signs like fibrinoid necrosis and thrombosis. The spectrum of histopathological features did not correlate with the duration of the lesions.

Vasculitis is defined as inflammation directed at vessels, which compromises or destroys the vessel wall leading to haemorrhagic and ischaemic events. The spectrum of vascular reaction to injury is variable.[16] Moderate-to-severe lymphocytic inflammation of vessel walls was noted in half of our cases, and inflammation was mild in the rest. In all of them, even in those with mild inflammatory infiltrate, indirect evidence of vascular damage and leakage was present in the form of extravasation of erythrocytes and dermal oedema.

The overall clinical and histopathological features in our cases are entirely in keeping with chilblains. Biopsies of chilblains show a moderate-to-dense superficial and deep perivascular lymphocytic infiltrate with exocytosis to the epidermis and acrosyringia and perieccrine accentuation. A few necrotic keratinocytes and mild vacuolar degeneration of the basal layer can be observed occasionally. Pronounced papillary dermal oedema and spongiosis are often present. Lymphocytic vasculitis is a common feature, and superficial thrombosis is present in some cases. Chilblains may be classified as primary (idiopathic, cold related) or secondary to underlying conditions. Some authors consider that primary and secondary chilblains cannot be reliably differentiated based only on microscopic features.[17] Perieccrine inflammation and marked dermal oedema have been described as features favouring idiopathic chilblains over chilblains associated with connective tissue diseases, particularly lupus erythematosus, which is favoured by more prominent changes of interface dermatitis.[18,19] Immunohistochemistry for lymphoid markers is not useful in the differential diagnosis of primary and secondary chilblains, as both show a predominance of mature T cells with only a minor proportion of B cells and some histiocytes. Increased numbers of large CD30+ cells have been described in some cases of idiopathic chilblains, with no clinical significance.[20,21]

Secondary causes of chilblains include connective tissue diseases (lupus erythematosus, Behçet disease, antiphospholipid syndrome, rheumatoid arthritis, Sjögren syndrome); cryopathies; haematoproliferative or neoplastic diseases; blood hyperviscosity states; genetic diseases (familial chilblain lupus, STING-associated vasculopathy of infantile onset, Aicardi–Goutières syndrome, and interleukin-1 receptor-associated kinase-4 deficiency); and anorexia or other disorders causing weight reduction.[22–24] None of these causes of secondary chilblains was present in our patients. Moreover, cold exposure was unlikely to be the trigger in our cases, as all lesions appeared during the mild spring climate in Madrid.

A causal relationship with COVID-19 has been considered doubtful in this outbreak of cases of chilblains. However, we are not surprised by the high rate of negative PCR tests in our cases considering the reported low rate of positive PCR tests in children with symptoms suggestive of COVID-19.[25] We have demonstrated the presence of viral particles within endothelial cells in lesional skin biopsies from patients presenting with chilblains during the COVID-19 pandemic. The positive immunohistochemistry was confirmed by the presence of viral particles in one case using transmission electron microscopy. Our results strongly support a pathogenetic role for SARS-CoV-2 in chilblains presenting during the pandemic. COVID-19 should be included in the list of secondary causes of chilblains, and we think these lesions can be denominated from now onwards as COVID-19 chilblains.

It has been proposed that SARS-CoV-2 uses angiotensin-converting enzyme (ACE)2 expressed by pneumocytes in the epithelial alveolar lining to infect the host cells and cause lung injury.[26,27] It is likely that SARS-CoV-2 uses ACE2 to enter vessels, as ACE2 is widely expressed by endothelial cells.[28,29] ACE2 is also present in epithelial cells of eccrine glands, explaining the positive immunolocalization of SARS-CoV-2 we have observed in these cells.[29]

Vascular injury and thrombosis could explain the clinical features of chilblains and related acroischaemic lesions seen in some patients with COVID-19. Different mechanisms of vascular damage in this setting have been proposed. Varga et al.[8] suggested that recruitment of immune cells, either by direct viral infection of the endothelium or immune mediated, can result in widespread endothelial dysfunction associated with apoptosis. Magro et al.[30] demonstrated a pattern of tissue damage consistent with complement-mediated microvascular injury in the lung and skin of five individuals with severe COVID-19. Their findings provide a foundation for further exploration of the pathophysiological importance of complement in COVID-19 and could suggest targets for specific intervention.

Histological features of COVID-19-related chilblains have been only scarcely described.[5,31] Kolivras et al.[5] reported a SARS-CoV-2 PCR-positive 23-year-old man with mild systemic symptoms and skin lesions suggestive of chilblains on his feet and toes. A skin biopsy showed features consistent with chilblains, and the authors suggested a possible role for interferon-1, based on the similarities with chilblain lupus erythematosus. The presence of SARS-CoV-2 particles in endothelial cells in our cases argues against this hypothesis, suggesting SARS-CoV-2 as a direct cause of the endothelial damage and thrombosis instead. A primary thrombotic mechanism appears unlikely for COVID-19 chilblains based on the limited presentation and normal coagulation tests.

Chilblains seen in young people may, to some extent, resemble the severe acral ischaemia caused by thrombosis in severely ill patients with COVID-19. These patients are usually elderly and admitted to intensive care units, and show coagulopathy with very elevated D-dimer levels.[32] A severity spectrum cannot be ruled out as widespread endothelial infection could contribute to the systemic multiorgan manifestations seen in patients with severe COVID-19. In fact, SARS-CoV-2 has been shown to infect endothelial cells in the skin and other organs in a few cases.[8,30,33]

Our study has some potential limitations. We acknowledge that the number of patients is small; furthermore, evidence of SARS-CoV-2 infection by PCR and serology could not be obtained. Also, the use of immunohistochemistry for SARS-CoV and SARS-CoV-2 is still restricted and larger studies are necessary to assess with accuracy the role of immunohistochemistry in the diagnosis of COVID-19. Finally, we only carried out electron microscopy in one case; even though the virus was detected, more cases are needed to support our conclusions further.

In conclusion, the presence of SARS-CoV-2 in the endothelium of dermal vessels in skin biopsies of children and adolescents with acute chilblains confirms that these lesions are a manifestation of COVID-19. Their clinical and histopathological features are similar to those of chilblains of other aetiologies, and virus-induced vascular damage could explain their pathophysiology. Our findings support the hypothesis that widespread endothelial infection by SARS-CoV-2 could have a role in the pathogenesis of severe forms of the disease. More studies are needed to understand the reasons why previously healthy children, adolescents and young adults present with limited skin forms of COVID-19, in contrast with the severe multiorgan presentations seen in older patients with background diseases.