Dermoscopy of Fire Coral (Millepora) Sting: The Clock Sign

This letter aims to illustrate the usefulness of dermoscopy in the diagnosis of fire coral (millepora) sting and describes a new dermoscopic sign related to fire coral anatomy. Fire corals, cnidarians belonging to the genus millepora, are hydrozoans frequently found in the Indian, Pacific, and Atlantic Oceans and the Caribbean Sea [1]. Predatory polyps protruding from the skeleton are numerous and highly toxic, for this reason millepora are popularly known as “stinging corals” or “fire corals” [2]. Direct skin contact with fire corals causes an immediate painful stinging reaction [1]. We report the case of a 34-year-old man consulting for pruritic, erythematous papules grouped in annular plaques on the medial malleolus of the right ankle (Figure 1A). The initial diagnostic impression was of annular granuloma. Surprisingly, dermoscopy showed fine black lines and dots arranged in multiple circular patterns. Moreover, scales and an erosion were present on an erythematous background (Figure 1B). Upon reevaluating the history, patient reported that a week earlier he had been scuba diving in the Dominican Republic. While underwater, he brushed against a hard object, experiencing pain from the impact but no burning sensation. Four days later, the skin lesions appeared. Dermoscopic and anamnestic findings resulted in the diagnosis of fire coral (millepora) sting. Fire corals (millepora) are a genus of colonial polypoidal hydrozoans (hydrocorals) located in shallow tropical seas [1, 2]. Millepora means “thousand pores”, because their skeleton is formed by numerous pores of two types: gastropores and dactylopores. Dactylopores are arranged in circles around individual gastropores [2] (Figure 2). The dactylopores contain the dactylozooids, predatory polyps used to capture prey and defend against predators [3]. Dactylozooids are covered with numerous fine, hair-like tentacles that enclose nematocysts containing stinging venom [1, 2]. A physical trigger, for instance the contact with a foreign body, causes the ejection of microscopic filaments that penetrate the target organism, releasing its toxic contents [1]. This circular arrangement of the dactytlozoids translates into the dermoscopic findings observed in the patient: several microscopic slender barbs (nematocysts) forming lines and dots arranged in multiple circular patterns, resembling each of them the face of a clock. Therefore, we propose the “clock sign” as the dermoscopic clue to diagnose millepora sting. Previously, dermoscopic features representing the penetration of jellyfish nematocysts have been reported, showing brown dots and lines irregularly distributed [4-6]. Other observed features include an erythematous background and erosions due to scratching when pruritus is the predominant clinical manifestation, as it was in our case. The patient's mild symptoms can be attributed to the small amount of venom injected, given the limited surface area exposed to contact. Additionally, it could be explained by a possible low potency of the venom, which may depend on the species of fire coral. Indeed, we could not precisely determine the exact fire coral species involved. Finally, the late-onset lesions may be due to a delayed-type reaction to the nematocysts left in the epidermis. The diagnosis of fire coral sting is clinical, considering the history of snorkelling or scuba diving in coral reef areas and characteristics symptoms. However, dermoscopy is an accessible tool in dermatology practices that complements the clinical diagnosis, becoming helpful when the history of contact with a cnidarian is unclear. The clock sign with its repeating circular pattern may help differentiate fire coral stings from other cnidarians stings like jellyfish and sea anemones, as well as other clinical mimickers such as insect bites, annular granuloma or contact dermatitis. Although the clock sign seems quite specific of fire coral stings, inferring a common dermoscopic pattern to every lesion cause by the contact with fire corals is limited by the single case design of the study. To the best of our knowledge, this is the first report describing the dermoscopic findings of a fire coral sting. Fátima Mayo-Martínez: conceptualisation, investigation, writing–original draft. Ruggero Moro: resources, writing–review and editing, supervision. The authors have nothing to report. All patients in this manuscript have given written informed consent for participation in the study and the use of their deidentified, anonymized, aggregated data and their case details (including photographs) for publication. The authors declare no conflicts of interest. The data that support the findings of this study are available from the corresponding author upon reasonable request.