Subungual Hyperkeratosis as a Dermoscopic Clue of Primary Fingernail Mycobacterium Marinum Infections

Sir, Mycobacterium marinum is a kind of ubiquitous acid-fast bacilli that often exists in contaminated fresh or salt water or marine animals. Patients ranging from the proportion of 25% to 87% have sporotrichoid distribution of nodular lesions, which indicates nodular lymphangitis[1] and a group of infectious diseases can express similar patterns of lesions. Here, we report a patient with M. marinum infection who presented with multiple nodules, and the primary lesion was found in the subungual area, which is rarely involved. It may provide some clues for the early clinical diagnosis of M. marinum infection with dermoscopic findings of primary lesions. A 39-year-old female presented with three-month history of pain and swelling of the distal right index finger and multiple asymptomatic nodules on the right upper extremity. She recalled that she could get her nail injured inadvertently when gutted fish. The patient initially took cephalosporins for a week under diagnosis of 'pyogenic paronychia' at clinics without improvement, and later, multiple lesions gradually appeared on the forearm. Physical examination showed redness, swelling and detachment of the nail plate of the right index finger and multiple disseminated, erythematous, fluctuant nodules distributed in a sporotrichoid pattern on the right upper extremity [Figure 1a]. No internal organs were involved. The laboratory evaluation revealed unremarkable results except for a positive interferon-γ release assay.Figure 1: Clinical and dermoscopic findings of the patient infected with Mycobacterium marinum. (a) Multiple disseminated, erythematous, fluctuant nodules distributed in a sporotrichoid pattern on the right upper extremity. (b) Dermoscopic image showing onycholysis, subungual orangish hyperkeratosis surrounded by white scales and fissures of the right index fingernail. (c) After four months, the lesions of the arm were almost healed, leaving residual dark erythema. (d) Advancement of the newly growing nail was observedDermoscopic view (AM7515MZT Handheld Digital Microscope, ×35) of the right index fingernail showed detachment of the partial distal nail plate from the nail bed and subungual orangish hyperkeratosis surrounded by white scales and fissures [Figure 1b]. The nodules exhibited pink background with diminishing dermatoglyphics. Histopathological analysis of skin biopsy showed chronic suppurative inflammation infiltrated with abscess formation [Figure 2a], Gomori methenamine silver (GMS) staining revealed bacilliform substances that could not be ruled out as the pathogen [Figure 2b] and Ziehl–Neelsen staining revealed small amounts of acid-fast bacilli [Figure 2c]. Periodic acid-Schiff staining and specific quantitative polymerase chain reaction for detecting Mycobacterium tuberculosis were both negative. The pus extracted from one of the non-open nodules on the right forearm was divided up for high-throughput sequencing and cultivation. High-throughput sequencing indicated the presence of a potential pathogen, Mycobacterium sp., and after two weeks, smooth, cream-yellow colonies were observed on solid Lowenstein–Jensen (LJ) media under 30°C [Figure 3a]. Ziehl–Neelsen staining of a colony revealed acid-fast bacilli [Figure 3b]. Then, M. marinum infection was confirmed by polymerase chain reaction (PCR; GenBank Number: OM649761).Figure 2: Skin biopsy from the proximal forearm. (a) Chronic suppurative inflammation infiltrated with abscess formation. (b) Gomori methenamine silver staining revealed bacilliform substances (arrow, original magnification ×400). (c) Ziehl–Neelsen staining revealed acid-fast bacilli (arrow, original magnification ×400)Figure 3: Pus culture and Ziehl–Neelsen staining. (a) Culture showed smooth, cream-yellow colonies (Lowenstein–Jensen media, 30°C, two weeks), which was confirmed as M. marinum by PCR-sequence (GenBank Number: OM649761). (b) Ziehl–Neelsen staining of a colony revealed acid-fast bacilli (original magnification ×1000)The patient was treated orally with rifampicin 450 mg and ethambutol 750 mg once daily. After four months of treatment, the lesions almost healed [Figure 1c and d], leaving residual scarring without recurrence after nearly two more months of treatment and five months of follow-up. M. marinum infections are often localised and primary lesions have varied manifestations, disseminated cases are rare and only affect immunosuppressive patients. A single papule or nodule can appear after an incubation period of two–three weeks, and sometimes, it presents as a psoriasiform or verrucous plaque. As the condition advances, multiple nodules can appear and spread in a sporotrichoid disposition, which indicates nodular lymphangitis. Infections including sporotrichosis, nocardiosis, leishmaniasis, atypical mycobacteriosis and many other diseases can express similar clinical patterns. Classic manifestations of primary lesions of lymphangitic sporotrichosis have been described as a single hard painless nodule with a central ulceration.[2] Nocardiasis may initially present with a nodule that ulcerates with mild to abundant purulent drainage and ragged undermined borders.[3] Leishmaniasis can primarily reveal small, nonpruritic papules that gradually ulcerate, with friable granulation tissue and raised indurated borders.[4] Primary lesions of M. marinum infection have their own features: since the pathogen has been associated with the exposure of damaged skin to polluted water, primary lesions are commonly located in fingers. The incubation site tends to be nodular as well as exhibiting redness and swelling instead of initially ulcerating, and it may slowly generate a suppurative lesion.[2] Nail changes caused by mycobacterial infection are extremely rare. Warren et al.[5] used to observe a case of M. marinum infection that the patient's nail fold pain and swelling after getting gravel from fish tanks underneath the nail, but they did not describe the signs of nail changes specifically. Despite the lack of a clear history of trauma, the subungual lesions of our patient gradually improved during treatment, which still proved that the primary lesion was located under the nail. We observed that the characteristics of the impaired nail that M. marinum invaded under dermoscopic examination are onycholysis, subungual orangish hyperkeratosis surrounded by white thick scales, which partially coincide with the previous findings of Conforti et al.,[6] they used to report the dermoscopic features in a case of M. marinum hand infection of orange-whitish central areas surrounded by polymorphous vessels and fine scaling. We attributed the differences in the dermoscopic features in the two cases to different locations and stages of the disease, and more cases are needed to confirm our findings. Some nail diseases (such as subungual wart and subungual exostosis) may have some similarities to it in the dermoscopic presentation: subungual wart usually presents with hyperkeratosis, vascular ectasia, thick adherent scales and subungual exostosis shows a hard nodule with hyperkeratosis and onycholysis, with or without vascular ectasia.[7] Unlike the latter two, we haven't found a certain correlation between dermoscopic and histopathological features of primary lesions of M. marinum infection. In conclusion, our case highlights the observation of a special primary lesion of M. marinum infection, and dermoscopy may be a promising method to assist early diagnosis. More cases and studies are needed to prove the common characteristics of primary lesions of M. marinum infection and clarify possible associated histopathological features. Declaration of patient consent The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient has given her consent for her images and other clinical information to be reported in the journal. The patient understood that the name and initials will not be published and due efforts will be made to conceal the identity, but anonymity cannot be guaranteed. Financial support and sponsorship This work was supported in part by the 1.3.5 project for disciplines of excellence (ZYJC18033) of West China Hospital, Sichuan University and the HX-Academician project (HXYS19003) of West China Hospital, Sichuan University. Conflicts of interest There are no conflicts of interest.