Kaposi Sarcoma of the Nail Unit: A Case Report and Review of the Literature

Introduction: Kaposi sarcoma is an angioproliferative neoplasm. Its manifestations are well known but nail involvement seems extremely underreported. Case Presentation: A 55-year-old man presented with a 6-month history of a growing subungual tumor affecting the third right toe. After surgical excision, histological examination revealed a Kaposi sarcoma. Discussion: We report a case of Kaposi sarcoma with nail involvement of only one toe as the first and unique manifestation, which is exceptional.


Introduction
Kaposi sarcoma (KS) is a rare malignant angioproliferative neoplasm associated with human herpesvirus 8 (HHV-8), an opportunistic viral infection.This condition generally occurs in immunodepressed subjects, for example, in case of HIV infection or with iatrogenic immunosuppression.
There are four well-known forms of KS [1].The classic form typically affects elderly men especially from the Mediterranean region.It is most often indolent and is restricted to lower limbs.The endemic African form affects children and sometimes adults of sub-Saharan origin.The infantile subtype is significantly more aggressive because of its frequent visceral and lymph node involvement.The adult subtype resembles the classic form.The iatrogenic form mainly affects solid graft recipients treated with immunosuppressants while the epidemic form affects HIV-seropositive persons.Recently, a fifth form affecting HIV-uninfected males having sex with males has been described [2].
The term "sarcoma" is a misnomer because the tumor is not originally mesenchymal but vascular.It can develop on soft tissues in multiple localizations, simultaneously.
Its cutaneous and systemic manifestations are well known.Skin lesions are often asymptomatic and begin with pink to dark blue macules evolving to papules and later on nodules, sometimes coalescing to form larger lesions.Ulceration of the nodules is not uncommon.Legs, feet, and face are the most common skin locations of KS.Mucosal lesions are frequently observed but KS may also develop in deeper tissues as lymph nodes, digestive tract, liver, spleen, or lungs in severe cases [3].Surprisingly, nail localizations of KS have almost never been reported.

Case Report
A 55-year-old Belgian man of African descent presented with a 6-month history of a progressively growing subungual painful tumor affecting the third right toe.There was no history of trauma.His medical history was unremarkable and he did not declare any medication.The patient was born in Congo-Brazzaville but was living in Belgium for more than 30 years.
Clinical examination revealed a hard hyperkeratotic nodular mass at the distal lateral tip of the toe, with a violaceous base, lifting the outer side part of the nail plate (Fig. 1).No lymphedema was present.The rest of the mucosal and cutaneous examination was normal.
X-ray of the affected toe did not show any bony alteration.At this stage, a diagnosis of pyogenic granuloma, superficial acral fibromyxoma, and KS were considered.A surgical excision was performed (Fig. 2).Histological examination revealed a vast vascular proliferation of nodular appearance that was made up of abundant and largely atypical endothelial cells mixed with intense extravasation of red blood cells.Capillaries had a very irregular structure.Some plasmatocytes were noted.HHV-8 immunostaining was intensely expressed (Fig. 3).
The diagnosis of endemic KS prompted us for complementary exams: the thoracoabdominal, the gastroscopy and colonoscopy were unremarkable.At 6-month follow-up, the patient showed no sign of recurrence.

Discussion
Nail localization of KS seems to be exceptional but probably underdiagnosed and underreported.To the best of our knowledge, only 6 other cases of KS involving the nail unit have been described [4][5][6][7][8][9].Other cutaneous lesions of KS were present in all cases except ours and the first case that was described in 1977.Four of these cases involved the great toe, one the third toe, one the ring finger, and one the small finger.Main characteristics of these cases are summarized in Table 1.
We report here an exceptional case with nail involvement of only one toe as presenting and exclusive manifestation of KS.Only 1 case in 1977 shared both characteristics.Nevertheless, histological diagnostic procedures in 1977 were different from today, especially immunohistochemical detection of HHV-8.It is nowadays a gold standard for the diagnosis of KS and misdiagnosis can happen without its use [10].Some other cases from our review (Table 1) do not specify its use for the diagnosis of KS but history or simultaneous skin lesions of KS have then supported the diagnosis.
Interestingly, only 2 cases have reported bony lesion associated with KS nail involvement [6,7].Aïm et al. [7] have hypothesized that bone involvement was secondary to contiguity with a cutaneous lesion, involving the nail unit.
Berkowitz et al. [6] have not only reported a case of nail KS with bone involvement but also a case of nail KS triggered by a traumatism.Indeed, Koebner phenomenon is occasionally reported in patients affected by KS, but their case was the second known case of nail KS with Koebner phenomenon after the case of 1977 [11].
Fingernail involvement seems to be less frequent than toenail involvement and only concerns HIVpositive subjects whereas toenail involvement is more common and not always HIV-related.These data are consistent with the fact that indolent forms of KS (classic and endemic, adult subtype) mostly affect lower limbs.Among these cases, Zinoune et al. [9] have proposed the first and only dermoscopic features of nail KS presenting as a longitudinal erythronychia and consisting of a combination of red-purplish band and a purplish spot on the proximal fold.Treatment will depend on the form of KS and the presence, or not, of localized or systemic involvement [1].
In case of iatrogenic KS, discontinuation or reduction of the immunosuppressive treatment is recommended.In case of untreated HIV-associated KS, antiretroviral therapy must be started.In refractory cases or other forms (classic and endemic), treatment will rely upon the extent of the disease.
For localized disease, abstention, radiotherapy, topical gels (alitretinoin, imiquimod, and timolol), intralesional injections (vinblastine or bleomycin), surgery, or cryotherapy are possible ways of therapy [12].Systemic treatments with antiproliferative agents (taxanes and liposomal anthracyclines or interferon alpha-2b) are generally indicated in the event of visceral or severe and symptomatic cutaneous involvement.In our case, surgical excision appeared as the best option for a unique well-delimited lesion.Solitary lesions of KS have already been described, in particular, as a pyogenic granuloma-like KS occurring on extremities mainly but exceptionally involving the nail unit [13].KS should be added to the differential diagnosis of nail pyogenic granuloma, exostosis, amelanotic melanoma but also to the one of tumors presenting as a longitudinal erythronychia like onychopapilloma, Bowen's disease, glomus tumor of the matrix, and subungual verrucous dyskeratoma.

Conclusion
We reported herein a unique case of KS at the nail unit on a toe as a first and unique manifestation, which has never been described anymore since the first case in 1977.Its clinical presentation is hard to recognize because of its resemblance to other pathologies.KS should be added to the list of differentials when facing a reddish to bluish nodule or a longitudinal erythronychia, even if its occurrence remains exceptional, in order to allow a rapid and adequate management.

Table 1 .
Review of main characteristics of nail KS cases