An Uncommon Species of Phaeohyphomycosis—A Rare Case Report of Rhytidhysteron Rufulum

Dear Editor,

Dematiaceous fungi are a group of fungi whose cell wall contains melanin complex and appear brown on microscopy. Based on histological findings, the infection patterns are grouped into chromoblastomycosis, phaeohyphomycosis, and eumycotic mycetoma. Phaeohyphomycosis is a broad term used for all mycoses, where the melanized fungal elements are present in host tissues caused by Fonsecaea, Exophiala, Cladophialophora, etc.[1] Phaeohyphomycosis involves the skin and subcutaneous tissue and disseminated infection can also occur in immunocompromised patients. Here, we report a case of subcutaneous phaeohyphomycosis in an old treated Hansen’s disease due to the rarity of the etiological agent. This is the first case report of subcutaneous phaeohyphomycosis caused by Rhytidhysteron rufulum in Tamil Nadu. Rhytidhysteron rufulum is a fungus of dead or decaying plant tissues and rarely infects humans.

A 60-year-old nondiabetic male was admitted for the treatment of a trophic ulcer and complained of pain from swelling over his left knee. He had developed this swelling 3 years ago, which progressively increased in size to attain the present size. He could not recall any history of accidental trauma or thorn prick. There was no history of similar lesions elsewhere in the body. He was a known case of old-treated Hansen’s disease, and had completed successful treatment of multibacillary multidrug therapy (MB-MDT) 12 years ago. Blood counts and other biochemical parameters were within normal range, and he was seronegative for human immunodeficiency virus. Clinical examination revealed a single cystic swelling of size 5 × 5 cm in the anterior aspect of the left knee joint [Figure 1a and b]. There was a purulent aspiration obtained from the swelling. The aspirate was subjected to 10% KOH examination, Gram stain, and bacterial and fungal cultures. Gram staining did not reveal any bacteria, and the bacterial culture was negative. KOH mount showed pigmented septate and thick-walled hyphae [Figure 2]. Grayish-white colonies were observed after one week of incubation on Sabouraud’s dextrose agar (SDA). On further incubation, the colonies turned grayish black, and black pigment was seen on the reverse [Figure 3a and b]. Lactophenol cotton blue tease mount showed thick, brown septate hyphae without any fruiting bodies [Figure 3c]. Excision in toto was done, and the patient was started on itraconazole 200 mg twice daily (BD). Histopathology showed pigmented septate hyphae in the dermis, and was confirmed by Gomori’s methenamine and silver staining [Figure 4a and b]. At 6-month follow-up, there was no recurrence of the lesion at the site. The isolate was submitted to Advanced Molecular and Diagnostic Research Centre for fungi (AMDRC), All India Institute of Medical Sciences, Bhubaneshwar. At AMDRC, the isolate was grown in Sabouraud’s dextrose broth for 15 days and DNA was extracted from a fungal ball using the phenol-chloroform isoamyl alcohol method. DNA obtained was subjected to PCR (Polymerase chain reaction) using the ITS4 and ITS5 primers and sequencing as described previously. On NCBI BLAST (National Centre for Biotechnology Information – Basic Local Alignment Search Tool) analysis, the sequence obtained showed 99.84% similarity with Rhytidhysteron rufulum (KP162181.1) and was identified as Rhytidhysteron rufulum submitted to NCBI GenBank with accession number OQ155021.

Figure 1.

(a and b) Cystic swelling over the left knee joint

Figure 2.

10% potassium hydroxide mount showing pigmented, septate hyphae (40x)

Figure 3.

(a) Grayish black colonies on SDA culture (b) On reversal of the culture showing black pigments (c) Lactophenol cotton blue tease mount showing thick and brown septate hyphae (10x)

Figure 4.

(a) Thick walled pigmented hyphae (black arrows) (H & E, 40x) (b) Gomori’s methenamine silver staining showing black fungal elements (white arrows) (40x)

Phaeohyphomycosis is caused by a heterogeneous group of fungi producing pigmented hyphae and yeast-like cells in tissue. The common presentations are solitary nodules, abscesses, or large cystic lesions occurring in the extremities.[2]

Other rare presentations so far reported are innocuous pustules in the hand, nodules on the surgical scar, erythematous nodules, tumid plaques over the face, erythematous indurated nodules in the lower limbs with a discharging sinus and cauliflower ear.[3] Totally, eleven cases of Rhytidhysteron rufulum were reported, till now. In India, by 2014, Mahajan et al. provided a detailed study on R. rufulum infection in a diabetic male and therapeutic challenges in the management.[4] A review of the literature of eight cases reported until 2018 was performed by Mudhigeti et al.[5] Details of the previous reported cases and the present case are discussed in Table 1. In all the previous case reports, only four species, R. rufulum, R. hysterinum, R. opuntiae, and R. dissimile, were described under the genus. According to the Fungorum Index (2023), the genus Rhytidhysteron includes forty-two species. It is a saprophytic dematiaceous fungus and is usually associated with dead or decaying plant tissues.[6] The fungus has a worldwide distribution and occurs particularly in the tropics and subtropics. Human infections are reported only with R. rufulum. It is commonly observed in patients with some degree of immunosuppression, resulting from diseases like tuberculosis, diabetes, acquired immunodeficiency syndrome (AIDS), hematological or solid organ malignancies, and transplantation patients. It is seen in tropical and subtropical environments worldwide. Rhytidhysteron rufulum has a putative orthologous gene set that code for a mycotoxin, dothistromin. They have the capacity to lyse human red blood cells (RBCs) in vitro.[7] This explains the pathogenic role of Rhytidhysteron on human cells. Clinical presentations may vary from a single nodule at the site of trauma to multiple spreading noduloulcerative lesions. Multilobular plaques have also been reported. At times, the overlying skin may have a few sinuses with crusts. Direct microscopy shows thick-walled, spherical, single-celled to two-celled or brownish muriform sclerotic bodies. Sometimes thick, brown hyphae with or without budding fungal bodies can also be seen. Lactophenol cotton blue wet mount of this fungus demonstrates septate, irregularly branched, smooth-walled, sterile (vegetative) dematiaceous hyphae without any spores. Sometimes toruloid hyphae and thick-walled terminal chlamydospores can be seen. The European Fungal Infection Study Group (EFISG) recommends surgical excision with the intent to cure localized subcutaneous nodules. The addition of itraconazole or voriconazole, at 400 mg daily, is recommended to prevent dissemination in immunocompromised patients.[8] These recommendations are generally for all phaeohyphomycosis and no specific recommendations for R. rufulum treatment exist.

Table 1.

Reports of old and current cases

Reports	Age/Gender	Comorbidities	Presentation	KOH mount	Biopsy	Management
Chowdary et al.[8]	50/M	Renal transplant	Noduloulcertive lesions on the left foot and a few smaller lesions over the shin and thigh	Thick-walled, spherical, single-celled, and two-celled, muriform sclerotic bodies with a brownish tinge and thick-walled chlamydospores	Pseudoepithe liomatous hyperplasia with an extensive dermal infiltrate	Itraconazole only
Mahajan et al.[4]	72/M	DM	Soft, painless, multiloculated, nontender swelling over the dorsum of the right foot with erythema and a few sinuses with crusts.	Multiple, broad, septate, irregularly branched, dematiaceous hyphae, toruloid hyphae (chains of yeast cells), and yeast-like cells	Multiple areas of neutrophilic abscess bounded by epithelioid cells and foreign body giant cells	Itraconazole, terbinafine, and liposomal amphotericin along with surgical excision
Mishra et al.[9]	65/M	None	Well-circumscribed, indurated, blackish, nontender, painless subcutaneous nodule on tendoachilles region in the left foot.	Thick brown branching septate hyphae. Hyphae were PAS-positive and had no spherical or sclerotic bodies.	Intense neutrophilic reaction	Terbinafine and itraconazole
Chander et al.[10]	45/M	None	Mobile, nontender swelling on the dorsal aspect of the right foot	Long, thick, septate, tortuous, dark brown hyphae and no sclerotic bodies.	Intense neutrophilic reaction with lymphocytes, macrophages, and few septate hyphae	Itraconazole only
Chander et al.[10]	50/M	None	Small, soft, nontender, movable nodule on the anterolateral aspect of the left knee	Long, thick, septate, dark brown hyphae	Intense neutrophilic reaction with lymphocytes, macrophages, and few septate hyphae	Itraconazole only
Yadav et al.[11]	54/M	None	Well-defined, painless subcutaneous swelling on the anterior aspect of the right leg	Thick, long septate hyphae	Acute and chronic inflammatory cells and necrotic background	Itraconazole only
Tejashree et al.[12]	59/M	DM	Painless, large, soft, slowly progressive, swelling, noduloulcerative lesions on his right leg.	Septate, branching phaeoid hyphae	Mononuclear inflammatory cell infiltrates	Itraconazole with surgical excision
Mudhigeti et al.[5]	40/F	None	Small, well-circumscribed, firm, painless, free mobile swelling on the base of 3rd phalanx of the right hand	Thick branching septate hyphae with constrictions and bulbous appearance which stained positive and occasional thick-walled chlamydospores	Inflammatory background showing neutrophils and necrotic debris	Details not available
Mangaiyarkarasi et al.[13]	55/M	DM/CKD	Swelling over left leg for one year	Thick, brown, septate, and branching hyphae.	Not available	Lost follow-up
Braue et al.[14]	40/M	Renal transplant	Painful, subcutaneous fluctuant nodule on his left dorsal hand	Pigmented spores and hyphae	Pseudoepithe liomatous hyperplasia with dermal fibrosis	Itraconazole with surgical excision followed by split-thickness skin graft.
Dingle et al.[15]	38/F	DM/PTB	Nodule on left leg for 2 years	Not available	Necrotizing granulomatous inflammation	Itraconazole with surgical excision
Present case	60/M	Old treated Hansen’s disease	Swelling in his left knee for 3 years	Pigmented, septate, and thick-walled hyphae	Neutrophilic suppurative granuloma	Itraconazole with surgical excision

KOH mount – Potassium hydroxide mount, PAS – Periodic acid Schiff, DM – Diabetes mellitus, CKD – Chronic kidney disease, PTB – Pulmonary tuberculosis

In recent times, many unusual melanized fungi as agents of phaeohyphomycosis have been recognized, and Rhytidhysteron rufulum is one among them. Through this case report, we hope to raise awareness among primary physicians about this emerging fungal infection in India.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.