Histopathological findings of dermatophytoma using the alginate method: Investigating the potential for mixed infections of fungi and cocci beneath the nail bed

Onychomycosis with dermatophytoma was first described by Roberts and Evans1 in 1998 as a rare clinical manifestation of onychomycosis caused by dermatophytes. It is a form of the disease that is resistant to oral antifungal therapy. The clinical presentation typically involves yellow to white circular or linear clouding of the nail sheath. Dermatophytomas, which do not adhere to either the nail plate or the nail bed, are found beneath the nail plate in the region of this clouding. Observation of the dermatophytoma using the potassium hydroxide-tube method2 shows that it consists of spore-like fungi that sometimes extend their mycelia around the dermatophyte. However, because of its granular or particulate nature, it is challenging to prepare a histopathological specimen. In the cytopathology field, the cell block method3 is used to process solitary, scattered cells into paraffin blocks, which can then be stained using the same techniques used in histopathology. By preparing pathological specimens, the same area can be stained and observed with various stains. Here we report the preparation and observation of pathological specimens of dermatophytomas using alginic acid,4 a reagent commonly used in cell block preparation, are reported. This study was approved by the institutional review board of Fukui Prefectural University (M2023026). Six pathology specimens from three patients with dermatophytomas were prepared as follows. Approximately 0.5 mL of 1% sodium alginate solution (FUJIFILM Wako Pure Chemical Corporation: 190-09991) was placed on a piece of Saran Wrap (Asahi Kasei Corporation) to which the dermatophytoma that was not adherent to the nail bed or nail plate was added (Figure 1a). To this, a few drops of 1 M calcium chloride were added, which instantly caused gelatinization (Figure 1b). The gel was placed in a 20% formalin solution and fixed for at least 6 h. Paraffin-embedded blocks were then prepared using standard procedures, and hematoxylin and eosin (H&E) staining, periodic acid-Schiff (PAS) staining, Grocott staining, anti-dermatophyte antibody immunostaining, and Gram staining were performed (Figure 1c–i). Thirteen clusters of dermatophytes were identified in six pathological specimens. PAS staining showed positive areas marked with red dotted lines, and strongly stained areas were highlighted with yellow dotted lines (Figure 1e). Grocott staining showed positivity in regions corresponding to the red and yellow dotted lines (Figure 1f). Immunostaining with anti-dermatophyte antibody also demonstrated overall positivity (Figure 1g). In contrast, Gram staining showed that the regions marked with yellow dotted lines were primarily Gram-positive, with Gram-positive cocci on higher magnification (Figure 1h,i). These findings indicate that most of the dermatophytoma was composed of dermatophytes that were PAS-positive, Grocott-positive, and positive for anti-dermatophyte antibody immunostaining. In addition, clusters of Gram-positive cocci were observed in the same areas. Why certain cases of tinea onychomycosis present as dermatophytomas remains unclear. Recently, the importance of the interrelationship between bacteria and fungi has been highlighted.5 If mixed infection is involved in the pathogenesis of dermatophytoma, it may be related to the cause of resistance to treatment of dermatophytoma with oral medications and requires further study. None declared.