Introduction
The dermatophytes are the fungi that reside exclusively on dead cornified layer of skin and skin appendages. The skin produces inflammatory and allergic response to them.1 Dermatophytes are divided into three genera- Tichophyton, Microsporum and Epidermophyton. Trichophyton infects hair, nails and skin, Microsporum infects skin and hair and Epidermatophyton infects skin and nails not hair. Infection by dermatophyets to smooth and non hairy skin of body is called Tinea corporis. Tinea unguium is the infection of nails, Tinea cruris involves groins and perineum, Tinea barbae affects face and neck, Tinea pedis affects foot and tinea capitis involves scalp.2, 3 The species Trichophyton rubrum and Trichophyton glabrosa are more common than Microsporum and Epidermophyton species.4 The treatment with antifungal drugs commonly fluconazole, ketoconazole, itraconazole, griseofulvin and terbinafine depends on the site of infection and may vary from one week to months. As treatment with these drugs may cause side effects like elevation of liver enzymes, headache, nausea, gynaecomastia etc, their dose dependent therapy should be optimized to the appropriate patient.5, 6 The skin scraping method for KOH mount is used commonly by dermatologist for dermatophytes identification.7 The dermatologist and dermatopathologist should be aware of mimickers of dermatophytes for proper selection and treatment of the patient.
Procedure
The routine method involves skin scraping, potassium hydroxide (KOH) mount and microscopic examination.7
Scraping
The skin surface to be sampled is thoroughly cleaned using alcohol. Then, the surface is stretched with one hand and scrapings are taken along the edge of the lesion using 24G scalpel blade with other hand. On hairy surface, epilating short hair along with scrapping obtains a better yield. For nails, nail clippings may be taken or scraping the undersurface of involved dystrophic nail may be sampled. Scrapings are collected on glass slide and covered with another slide and sent to lab for KOH mount and microscopic examination.
KOH mount
A drop of 10% KOH is added on the slide with scrapings and covered by a cover slip made of fragile glass and gently heated to bring about clearing.
Microscopic examination
The slide is observed under direct microscope to find out spores and hypae by lowering down the condenser. The examination is done initially at low power (10X) and then at higher magnification (40X) to study the morphology of the fungus. Microscopy reveals branched septate hyphae with or without fungal spores.4
Discussion
The dermatophytes are a group of closely related filamentous fungi that infect only superficial keratinized tissues like skin, hair and nails. They cause Tinea or ‘ringworm’ infection on the body. ‘Favus’ is a chronic type of ringworm in which dense crust is developed in the hair follicles. Clinically, tinea may manifest as pruritis, blistering, fissures, spots and scales.1, 8, 9 In lesions, dermatophytes appear as septate hypae and arthospores. The three types of dermatophytes are of different shapes: Epidermophyton are club shaped, Microsporum are fusiform shaped and Trichophyton are cylinder shaped.4 The routine method of diagnosis is by examination of KOH mount of the skin scraping, however, species identification is possible only by culture examination.4 Usually, it is not difficult to identify dermatophytes on KOH. However, sometimes the diagnosis becomes difficult as some normal structures mimic dermatophytes.7 This may cause erroneous diagnosis and inappropriate treatment by the treating clinician. The several mimickers of dermatophytes which may commonly encountered and their differentiating features are discussed below precisely.
Cotton fibres/synthetic fibres
Cotton fibres/synthetic fibres are the most common mimickers of the fungal hypae.7 They may be present due to cleaning of glass slides with either tissue papers,cotton or any other similar material. These fibres may even look branching along their length. However, they show no septation and are seen with taper ends (Figure 1 a). The true dermatophytes are always septate and their ends are not tapered (Figure 1b).
Epidermal cells edges
The epidermal polygonal cells edges commonly mimic dermatophytes7 due to refraction under the microscope when condenser is lowered down (Figure 2 a). They can be differentiated from dermatophytes as they don’t show beaded appearance and branching which are seen in dermatophytes (Figure 2b).
Broken hair shaft
At low power, villous hair shaft can be mistaken for dermatophytes, however, they are always pigmented and and show no septation. They can also be differentiated from dermatophytes due to their broken ends (Figure 3 ).
Gram negative bacteria
Gram negative bacterial colonies may mimic dermatophytes belonging to the genera Trichophyton soleniecii (Favic chandelier) at low power. However, at high power, numerous small side branches can be traced out in true dermatophytes (Figure 4 a) which may not be seen in gram negative bacterial colonies (Figure 4b).
Skin epithelial debris
Epithelial debris from the skin scrapings may look fungal hypae under microscope (Figure 5). They may mimic Trichophyton rubrum. However, they give an impression of thick cast like appearance and are not branched. They may be avoided by thorough cleaning of the skin surface by alcohol before sampling.
Seeping of KOH at edges
Lastly,the seeping of KOH at the edges of coverslip along the imaginary lines may mimic to fungal hypae especially to Trichophyton mentagrophytes. It may happen due to overflow of KOH drop or improper placement of the glass coverslip This happens near or at the edges of coverslip and don’t show true septation (Figure 6).
Conclusion
KOH mount is a rapid and easy techinique to identify dermatophytes. However, some of normal structures may mimic dermatophytes on KOH. It is therefore important to be aware of their existence and to recognize their differentiating features. It sometimes becomes a challenge to make a correct diagnosis especially in already treated or modified tinea infection. It is noteworthy that there are various morphological differences between true dermatophytes and its mimickers. Usually, the presence of features such as septation, branching and presence of spores establish the diagnosis of dermatophytes without any doubt. Few steps like cleaning of the skin surface with alcohol before sampling, avoidance of using cotton materials for wiping the slides, proper placement of KOH drops over the slide in addition to clinical correlation may help to reduce the mimickers and may prevent misdiagnosis so that inappropriate treatment with antifungal drugs to the patient who do not require them may be avoided. This article has been communicated to make dermatopathologist aware of the routinely encountered mimickers of dermatophytes. However, a detailed research work with statistical value to each mimicker needs to be done in future.
Article DOI
