Ringworm, medically known as tinea, is a common superficial fungal infection that affects the skin, hair, and nails. Despite its name, this condition is not caused by a worm but by a group of fungi called dermatophytes. These fungi thrive by consuming keratin, the hard protein found in the outer layer of human skin, hair, and nails. When a clinician suspects a ringworm infection, a rapid and definitive diagnosis is often achieved through a microscopic examination of a skin sample. This procedure, known as a potassium hydroxide (KOH) preparation, is a standard tool used to confirm the presence of the causative fungal elements.
Preparing the Sample for Microscopic Examination
The process of microscopic diagnosis begins with collecting a small sample from the infected area. For a skin infection, a healthcare provider typically uses the blunt edge of a scalpel or a glass slide to gently scrape scales from the advancing border of the rash. If the infection involves the hair, infected hairs are carefully plucked rather than cut. Samples from nails are usually collected as clippings or scrapings from the discolored or thickened portion.
Once the sample is collected, it is placed onto a glass slide and treated with a solution of potassium hydroxide (KOH). The primary role of the KOH solution is to dissolve the dense cellular material that makes up the sample, which is composed primarily of keratin. Since the rigid cell walls of fungi are composed of chitin, they are resistant to the alkaline KOH solution and remain intact. The KOH treatment essentially “clears” the specimen, eliminating the background of human cells and debris that would otherwise obscure the view of the fungus.
The slide may be gently heated to accelerate the dissolving action of the KOH, though this is not always necessary. A coverslip is then placed over the preparation, which may be left to stand for several minutes to allow the clearing process to complete. This chemical preparation is what makes the fungal elements visible against the now-transparent background of the host tissue when viewed under the microscope. The prepared slide is then ready for examination, typically using a low-power objective lens to scan the field.
Visual Characteristics of Dermatophytes
Under the microscope, the primary sign of ringworm infection is the presence of hyaline, septate hyphae. Hyphae are long, branching, thread-like filaments that represent the main body of the fungus. These structures are described as septate because they are divided into individual cells by cross-walls (septa), a feature that helps distinguish them from other types of fungal contamination.
These hyphal threads appear as translucent, parallel-walled structures that may twist and turn across the microscopic field. They often show true branching, giving them a distinct and irregular pattern within the cleared keratin. The hyphae may also fragment into infectious, barrel-shaped or rectangular structures known as arthrospores. These arthrospores are highly resistant and are the main way the infection spreads.
When a hair is infected, the hyphae and arthrospores will be observed either within the hair shaft (endothrix invasion) or on the outside surface (ectothrix invasion). Ectothrix invasion, for example, can show a distinct “cuff” of spores coating the outer layer of the hair. While the KOH preparation confirms the presence of a dermatophyte infection by revealing these characteristic structures, it does not allow for the identification of the specific fungal species.
Other Methods for Identifying Ringworm
Microscopic examination is a fast and simple diagnostic tool, but it is often used in combination with or followed by other methods to confirm the diagnosis or identify the species. One complementary technique is the Wood’s lamp examination, which involves shining a specific wavelength of ultraviolet light onto the affected area.
Wood’s Lamp Examination
Certain species of dermatophytes produce metabolites that cause the infected hair shafts to glow a bright apple-green color under the lamp. However, not all dermatophytes fluoresce, so a negative result from a Wood’s lamp does not rule out an infection.
Fungal Culture
The most definitive method for confirming ringworm and identifying the specific fungal species is a fungal culture. A sample from the lesion is placed on a specialized growth medium, such as Sabouraud agar, and incubated over several days or weeks. This culture allows the fungus to grow into a visible colony, which can then be examined for its color, texture, and microscopic features to determine the exact species. Fungal culture is often considered the “gold standard” because it provides a precise species identification, which can be important for guiding treatment, especially in persistent or widespread infections. Polymerase chain reaction (PCR) testing is also becoming more common as a fast and highly accurate method to identify the fungal DNA.