Chapter 31 Superficial fungal infections
2. How are superficial fungal infections diagnosed?
Superficial fungal infections can usually be suspected clinically, but definitive diagnosis requires the demonstration of fungal pathogens by microscopic examination or culture of skin, nail, or hair scrapings from the suspected lesion. During microscopic examination, hyphae are sought in the material. The material is first placed on a glass slide, and then 1 or 2 drops of 10% to 20% potassium hydroxide (KOH) are added. A fungal stain such as chlorazol black E may be added to the preparation to aid visualization of the fungal elements. The hyphae of dermatophytes will be septate and typically demonstrate branching (see Figure 3-1). Skin scrapings can also be placed on culture media. Culturing the organism, in addition to being a diagnostic aid, permits speciation of the organism.
3. On a KOH examination, hyphal-like structures arranged in a mosaic pattern are noted. Does this indicate the presence of a dermatophyte?
“Mosaic hyphae” are not really hyphae and do not indicate the presence of a dermatophyte. If you vary the microscope’s focus, the pattern can be observed to conform to the cell walls. Mosaic hyphae actually represent thickened stratum corneum cell walls. True hyphae cross the cell walls of keratinocytes and do not conform to the contour of keratinocytes.
4. What are the three most commonly used culture media for the growth of dermatophytes?
• Sabouraud’s dextrose agar: A nonselective culture medium consisting of peptone, dextrose, agar, and distilled water. It allows the growth of bacteria as well as pathogenic and nonpathogenic yeast and molds.
• Mycosel or mycobiotic agar: A selective growth medium for dermatophytes. It consists of Sabouraud’s agar with cycloheximide (suppresses saprophytic fungi) and chloramphenicol (suppresses bacteria). Dermatophytes and Candida albicans grow readily on this media, while the growth of contaminant bacteria, some yeast, and many opportunistic fungi is inhibited.
5. Describe some of the presentations of superficial fungal infections caused by dermatophytes.
The superficial dermatophyte infections are classified according to their location on the affected person. This location does not necessarily reveal the identity of the offending organism. The infection will cause the production of scale. The scale may or may not be associated with erythema, vesicles, or annular plaques (Table 31-1, Fig. 31-1).
6. Which dermatophyte causes the most fungal infections of skin?
Trichophyton rubrum.
Table 31-1. Clinical Presentations of Dermatophyte Infections
| INFECTION | LOCATION |
|---|---|
| Tinea capitis | Scalp |
| Tinea faciei (see Fig. 31-1A) | Face |
| Tinea barbae | Beard |
| Tinea corporis (see Fig. 31-1B) | Trunk, extremities |
| Tinea cruris (see Fig. 31-1C) | Groin |
| Tinea manuum (manus) | Hands |
| Tinea pedis | Feet |
| Tinea unguium | Nails |
7. What is the most common cause of tinea capitis in the United States?
Until the mid-1950s, Microsporum audouinii was the most common cause of endemic tinea capitis in the U.S., but it has since been replaced by Trichophyton tonsurans. Several theories have been proposed to explain the almost total disappearance of M. audouinii from the U.S., but the most plausible theory is that it was eradicated by the widespread use of griseofulvin. At the same time that M. audouinii disappeared, T. tonsurans, formerly an uncommon cause of tinea capitis, quickly spread. This species was probably introduced into the U.S. from Central or South America.
Foster KW, Ghannoum MA, Elewski BE: Epidemiologic surveillance of cutaneous fungal infections in the United States from 1999 to 2002, J Am Acad Dermatol 50:748–752, 2004.
8. Name the four clinical patterns of tinea capitis.
1. The seborrheic pattern has a dandruff-like scaling of the scalp and should be considered in prepubertal children with suspected seborrheic dermatitis (Fig. 31-2A).
2. In the black-dot pattern, hairs are broken off at the skin line, and black dots are seen within the areas of alopecia (Fig. 31-2B). In the U.S., this pattern is primarily associated with T. tonsurans infections.
3. A kerion is an inflammatory fungal infection that may mimic a bacterial folliculitis or an abscess of the scalp (Fig. 31-2C). The scalp is tender to the touch, and the patient usually has posterior cervical lymphadenopathy.
4. Favus is a rare form of inflammatory tinea of the scalp presenting with sites of alopecia that have cup-shaped, honey-colored crusts, which are called scutula and are composed of fungal mats.
Tinea capitis is one of the most commonly misdiagnosed skin infections. Any prepubertal child who presents with a scaly scalp dermatitis or carries a diagnosis of seborrheic dermatitis should be presumed to have a dermatophyte infection of the scalp until proven otherwise. Similarly, any child who presents with one or more scalp abscesses most likely has a kerion. Kerions are frequently secondarily infected with Staphylococcus aureus, and unsuspecting health care providers often mistakenly treat kerions as bacterial abscesses.
Figure 31-2. Tinea capitis. A, Seborrheic pattern. B, Black-dot pattern. C, Kerion presenting as a tender boggy mass in the scalp.
9. What are the types of hair invasion in tinea capitis? What dermatophytes are associated with each type?
1. Endothrix infections are produced by fungi that invade the inside of the hair shaft and are composed of fungal arthroconidia and hyphae (Fig. 31-3). A helpful mnemonic to remember the organisms that cause endothrix invasion is: “TVs are in houses.”—T is Trichophyton tonsurans, V is violaceum, and S is soudanense.
2. Ectothrix infections are produced by fungi that primarily invade the outside of the hair shaft. Some agents of small-spore ectothrix cause a fluorescent tinea capitis.
10. What is a Wood’s light? What organisms are detected by this exam?
A Wood’s light is an ultraviolet light source that emits in the spectrum of 325 to 400 nm. This light was used extensively for the diagnosis of tinea capitis when Microsporum audouinii was the major cause of this disorder. However, it is of limited usefulness today because most cases are now produced by Trichophyton tonsurans, which is not fluorescent. The fluorescence is caused by pteridine. The fungi responsible for fluorescent tinea capitis can be remembered by the mnemonic “See Cats and Dogs Fight.”
