What is the role of Wood lamp exam in the diagnosis of tinea capitis (scalp ringworm)?

Updated: Feb 21, 2020
  • Author: Marc Zachary Handler, MD; Chief Editor: Dirk M Elston, MD  more...
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Answer

In 1925, Margarot and Deveze observed that infected hairs and some fungus cultures fluoresce in ultraviolet light. The black light commonly is termed Wood lamp. Light is filtered through a Wood nickel oxide glass (barium silicate with nickel oxide), which allows only the long ultraviolet rays to pass (peak at 365 nm). Wood lamp examination is useful for certain ectothrix infections (eg, those caused by M canis,M audouinii,Microsporum rivalieri). In cases with endothrix infection, such as T tonsurans, however, negative Wood lamp examination findings are of no practical value for screening or monitoring infections. [23]

Hairs infected by M canis, M audouinii, M rivalieri, and M ferrugineum fluoresce a bright green to yellow-green color (see the image below).

Wood lamp examination of a gray-patch area on the Wood lamp examination of a gray-patch area on the scalp. In Microsporum canis infection, scalp hairs emit a diagnostic brilliant green fluorescence. Trichophyton tonsurans does not fluoresce with Wood lamp.

Hairs infected by T schoenleinii may show a dull green or blue-white color, and hyphae regress leaving spaces within the hair shaft. T verrucosum exhibits a green fluorescence in cow hairs, but infected human hairs do not fluoresce. The fluorescent substance appears to be produced by the fungus only in actively growing infected hairs. Infected hairs remain fluorescent for many years after the arthroconidia have died.


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