Wood’s Light Examination

CHAPTER 43 Wood’s Light Examination



Roberta E. Gebhard


The Wood’s lamp (“black light”) produces ultraviolet rays with a wavelength of 365 nm and above by projecting a beam of light through a filter of glass containing nickel oxide. Invisible light in the long-wave ultraviolet range and visible blue-white light are created. The fluorescence produced as the light hits objects varies in color depending on qualities of the surface itself. Characteristic appearances have been described for several dermatologic conditions. The Wood’s light examination provides a quick, inexpensive, and useful adjunct in their diagnosis (Fig. 43-1).



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Figure 43-1 A and B, Examples of Wood’s lamps (ultraviolet light).


(B, Courtesy of Dennis Babel, PhD.)


Diagnostic uses for the examination include the following:


• Tinea capitis

• Erythrasma

• Vitiligo, albinism, tuberous sclerosis, and other pigmentary conditions

• Pseudomonas infections

• Porphyria cutanea tarda

• Tinea versicolor

• Detection of some chemicals that are applied to the skin or taken systemically

• Adjunct in finding corneal abrasions or herpetic corneal lesion

• Identification of ejaculate in rape/sexual assault


Indications



• Any dermatitis in body folds such as the inguinal, perianal, interdigital, axillary, or inframammary areas (e.g., erythrasma, Pseudomonas infections)

• Patches of scalp scaling and partial hair loss, especially when the hairs are broken or shorter than normal (tinea capitis)

• Pigmentary conditions

• Blisters or punctate erosions on the exposed portions of the hands and forearms, with follow-up urine examination (porphyria cutanea tarda)

• Patches of scaling and altered pigmentation of the skin (tinea versicolor)


Technique


Let the light warm for a few minutes. From a distance of approximately 8 inches, focus the Wood’s light on the area of interest. Darken the room to improve visualization of the resultant fluorescence. Observe and record findings carefully. It is crucial to observe the specific color of fluorescence, not simply its presence. Soaps, lotions, cosmetics, urine, other chemicals, and fragments of scaling skin may themselves yield fluorescence. In addition, patients should not bathe 24 hours before the examination. Otherwise the examination may show little or no fluorescence.



Common Findings



• Tinea capitis: The majority of cases in the United States are caused by Trichophyton tonsurans, which does not fluoresce. Trichophyton verrucosum does not fluoresce either. Hair (but not the skin of the scalp) may fluoresce yellow-green if infected with Microsporum canis or Microsporum audouinii, or a pale white-green in the rare event of Trichophyton schoenleinii infection. In these cases, sensitivity may be relatively low. Sensitivity of approximately 50% has been reported for M. canis (Kefalidou and colleagues, 1997). Fluorescence of affected hair should be sought, particularly in the follicular portion of the hair. Potentially infected, broken-off hair may be plucked and the subepidermal portion then viewed under the Wood’s lamp (Fig. 43-2).

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May 14, 2017 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on Wood’s Light Examination

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