A Wood’s lamp is a mercury vapor ultraviolet lamp with a filter of barium silicate glass with 9% nickel oxide. The filter allows UV light in the wavelength of 320 to 400 nanometers only while blocking out all other wavelengths. Wood’s lamp mainly emits ultraviolet light in the wavelength of 360 nanometers.
Wood’s Lamp in the Diagnosis of Fungal Infection of the Scalp
Tinea capitis, the fungal infection of the scalp, is caused by different species of fungi. Examination of the scalp in a dark room with Wood’s lamp produces a greenish fluorescence when the infection is caused by certain species like the Microsporum and Trichophyton schoenleini. Wood’s lamp examination is particularly useful in rapidly diagnosing outbreak of tinea capitis in large number of cases as in community schools and boarding. Earlier diagnosis and management prevents further spread of the disease.
The disadvantage is that, at times, the fungal infection is caused by nonfluorescing species like Trihophyton violaceum. Hence a negative Wood’s lamp examination does not rule out presence of fungal infection.
Wood’s Lamp in Pigmentary Disorders of the Skin
In light-skinned individuals, it may be difficult to distinguish between normal skin and slightly hypopigmented (light white color) areas in diseases like pityriasis versicolor, pityriasis alba and ash leaf macules present in the genetic disease tuberous sclerosis. These are accentuated when examined under the Wood’s lamp. The extent of vitiligo patches also can be clearly delineated in the fair skinned with the help of the Wood’s lamp.
In pigmentary disorders like melasma and freckles, the level of melanin deposition can be differentiated using Wood’s lamp. Dark color due to pigments in the outer epidermal layer of the skin is accentuated while the color of the deeper dermal pigments is decreased during Wood’s lamp examination.
Wood’s Lamp Examination in the Diagnosis of Bacterial Infections of the Skin
Wood’s lamp fluoresce a coral pink color in eythrasma, a superficial bacterial infection causing brownish red patches on the skin. A yellowish green fluorescence is typical of Pseudomonas pyocyanea infection.
Wood’s Lamp in the Diagnosis of Porphyrias
Porphyrias are a group of inherited and acquired disorders resulting from certain enzyme defects in the body. These could range from life threatening disease to disease limited to skin. In porphyrias, the urine is high-colored and the skin is highly photosensitive with other changes in hair, nails and other internal organs. A pinkish or orange red fluorescence of the urine accentuated by the addition of dilute hydrochloric acid helps in making a presumptive diagnosis of porphyiria cutanea tarda using Wood’s lamp. The same fluorescence is present in the stools and blister fluids of these patients.
Wood’s Lamp to Detect Scabies Burrows
Fluorescine or tetracycline rubbed into the skin allows the identification of scabies burrows under Wood’s light.
Similarly, drug deposits like tetracycline in the teeth and mepacrine in the nails can be diagnosed from the yellow fluorescence under Wood’s lamp examination.
Wood’s lamp, thus, is a simple and cost effective tool in the diagnosis of a variety of skin diseases.
Source:
Bedi BMS. Principles of Clinical Diagnosis in IADVL Textbook of Dermatology, 3rd Ed, 2008
Disclaimer
The information given in this article is for educational purpose only so that patients are aware of the options available. No diagnosis should be made or treatment undertaken without first consulting your doctor. If you do so, the author or suite101 will not be responsible for any consequences. The images provided are for illustration purpose only.
Hanish Babu - Dr.Hanish Babu, MD is a dermatologist and a feature writer on Suite101.com with more than 200 articles related to skin diseases on the ...
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