Aids to Dermatologic Diagnosis: Clinical, Instrumental, and
Laboratory
Magnification: To examine the skin surface critically and to
detect the fine morphologic detail of skin lesions it is necessary to use a
hand lens magnifier (preferably 7×); also, a better image is obtained following
application of a drop of mineral oil to the lesion. Magnification is especially
helpful in the diagnosis of lupus erythematosus (follicular plugging and atrophy),
lichen planus (Wickham's striae), basal cell carcinomas (translucence and
delicate telangiectasia), and early malignant melanoma (subtle changes in
color, especially gray, slate, or blue). Hand-held magnifying instruments with
built-in lighting and a magnification of 10× to 30× have recently become
available and these permit better visualization of lesions when used with a
drop of oil. Using this small optical instrument or the larger binocular
microscope—this technique is called epiluminescence microscopy—facilitates the
distinction of benign and malignant pigmented neoplasms.
Wood's lamp (longwave ultraviolet light, “black” light) is
essential for the clinical diagnosis of certain skin and hair diseases and of
porphyria. Longwave ultraviolet radiation is obtained by fitting a
high-pressure mercury lamp with a specially compounded filter made of nickel
oxide and silica (Wood's filter); this filter is opaque to all light except for
a band between 320 and 400 nm. When using the Wood's lamp, it is essential for
the examiner to become dark-adapted in order to see the contrasts clearly. When
the ultraviolet waves emitted by Wood's lamp impinge on the skin, a visible
fluorescence occurs. Wood's lamp is particularly useful in the detection of the
fluorescence of dermatophytosis ( Microsporum) in the hair shaft (green) and of
erythrasma (coral red) on the skin. Wood's lamp also helps to estimate the
variation in the “whiteness” of lesions in relation to the normal skin color,
in dark-skinned and especially in fair-skinned persons; for example, the
lesions seen in hypomelanotic macules in tuberous sclerosis and in tinea
versicolor are not as white as the macules present in vitiligo, which are
typically amelanotic. Circumscribed hypermelanosis, such as ephelides and
melasma, is much more evident under Wood's lamp, and in lentigo maligna
melanoma and acrolentiginous melanoma the Wood's lamp can be used to detect the
total extent of the lesion as a guide to total excision. Melanin in the dermis,
as in a Mongolian sacral spot, does not become accentuated under Wood's lamp. Therefore,
it is possible to localize the site of melanin (epidermal or dermal) by use of
the Wood's lamp; this phenomenon is not evident in patients with brown or black
skin. The technique is as follows: a grading (minimal, moderate, marked) of the
degree of pigmentation is made with visible light and compared with a grading
of the degree of color change when examined with Wood's lamp. In epidermal
melanin pigmentation the pigment grade increases from minimal to marked, but
dermal melanin has the same degree of pigment in both visible light and Wood's
lamp illumination.
Diascopy consists of firmly pressing a transparent, hard,
flat object (such as a hand lens or two microscope slides) over the surface of
a skin lesion. The examiner will find this procedure of special value in
determining whether the red color of a macule or papule is due to capillary
dilatation (erythema) or to extravasation of blood (purpura). Diascopy is also
useful for the detection of the hyaline yellowish-brown color of papules or
nodules in sarcoidosis, tuberculosis of the skin, lymphoma, and granuloma
annulare.
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