Fitzpatrick's Color Atlas and Synopsis of Clinical Dermatology, Seventh Edition

SPECIAL CLINICAL AND LABORATORY AIDS TO DERMATOLOGIC DIAGNOSIS

SPECIAL TECHNIQUES USED IN CLINICAL EXAMINATION

Magnification with hand lens. To examine lesions for fine morphologic detail, it is necessary to use a magnifying glass (hand lens) (7×) or a binocular microscope (5× to 40×). Magnification is especially helpful in the diagnosis of lupus erythematosus (follicular plugging), lichen planus (Wickham striae), basal cell carcinomas (translucence and telangiectasia), and melanoma (subtle changes in color, especially gray or blue); this is best visualized after application of a drop of mineral oil. Use of the dermatoscope is discussed below (see “Dermoscopy”).

Oblique lighting of the skin lesion, done in a darkened room, is often required to detect slight degrees of elevation or depression, and it is useful in the visualization of the surface configuration of lesions and in estimating the extent of the eruption.

Subdued lighting in the examining room enhances the contrast between circumscribed hypopigmented or hyperpigmented lesions and normal skin.

Wood lamp (ultraviolet long-wave light, “black” light) is valuable in the diagnosis of certain skin and hair diseases and of porphyria. With the Wood lamp (365 nm), fluorescent pigments and subtle color differences of melanin pigmentation can be visualized; the Wood lamp also helps to estimate variation in the lightness of lesions in relation to the normal skin color in both darkskinned and fair-skinned persons; e.g., the lesions seen in tuberous sclerosis and tinea versicolor are hypomelanotic and are not as white as the lesions seen in vitiligo, which are amelanotic. Circumscribed hypermelanosis, such as a freckle and melasma, is much more evident (darker) under the Wood lamp. By contrast, dermal melanin, as in a Mongolian sacral spot, does not become accentuated under the Wood lamp. Therefore, it is possible to localize the site of melanin by use of the Wood lamp; however, this is more difficult or not possible in patients with brown or black skin.

Wood lamp is particularly useful in the detection of the fluorescence of dermatophytosis in the hair shaft (green to yellow) and of erythrasma (coral red). A presumptive diagnosis of porphyria can be made if a pinkish-red fluorescence is demonstrated in urine examined with the Wood lamp; addition of dilute hydrochloric acid intensifies the fluorescence.

Diascopy consists of firmly pressing a microscopic slide or a glass spatula over 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) that does not blanch. Diascopy is also useful for the detection of the glassy yellow-brown appearance of papules in sarcoidosis, tuberculosis of the skin, lymphoma, and granuloma annulare.

Dermoscopy (also called epiluminescence microscopy). A hand lens with built-in lighting and a magnification of 10× to 30× is called a dermatoscope and permits the noninvasive inspection of deeper layers of the epidermis and beyond. This is particularly useful in the distinction of benign and malignant growth patterns in pigmented lesions. Digital dermoscopy is particularly useful in the monitoring of pigmented skin lesions because images are stored electronically and can be retrieved and examined at a later date to permit comparison quantitatively and qualitatively and to detect changes over time. Digital dermoscopy uses computer image analysis programs that provide (1) objective measurements of changes; (2) rapid storage, retrieval, and transmission of images to experts for further discussion (teledermatology); and (3) extraction of morphologic features for numerical analysis. Dermoscopy and digital dermoscopy require special training.

CLINICAL SIGNS

Darier sign is “positive” when a brown macular or a slightly papular lesion of urticarial pigmentosa (mastocytosis) becomes a palpable wheal after being vigorously rubbed with an instrument such as the blunt end of a pen. The wheal may not appear for 5–10 min.

Auspitz sign is “positive” when slight scratching or curetting of a scaly lesion reveals punctate bleeding points within the lesion. This suggests psoriasis, but it is not specific.

The Nikolsky phenomenon is positive when the epidermis is dislodged from the dermis by lateral, shearing pressure with a finger, resulting in an erosion. It is an important diagnostic sign in acantholytic disorders such as pemphigus or the staphylococcal scalded skin (SSS) syndrome or other blistering or epidermonecrotic disorders, such as toxic epidermal necrolysis.

CLINICAL TESTS

Patch testing is used to document and validate a diagnosis of allergic contact sensitization and identify the causative agent. Substances to be tested are applied to the skin in shallow cups (Finn chambers), affixed with a tape and left in place for 24–48 h. Contact hypersensitivity will show as a papular vesicular reaction that develops within 48–72 h when the test is read. It is a unique means of in vivo reproduction of disease in diminutive proportions, for sensitization affects all the skin and may therefore be elicited at any cutaneous site. The patch test is easier and safer than a “use test” with a questionable allergen, that for test purposes is applied in low concentrations in small areas of skin for short periods of time (see Section 2).

Photopatch testing is a combination of patch testing and UV irradiation of the test site and is used to document photo allergy (see Section 10).

Prick testing is used to determine type I allergies. A drop of a solution containing a minute concentration of the allergen is placed on the skin and the skin is pierced through this drop with a needle. Piercing should not go beyond the papillary body. A positive reaction will appear as a wheal within 20 min. The patient has to be under observation for possible anaphylaxis.

Acetowhitening facilitates detection of subclinical penile or vulvar warts. Gauze saturated with 5% acetic acid (or white vinegar) is wrapped around the glans penis or used on the cervix and anus. After 5-10 min, the penis or vulva is inspected with a 10x hand lens. Warts appear as small white papules.

LABORATORY TESTS

Microscopic Examination of Scales, Crusts, Serum, and Hair

Gram stains of smears and cultures of exudates and of tissue minces should be made in lesions suspected of being bacterial or yeast (Candida albicans) infections. Ulcers and nodules require a scalpel biopsy in which a wedge of tissue consisting of all three layers of skin is obtained; the biopsy specimen is divided into one-half for histopathology and one-half for culture. This is minced in a sterile mortar and then cultured for bacteria (including typical and atypical mycobacteria) and fungi.

Microscopic examination for mycelia should be made of the roofs of vesicles or of scales (the advancing borders are preferable) or of the hair in dermatophytoses. The tissue is cleared with 10-30% KOH and warmed gently. Hyphae and spores will light up by their birefringence (Fig. 26-24). Fungal cultures with Sabouraud medium should be made (see Section 26).

Microscopic examination of cells obtained from the base of vesicles (Tzanck preparation) may reveal the presence of acantholytic cells in the acantholytic diseases (e.g., pemphigus or SSS syndrome) or of giant epithelial cells and multinucleated giant cells (containing 10-12 nuclei) in herpes simplex, herpes zoster, and varicella. Material from the base of a vesicle obtained by gentle curettage with a scalpel is smeared on a glass slide, stained with either Giemsa or Wright stain or methylene blue, and examined to determine whether there are acantholytic or giant epithelial cells, which are diagnostic (Fig. 27-33). In addition, culture, immunofluorescence tests, or polymerase chain reaction for herpes have to be ordered.

Laboratory diagnosis of scabies. The diagnosis is established by identification of the mite, or ova or feces, in skin scrapings removed from the papules or burrows (see Section 28). Using a sterile scalpel blade on which a drop of sterile mineral oil has been placed, apply oil to the surface of the burrow or papule. Scrape the papule or burrow vigorously to remove the entire top of the papule; tiny flecks of blood will appear in the oil. Transfer the oil to a microscopic slide and examine for mites, ova, and feces. The mites are 0.2-0.4 mm in size and have four pairs of legs (see Fig. 28-16).

Biopsy of the Skin

Biopsy of the skin is one of the simplest, most rewarding diagnostic techniques because of the easy accessibility of the skin and the variety of techniques for study of the excised specimen (e.g., histopathology, immunopathology, polymerase chain reaction, and electron microscopy).

Selection of the site of the biopsy is based primarily on the stage of the eruption, and early lesions are usually more typical; this is especially important in vesiculobullous eruptions (e.g., pemphigus and herpes simplex), in which the lesion should be no more than 24 h old. However, older lesions (2-6 weeks) are often more characteristic in discoid lupus erythematosus.

A common technique for diagnostic biopsy is the use of a 3- to 4-mm punch, a small tubular knife much like a corkscrew, which by rotating movements between the thumb and index finger cuts through the epidermis, dermis, and subcutaneous tissue; the base is cut off with scissors. If immunofluorescence is indicated (e.g., as in bullous diseases or lupus erythematosus), a special medium for transport to the laboratory is required.

For nodules, however, a large wedge should be removed by excision including subcutaneous tissue. Furthermore, when indicated, lesions should be bisected, one-half for histology and the other half sent in a sterile container for bacterial and fungal cultures or in special fixatives or cell culture media, or frozen for immunopathologic examination.

Specimens for light microscopy should be fixed immediately in buffered neutral formalin. A brief but detailed summary of the clinical history and description of the lesions should accompany the specimen. Biopsy is indicated in all skin lesions that are suspected of being neoplasms, in all bullous disorders with immunofluorescence used simultaneously, and in all dermatologic disorders in which a specific diagnosis is not possible by clinical examination alone.