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4 When, Where, and How to Biopsy Skin

Dawn Logas

KEY POINTS

1 Skin biopsies should be taken when a lesion appears atypical or is not responding as expected to therapy.
2 Most nodular lesions should be biopsied for both histopathology and culture.
3 Be sure to use a veterinary dermatopathologist.
4 Histopathology is just a diagnostic test and may need to be repeated.

When

Although skin biopsies are not a common procedure, they are essential for the diagnosis of various dermatologic conditions. A biopsy should be performed when a probable diagnosis is not clear based on signalment, history, physical exam, or less invasive diagnostic tests. An easy rule of thumb is that a skin biopsy should be taken when the skin lesions appear atypical or the skin is not responding the way you think it should to current therapy. A biopsy should be taken for histopathology if you suspect neoplasia, pre‐ or para‐neoplastic disease, systemic disease with dermatologic markers/manifestations, autoimmune disease, or noninflammatory alopecia (Figure 4.1). Nodular lesions should usually be biopsied for both culture and histopathology, since clinically it can be impossible to tell whether the lesions are infectious or sterile (Figure 4.2). Culturing for aerobic bacteria, anaerobic bacteria, mycobacteria, fungi, and oomycetes may be indicated.

Where

Take multiple samples, preferably at least three to four that represent the variations in the lesions, including early to mature lesions. Try to biopsy primary lesions such as macules, papules, plaques, pustules, vesicles, wheals, nodules, and tumors (Figure 4.3). Secondary lesions such as alopecia, crusting, comedones, follicular casts, and erosions/ulcers should be biopsied if they are representative of the disease (Figure 4.4). Only abnormal tissue should be collected when using punch biopsies (Figure 4.5), otherwise when the biopsy is cut in the lesion may be missed and only normal tissue viewed. For ulcerative or erosive lesions, be sure a little intact epithelium on the edge of the lesion is included in the biopsy if an elliptical biopsy is taken. If punch biopsies are used, one sample should be just of the erosion/ulcer and another sample should be of the lesional area immediately adjacent to and including a small rim of intact epithelium. If a thick crust becomes separated from the underlying tissue of the biopsy sample, place it in the jar of 10% formalin and notify the lab that the specimen is in two pieces. For very crusty lesions, a piece of crust can be included along with the biopsy in the formalin jar, but be sure to inform the pathologist so they will cut it in.

How

The most common biopsy technique used in veterinary dermatology is a punch biopsy (Figure 4.6). Punch biopsies are simple procedures that require minimal instrumentation (Figure 4.6). The biopsy punches range from 2 to 10 mm in diameter, but for most lesions a 6–8 mm punch will suffice. The area chosen for biopsy should consist entirely of affected skin, with little to no normal tissue included. Once you have selected a lesion, gently remove excess hair with scissors or clippers, but do not shave close and do not scrub or prep the area. The only time a lesion should be disinfected is when the biopsy is for a culture. Most punch biopsies can be performed with local anesthesia (lidocaine) only. If the patient is fractious or a sensitive part of the body such as the face, paws or genitals is being biopsied, heavy sedation or general anesthesia is recommended. Lidocaine (2%) without epinephrine should be used and a maximum dose of 1.0 mL per 4.5 kg up to a total of 3.0 mL should not be exceeded. The lidocaine should be injected subcutaneously directly underneath the area to be biopsied. The punch biopsy instrument is placed perpendicular to the lesion and then rotated with gentle pressure until the punch drops into the subcutaneous space. Grasp the subcutaneous fat with thumb forceps to avoid crushing the delicate dermal and epidermal layers of skin and place the biopsy upright on a piece of wooden tongue depressor. This maintains the normal anatomic orientation of the biopsy sample. Place the sample in 10% formalin with a 1:10 ratio of tissue to formalin.

Figure 4.1 Noninfectious diseases that should be biopsied.

Figure 4.2 Nodular lesions that should be biopsied and cultured.

Figure 4.3 Primary lesions.

Figure 4.4 Secondary lesions.

Figure 4.5 Appropriate sampling. (a) Inappropriately taken biopsy punch with half normal tissue (outlined in white) and half abnormal tissue (outlined in black). (b) Biopsy appearance once in formalin. The loss of color makes it very difficult to differentiate the lesion from normal tissue. (c) If the biopsy is cut in following the green lines, then the lesion is included and can be examined by the pathologist. If the biopsy is cut in following the red line, the lesion has been completely missed and cannot be examined by the pathologist.

Figure 4.6 Punch biopsy. (a) Instrumentation for punch biopsy. (b) Inject lidocaine subcutaneously. (c) Hold the biopsy punch perpendicular to the skin and gently twist until it drops into the subcutaneous space. (d) Be sure to grasp the tissue sample in the subcutaneous fat to minimize artifacts. (e) Place the tissue sample on a wooden stick so it will form a cylinder.

Figure 4.7 Double punch. (a) Rapidly growing mycobacteriosis. (b) Lateral view of mass: one 6 mm plug removed. (c) Lateral view of mass: several deeper plugs removed from the same opening in the skin.

Another biopsy technique used in veterinary dermatology is the double punch (Figure 4.7). This method is very useful for large subcutaneous lesions with little epidermal involvement that are being biopsied for both histopathology and culture. Unlike biopsies of lesions affecting the epidermis and dermis, these lesions should be scrubbed and disinfected before the biopsy is taken. This procedure is normally performed under heavy sedation or general anesthesia. A 6 mm punch is used to remove a core of tissue from the surface of the nodule. The punch is then placed back into this same area and is pushed deeper into the lesion, usually in several directions. Then small forceps and iris scissors are used to remove pieces of tissue for both culture and histopathology. The original site is closed with one or two simple interrupted sutures.

Where to Send

Where the biopsy is sent is as important as taking appropriate samples. Whenever possible, biopsies should be sent to a veterinary dermatopathologist. If a facility has multiple pathologists on staff, be sure to request the dermatopathologist for skin biopsies. A short history of the case including progression, response to therapy, and current therapies should be included. A description of the lesions that were biopsied, where they were obtained, and a differential diagnosis list should also be included. If possible, include several photographs of the lesions. This information will help the pathologist narrow their differential list and may assist them with keying in on specific structures. The report from the pathologist should include a complete microscopic description of the lesions and a comment on what disease the histopathologic changes best fit, plus other possibilities if the histopathology is not pathognomonic for a specific condition. There should also be recommendations from the pathologist on special stains and immunohistochemistry if indicated.

Remember that biopsies are just a diagnostic tool. The more complete information supplied to the pathologist about the patient and the clinical picture, the better the information the pathologist will be able to provide in return. If the results do not fit with the clinical picture or response to therapy, another biopsy should be taken.