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33 JULY/AUGUST 2017 ■ TVPJOURNAL.COM DERMATOLOGY DETAILS Biopsy sites are typically outlined with a permanent marker as a guide for where lidocaine should be injected ( Figure 1 ). The same marker may be used to draw a line in the direction of the hair growth to ensure proper orientation of the hair follicles when the sample is cut. This is most important for alopecic lesions, where the pathologist cannot determine the direction of hair growth by sight. Biopsy sites should never be scrubbed because diagnostic material may be removed. Gentle clipping may be necessary in long-haired patients; care should be taken not to let the clippers touch the skin surface. Lidocaine 2% is typically used for local anesthesia. This may be mixed with sodium bicarbonate in a 10:1 ratio to reduce the stinging sensation that lidocaine can cause. Inject lidocaine subcutaneously with a 25-gauge needle into the skin surrounding the lesion. Avoid injection into the dermis because it can cause an artifactual appearance of dermal edema. Do not inject lidocaine directly into lesions of the panniculus or if tissue cultures are to be performed. In these cases, use regional or general anesthesia. The total dose of lidocaine should not exceed 5 mg/kg in dogs or 2.5 mg/kg in cats. 1 In small patients with many biopsy sites, the lidocaine may be mixed 1:1 with sterile saline to reduce the amount used, creating a 1% solution, which has also been shown to be effective. 2 TECHNIQUE The sample may be obtained with a punch, incisional, or excisional biopsy. Punch Biopsy Punch biopsies are most often performed, and typically a 6-mm punch is used. Reserve 4-mm punches for pinnae, the nasal planum, or footpads of small dogs and cats. Obtain punches from the center of a lesion unless it is an ulcer, and center small lesions within the punch. Do not include any substantial amount of normal skin within the specimen because when the tissue section is cut in half at the lab the lesion may be missed. Use a fresh punch for each animal, and if the punch dulls during sample collection use a new one for subsequent punches. Dull punches cause tissue compression and artifact. Rotate the punch in one direction to prevent shear artifact. Decreased resistance is felt once the punch has reached the subcutis. Use a gauze square to blot any excess blood from the edge of the excised tissue; do not blot on top of the lesion. Once the subcutis is reached, remove the punch and gently grasp the tissue section (do not squeeze) with tissue forceps or a 25-gauge needle, and cut the attachment with iris scissors ( Figure 2 ). A single cruciate suture easily closes the defect left by a 6-mm biopsy punch. Incisional and Excisional Biopsy An incisional elliptical biopsy is preferred for ulcers, vesicles, and bullae. The shearing force of a punch may damage vesicles and bullae, and the adjacent unaffected skin must be included for these lesions because the diagnostic tissue resides in the margin between affected and unaffected areas. Keep in mind that samples are cut in half longitudinally during processing. Incisional or excisional biopsies are performed for suspected disease of the panniculus and for tumors. A punch often does not reach deep enough to obtain the diagnostic sample for diseases of the panniculus. Biopsy for Culture This article is primarily focused on biopsy for histopathology. However, in some cases, such as nodular disease, cellulitis, and diseases with fistulous tracts, biopsy for aerobic, anaerobic, and/or fungal culture may be needed. When fungal or unusual bacterial diseases (eg, mycobacteriosis, bacterial pseudomycetoma, actinomycosis, actinobacillosis, nocardiosis) are suspected, tissue biopsy specimens should be submitted and the lab should be notified of the suspected disorder. In these cases, disinfect the surface to avoid contamination with surface bacteria and observe sterile technique. Submit tissue in a sterile container, typically a plain blood FIGURE 1. Permanent marker is used to outline the location of lidocaine injection for a punch biopsy.