Diagnosis of Skin Disease – Skin Biopies

By Pauline M. Rakich, DVM, PhD, DACVP

One of the most common reasons for bringing pets to a veterinarian is skin disease. In many cases, the diagnosis is obvious and treatment is straightforward and successful. However, diagnostic tests are necessary to determine the cause when lesions are unusual; suggestive of serious diseases that require expensive, dangerous, or long term treatment; or do not respond to treatment as expected. Skin biopsies are frequently the most direct means of making a diagnosis because they are relatively easy to do, rapid, cost-effective, and safe for the patient.

To maximize the information obtained from histopathology there are some basics to follow. Most importantly, unless a lesion is very small, multiple biopsies are much more likely to provide a diagnosis than a single biopsy; because of this, the AVDL has a single charge for a patient’s dermatology biopsy regardless of the number of skin punches submitted.

A single lesion is rarely representative of the entire disease process. Since skin lesions are in various stages of progression at any single time, biopsying multiple lesions makes it possible to determine the disease course and make a diagnosis. The easiest way to understand this concept is to imagine a still photo from a movie. It is not possible to learn the plot of a movie from a single frame. In contrast, by viewing multiple still frames from different points of the movie, it is possible to get some idea of the progression of the action. The same is true of skin lesions.

The very earliest lesions are frequently similar for a number of conditions. It is the fully evolved primary lesions that are most diagnostic of the condition because they develop spontaneously as a direct reflection of the disease condition. Primary lesions include macules, patches, papules, wheals, vesicles, bullae, pustules, plaques, and nodules. With time, these primary lesions regress, degenerate, or become traumatized, evolving into secondary lesions which are less diagnostic because they can develop from multiple primary lesions. These secondary lesions include epidermal collarettes, scars, excoriations, erosions, ulcers, fissures, lichenification, and calluses. Some lesions are not as easily categorized as primary or secondary and may be either.

If the skin condition is characterized only by these lesions, they may be the primary lesions and biopsy could be diagnostic. These include alopecia, scales, crusts, comedones, and pigmentary changes. Since some disease conditions wax and wane, it is better to postpone biopsies until primary lesions develop rather than taking biopsies of regressing or healing lesions. If the animal has been treated with corticosteroids, treatment should be discontinued (for two to three weeks for oral steroids and four to six weeks for injected steroids) prior to taking biopsies so that the anti-inflammatory effects of the steroid will not alter the histologic appearance of the lesions. Biopsy of as many primary lesions as possible is most likely to yield a diagnosis.

In general, physical restraint and local anesthesia are all that is required to take skin biopsies. The skin should not be cleaned or prepared in any manner because surface material may be diagnostic and thus it should not be disturbed at all. If the hair is long and interferes with the biopsy, it can be trimmed down to the skin surface with scissors.

The easiest means of obtaining skin biopsies is with punches. However, punches are not suitable for obtaining samples of subcutaneous lesions because they do not extend deep enough to obtain sufficient tissue to adequately evaluate a subcutaneous lesion. If the predominant lesion is suspected to be in the subcutaneous fat, a larger biopsy such as a wedge biopsy is necessary to adequately examine the subcutis.

Punches are plastic instruments with a sharp round metal blade that come in a number of sizes, and it is important to use the larger punches to obtain the most diagnostic specimens. The smaller punches (4 mm) should be reserved only for biopsying facial lesions of cats and small dogs because they provide such a limited sample. For lesions involving other areas, 6 or 8 mm punches should be used.

Although the difference in size seems small, the larger biopsies provide significantly more material for histologic examination without producing a skin lesion that takes any longer to heal. A single punch can be used to obtain all of the biopsies from a patient. The punches are disposable and supplied individually packaged in quantities of 25 or 50 for about $2 each. They can be bought from a variety of sources, including HSB Veterinary Supply, Inc. (800-526-8981), Moore Medical (800-234-1464), and TW Medical Veterinary Supply (888-787-4483).

Depending on the variability in the appearance of the skin lesions, three to six biopsies should be taken. In order to encourage submission of multiple biopsies for dermatopathology cases to the AVDL, the dermatopathology fee is fixed regardless of the number of biopsies submitted and the number of routine special stains required. As the lesions are chosen to biopsy, they are each marked (e.g., with a dot from a marker) and locally anesthetized with lidocaine injected subcutaneously. By the time the last lesion is anesthetized, the first lesion is sufficiently anesthetized to proceed with biopsy. The punch is centered over the lesion without any significant amount of normal skin included. Only lesional skin should be biopsied because the margin of a lesion is typically an evolving lesion so it is neither completely normal nor completely affected and consequently its diagnostic usefulness is usually limited. As downward pressure is placed on the biopsy punch, it is rotated in a single direction to prevent creating shear forces that would rupture delicate lesions such as pustules or vesicles. Pustules and vesicles tend to be easily disrupted and must be biopsied with a punch that is larger than the lesion or by excision with scalpel blade so that they remain intact. Once a vesicle (or bulla or pustule) is ruptured, its diagnostic usefulness has been destroyed. The biopsy should extend into the subcutaneous fat which, when using a punch, can be felt as a sudden easing in the amount of pressure required to rotate the punch.

A common artifact results from crushing the biopsy when picking it up with thumb forceps to transfer it to formalin. Crush artifact destroys any subtle changes in the skin and frequently makes it impossible to identify cell types in an inflammatory reaction, which limits the diagnostic usefulness of the specimen. A better means of transferring the biopsy to 10 percent buffered formalin is to pick it up by the deep subcutaneous fat using the needle with which the lidocaine was injected. Each biopsy should be placed in formalin as soon as it is collected, rather than taking all biopsies first and then fixing them all at once, because autolysis of skin occurs very rapidly. The changes associated with autolysis can mimic or interfere with subtle changes that are diagnostic for some conditions and make interpretation of the biopsy more difficult, if not impossible. If the skin is very thin, it can be placed on a small piece of cardboard or wooden tongue depressor (the subcutaneous tissue will adhere to the flat surface) to prevent folding of the biopsy prior to submerging it in formalin. A single suture can be placed at the biopsy site and healing is usually rapid and uncomplicated. Remember that each biopsy heals independently of other biopsies; so whether one biopsy or six are taken, healing occurs in the same amount of time.

Submitting the biopsies properly is just as important as taking them. Successful diagnosis of skin disease requires collaboration between the clinician and pathologist. Skin biopsies taken for diagnosis of a disease condition (i.e., not tumors) should be submitted to a dermatopathologist. Dermatopathology is a specialized area of pathology that requires additional knowledge and an interest in clinical dermatology. The dermatopathologist not only evaluates the histologic changes in the biopsies, but attempts to correlate the histologic findings with signalment, history, and physical examination findings. Without this information, the pathologist can only provide a limited interpretation of the skin changes. For this reason, when submitting skin biopsies for histopathology, a complete history should be included. Signalment (age, breed, and sex) is important because certain breeds are predisposed to certain disease conditions (e.g., some diseases are congenital and occur in young animals). Other diseases are acquired and develop with age. Description of the clinical lesions, their distribution, presence or absence of pruritus, results of any other diagnostic tests performed, and treatment and response are all helpful in deciding which diseases are more likely than others. In many if not most cases, special stains are used (e.g., to examine the tissue for infectious agents).

The pathology report consists of a morphologic diagnosis which may or may not be a clinical diagnosis. In the majority of cases, the dermatopathologist also provides a comment in which the histologic changes are interpreted in light of the clinical information provided. In some instances, a definitive diagnosis may not be possible histologically, but the histologic changes can suggest a group of diseases and exclude other conditions. For example, histologic changes can be suggestive of allergic skin disease but they cannot indicate the specific allergy involved. In such instances, a dermatopathologist can suggest additional diagnostic tests to make the diagnosis.

Communication between the clinician and pathologist is the most important factor in successfully diagnosing skin disease. If the pathologist does not see changes consistent with the clinical description of the condition, (s)he should communicate this to the clinician. Likewise, if the clinician is confused about the pathology report, (s)he should not hesitate to contact the pathologist. In fact, when there is any question concerning collection or submission of specimens, a pathologist should be consulted. I would be happy to consult with a clinician before any biopsies are taken to be certain of choosing appropriate lesions, handling them correctly and requesting any additional diagnostic procedures that may be helpful in determining the cause of a skin condition. A small amount of time taken to establish communication between the clinician and pathologist may make the difference between a clinically useful skin biopsy and one that leaves the clinician, patient, and owner no better off than they were before a biopsy was performed.

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