Dermatohistopathology

Histopathology is valuable in investigation of skin disease, especially where initial investigations such as skin scrapes have failed to reveal a specific cause; where hormonal, autoimmune or neoplastic disease is suspected; or where there has been failure of response to treatment for the initial clinical diagnosis. These brief notes are designed to assist the clinician in getting the best from histological investigation of skin problems.

CLINICAL HISTORY

The clinical history is particularly important in appraisal of skin biopsies and greatly assists the pathologist in giving a helpful interpretation of any changes that are present. Photographs of the animal are welcomed. The history should include:

• Animal details—species, breed, age and sex
• Appearance and distribution of lesions
• Progression of lesions over time
• Previous or current therapy
• Details of any known intercurrent disease

Sampling Techniques

Skin preparation

Minimal surgical preparation is required: clip rather than shave hair; use alcohol swabbing rather than surgical scrubbing.

Site selection

Primary lesions should always be included. Secondary lesions should also be included, especially if lesions are progressive. Multiple biopsies are always advantageous, especially where there is widespread and variably severe disease. Any detaching crusts or debris should be included with the samples.

With most expanding lesions, the lesion margin should be included since diagnostic early changes may be most likely at this site. With patches of alopecia, however, the central, well-established areas must also be sampled.

Where vesicular or bullous disease is suspected, close observation may reveal small early intact lesions, which are most likely to give a specific diagnosis. Longstanding, ulcerated or secondarily infected lesions may be non-diagnostic.

Ellipse biopsy

This type of biopsy has the advantage of making inclusion of subcuticular tissues easier and is preferable where deep lesions are present. Superficial vesicles and bullae included in the sample are less likely to be damaged during sampling. Junctions of normal and abnormal tissues can be readily included.

• Take full thickness wedges not more than 2cm wide
• Sample with the long axis parallel to the direction of hair growth—this allows good orientation of hair follicles and adnexal glands

Punch biopsy

Punch biopsy is very useful where multiple lesions are present as samples can be taken from several sites and examples of early, established and late lesions included. The technique is also useful at sites where skin mobility is limited.

• 6 mm or larger punches are recommended for routine HE preparations.
• Avoid excess shearing force when taking the biopsy—always use a sharp punch.
• Only include abnormal tissue, since section preparation may obliterate small lesions if the normal/abnormal junction is included.
• Include a separate punch of normal skin if necessary.
• Centre small vesicles or lesions carefully to avoid trauma.

Sample fixation

Formalin fixation (10% formol saline) is adequate for routine examination. Buffered formol saline should be used where possible. Sample containers containing fixative are available from the laboratory on request.

• Samples should be fixed as soon as possible to avoid desiccation artefact.
• Samples can be placed subcutis down on a portion of stiff card and allowed to dry for not more than a minute before immersion in fixative. This helps to prevent curling and distortion of small tissue samples and long ellipses. Samples may subsequently float off in transit, so this should not be relied on for identification purposes.
• Avoid crushing of samples with forceps.
• If site identification is important, place similar samples in separate containers. Do not staple, pin or suture samples to card.

Michel's fixative is used for immunofluorescence examination of tissues in suspected autoimmune disease. Containers of this fixative are available from the laboratory on special request.

• Small samples (4-6 mm punches or thin ellipses) are required to allow penetration of this fixative.
• Duplicate samples in formalin should be taken at the same time for routine HE examination. Immunofluorescence examination can then proceed if autoimmune disease is suspected.

Combination with Other Tests

Skin biopsy can be combined with other examinations for more extensive investigation. Depending on circumstances, these can be performed before, concurrently or subsequent to biopsy. For some tests, we offer a preferential discount where combinations of tests are required.

• Microbiology—culture from fresh tissue or scrapings
• Parasitology—skin scraping examination
• Immunology—examination for IgE specific antibody to specific allergens
• Haematology—blood in EDTA
• Biochemistry—spun gel, plasma or heparin samples
• Endocrinology—spun gel sample