In order to get the best out of a (non-neoplastic) skin biopsy, one needs a combination of both clinical and pathological knowledge. A little bit of basic micro-anatomy, a little bit of immunology and a big list of lesion-based, clinical differential diagnoses is extremely helpful. What we attempt here in this lecture is to enable a better interpretation of skin biopsy reports in practice. We will not attempt to compact a "post-boards, specialist dermatopathology training" into a one hour lecture!
How does a dermatopathology report, which is not "specifically diagnostic" help us to make a diagnosis? The answer varies with the clinical lesions, what is on the differential diagnosis list and what has been ruled OFF that list as a result of the biopsy!
What should we expect from a skin biopsy report? The answer is similar to that for what should we expect for a serum biochemistry and complete blood examination?
Results can be useful even if there are no abnormalities present, as a "normal" result has almost surely ruled out some clinical differential diagnoses. But we can only make use of a biopsy report if we understand the basic terminology.... Dermatopathology is a new and rapidly evolving area in veterinary medicine. It is taught in relatively few vet schools around the world, even to pathologists and so really the veterinary profession as a whole is learning how to interpret skin biopsies. It helps to think of it in the same way that the diagnosis of an "eosinophilia" does not give a diagnosis, but rather a list of possible aetiological causes (e.g., allergies, parasites and rarely unusual immune/neoplastic conditions). We know this not because we are intrinsically smart! Veterinarians, but because we have all read many textbooks and attended many lectures discussing the interpretation of basic haematology and serology. Which lesions should be sampled? Are there any clinicopathology expectations and correlations? There are some very clear guidelines regarding the lesion type:
Papule or crusted papule. This clinical lesion represents a small swelling of the dermis with or without associated epidermitis. We are typically interested in sampling this to know whether the cause of the swelling is an inflammatory infiltrate or a tumour. If it is inflammatory, we wish to know the cell type (as that will give us a clue to the cause) and we wish to know if there is any evidence of an infection. Best are freshly erupted lesions.
Pustule/vesicle. This clinical lesion represents a collection of fluid and or inflammatory cells within the epidermis. Although the type of inflammatory cell is important, we are also interested in clues for immune mediated disease. Any intact (with the roof still on) sized pustule can be sampled. Any time they are present, they should be included as at least one of the specimens.
Depigmenting areas. Areas in which the skin is actively depigmentING are more likely to be helpful than those which have already depigmentED. Clinically this translates to taking a biopsy sample form the grey and not the white area
Scale / crust. The type of epidermal hyperkeratosis and the presence of inflammatory cells are clues we look for in scale and crusts. Organisms such as yeast and dermatophytes may also be identified there, when they are not found in the deeper parts of the biopsy and sometimes are identified on biopsy in cases where a culture was negative. This should never be washed or clipped away when preparing the skin for a biopsy.
Alopecia. The centre and the leading edges of areas of alopecia should be sampled. Possibly the best method to use with alopecia is the ellipse, oriented from the centre to the edge of the lesion. A biopsy from alopecic lesions is taken to assess the growth stages of the hair follicle, for example with a suspicion of hormonal disease or follicular dysplasia and to look for a possible disease affecting the hair follicle or bulb or for an infection of the hair follicle.
It is important in this case to include as many hair follicles as possible, so if there is a choice of affected sites, one should choose the one with the most hairs. For example when sampling the ventral abdomen or axillae one does not see as many hair follicles as when sampling the dorsal back. If the lesions are exclusively on the ventrum of the animal, then a larger piece of skin may need to be sampled to provide an adequate number of hair follicles for evaluation. In this case, one would anticipate that the clinically "normal" areas would have histologically "normal" hair follicles and these are used as a "control" specimen. Some diseases--in particular immune-mediated diseases- target the follicle wall, the glands or the hair bulb. In many cases the active pathology may be found in only one of 20 or 100 follicles. To look for those diseases one needs not only as many hairs as possible, but also the "active" part of the disease. This is found sometimes at the "leading or outer edge" of a focal alopecia and sometimes in the centre (where the pathology started!). Haired skin is needed when we have a differential diagnosis of an infection of the hair follicle (of which the big 3 are Demodex, dermatophytes and staphylococcal folliculitis). In the case of dermatophytes, the sample should be taken at the leading edge of the alopecia. For Demodex and staph pyodermas, it may be more useful to select an older (more central) area.
Planum nasale and / or foot pads is needed whenever there are lesions which directly involve these sites. Although these sites are difficult to close surgically, they should not be avoided. When one suspects immune mediated disease, these are good sites to biopsy.
Mostly a clinical pustule will not be seen in these sites as the epidermis is simply too thick. That works for us in a positive way in pathology as pustules are especially well retained within the thicker epidermis of these sites. What this means is that a diagnosis of pemphigus foliaceus may be obtained from a footpad or planum nasale biopsy even when there are no visible pustules. A (rare) clinical clue for a possible pustule is when the footpad feels "spongy" on palpation. A number of non-immune, specific diseases affecting only these sites are recognised, but even when there are many haired skin lesions, these sites should also be sampled.
Dermatitis of the mucocutaneous area typically starts the clinicians' internal "alarm bells" ringing for immune mediated disease. There may be disease clues at the mucocutaneous area and this should be biopsied when clinical lesions are present.
However, these areas also have their own special problems with biopsy interpretation, a lymphocytic plasmacytic sub-epidermal band-like infiltrate is frequently seen regardless of whether the disease is immune--mediated or a superficial pyoderma. This is a very important point to recognise. It is also important to read carefully in the biopsy report exactly what type of associated epidermal pathology is present. Where possible, do not take just mucocutaneous lesions, sample lesions at the mucocutaneous area, along with those of haired skin and footpad / planum.
Reaction Patterns--Description and Significance
The skin has only a limited number of ways to react to the infections, parasites and allergens to which it is exposed. This is an important concept to remember because many conditions share the same microscopic changes. The accurate interpretation of the biopsy often depends on the clinical appearance and history of the case.
A reaction pattern is a term used by dermatopathologists to describe the similar histopathologic appearance despite a differing aetiology. For example the eosinophilic granuloma complex in cats can be seen in response to atopic dermatitis, food and flea allergy. The histopathologic appearance of palisading eosinophilic granulomas is not specific for any of these diseases but rather regarded as a 'reaction pattern'.
1. Hyperplastic Perivascular Dermatitis
This tissue reaction pattern is characterised by thickening of the epidermis and stratum corneum. The predominant inflammatory reaction is centered around the superficial and/or deep dermal blood vessels. Superficial perivascular dermatitis is the most common. This is a common, non-diagnostic, chronic dermatitis reaction seen with hypersensitivities, seborrhoeic disorders and ectoparasites as well as other diseases. Occasionally, the type of inflammatory cells can give us clues about the pathogenesis. Examples: eosinophils point to ectoparasite involvement, neutrophils to an acute infection or severe superficial inflammation and plasma cells to chronic bacterial infection.
2. Interface Dermatitis
This refers to a reaction pattern in which the junction between the epidermis and the dermis is obscured by either degeneration of basal cells or a cellular infiltrate or both. The term "lichenoid" infiltrate is a frequently misused one. It describes a band-like collection of cells which hug and may obscure the dermo-epidermal junction and many pathologists interpret this as diagnostic for or synonymous with immune mediated disease. There are however many band-like infiltrates associated with infections and we (as well as many other veterinary dermatopathologists) choose to use that description "band-like" and not lichenoid so as to avoid a clinical misinterpretation. If your pathologist does use it, then be aware that it may not mean "immune". An interface dermatitis is seen with drug eruptions, lupus erythematosus, pemphigus, bullous pemphigoid, erythema multiforme, uveodermatological (Vogt-Koyanagi-Harada-like) syndrome, idiopathic lichenoid dermatoses and cutaneous T-cell lymphoma.
3. Vasculitis
Vasculitis is an inflammatory process in which the inflammatory cells are within and around the blood vessel walls and there are associated signs of damage to the vessels, such as hyalinisation of and necrotic nuclear debris within the vessel walls. They are classified on the basis of the predominant cell type (neutrophilic, lymphocytic) which in turn may give clues to the possible aetiology. In humans there is a definite clinicopathologic correlation between the type of vasculitis and response to therapy. This possibility remains controversial in veterinary dermatology.
4. Granulomatous Inflammation
This may be described as a circumscribed tissue reaction in which the histiocyte or macrophage is the predominant cell type. Granulomatous infiltrates that contain large numbers of neutrophils are frequently called pyogranulomatous. The most common causes of pyogranulomatous dermatitis in dogs are furunculosis, dermatophytosis or other fungi, mycobacterial infections, sterile panniculitis and ruptured keratinous cysts. In cats, mycobacterial infections, fungal infections and sterile processes may cause this type of inflammation. With this pattern, we may be in a dilemma. If special stains do not identify specific organisms, then we either deal with sterile disease (and need immunosuppressive treatment) or have missed an organism (in which case the immunosuppressive treatment will kill our patient).
5. Vesicular and Pustular Dermatitis
Vesicular dermatitides in dog and cats frequently appear pustular both clinically and microscopically as the vesicles accumulate leukocytes early in the life of the lesion.
The level of vesicle formation is important as a diagnostic clue, as are associated epidermal and dermal inflammatory changes. The anatomic description of intra-or subepidermal is used as the initial classification and features such as hydropic degeneration of basal cells, presence of microorganisms, acantholysis and inflammatory cell type then lead to the diagnosis of, for example pemphigus, lupus, dermatomyositis or drug eruption.
6. Perifolliculitis, Folliculitis and Furunculosis
These terms all relate to the hair unit. Periappendageal or periadnexal is a term which relates to the pilosebaceous unit and apocrine glands. Perifolliculitis defines the accumulation of inflammatory cells around hair follicles rather than the lands or the unit. When associated with bacteria, parasites and fungi, the infiltrate is typically suppurative. Mural folliculitis defines the exocytosis of these cells through the follicular epithelium. Luminal folliculitis is the accumulation of these cells within the follicular lumen. Bacterial folliculitis is a common secondary complication in pruritic dermatoses, seborrhoeic dermatoses and endocrinopathies.
Furunculosis refers to follicle rupture which results in the release of keratin into the dermis inciting a pyogranulomatous reaction. Furunculosis, regardless of the cause, is frequently associated with tissue eosinophilia.
7. Panniculitis
Is commonly associated with deep dermal inflammation. It is classified on an anatomic basis into septal (principally involving the interlobular connective tissue septae), lobular (involving the fat lobules) and diffuse. Cats most commonly develop septal, dogs develop diffuse panniculitis. Septal panniculitis is seen with vasculitides. Lobular panniculitis is associated with microbial infections, feline nutritional pansteatitis and nodular panniculitis.