Chapter from BSAVA Manual of Canine and Feline. Dermatology. 3rd edition. Edited by Hilary Jackson and Rosanna Marsella BSAVA 2012.

Transcription

1 Chapter from BSAVA Manual of Canine and Feline Dermatology 3rd edition Edited by Hilary Jackson and Rosanna Marsella BSAVA

2 4 Dermatopathology David H. Shearer Dogs and cats presented for the investigation and treatment of skin disease can show a variety of clinical signs. The investigation into the cause of the dermatosis can be planned using a problem-solving approach based on the signs present at examination (see Chapter 2). The cutaneous signs include pruritus, erythema, papules, pustules, alopecia, crusts, scales, erosions, ulcerations, nodules and pigment changes. One or more of these signs may be present and the balance of clinical features, along with the signalment and history, allows the dermatologist to create a list of differential diagnoses and diagnostic tests, and/or to prescribe treatment. Gross lesions Gross lesions are divided into primary and secondary lesions, with some overlap between the types (Figure 4.1; see also Chapter 2). It is important to identify lesions accurately, to describe the lesions in the clinical notes, and to add the description to any pathology submission form. The interpretation of the histological features of skin biopsy samples can be dependent upon the presenting signs and the appearance and site of the lesions. Primary Primary or secondary Secondary Macule/patch Alopecia Epidermal collarettes Papule/plaque Scale Excoriation Pustule Crust Erosions/ulcer Vesicle/bulla Follicular casts Lichenification Wheal Comedo Fissure Nodule Pigment changes Callus Tumour 4.1 Gross skin lesions. Histological changes Scar Skin biopsy may be indicated as part of the investigation of a skin disease. If biopsy and histopathology are required, a few simple rules should be observed, including: 1. Apart from careful removal of hair, do not clean the biopsy site(s) or disturb surface crust. 2. In generalized skin diseases take multiple 6 mm punch biopsy samples (between three and four). 3. Try to get the opinion of a pathologist with specific expertise in skin pathology, e.g. a member of the International Society of Veterinary Dermatopathology (ISVD). The gross appearance of the cutaneous lesions should correlate with the histological features, and in most cases the microscopic features present are predictable from the macroscopic lesions (Figure 4.2). If the histological features described by a dermatopathologist do not correlate with the gross appearance of the lesions then the following poss ibilities should be considered: The biopsy specimens are not representative of the lesion. The reasons for this include the age of the lesion; most lesions have a normal progression from formation to resolution or repair. Multiple biopsy samples are more likely to reveal the progression of the lesion, and one of them may contain the diagnostic features required. The area containing the diagnostic features may be lost during biopsy or processing; an example is the superficial crust or pustule in pemphigus foliaceus, which can be removed during preparation of the skin for biopsy or lost during histological processing (i.e. it separates from the skin). It is for this reason that the skin surface should never be cleaned before skin biopsy (see Chapter 3) The appearance of the gross lesions and their clinical interpretation has been incorrect. The most common assumption made by general practitioners is it looks bad therefore it must be autoimmune. Although some autoimmune dermatoses are severe, infectious agents are the most common cause of severe skin disease. In addition, although neoplastic diseases can produce severe skin lesions this is not necessarily so; for example, the early lesions of epitheliotropic lymphoma can consist of rather benign scaling and alopecia. In this case the clinician often contacts the pathologist and asks, Are you sure that this is a neoplasm? 31

3 Clinical sign Pruritus Alopecia Scaling and crusting Pustules Ulcers Hyperpigmentation Hypopigmentation Histological patterns Vasculitis Subepidermal vesicular/pustular dermatitis /furunculosis Panniculitis Subepidermal vesicular/pustular dermatitis /perifolliculitis/furunculosis Panniculitis. The biopsy sections are then examined in detail to identify dermatopathological changes in each part of the skin, usually beginning with the epidermis, followed by the adnexae and dermis. The pathologist then makes an interpretation of the histological features in light of the clinical information. The pathologist s report usually includes a morphological diagnosis, aetiological diagnosis (if possible), description of changes and a discussion of the findings (Figure 4.3). The pathology report should include a summary of the histological features present, a statement of the patterns seen, an aetiological diagnosis (if possible) and a commentary outlining the conclusions that can be made. A typical report might be set out as given below. Morphological diagnosis: This states the major and minor histological patterns present. Aetiological diagnosis: This is stated if a causative agent is seen in the sections examined. Description: This is a summary of the histological features seen and usually starts with the epidermis followed by the dermis and adnexae. Comments: The pathologist states what can be concluded from the histological features and relates them to the clinical features. The pathologist should give a comment as to the clinicopathological correlation in this section, and for this reason an accurate history and description of the gross changes is important. 4.3 Dermatohistopathology report. Nodules 4.2 /furunculosis Panniculitis Vasculitis Histological patterns associated with major clinical signs. In general, if the clinical features cannot be correlated with the dermatohistopathology report, the pathologist should be contacted and the case discussed. It is important to describe the clinical features accurately on the submission form and to give a brief history. The biopsy sites should also be recorded because the pathologist s interpretation of the histology may be affected by the sites of origin. Generally speaking, skin sections are examined by the pathologist under low power initially and before reading the clinician s notes on the submission form. This allows an unbiased assessment and interpretation of the histology. After examination under low power the changes seen are categorized into a pattern based on the analysis (Ackerman, 1978; Yager and Wilcock, 1994). Examples of histological patterns include: (the dermatitis reaction ) Vasculitis Nodular and/or diffuse dermatitis Basic dermatohistopathological changes Epidermal atrophy: This is a reduction in the thickness of the epidermis. As a general rule the thickness of the epidermis is inversely proportional to the hair density. This change is seen in cases with alopecia and skin thinning. Acanthosis (epidermal hyperplasia): This represents keratinocyte hyperplasia and occurs in various types: Regular acanthosis is a uniform increase in the stratum spinosum with no rete ridges Irregular acanthosis has rete ridge formation and is the commonest form in the dog and cat Psoriasiform acanthosis has regular rete ridge formation with or without club-shaped tips. This is rare in the cat and dog Papillated acanthosis has folding and projection above the surface. This is acanthosis with upward proliferation of the dermis. It is a non-specific term Pseudoepitheliomatous/carcinomatous acanthosis has an invasive appearance resembling squamous cell carcinoma. It occurs at the edge of ulcers Acanthosis is seen in lesions with lichenification and inflammatory alopecia, typically in dogs with chronic dermatitis, pyoderma or Malassezia dermatitis. 32

4 Crusts: These are surface accumulations of squames, serum proteins, red blood cells and white blood cells. They are classified as serous, keratinous, cellular or haemorrhagic. Crusts often contain microorganisms such as bacteria, yeasts (Malassezia) and occasionally dermatophytes. Crusts are most often seen in cases of pyoderma, dermatophytosis and parasitic disease, but can be seen in the autoimmune disease pemphigus foliaceus. Hyperkeratosis: This is an increase in the stratum corneum. It can be orthokeratotic or parakeratotic, basket-weave or compact. Compact hyperkeratosis is associated with chronic trauma. This is seen grossly as scale and forms part of the crusts. Parakeratosis: This is where there is retention of a pyknotic nucleus in the squames in the stratum corneum. It reflects increased cell turnover and a rapid response to injury. It occurs as a focal change either vertically (in episodic injury) and/ or horizontally (in focal injury). Generalized parakeratosis is associated with zinc-responsive dermatosis and metabolic dermatoses. This is seen grossly as scale, and parakeratosis forms part of the crusts. Hypergranulosis: This is an increase in the thickness of the stratum granulosum and is usually associated with compact hyperkeratosis due to chronic trauma. Areas of lichenification can have hypergranulosis. Hypogranulosis: This is a decrease in or loss of the stratum granulosum and is usually associated with parakeratosis. Papillomatosis: This is epidermal growth caused by papilloma virus infection. This is a specific term. These are usually exophytic lesions described as warts but can be endophytic in verruca-like lesions. Dyskeratosis: This is premature, faulty keratinization, which can occur in the surface or adnexal epithelium. It can be a benign change seen in various dermatoses associated with abnormal keratinization. It can also be associated with malignant lesions such as squamous cell carcinoma. Spongiosis: This is epidermal intercellular oedema, which gives the epidermis a spongy appearance. This change is usually associated with exocytosis of inflammatory cells and if severe it can lead to intraepidermal vesicle formation. Exocytosis: This refers to the migration of inflammatory cells from the dermis into the epidermis and adnexae. It is usually associated with spongiosis. Intracellular oedema: This appears as intracytoplasmic vacuolation and occurs in hydropic degeneration of basal keratinocytes and in ballooning degeneration. Hydropic degeneration (vacuolar alteration/ liquefactive degeneration): This refers to the presence of vacuoles within the stratum basale cells. This may lead to intrabasal or subepidermal clefts. This change is seen in some ulcerative skin lesions such as dermatomyositis. Ballooning degeneration: This type of degeneration is specifically seen in herpesvirus infection. Reticular degeneration: This is multilocular/ intraepidermal vesicle formation associated with severe epidermal oedema. Severe keratinocyte swelling can also occur in metabolic dermatosis (superficial necrolytic dermatitis/ hepatocutaneous syndrome) or acute contact dermatitis. Acantholysis: This is separation of keratinocytes. Primary acantholysis is separation of normal keratinocytes as seen in pemphigus foliaceus. Secondary acantholysis occurs as the result of damage to the keratinocytes and occurs in dyskeratosis, viral infections and in association with inflammatory cells in staphylococcal pyoderma. Pigment incontinence: This refers to melanin that drops from the epidermis/adnexae into the dermis, where it is phagocytosed by dermal monocytes/macrophages. This is usually associated with stratum basale cell oedema. This is a feature of an interface dermatitis as seen in cutaneous lupus. Epidermal necrosis: This can be caseous or coagulative. Caseous necrosis appears as an eosinophilic and basophilic mush. This is the most common and least specific form of necrosis and usually reflects trauma (especially scratching). Coagulation necrosis appears eosinophilic with pyknotic nuclei and the superficial dermis is often involved. Coagulation necrosis usually occurs as the result of physical damage (burning/freezing/chemical), ischaemia or immunological mechanisms. Lesions with epidermal necrosis present clinically with ulceration. Necrolysis: This is used to describe epidermal coagulation necrosis with no dermal involvement and minimal inflammation (e.g. toxic epidermal necrolysis). Apoptosis: This is individual cell death and involves intracellular processes that require energy (compared with necrosis by oncosis, which is not energy dependent and occurs as a result of catastrophic cell damage). This can be a physiological or pathological process. These processes can be triggered in a variety of ways, one of which is cytotoxic attack by lymphocytes (satellitosis). Histologically, apoptosis appears as eosinophilic bodies that are phagocytosed by adjacent cells. Satellitosis: This refers to the appearance of cytotoxic lymphocytes surrounding an apoptotic cell; this indicates a cell-mediated immune response. Civatte bodies: These are apoptotic cells in the stratum basale of the epidermis. 33

5 Patterns used in dermatopathological interpretation In this condition there are prominent blood vessels (high endothelial venules), oedema of the dermis, and exocytosis of leucocytes into the dermis around the high endothelial venules (Figures 4.4 and 4.5). In addition to these dermal changes, there are epidermal changes that may reflect the aetiology or age of the lesions. The epidermal changes include acanthosis (hyperplasia), spongiosis, intracellular oedema (in the basal layer of an interface dermatitis), hyperkeratosis (basket-weave or compact), para keratosis (focal or diffuse), epidermal necrosis (erosions/ulcerations) and crusts. 4.4 (the dermatitis reaction ) dermoepidermal junction. A cell-rich interface dermatitis is seen in cutaneous lupus and erythema multiforme (lymphocytic). A cell-poor interface occurs in dermatomyositis. There is pigment incontinence. Although in some circumstances this pattern can be indicative of pathogenesis, it can be seen in many different diseases and its significance needs careful interpretation. It is a common pattern at mucocutaneous junctions and on the nasal planum, where it is diagnostically weak compared with elsewhere. Vasculitis In this condition there is specific inflammation of the blood vessels. Histologically, there are tight perivascular cuffs of inflammatory cells (Figure 4.7), with evidence of degeneration of the vessel wall. In some circumstances there is necrosis of the inflammatory cells, represented by the presence of nuclear dust. A variety of cell types can be seen in cutaneous vasculitis. Microhaemorrhages should alert the pathologist to the possible presence of vasculitis. Other changes seen in vasculitis include panniculitis, dermal necrosis and atrophy of hair follicles. 4.7 Vasculitis. 4.5 Superficial perivascular dermatitis. is further classified acc ording to the vascular plexuses involved: zone 1 (superficial dermal); zone 2 (mid-dermal and peri - fo llicular); and zone 3 (deep dermal). The type of cellular infiltrate (neutrophilic, lymphocytic or eosinophilic) may also reflect the aetiology or pathogenesis. An eosinophilic infiltrate tends to indicate type I hypersensitivity and a parasitic or allergic aetiology. This is a common reaction pattern, which is weak diagnostically. In this condition there is hydropic degeneration of basal keratinocytes (Figure 4.6), with or without individual cell necrosis (apoptosis). There is a cell-rich or cell-poor mononuclear infiltrate at, and crossing, the Nodular and diffuse dermatitis (Figure 4.8) are considered to be one pattern, but it is important to differentiate a disease of the hair follicles from non-follicular nodular dermatitis. The diffuse pattern reflects convergence of nodules. The cellular infiltrate gives some indication of the likely cause. A neutrophilic infiltrate occurs in response to pyogenic agents. Histiocytes/macrophages occur in response to foreign bodies and mycobacteria. The presence of neutrophils and macrophages should alert the pathologist to the possibility of furunculosis. An eosinophilic nodular and diffuse dermatitis raises the possibility of a parasitic aetiology. A lymphocytic infiltrate can be seen with vaccine reactions and insect bites. 34

6 4.8 Nodular and/or diffuse dermatitis /perifolliculitis/ furunculosis. Clefting that leads to vesicles or pustules within the epidermis (Figure 4.9) can occur as a result of spongiosis/epidermal inflammation (parasites or infection), acantholysis (due to infection or autoimmune disease), intracellular oedema and mechanical forces (friction). The clefts may be subcorneal (pemphigus foliaceus/pyoderma) or suprabasilar (pemphigus vulgaris), or may be with the follicular external root sheath (pemphigus foliaceus). A variety of cells can be present: neutrophils (pemphigus foliaceus/pyoderma), eosinophils (pemphigus foliaceus/parasitic disease) or mononuclear cells (macrophages in bacterial pyoderma). 4.9 Intraepidermal vesicular/pustular dermatitis. or the epitrichial (apocrine) glands (hidradenitis). Furunculosis is identified by rupture of the hair follicle. The causes of follicular inflammatory disease include bacteria (polymorphonuclear neutrophils and plasma cells predominate), dermatophytes (lymphocytic folliculitis), demodicosis (typically a lymphoid mural folliculitis), parasites (mosquito/insect stings; eosinophilic folliculitis/furunculosis) and immune-mediated conditions (alopecia areata; (lymphocytic bulbitis). Panniculitis Inflammation of the subcutaneous adipose tissue (Figure 4.12) can be septal or lobular. It can also be an extension of follicular disease. Histological examination may indicate the cause and pathogenesis, e.g. infectious agents, vasculitis or a foreign body may be apparent Panniculitis. Subepidermal vesicular/pustular dermatitis This is an uncommon pattern (Figure 4.10). It occurs with autoimmune diseases such as bullous pemphi goid, thermal trauma (burns), severe dermal oedema, and severe interface dermatitis (cutaneous lupus). It can also be a histological artefact Subepidermal vesicular/ pustular dermatitis. In this condition there is atrophy of the epidermis, hair follicles and sebaceous glands (Figure 4.13). Orthokeratotic hyperkeratosis and follicular keratosis may be present. In cases of hyperadrenocorticism there may be calcinosis cutis. The mineralized collagen is eliminated by transepidermal extrusion. This pattern is seen in a variety of endocrine dermatoses, which require a variety of hormonal assays to confirm their exact aetiology (see Chapter 3) /perifolliculitis/furunculosis Inflammation associated with the hair follicles (Figure 4.11) can affect the perifollicular vascular plexus (perifolliculitis), the outer root sheath (mural folliculitis), the entire hair follicle (luminal folliculitis), the bulb (bulbitis), the sebaceous glands (sebaceous adenitis) 35

7 Conclusion The histological diagnosis of a dermatological condition depends upon identification of the reaction patterns present and their order of importance, combined with the detailed features of each component of the skin. This information is then compared to and correlated with the clinical features and gross lesions to arrive at a diagnosis, which should encompass both the aetiology and the pathogenesis of the skin disease. Using a problem-solving approach to the individual case on the basis of the main clinical signs present, it should be possible to predict the most likely histological patterns present (see Figure 4.2). The appearance of the gross lesions and histological features should correlate, and if they do not the clinician should discuss the possible reasons with the dermatohistopathologist. References and further reading Ackerman AB (1978) Histologic Diagnosis of Inflammatory Skin Disease. A Method by Pattern Analysis. Lea & Febiger, Philadelphia Goldschmidt MH and Shofer FS (1992) Skin Tumors of the Dog and Cat. Pergamon Press, Oxford Gross TL, Ihrke PJ, Walder EJ and Affolter VK (2005) Skin Diseases of the Dog and Cat. Clinical and Histopathological Diagnosis, 2nd edn. Blackwell Science, Oxford Scott DW, Miller WH and Griffin CE (2001) Muller & Kirk s Small Animal Dermatology, 6th edn. WB Saunders, Philadelphia Yager JA and Wilcock BP (1994) Colour Atlas and Text of Surgical Pathology of the Dog and Cat. Mosby-Year Book Europe, London 36