Dermatological diagnostics Flashcards
What are the key steps in dermatological diagnostics?
Choose tests wisely – prioritise those that provide quick, non-invasive & cost-effective information
Take high-quality samples – ensure adequate quantity & correct technique
Examine & interpret samples correctly – use proper microscopy techniques
Consider limitations – be aware of false negatives & test sensitivity
What are the most commonly used dermatological diagnostic tests and what are they used for?
Coat brushing – Surface parasites/fleas
Acetate tape strip unstained - Surface parasites, objective x4-10
Skin scrapings – superficial & deep parasites, objective x4-10
Trichogram – Demodex, nits, dermatophytosis, hair cycle abnormalities, objective x4-40
Cytology – Bacteria, yeasts, cells, objective x4-100
Wood’s lamp - Micosporum canis
McKenzie coat brush - Dermatophytes
Culture - bacteria, yeasts
What are these parasites?
1= Cheyletiella
2 = Sarcoptes
3 = Demodex
What can be identified in hair plucks?
Lice & Cheyletiella eggs are found attached to hair shafts
In follicular diseases (demodex, Dermatophytosis & sebaceous adenitis) may see follicular casts
Demodex canis, D. cati & D. injai may be seen on hair plucks
What can be identified in superficial skin scrapes?
Non-burrowing mites: Demodex gatoi (cats), Cheyletiella spp. (dogs, cats & rabbits)
Burrowing mites: Sarcoptes scabiei (dogs) & Trixicarus caviae (guinea pigs)
What can be identified in deep skin scrapes?
Follicular mites: Demodex canis, D. cati & D. injai
What kind of lesions are sampled and what staining is used for direct impression smears (cytology)?
Moist/greasy lesions
Ruptured pustules
Skin under crusts
Accessible sites
All 3 Diff-quick solutions: A fixative + B eosin + C methylene blue
What kind of lesions are sampled and what staining is used for indirect impression smears (cytology)?
Ear canals
All 3 Diff-quick solutions: A fixative + B eosin + C methylene blue
What kind of lesions are sampled and what staining is used for acetate tape strip (cytology)?
Dry lesions
Less accessible sites
B eosin + C methylene blue
What microbe can you see in this cytology example?
Rods (x620)
What microbe can you see in this cytology example?
Neutrophils with intracellular cocci (x1000)
What microbe can you see in this cytology example?
Malassezia, corneocytes (x1000)
Label these WBCs (simplified)
Label these WBCs (simplified)
Why perform a skin biopsy?
To obtain definitive diagnosis when other methods are inconclusive
To identify deep infections, autoimmune diseases, neoplasia, vasculitis
Which tests are carried out on skin biopsies?
Histopathology
Tissue culture
How are skin biopsies stained?
Usually haematoxylin & eosin (H&E)
Special stains:
- Periodic Acid Schiff (PAS) for fungi
- Giemsa for bacteria
- Ziehl-Neelsen (ZN) for mycobacteria
- Immunohistochemistry
Is sedation or GA required for skin biopsies?
In calm animals, biopsies usually taken using sedation & local anaesthesia
General anaesthesia usually required for biopsies of feet, pinnae, lips & noses
What needs to be sampled in skin biopsies?
Sample representative range of lesions
Take multiple samples (min. 3, unless solitary lesion)
Sample fully developed primary lesions where possible, avoiding traumatised skin/necrotic crust
Where would you sample this site (for alopecia)?
- Across margin of alopecic area
- Area of maximum hair loss
- Normal haired skin wedge
Wedge biopsy
Where would you sample this site (for ulcerated skin)?
Skin just adjacent to ulcer, where epidermis is still intact
Where would you sample this site (for pustules, vesicles or bullae)?
Remove whole lesion without disruption (Often very delicate)
Difficult with punch not to cause damage so wedge biopsy best
How do you prepare a sample site for skin biopsy?
Avoid disturbing skin surface!
- Even gentle cleaning can remove many layers of stratum corneum
Clip hair, but not too short – scissors often preferable to clippers
Don’t disturb crusts or skin surface – include crusts!
Don’t prep or scrub skin (unless excisional biopsy of nodules)
How do you mark and anaesthetise the site for a skin biopsy?
If infiltrating local anaesthetic:
- Draw circle around lesion in indelible marker
- Infiltrate LA into subcutis around periphery of circle
- Care not to exceed max volume of LA for patient’s weight
- Check efficacy of analgesia by pricking with a needle
Can draw orientation line along line of hair growth for cases of alopecia as better chance of longitudinal hair sections
Describe punch biopsies
Quick and convenient
Ideal for superficial lesions
Use 6mm or 8mm biopsy punches routinely, 3mm/4mm only for delicate structures
Hold perpendicular to skin surface
Rotate in one direction, not back & forth
Don’t reuse blunt biopsy punches
Describe wedge biopsies
Tissue excised with scalpel
Excisional for:
- Excision of solitary nodules –> histopathology
- Vesicles – minimal disruption
Incisional for:
- Transition from normal to lesional skin
- Biopsy of cutaneous masses
- If pathology suspected in deep dermis/subcutis, e.g. panniculitis (inflammation of s/c fat)
What are the key steps in preparing a skin biopsy for histopathology?
- Blot blood gently from underside of sample
- Place promptly into 10% formalin – use min. 10x volume of tissue sample
- For thin samples, prevent curling by placing them on stiff card, then immerse in formalin
- Separate ‘normal’, marginal & central lesions in different pots
Help pathologist by providing brief history & differential diagnoses
When should bacterial and fungal tissue culture be performed?
Deep & superficial pyoderma (Less affected by environmental contamination than surface sample)
Subcutaneous & deep fungal infections
How should tissue be prepared for culture?
- Withdraw antibiotics (5–7 days) & topical antimicrobials (≥3 days) before sampling
- Gently blot surface with alcohol swab to remove contamination & let dry
- Punch biopsy & place it in sterile glass tube +- sterile saline
Avoid formalin exposure, as it kills organisms
Store appropriately – many organisms survive in fridge or freezer & can be stored post-histopathology
How should cutaneous masses be biopsied?
Incisional biopsy (wedge) → preferred for invasive neoplasms to guide treatment
Excisional biopsy → for removal of entire mass with margins for histopathology
Max. 1cm tissue thickness for adequate fixation
Why is an incisional biopsy preferred before excising a suspected neoplasm (cutaneous masses)?
Helps determine tumour type before removal
Ensures adequate margins during final excision
Biopsy tracts should be removed with the tumour to prevent spread
What are common reasons why skin biopsy results may not be conclusive?
Poor technique
Sample examined not representative of lesion
- Ask pathologist to cut further sections if suspect this
- Put visible lesions centrally in biopsy
Lesion altered by treatment
Biopsy taken too late – early lesions are better for vasculitis
Unrealistic expectations – some diseases can’t be distinguished on biopsy
What is pattern analysis in dermatology?
Method of categorising skin lesions based on histological patterns
Helps narrow down differential diagnoses & guide further testing
What are the main histopathological patterns seen in skin disease?
- Perivascular Dermatitis – Inflammatory cells around blood vessels
- Interface Dermatitis – Cells abutting basement membrane
- Vasculitis – Inflammation in & around blood vessels
- Nodular/Diffuse Dermatitis – Large lumps of inflammatory cells (nodular) or spread out (diffuse)
- Vesicular/Pustular Dermatitis – Blisters or pustules on skin
a) Intraepidermal
b) Subepidermal - Folliculitis/Furunculosis/Adenitis – Inflammation of hair follicles or glands
- Panniculitis – Affects subcutaneous fat
- Atrophic Dermatosis – Thin, weakened skin due to hormones or blood supply issues
What is perivascular dermatitis, and what conditions cause it?
Inflammatory cells (neutrophils, lymphocytes, eosinophils (Type 1 hypersensitivity)) exit blood vessels & move into tissue
Clinical signs:
- Prominent blood vessels
- WBCs around vessels
- Oedema of dermis
Classified according to depth
- Superficial dermal
- Mid-dermal/perifollicular
- Deep dermal
Seen in allergic diseases (atopy, flea allergy dermatitis), bacterial pyoderma
What is interface dermatitis, and what conditions cause it?
Band-like infiltrate of immune cells at the dermo-epidermal junction
Basal keratinocyte degeneration, pigment incontinence, apoptosis
Can cause erosions & ulcerations through clefting
Seen in immune-mediated diseases
What is vasculitis, and what conditions cause it?
Inflammation of blood vessels
- Inflammatory cells tightly surround blood vessels, causing vascular wall degeneration
Leads to microhemorrhages, dermal necrosis, panniculitis, alopecia
Either primary or secondary to inflammation, infection, drug reactions, neoplasia, vaccination so look for primary cause
Can be difficult to find as lesions short-lived, need to biopsy early lesion & take multiple samples
What is going on here?
(Post-vaccination) vasculitis with well-demarcated areas of necrosis and alopecia
What cells are seen in nodular/diffuse dermatitis?
Convergence of nodules –> diffuse pattern
Very common in dogs
Cells vary:
- Neutrophils - pyogenic agents
- Macrophages (granulomatous inflammation)– e.g. foreign bodies, mycobacteria
- Neutrophils & macrophages (pyogranulomatous inflammation) – e.g. fungi
- Eosinophilic – parasitic?
- Lymphocytic – insect bites, vaccine reactions
What histological pattern can be seen here?
Nodular granulomatous dermatitis of cat – due to deep Microsporum canis infection
What causes vesicles or pustules in intraepidermal dermatitis?
Epidermal inflammation (spongiosis) – Intercellular oedema (parasites, infection)
Acantholysis – Loss of cell adhesion due to infection or autoimmune disease
Intracellular oedema – Due to mechanical forces
How are intraepidermal vesicles/pustules classified?
Subcorneal (superficial) – Pemphigus foliaceus, pyoderma
Suprabasal (deeper) – Pemphigus vulgaris
Follicular external root sheath – Pemphigus foliaceus
What cellular infiltrate is often seen in intraepidermal dermatitis?
Neutrophils
- Bacterial pyoderma, PF
Eosinophils
- PF, parasite
What is the difference between pemphigus foliaceus and pemphigus vulgaris?
Pemphigus foliaceus (PF) – Superficial pustules under stratum corneum, fragile, ruptures easily
Pemphigus vulgaris – Deeper pustules, more tense & less likely to burst
What histological features are seen in pemphigus foliaceus?
Pustules under stratum corneum filled with neutrophils & acantholytic cells
Thickened epidermis due to chronic inflammation
Diffuse dermal inflammation beneath pustules
What causes the acantholytic cells in pemphigus foliaceus?
Type II hypersensitivity targeting desmosomes, leading to loss of cell adhesion (acantholysis)
Results in floating keratinocytes (acantholytic cells) in pustules
What is subepidermal vesicular/pustular dermatitis?
Separation of epidermis from dermis
Causes severe clinical effects & is difficult to biopsy accurately
What are the causes of subepidermal vesicular dermatitis?
Autoimmune diseases: Bullous pemphigoid, epidermolysis bullosa
Thermal burns
Severe dermal oedema & interface dermatitis.
Occasionally artefact in biopsy processing
What are the different types of folliculitis/furunculosis/ adenitis?
Perifolliculitis – inflammation around hair follicle plexus (early stage)
Mural folliculitis – inflammation within follicle wall (Pemphigus foliaceus, demodicosis)
Luminal folliculitis – infection inside follicle (Demodex, dermatophytosis)
Bulbitis – affects hair bulb (Alopecia areata – rare)
Sebaceous adenitis - affects sebaceous glands (auto-immune & Leishmaniasis)
Furunculosis - rupture of hair follicle with release of keratin into dermis –> marked inflammatory response (deep pyoderma, Demodex)
Label the histological patterns
Label the histological patterns
What is panniculitis, and what causes it?
Inflammation of subcutaneous fat
Sometimes extension of follicular disease
Causes: Infectious agents, vasculitis, trauma, foreign bodies, pancreatic disease
May be sterile idiopathic, but infection must be ruled out first
What causes atrophic dermatosis, and how does it appear histologically?
Caused by endocrine diseases (Cushing’s, hypothyroidism) or chronic illness/malnutrition
Histology:
- Epidermal, follicular, sebaceous gland atrophy
- Orthokeratotic hyperkeratosis.
- Follicular keratosis +/- calcinosis cutis (Cushing’s disease)
Define acanthocyte & acantholysis
Acanthocyte: epidermal cell free in vesicle/pustule, caused by acantholysis
Acantholysis: loss of cohesion between cells of living epidermis
Define Dyskeratosis
Abnormal, premature or imperfect keratinisation of keratinocytes
Define Exocytosis
Migration of inflammatory cells from dermis to epidermis
Define Hyperkeratosis
Increase in stratum corneum
Define orthokeratosis and parakeratosis
Orthokeratosis: excessive cornification – keratinocytes lose nuclei
Parakeratosis: excessive cornification – keratinocytes retain nuclei
Label the histological pattern
Label the histological pattern
Label the histological pattern
Label the histological pattern
Label the histological pattern
