Dermatologic Examination

A good dermatologic examination requires adequate lighting, a systematic and thorough approach, and should always include a general physical examination. Observation from a distance should be followed by a close inspection of skin and mucous membranes. I start at the head, look at the lips, mouth, ears, run my hands through the coat of the trunk, lift up the tail to inspect the perianal area, and then examine the legs and feet with pads and claws. Next, the patient is rolled on his back - reluctant small pets are made to sit up in the lap of the owner; with larger dogs the front paws are lifted up for a short moment, which gives me the opportunity to examine the animal's ventral aspects from the axillae to the groin.

General Observation

Localized or Generalized Problem

> Localized problems may be due to infectious organisms that gained entry at a certain site and spread only locally such as atypical mycobacterial or fungal infections.

> Neoplastic disease is commonly localized, at least initially.

> Generalized disorders are more commonly due to hypersensitivities or systemic conditions such as endocrine disorders and immune-mediated or metabolic skin diseases.

Symmetry

> Bilaterally symmetrical lesions are typically caused by internal disease such as hypothyroidism, hyperadrenocorticism, or pemphigus foliaceus. Allergies may also present with bilaterally symmetric symptoms.

> Asymmetric lesions more commonly have external causes such as ectoparasites (e.g., demodicosis) or fungi (e.g., dermatophytosis).

Haircoat Quality, Color, and Shine

> Is the haircoat dull or shiny? A dull haircoat may be due to metabolic or hormonal diseases, nutritional deficiencies, or chronic skin disease.

> Are there color abnormalities or changes and if so, did they occur before or concurrent with the onset of skin disease or as a consequence of the disease. Hair color changes may be associated with hormonal disease or follicular dysplasia.

> Changes in the hair quality (either to a coarse coat or to a fine puppy coat) may again point to hormonal disease or follicular dysplasia.

Close inspection of the skin and mucous membranes follows the general observation. Pay special attention to any individual lesions. Primary lesions are initial eruptions that are caused directly by the underlying disease process. Secondary lesions evolve from primary lesions or are caused by the patient (self-trauma) or environment (medications). It is important that the clinician be able both to differentiate between primary and secondary lesions and to understand the underlying pathomechanism because this helps in the formulation of a relevant list of differential diagnoses. I next discuss the individual lesions and their implications and give the most common differential diagnoses for each lesion.

Primary Lesions

Macule

Figure 1-1a. Macule - Definition: A focal, circumscribed, nonpalpable change in color <1 cm (when it is larger, it is termed a patch). Pathogenesis: Pigmentation change due to decreased or increased melanin production, erythema due to inflammation or local hemorrhage caused by trauma or vasculitis.

Figure 1-1b. Macule - Differential diagnoses-depigmentation: Vitiligo, discoid lupus erythematosus, uveodermatologic syndrome, mucocutaneous pyoderma. Differential diagnoses -hyperpigmentation: Lentigo, hormonal diseases or post-inflammatory with a multitude of possible underlying causes of lentigo. Differential diagnosis-erythema: Inflammation due to a variety of underlying diseases or hemorrhage due to vasculopathies or coagulopathies.

Papule

Figure 1-2a. Papule - Definition: A solid elevation of up to 1 cm in diameter. Larger lesions are called plaques. Pathogenesis: Influx of inflammatory cells into the dermis, focal epidermal hyperplasia, early neoplastic lesions.

Figure 1-2b. Papule - Differential diagnoses: Bacterial folliculitis, demodicosis, fungal folliculitis, flea-bite and mosquito-bite hypersensitivity, scabies, contact allergy, autoimmune skin disease, drug eruption.

Pustule

Figure 1-3a. Pustule - Definition: A small circumscribed area within the epidermis filled with pus. Pathogenesis: Most pustules are filled with neutrophils, but eosinophilic pustules may also be seen. Aspiration cytology and biopsy are indicated (Courtesy of Dr. Thierry Olivry).

Figure 1-3b. Pustule - Differential diagnoses-neutrophils: Bacterial infection, fungal infection, autoimmune skin disease. Differential diagnoses-eosinophils: Insect or contact hypersensitivity, parasites, immune-mediated skin disease.

Vesicle

Figure 1-4a. Vesicle - Definition: A small circumscribed area within or below the epidermis filled with clear fluid. Larger vesicles are called bullae. Vesicles are very fragile and thus transient. Pathogenesis: Spongiosis and extra-cellular fluid collection due to inflammation and loss of cohesion (Courtesy of Dr. Thierry Olivry).

Figure 1-4b. Vesicle - Differential diagnoses: Immune-mediated and congenital skin diseases, viral diseases, or irritant dermatitis.

Wheal

Figure 1-5a. Wheal - Definition: A sharply circumscribed, raised, edematous lesion that appears and disappears within minutes to hours. Pathogenesis: Subcutaneous edema.

Figure 1-5b. Wheal - Differential diagnoses: Urticaria, insect bites, other hypersensitivities, drug eruption.

Nodule

Figure 1-6a. Nodule - Definition: A circumscribed, solid elevation more than 1 cm in diameter that extends into deeper layers of the skin. Pathogenesis: Massive infiltration of inflammatory or neoplastic cells into the dermis and subcutis or deposition of fibrin and crystalline material.

Figure 1-6b. Nodule - Differential diagnoses: Sterile granulomatous diseases, bacterial or fungal infections, neoplastic diseases, calcinosis cutis.

Tumor

Figure 1-7a. Tumor - Definition: A large mass involving skin or subcutaneous tissue. Pathogenesis: Massive influx of inflammatory or neoplastic cells.

Figure 1-7b. Tumor - Differential diagnoses: Sterile granulomatous diseases, bacterial or fungal infections, neoplastic diseases.

Primary or Secondary Lesions

Alopecia

Figure 1-8. Alopecia - Definition: Partial to complete loss of hair. Pathogenesis: Self-trauma, damage to the hair or hair follicle due to dysplasia, inflammation and/or infection, lack of hair regrowth often due to hormonal disease.

Differential diagnoses: Primary lesion in follicular dysplasias, endocrine disorders, telogen effluvium, anagen defluxion. Secondary lesion in pruritic skin diseases, bacterial or fungal folliculitis, demodicosis.

Scale

Figure 1-9a. Scale - Definition: An accumulation of loose fragments of the horny layer of the skin. Pathogenesis: Increased production of keratinocytes (often associated with abnormalities of the keratinization process) or increased retention of corneocytes.

Figure 1-9b. Scale - Differential diagnoses: Primary lesion in follicular dysplasias, idiopathic seborrheas and ichthyosis. Secondary lesion in diseases associated with chronic skin inflammation.

Crust

Figure 1-10a. Crust - Definition: Adherence of dried exudate, serum, pus, blood, scales, or medications to the skin surface.

Figure 1-10b. Crust - Differential diagnoses: Primary lesion in idiopathic seborrhea, zinc-responsive dermatitis, metabolic epidermal necrosis. Secondary lesion in a variety of skin diseases.

Follicular Cast

Figure 1-11a. Follicular Cast - Definition: An accumulation of keratin and follicular material to the hair shaft.

Figure 1-11b. Follicular Cast - Differential diagnoses: Primary lesion in vitamin A-responsive dermatosis, idiopathic seborrhea, and sebaceous adenitis. Secondary lesion in dermatophytosis and demodicosis.

Pigmentary Abnormalities

Hyperpigmentation

Figure 1-12a. Hyperpigmentation - Definition: Increased epidermal and/or dermal melanin. Pathogenesis: Increased production, size, or melanization of melanosomes or increased number of melanosomes due to a variety of intrinsic or extrinsic factors. Most common cause: Chronic inflammation.

Figure 1-12b. Hyperpigmentation - Differential diagnoses: Primary lesion in endocrine dermatoses, secondary post-inflammatory change due to a variety of skin diseases.

Comedo

Figure 1-13a. Comedones - Definition: A dilated hair follicle filled with corneocytes and sebaceous material. Pathogenesis: Primary keratinization defects or hyperkeratosis due to hormonal abnormalities or inflammation.

Figure 1-13b. Comedones - Differential diagnoses: Primary lesion in feline acne, some idiopathic seborrheas, Schnauzer comedo syndrome, endocrine diseases. Secondary lesion in demodicosis, and less commonly dermatophytosis.

Secondary Lesions

Epidermal Collarette

Figure 1-14a. Epidermal Collarette - Definition: Scale of loose keratin flakes or "peeling" keratin arranged in a circle. Pathogenesis: Remnant of a pustule or vesicle after the top part (the "roof") has been lost, or caused by a point source of inflammation, such as a papule.

Figure 1-14b. Epidermal Collarette - Differential diagnoses: Most likely bacterial infection, less commonly fungal infection, immune-mediated skin disease, insect-bite reaction, or contact hypersensitivity.

Erosion

Figure 1-15a. Erosion - Definition: A shallow epidermal defect that does not penetrate the basal membrane. Pathogenesis: Trauma or inflammation leads to rapid death and/or loss of keratinocytes.

Figure 1-15b. Erosion - Differential diagnoses: Various skin diseases associated with self trauma such as infections or allergies.

Ulcer

Figure 1-16a. Ulcer - Definition: Focal loss of epidermis with exposure of underlying dermis. Pathogenesis: Severe trauma and/or deep and severe inflammation.

Figure 1-16b. Ulcer - Differential diagnoses: Various skin diseases associated with trauma such as infections and allergies, also immune-mediated diseases.

Lichenification

Figure 1-17a. Lichenification - Definition: Thickening and hardening of skin characterized by exaggerated superficial skin markings. Pathogenesis: Chronic trauma such as friction or rubbing.

Figure 1-17b. Lichenification - Differential diagnoses: All chronic and pruritic skin diseases.