The skin of domestic animals has a limited range of responses to insult or injury, and these manifest as clinical signs or lesions. The main presenting signs of dermatologic disease are things that are noticed by the owner, such as onset of pruritus, change in smell, crusting or flakiness, loss of hair, or change in pigmentation. However, all these are secondary signs, and may likely be the effect of the cutaneous response to chronic inflammation or self-excoriation due to a number of causes. Because of this, history, signalment, and thorough physical examination are extremely important in arriving at a correct diagnosis.
Primary lesions occur de novo in the skin and often reflect the underlying etiology. While it is often not pathognomonic, a limited number of diseases may cause it. It must be kept in mind that lesions in skin disease may undergo a sequential evolution, resulting in a variety of identifiable changes in the skin, that when observed together can assist in deducing the underlying pathologic changes, and formulating a differential diagnosis.
In the glossary listings below the lesion name will be followed by a P if it may appear as a primary lesion and/or an S if it may appear as a secondary lesion.
Loss of hair that may vary from partial to complete. It may be primary, as seen in alopecia of
endocrine disease and follicular dysplasias, or secondary due to trauma or inflammation.Bullae are simply large vesicles. They may arise from cleft formation in or below the epidermis, or following coalescence of adjacent foci of intercellular epidermal edema.
Dilated hair follicle containing a pigmented impaction of lipid and keratinaceous debris (a "blackhead). Often seen in keratinization defects; can be seen with demodicosis, and are a frequent finding on the abdomen of dogs with hyperadrenocorticism.
A crust is composed of dried exudate mixed with debris on the skin surface. The exudate may be serum, blood, inflammatory cells; the composition will determine the color.
Cysts are epithelium-lined cavities within the skin. Most commonly they are of adnexal or follicular origin. The content is a reflection of the epithelial lining; squamous lined cysts are usually keratin-filled, sebaceous or apocrine gland cysts are filled with secretions typical of those glands.
These are often regarded as secondary lesions, but can be thought of as ruptured, peripherally spreading pustules, vesicles or bullae, giving rise to the appearance of a ring of scale. There are often spreading zones of accompanying erythema, and central healing and post-inflammatory pigmentation is common.
Loss of epidermis with the basal layer remaining intact. Often occur when intraepidermal vesicles and bullae rupture. Healing occurs without scar formation.
This is superficial loss of epidermis often due to self trauma secondary to pruritus. They are often linear.
Hyperemia occurs when arterial and arteriolar dilatation produces an increased flow of
blood into capillary beds. In other words, hyperemia is an active process.
As would be expected, there is increased redness in the affected area. The arterial and
arteriolar dilatation is brought about by sympathetic neurogenic mechanisms or the release
of vasoactive substances. Cutaneous hyperemia occurs whenever excess body heat must be
dissipated, such as in exercise and febrile states.
Blushing is another example of hyperemia induced by neurogenic mechanisms. In veterinary
medicine, the most common cause of hyperemia is that associated with vasoactive substances
released in inflammatory processes.
Usually a consequence of melanin excess, either increased numbers of melanocytes, or increased production of melanin and melanin deposition in the basal epithelium. Can be
secondary to inflammation, or idiopathic (primary).May occur as a
post-inflammatory event, or may be idiopathic (vitiligo).The thickening of the epidermis with accentuation of normal skin folds and markings. It is often accompanied by hyperpigmentation. It is a response to chronic trauma or inflammation.
Macules P and Patches PThese are characterized by discoloration of the skin. Macules are focal, well circumscribed, and less than 1 cm; patches are less well circumscribed, and are larger. Color changes can be due to any number of causes, including erythema, hemorrhage, or pigment changes. Erythema is often secondary to inflammation; hemorrhage may be due to clotting defects or trauma. A quick way to distinguish is to apply pressure...if there is blanching, it is erythema, if no blanching, there is likely hemorrhage into the skin. Changes in pigment may be due to increases or decrease in normal pigmentation.
A nodule is a circumscribed elevation of the skin larger than a papule (1 cm). They often extend into the dermis, and may be due to inflammatory, neoplastic, or metabolic in origin.
Papules are circumscribed, solid elevations of the skin, up to 1 cm in diameter. The elevation may be due to accumulation of cells, fluid, debris or metabolic deposits and may be follicular or interfollicular in orientation. Papules often begin as erythematous macules.
Patches P (see Macule)
A plaque is an elevation of the skin that is usually flat and wider than it is tall. These can result from edema (wheals), coalescence of adjacent papules, or be of neoplastic origin.
Pustules are well circumscribed elevations of the superficial layers of the epidermis. Like papules, they may be follicular or interfollicular in distribution. The most common cause is bacterial infection, where the pustules are filled with neutrophils, bacteria, debris, and possibly a few loose keratinocytes (acantholytic cells). It must be kept in mind that pustules, due to the thinness of the epidermis are usually transient. What is often observed is a transition from erythematous macules to papules to relatively few pustules to small crusts and scale.
Under normal conditions, the process of epithelial maturation and keratinization provides a steady replacement of the epidermis. This takes about 21 days in the dog. In keratinization defects (primary lesion), the epithelial turnover rate may decrease to 3-4 days, giving rise to increased numbers of poorly differentiated cells shed from the skin surface. Aggregates of these are visible as scale. The same keratinization defects may affect the sebaceous glands, giving rise to increased oiliness as well. Scaliness and oiliness are secondary features of many diseases, including bacterial skin disease, ectoparasitism, endocrine dermatoses, and hypersensitivity dermatoses.
Occurs when the basal layer of the epidermis has been breached and the underlying dermis is damaged. As you hopefully will recall, the defect is filled with granulation tissue, the wound contracts, with the resultant formation of a fibrous scar.
Tumors are masses of neoplastic origin. They may be benign or malignant; mobile or locally infiltrative, ulcerated or domes; plaque-like or pedunculated.
Loss of epidermis with loss of integrity of the basement membrane. By definition, at least the superficial dermis is involved, and healing occurs by granulation; often with scar formation.
A vesicle is a well demarcated elevation of the superficial layers of the skin less than 1 cm in diameter. The skin elevation is due to accumulation of intercellular fluid beneath the roof of the vesicle. The fluid is usually serum or an inflammatory exudate. Vesicles as seen in viral diseases, irritant contact dermatitis and autoimmune disorders are often superficial; therefore the vesicles are often transient, with crusting and erosions common.
A usually well defined elevation of the skin due to edema. They arise rapidly, and often disappear quickly. They are due to accumulation of edema fluid within the dermis, and cause no pathologic changes in the epidermis.