Skin lesions: what do they mean? (Proceedings)

While some people claim that all skin diseases look alike, the fact is to the trained eye, even subtle difference in skin changes can offer clues to the underlying disease process.  One of the first steps in appreciating and understanding the differences of skin lesions is to learn what primary and secondary skin lesions actually are, and what they represent.  Primary lesions are generally considered a result of the disease process, and are the first lesion seen.  Examples of primary lesions are papules, pustules, vesicles, nodules, plaques, macules, patches, and sometimes alopecia.  Secondary lesions may be the result of the evolution or change that occurs to a primary lesion, or are the result of forces outside the skin such as trauma (scratching, chewing etc). Examples of secondary lesions include erosions, ulcers, crusts, epidermal collarettes, hyperpigmentation and lichenification.  A pustule that dries into a crust, which then might fall off leaving an epidermal collarettes with central hyperpigmentation are examples of how primary lesions could evolve into secondary lesions.  In general, primary lesions offer a better chance of obtaining an accurate diagnosis when performing any type of diagnostic test.

Macules are color changes of the skin less in one centimeter in diameter that do not have any substance or “mass”.  Patches are larger than a centimeter.  Most macules and patches are inflamed (erythematous) but may be hyper or hypopigmented.

Papules are solid, usually firm raised areas of the skin smaller than a centimeter.  They are usually the result of a local irritation to the skin. Papules will develop due to skin infections (Staphylococcus), parasites (flea bites or scabies mites), environmental or contact allergy.  Sometimes papules are seen which then develop into a pustule.  Papules are usually scattered and random within an inflamed area on the skin, but sometimes they are grouped into patterns such as circles.

Pustules are “pus filled” skin lesions smaller than a centimeter.  The contents of a pustule are usually neutrophils, but can also include eosinophils.  Staphylococcal infections of the skin are the most common cause of pustules, although dermatophytes (ringworm) and Demodex mites can also cause pustule.  Pemphigus foliaceus will cause pustule formation and is often misdiagnosed as a “resistant Staph infection” when the patient with a pustule fails to improve when treated with antibiotics.  Close examination of the pustule to determine if it is centered around a hair follicle (folliculocentric) is warranted.  Staph infections are usually folliculocentric, where as pustules caused by pemphigus can “bridge” across several follicles.  When pustules are large and approaching one centimeter in size, they are usually associated with pemphigus or an immune suppressed patient with a Staph infection.  Cytology of the contents of a pustule is invaluable in helping to determine the cause.  The presence of bacteria and acantholytic cells can be diagnostic.

Nodules are raised lesions larger than a centimeter.  It helps to realize they usually represent the accumulation of either inflammatory or neoplastic cells.  Inflammatory nodules are called granulomas, and are often the result of a deeper infection of the skin.  However, some granulomas are sterile, the result of an “overactive” immune response.  A large variety of infectious organisms can cause a granuloma, including bacterial, fungal, or parasitic causes.  Cytology of a granuloma can be a useful in-house test to determine if the nodule is likely neoplastic or not, but most nodules should be biopsied and possibly cultured in order to determine their cause.

Vesicles are fluid filled lesions (instead of solid lesions) with a clear (non-purulent) fluid and are rare in veterinary patients.  Serous (clear) filled vesicles are often one of the first lesions seen with a burn.  Hemorrhagic nodules could be due to hemangioma, hemangiosarcoma, trauma, or bleeding disorders.  Interdigital infections in some breeds such as bulldogs can result in papules and nodules that appear hemorrhagic, but cytology will show a pyogranulomatous reaction or infiltrate with red blood cells.

Plaques are formed by the accumulation of cells or material causing a raised or elevated lesion greater than a centimeter.  When plaques occur, neoplasia should always be a concern.  T-cell lymphoma, histiocytoma, squamous cell carcinoma, and mast cell tumors are some of the common neoplastic causes of plaque formation.  Other causes include calcinosis cutis which is the result of spontaneous or iatrogenic Cushing's disease.  Chronic infection (Staphylococcus, Malassezia) and chronic pruritus can also cause plaque formation.  As with all solid lesions, aspirate cytology may give initial clues as to the cause, but biopsies should usually be performed if there is any doubt as to the etiology of the plaque.

Alopecia can be a primary or secondary lesion.  Primary alopecia occurs as the result of an endocrinopathy.  Rare causes of follicle destruction such as alopecia areata could also lead to alopecia as a primary lesion. The majority of the patients with alopecia will be secondary to underlying infections of the follicle, or trauma to the hair shaft (such as pruritus).  The pattern of alopecia is usually patchy, unlike the symmetrical pattern of alopecia with an endocrine disorder. The big three causes of folliculitis in dogs are Staphylococcus, dermatophytes, and Demodex mites.  All three causes should always be ruled out when a patient presents with patchy alopecia. 

Crust is another secondary lesion, and is usually formed due to either pustules which have “dried out” or exudate that has “oozed” or drained through the epidermis to the skin surface. The color of the crust can be a somewhat useful clue as to the depth of the skin lesion.  If the crust is golden colored (serous) it indicates the basement membrane zone of the epidermis is likely intact, and lesions are confined to the top layers of the skin and inside the follicles.  If the crust has a darker or red color (hemorrhagic crust) then it indicates the skin disease extends past the epidermis and follicles into the deeper dermis, and that the follicles have likely been ruptured.

Erosions are partial thickness defects where the top layers of the epidermis has been damages or lost but the basement membrane zone is still present.  These occur as the result of some form of damage to the skin cells.  Causes can include proteolytic enzymes from bacteria, chewing or scratching due to pruritus, or immune mediated destruction of the epidermis.  An ulcer represents full destruction or loss of the full thickness of the skin, with the basement membrane zone destroyed.

Lichenification which is incorrectly referred to as “elephant skin” represents skin which has thickened as a result of chronic trauma, usually scratching.  Lichenified skin can frequently be secondarily infected with Malassezia or Staphylococcus, which exacerbates the pruritus and leads to even more scratching and lichenification.  Lichenified skin frequently is also hyperpigmented. 

Post-inflammatory hyperpigmentation is another secondary lesion of the skin and as the name suggests is typically the result of inflammation of the skin.  This pigment may fade but may be present permanently in some patients.  It can appear to be lattice shaped (spider web like).