Nodules, Tumors, and Swellings

Definition

A nodule is a circumscribed, solid elevation larger than 1 cm in diameter that does not deform when palpated. Nodules extend into the deeper layers of the skin and are usually the result of cellular infiltrates in the dermis or subcutis.

Mechanisms of Nodule, Tumor, and Swelling Formation

Nodular lesions can be subdivided into inflammatory and neoplastic lesions. Inflammatory nodules are composed of a massive mixed cellular infiltrate involving the dermis, the subcutis, or both. The inflammatory infiltrate may contain variable numbers of neutrophils, histiocytes, lymphocytes, plasma cells, and eosinophils. Cellular infiltration is usually stimulated by the presence of foreign material, and the nature of that material influences the composition of the inflammatory infiltrate. The foreign material may be infectious (parasite, bacteria, or fungi) or noninfectious (fibrin, crystalline material, or other inert substances). Grossly visible nodules develop as the masses of inflammatory cells accumulate in the tissues to phagocytize or “wall off” the foreign material. As the lesion enlarges, the dermis and subcutis are obliterated by the inflam­matory infiltrate and the overlying epidermis may become atrophic, resulting in ulceration of the nodule's surface.

Most cutaneous and subcutaneous neoplasms form nodular lesions. Cutaneous and subcutaneous neoplasms may either arise from a cell type of the epidermis, dermis, or subcutis or less commonly metastasize from another tissue of origin.

Swellings include solid lesions such as nodules and tumors, as well as urticaria, cysts, and abscesses. Urticarial lesions (wheals, hives) are often transient, localized, inflammatory lesions caused by a vascular reaction in the dermis in which vasodilatation results in fluid transudation with or without erythema. The fluid is not compartmentalized but dispersed evenly throughout the dermal tissue. The result is an elevated lesion that, unlike a nodule, pits with pressure and often dissipates within minutes to hours as the fluid is resorbed. Typically a sparse, perivascular infiltrate that is usually lymphocytic is seen, although the infiltrate may be dense and intermingled with eosinophils. Urticaria is usually well circumscribed, although with confluence the edema may assume geometric shapes.

A cyst (Latin for sac) is an epithelium-lined cavity containing fluid or semisolid material.⁷ A cyst usually presents as an elevated, smooth, well-circumscribed, fluctuant mass. Cutaneous cysts are usually lined by adnexal epithelium (hair follicle, sebaceous or apocrine epithelium) and are filled with cornified cellular debris and sebaceous or apocrine secretions.

An abscess is a localized, fluid-filled, fluctuant lesion; if large enough, it may be ballotted. It results from a dermal or subcutaneous accumulation of the debris of dead cells and tissue elements liquefied by the proteolytic and histolytic enzymes elaborated by polymorphonuclear cells (e.g., pus). Abscesses most commonly result from localized infection, although they occasionally result from septicemia or may be sterile.

Approach to Diagnosis of Nodules,

Tumors, and Swellings

Nodules, tumors, and swellings may arise from a variety of cutaneous disorders and, in rare cases, are signs of a systemic

■ BOX 11.2

Most Common Causes of Nodules, Tumors, or Swellings in Horses and Ruminants

Horses

Infectious

Corynebacterium pseudotuberculosis Habronema species

Sporotrichosis

Neoplasia

Sarcoid

Squamous cell carcinoma

Melanoma

Sterile, Nonneoplastic

Eosinophilic granuloma

Exuberant granulation tissue (proud flesh) Urticaria (hives)

Ruminants

Infectious

C.

Hypoderma species (warbles)

Sporotrichosis

Actinobacillosis

Actinomycosis

Neoplasia

Squamous cell carcinoma

Fibroma or fibrosarcoma

Epidermal inclusion cysts

Sterile, Nonneoplastic

Urticaria (hives)

disease. The major categories of diseases that should be considered when forming a differential diagnosis include hypersensitivity reactions, infectious diseases, sterile inflam­matory diseases, and neoplasia (Box 11.2). The primary systemic diseases that should be considered are amyloidosis, lympho­sarcoma, and anaphylaxis.

The following steps are a guide to the diagnosis of nodules, tumors, and swellings in horses and ruminants:

1. History (see Fig. 11.1). Pay particular attention to:

a. Signalment. Older animals are at greater risk for cutaneous neoplasia (e.g., gray horses and melanomas). However, neoplasia is not restricted to older animals; equine sarcoids are frequently recognized in horses as young as 3 years of age.

b. Number and progression of lesions. If one or only a few lesions are present, hypersensitivity to arthropod bites should be considered, particularly if there is rapid onset of the lesion. Rapid onset of generalized lesions such as urticaria suggests a differential diagnosis of drug, flying insect, environmental (pollen), or (rarely) food allergies.

c. A recent history of systemic illness, which might suggest that the lesion is a bacterial abscess.

2. Physical examination (see Fig. 11.2). In particular:

a. Determine by palpation if the cutaneous lesions are nodules or swellings.

b. Inspect the lesion or lesions closely for evidence of cutaneous parasitism (ticks, breathing pores associated with Hypoderma larvae, yellow granules associated with cutaneous habronemiasis).

c. Determine if the lesions are painful or pruritic (e.g., evidence of excoriations).

3. Perform fine-needle aspiration for cytologic studies.

4. Perform biopsy for histopathologic examination.

5. Perform bacterial culture and sensitivity.

6. Perform subcutaneous and deep fungal cultures.

7. Perform dermatophyte culture and KOH preparation.