JPC SYSTEMIC PATHOLOGY
INTEGUMENTARY SYSTEM
October 2022
I-N29 (NP)
Signalment (JPC# 2328486): A military working dog
HISTORY: This dog had a raised, leathery, non-pruritic area of lichenified hyperpigmented skin over the sternum that recurred post surgically.
HISTOPATHOLOGIC DESCRIPTION: Haired skin: Expanding the dermis and subcutis and elevating the overlying epidermis is a 2.5 cm diameter, ovoid, cystic cavity that lacks epithelial lining (pseudocyst) and contains abundant clear space with aggregates of neutrophils and macrophages, moderate amounts of fibrin, and scant scattered environmental debris. The pseudocyst is lined by plump fibroblasts and small caliber blood vessels with hypertrophied endothelium (granulation tissue) that form irregular villous projections into the lumen; the granulation tissue is admixed with numerous neutrophils and macrophages that often have foamy cytoplasm and occasionally contain phagocytized erythrocytes (erythrophagocytosis). The granulation tissue lining progressively matures into an outer thick band of dense fibrous connective tissue that is admixed with many neutrophils, lymphocytes, plasma cells, and few macrophages. Similar inflammatory infiltrates surround vessels and adnexa within the superficial dermis, and the superficial dermis contains occasional dilated lymphatics (edema). Within the overlying epidermis, there is mild multifocal acanthosis and orthokeratotic hyperkeratosis as well as moderate amounts of melanin in all layers of the epidermis (hyperpigmentation).
MORPHOLOGIC DIAGNOSIS: Haired skin: Pseudocyst (hygroma), focal, with granulation tissue, multifocal moderate chronic-active dermatitis, mild epidermal hyperplasia, and dermal edema, breed unspecified, canine.
SYNONYM: Adventitious bursa
CONDITION: Hygroma
GENERAL:
- A hygroma, or adventitious bursa, is a false or acquired bursa that develops subcutaneously over bony prominences, pressure points (e.g. lateral aspect of the elbow, the greater trochanter of the femur, tuber coxae, and sternum of deep chested dogs), or areas of chonic trauma
- Common in giant breed dogs
PATHOGENESIS:
- Repeated pressure and/or trauma-induced necrosis and inflammation in the subcutaneous tissues may initially form a callus; eventually the accumulation of inflammation and fluid may form a false bursa or pseudocyst
- Pseudocyst has no epithelial lining, vs. cyst that has epithelial lining
- Persistent decubital ulcers or recurrent hematomas can result in hygromas
TYPICAL CLINICAL FINDINGS:
- Non-painful, subcutaneous swelling that is initially soft/fluctuant but may develop into an abscess or granuloma especially if it becomes secondarily infected
- Cytology: aspirates are poorly cellular, primarily macrophages, reactive fibrocytes or reactive mononuclear cells, but no tissue cells
TYPICAL GROSS FINDINGS:
- Variably sized cystic cavity separated from the skin by loose connective tissue and filled with yellow to red mucinous contents
- Surrounded by a dense connective tissue wall; smooth or villous inner lining
TYPICAL MICROSCOPIC FINDINGS:
- Wall composed of granulation tissue of variable maturity; there is no epithelial lining; typically lined by flattened layer of mesenchymal cells that may resemble synoviocytes
- Pseudocyst may contain clumps of fibrin at the cavity margins that give rise to the gross villous appearance; these may occasionally undergo cartilaginous metaplasia
- Often many inflammatory cells (secondary to necrosis or infection)
- Usually no connection with joints or tendon sheaths
DIFFERENTIAL DIAGNOSIS:
- Seroma:
- Less granulation tissue
- Often associated with surgical or injection sites
- Callus:
- Also occur over bony prominences secondary to pressure or irritation but no cystic space
- Hyperkeratosis, dermal fibrosis
- Well-circumscribed, raised, alopecic
COMPARATIVE PATHOLOGY:
- Goats: Unilateral or bilateral carpal hygromas are a characteristic clinical feature of caprine arthritis-encephalitis virus (small ruminant lentivirus)
- Horses:
- “Capped elbow” and “capped hock” are adventitious bursae due to trauma (e.g. kicks, falls, prolonged recumbency)
- Pigs: Adventitious bursae of the tuber calcaneous (“capped hocks”) or distolateral hock (“adventitious bursitis”) are common in finishing pigs, especially those kept on hard, slippery surfaces
- Cattle, sheep, goats: Carpal bursitis (AKA carpal hygroma); these distended bursae are sometimes filled with fibrin concretions
- Cervids: Hygromas are associated with Brucella abortus, characterized by moist, rough, red-brown internal linigns with opqaque, straw-colored fluid and fibrinous debris.
- Lions: Hygromas are associated with Mycobacterium bovis characterized by distended, sac-like structures, over elbow and hock joints
REFERENCES:
- Craig LE, Dittmer KE, Thompson KG. Bones and Joints. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. St. Louis, MO: Elsevier Limited; 2016:154 - 156.
- Fisher DJ. Cutaneous and subcutaneous lesions. In: Valenciano AC, Cowell RL, eds. Diagnostic Cytology and hematology of the dog and cat. 5th ed. St. Louis, MO: Elsevier; 2020: 108.
- Howert EW, Nemeth NM, Ryser-Degiorgis MP. Cervidae. In: Terio K, McAloose D, Leger J, eds. Pathology of Wildlife and Zoo Animals, San Diego, CA: Elsevier 2018: 164.
- Mauldin EA, Peters-Kennedy J. Integumentary System. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. St. Louis, MO: Elsevier Limited; 2016:559.
- Raskin RE, Conrado FO. Integumentary system. In: Raskin RE, Meyer DJ, eds. Canine and Feline Cytopathology: A Color Atlas and Interpretation Guide. 4th ed. St. Louis, MO: Elsevier; 2023: 118.
- Terio KA, McAloose D, Mitchell E. Felidae. In: Terio K, McAloose D, Leger J, eds. Pathology of Wildlife and Zoo Animals, San Diego, CA: Elsevier 2018: 274-275.
- Welle MM, Linder KE. The integument. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed., St. Louis, MO; Elsevier; 2022:1120, 1160-1161.
