JPC SYSTEMIC PATHOLOGY
Signalment (JPC # 21474-29): Dog, breed not specified.
HISTORY: This dog had bilaterally symmetrical, rugose thickening, lichenification, and gray-black hyperpigmentation of the axillary skin.
HISTOPATHOLOGIC DESCRIPTION: Haired skin, axilla (per contributor): Diffusely, the epidermis is moderately to markedly hyperplastic with prominent rete ridges, thickening of the stratum spinosum (acanthosis), intercellular edema (spongiosis) and moderate parakeratotic hyperkeratosis. Multifocally there are increased intracytoplasmic melanin granules within the stratum spinosum, stratum granulosum, and stratum corneum (hyperpigmentation). Within the superficial dermis, there are numerous melanomacrophages and free melanin granules (pigmentary incontinence) as well as infiltration by low numbers of neutrophils, lymphocytes, plasma cells, and macrophages. Apocrine glands are ectatic, frequently contain amphophlic secretory product and are lined by hyperplastic epithelial cells with abundant microvacuolated cytoplasm.
MORPHOLOGIC DIAGNOSIS: Haired skin, axilla (per contributor): Hyperplasia, epidermal, diffuse, marked, with hyperkeratosis, hyperpigmentation, and pigmentary incontinence, breed unspecified, canine.
CONDITION: Acanthosis nigricans
- Primary or idiopathic form: Rare disease of dachshunds, which is most likely inherited, and seen in dogs less than 1 years old of either sex
- Secondary form or pseudoacanthosis nigricans: Lesions virtually identical to primary acanthosis nigricans occur secondarily to other diseases or conditions such as hypothyroidism, hyperadrenocorticism, sex hormone related dermatoses, friction or intertrigo with a secondary bacterial or Malassezia infection, and hypersensitivities
- Idiopathic form - unknown, suggestive of genodermatosis
TYPICAL CLINICAL FINDINGS:
- Bilaterally symmetric axillary or inguinal hyperpigmentation and lichenification
- Edges of lesions often erythematous secondary to bacterial and/or yeast pyoderma
- Pruritus is variable and may be caused by the underlying disease or a secondary infection
- As the disease progresses, there is secondary alopecia, seborrheic dermatitis, and infections (staphylococcal or Malassezia dermatitis) may develop
TYPICAL GROSS FINDINGS:
- Bilateral axillary hyperpigmentation progressing to lichenification and alopecia
- Can affect the forelimbs, neck, chest, abdomen, groin, periocular region, pinnae
- Common sequelae include seborrhea, bacterial pyoderma and Malassezia dermatitis
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Marked acanthosis and rete ridge formation
- Mild to moderate superficial perivascular dermatitis with lymphocytes, macrophages, few neutrophils, plasma cells
- Mild to moderate orthokeratotic and parakeratotic hyperkeratosis
- Melanosis of the stratum spinosum, stratum granulosum and stratum corneum
- Pigmentary incontinence
ADDITIONAL DIAGNOSTIC TESTS:
- Histopathologic lesions alone are not diagnostic, just suggestive
- Definitive diagnosis from signalment, history, physical exam, skin biopsy and therapeutic response
- For microscopic findings:
- Superficial bacterial folliculitis: Pustules within follicles
- Atopic skin disease and food allergy: Acanthosis, dermal edema, superficial, perivascular infiltrates of mast cells and eosinophils
- Hypothyroidism: Epidermal and sebaceous gland atrophy, epidermal melanosis, myxedema, arrest of follicular growth cycle
- Hyperadrenocorticism: Epidermal atrophy, comedones, arrest of follicular growth cycle
- For gross findings:
- Nearly any chronic skin condition can lead to hyperpigmentation, which is a common, nonspecific response by the skin to chronic irritation
- Superficial pyoderma: Erythematous, papules, collarettes
- Atopic skin disease: Will involve more sites than acanthosis nigricans
- Sex hormone related dermatoses: Inguinal or genital area as well as the nipples, scrotum, prepuce, or vulva affected, accompanying signs such as pseudopregnancy, male infertility, gynecomastia
- Humans - acanthosis nigricans is associated with underlying cancer (gastric, uterine, or pulmonary), diabetes mellitus, genetic predisposition and obesity
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- Miller WH, Griffin CE, Campbell KL. Congenital and hereditary defects. In: Muller and Kirk’s Small Animal Dermatology. 7th ed. WB Saunders Company, Philadelphia, PA: WB Saunders Company, 2013: 600-601.