PC SYSTEMIC PATHOLOGY
INTEGUMENTARY SYSTEM
August 2022
I-M06

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 and thickening of the stratum spinosum (acanthosis), intercellular edema with prominent intercellular bridging (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):  Epidermal hyperplasia, diffuse, marked, with hyperkeratosis, hyperpigmentation, and pigmentary incontinence, breed unspecified, canine.

CONDITION:  Acanthosis nigricans

GENERAL DISCUSSION:

• An idiopathic dermatitis characterized by progressive hyperpigmentation, alopecia, and lichenification. Generally, this is bilaterally symmetric and generally starts in the axilla before spreading to the proximal limbs ventral abdomen neck and inguinal area.
• Primary or idiopathic form: inherited disease of dachshunds seen in dogs less than one year old with no sex predilection.
• 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

PATHOGENESIS:

• 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, 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, rete ridge formation with orthokeratotic and parakeratotic hyperkeratosis.
• Mild to moderate superficial perivascular dermatitis with lymphocytes, macrophages, few neutrophils, plasma cells
• 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

DIFFERENTIAL DIAGNOSIS:

• For microscopic findings:
  • Superficial bacterial folliculitis:  Pustules within follicles
  • Atopic skin disease and food allergy (I-M02):  Acanthosis, dermal edema, superficial, perivascular infiltrates of mast cells and eosinophils
  • Hypothyroidism (I-M22):  Epidermal and sebaceous gland atrophy, epidermal melanosis, myxedema, arrest of follicular growth cycle
  • Hyperadrenocorticism (I-M23):  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 (I-M24): Inguinal or genital area as well as the nipples, scrotum, prepuce, or vulva affected, accompanying signs such as pseudopregnancy, male infertility, gynecomastia

COMPARATIVE PATHOLOGY:

• Humans - acanthosis nigricans is associated with underlying cancer (gastric, uterine, or pulmonary), diabetes mellitus, genetic predisposition, and obesity

REFERENCES:

1. Gross TL, Ihrke PJ, Walder EJ, Affolter VK. Hyperplastic diseases of the epidermis. In:  Skin Diseases of the Dog and Cat. 2nd ed. Ames, IA: Blackwell Publishing Company; 2005: 151-152.
2. Lazar AJF, Murphy GF. The skin. In: Kumar V, Abbas AK, Aster JC. eds. Robbins and Cotran Pathologic Basis of Disease. 9th ed., Philadelphia, PA: Elsevier Saunders Company; 2015:1151.
3. 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; 2016: 555.
4. 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.

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