JPC SYSTEMIC PATHOLOGY
Signalment (JPC # 3054993): 9-year-old male Dutch shepherd
HISTORY: Swollen right rear fourth digit
HISTOPATHOLOGIC DESCRIPTION: Haired skin, digit: Expanding the deep dermis, compressing adnexa, and elevating the overlying moderately hyperplastic epidermis is a 5 X 2 mm cyst lined by a thick wall of squamous epithelium which undergoes gradual keratinization and contains deeply basophilic keratohyalin granules within the stratum granulosum and melanin pigments throughout all cell layers. The cyst lumen is filled with laminations of keratin admixed with hemorrhage and cellular debris. The dermis adjacent to the cyst is thickened with increased fibrous tissue (fibrosis) along with a cellular infiltrate composed of numerous macrophages and neutrophils, few lymphocytes and plasma cells, free melanin pigment and clusters of melanomacrophages (pigmentary incontinence), and hemorrhage. Surrounding the third phalanx are multifocal to coalescing foci of fibrosis surrounding the previously described cellular infiltrate along with eosinophilic cellular and karyorrhectic debris (necrosis) and abundant hemorrhage. Along the phalanx there are rare areas of scalloped bone with increased numbers of osteoclasts within Howship’s lacunae (bone resorption) as well as foci of woven bone (bony remodeling). The adjacent epidermis is moderately hyperplastic with prominent anastomosing rete ridges and parakeratotic hyperkeratosis. Multifocally, hair follicles contain laminations and fragments of keratin and aprocrine glands are mildly ectatic.
MORPHOLOGIC DIAGNOSIS: Haired skin, digit: Nail bed epidermal inclusion cyst with pyogranulomatous dermatitis, hemorrhage and necrosis, Dutch shepherd, canine.
- Non-neoplastic, rare transformation to neoplasm (one case)
- Most likely caused by trauma or surgical implantation of epidermal tissue into subjacent connective tissue
- Congenital malformation of ungual or periungual epithelium
- Previous onychectomy sites and dewclaws are most often affected
- Often grouped with other infundibular cysts such as epidermoid cysts, epidermal cysts and epidermal inclusion cysts that can be found on other areas of the body
- Can cause pyogranulomatous inflammation if ruptured (foreign body response to free keratin)
- Proliferation of epidermal cells within a circumscribed space of the dermis
TYPICAL CLINICAL FINDINGS:
- Single swollen digit with or without cracked or irregular nail
- Radiographically, there can be partial to total bone loss of the third phalanx with little periosteal response
TYPICAL GROSS FINDINGS:
- Nodule size difficult to assess until sectioned, but generally 0.5 - 2 cm
- Cyst contents may be semi-fluid, caseous or composed of dry granular material that can be pigmented
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Similar histologic appearance to follicular cysts of infundibular origin
- Cyst lined by normal squamous epithelium that undergoes gradual keratinization
- Cyst contains laminations of keratin
- Layers may be pigmented, especially in animals with dark coat and nails
- Mild infiltration of lymphocytes and plasma cells surrounding connective tissue
- Pressure lysis of third phalanx may be present depending on size
- If ruptured may see granulamatous response to include periosteal fibrosis or periosteal bone proliferation if severe
- Subungual keratoacanthoma: Contiguous with epidermis; inverted or cup shaped; central core of keratin that may open onto the skin surface directly ventral or adjacent to the nail; abrupt keratinization (no granular cell layer)
- Reported case of systemic lupoid onychodystrophy in nailbed of dog
- Epidermal cysts can occur in a variety of places in multiple species
- Gross TL, Ihrke PJ, Walder EJ, Affolter VK. Skin Diseases of the Dog and Cat: Clinical and Histopathologic Diagnosis. 2nd ed. Ames, IA: Blackwell Science; 2005: 695-696.
- Stern AW, Pieper J. Pathology in practice. Symmetric lupoid onychodystrophy of the claw bed epithelium; J Am Vet Med Assoc. 2015; 246(2):197-9.
- Wilcock BP, Njaa BL. Special senses. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. Philadelphia, PA: Elsevier Saunders; 2016: 500.
- Wobeser BK, Kidney BA, Powers SJ, Withrow SJ, Mayer MN, Spinato MT, Allen AL. Diagnosis and clinical outcomes associated with surgically amputated canine digits submitted to multiple veterinary diagnostic laboratories. Vet Pathol. 2007; 44: 355-361.