JPC SYSTEMIC PATHOLOGY

SPECIAL SENSES SYSTEM

April 2024

S-M14 (NP)

 

Signalment (JPC #1690931): A ten-year-old Siamese cat

 

HISTORY: This cat had a black spot on one eye for approximately one year. The spot was surgically removed. A similar spot had been removed three years previously.

 

HISTOPATHOLOGIC DESCRIPTION: Cornea: Over 90% of the stroma is composed of hyalinized, golden brown to eosinophilic, acellular material that retains tissue architecture (stromal coagulative necrosis). In the affected area there is diffuse loss of epithelium and endothelium and multifocal loss of Descemet’s membrane. There are multifocal depressions along the surface that occasionally extend into the deep stroma. In the remaining area of the cornea, there is a focal loss of epithelium (ulcer) and irregular clefting and clear space separating stromal fibers (edema).

 

MORPHOLOGIC DIAGNOSIS: Cornea: Necrosis, coagulative, focally extensive, with pigmentation and loss of epithelium (sequestrum), and ulceration, Siamese, feline.

 

CAUSE: Unknown

 

CONDITION: Corneal sequestrum

 

SYNONYMS: Corneal black spot, corneal mummification, corneal nigrum, superficial stromal sequestration, focal corneal degeneration

 

GENERAL DISCUSSION: 

• Central or paracentral focal degeneration of the corneal stroma, often with accumulation of a brown pigment surrounded by variable amounts of inflammation 
• Flat, initially non-ulcerated, brown to black necrotic spot in the central cornea, usually unilateral, characterized by lack of epithelial attachment

• Brown discoloration unique to cats but not all cases have this discoloration; results from absorbed porphyrins from tear film

• Corneal sequestrum presumed to be an uncommon sequel to corneal ulceration but exact pathogenesis is unknown
• Persian and Himalayan cats overrepresented

 

PATHOGENESIS:

• Controversial; contributing factors likely include:

• Corneal ulceration due to chronic desiccation suspected in flat-faced breeds
• Feline Herpes virus (FHV-1) infection in non-predisposed breeds
• Any corneal ulceration in cats may lead to sequestrum

• Persistence of degraded corneal stroma from repeated ulceration prevents hemidesmosomal adhesion; this is believed to be true for corneal sequestrum formation in cats, dogs, and horses
• Sequestrum will eventually slough with replacement by granulation tissue; takes weeks to months; usually surgery is elected prior to this stage of disease

 

TYPICAL CLINICAL FINDINGS:

• Blepharospasm, lacrimation, protrusion of the third eyelid
• May be asymptomatic

 

TYPICAL GROSS FINDINGS:  

• Discrete central, flat superficial orange-brown discoloration of the central cornea
• Usually unilateral, occasionally bilateral
• Adjacent cornea may be ulcerated, edematous, and vascularized

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Bland corneal epithelial desiccation – easily mistaken for artifact 
• Necrosis of stromal keratinocytes with pallor, hyalinization, and vague orange-brown discoloration of affected stroma without inflammation; mineralization is rare

• Discoloration may be absent in early lesions

• Overlying epithelium may be ulcerated or intact, with growth over the surface of dead stroma (if intact usually evidence of previous ulceration)
• Older lesions may have a deep margin of mononuclear leukocytes with occasional macrophages and giant cells

 

DIFFERENTIAL DIAGNOSIS:

• Corneal ulcer
• Foreign bodies
• Melanocytoma or melanoma
• Corneal pigmentation
• Mineral deposits

 

COMPARATIVE PATHOLOGY:  

• Dog: Canine persistent (recurrent) ulcer syndrome
  • Boxers predisposed; previously referred to as “Boxer ulcer”
  • Clinical syndrome is distinctive: Characterized by shallow central corneal erosion with minimal edema and no neovascularization (initially); repeated re-ulceration follows due to poor adhesion of epithelium to underlying stroma
  • Basic defect is degeneration of the superficial stroma
    • Epithelium attempts to adhere to zone of hypocellular, pale-staining stroma (shallow sequestrum)
    • Pyknotic and lytic keratocyte nuclei are observed within this superficial zone
    • Epithelial hemidesmosomes and anchoring collagen fibers have no substrate to which they can anchor
  • Corneal injury > abnormal/degenerate superficial corneal stroma > regenerating corneal epithelial hemidesmosomes unable to anchor properly to abnormal stroma > regenerating corneal epithelium slides off easily > non-healing or recurrent shallow central corneal erosion/ulcer 
  • Corneal epithelial basal lamina is often not apparent microscopically; multiple clefts separate hyperplastic epithelium from underlying stroma; pyknotic and lytic keratocyte nuclei (apoptosis) and pallor within superficial stroma; usually less severe than the stromal devitalization associated with feline corneal sequestrum
• Horse: Less frequent and less well characterized than dogs or cats; often associated with superimposed fungal infection.

 

REFERENCES:

1. Labelle P. The eye. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7^th ed. St. Louis, MO: Elsevier; 2022: 1411-1412.
2. Wilcock BP, Njaa BL. Special senses. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6^th ed. St. Louis, MO: Elsevier; 2016:434-435.

 

 

Click the slide to view.

---