Pathology: Eye + Ear

Anatomy and Histology of the Eye

• The eye comprises various structures; understanding normal anatomy is essential for diagnosing diseases.
• Macroscopic evaluation includes the globe, eyelids, Meibomian glands, conjunctiva, third eyelid, lacrimal glands, and extraocular muscles.
• The cornea consists of an outer stratified squamous epithelium, avascular stroma, and endothelial layer with Descemet's membrane.
• The uvea includes the iris, ciliary body, and choroid, contributing to vascular support.
• The lens is biconvex, avascular, and anchored by zonular fibers.
• The retina, an outpouching of brain tissue, features three layers of neurons and is crucial for vision.
• Aqueous humor production occurs in the ciliary body, flowing through the pupil to the trabecular meshwork, contributing to intraocular pressure regulation.
• The blood-eye barrier protects internal structures from immune system exposure.

Postmortem Examination of the Eye

• Rapid postmortem changes necessitate immediate fixation in formalin, Bouin’s, or Davidson’s solution for quality histologic sections.
• Ocular tissues, especially the retina, are highly sensitive to anoxia; mishandling can impair diagnostic efforts.

Diseases of the Globe

• Glaucoma: Characterized by increased intraocular pressure, leading to vision loss and blindness; classified as primary or secondary.
• Primary Glaucoma:
  • Goniodysgenesis: Maldevelopment of the filtration angle, prevalent in purebred dogs.
  • Primary open-angle glaucoma: No visible trabecular meshwork abnormalities, common in laboratory Beagles.
• Secondary Glaucoma: Caused by obstructions like lens luxation, synechiae, or neoplasia.
• Pathologic changes from increased intraocular pressure primarily affect the optic nerve and retina, leading to retinal and optic nerve atrophy, ganglion cell death, buphthalmia, and corneal changes.

Diseases of the Eyelids and Ocular Adnexa

• Entropion: Eyelid inversion that may cause secondary trichiasis; classified into primary, spastic, and cicatricial types.
• Ectropion: Eyelid eversion, presenting with conjunctival visualization, hyperemia, and keratitis.
• Distichiasis and Trichiasis: Eyelash abnormalities that may necessitate treatment if causing corneal irritation.
• Neoplastic eyelid diseases include:
  • Meibomian adenomas: Common benign tumors in dogs.
  • Melanomas: Typically benign in dogs, but more aggressive in cats.
  • Squamous cell carcinoma: Most common in cats, locally invasive, with metastatic potential.

Diseases of the Conjunctiva

• Conjunctival hyperemia: A response to various local and systemic conditions, not synonymous with conjunctivitis.
• Conjunctivitis: Rare in dogs, often linked to allergies or irritation; common in cats and associated with infectious agents like Herpesvirus and Chlamydia.

Nictitating Membrane and Corneal Diseases

• Prolapse of the gland of the nictitating membrane (“cherry eye”): Associated with weak connective tissue in certain breeds.
• Corneal diseases include keratitis, erosion, ulceration, and associated complications such as vascularization and pigmentation.
• Melting corneal ulcers lead to progressive corneal tissue loss and may create descemetoceles.

Diseases of the Sclera and Uveal Tract

• Nodular granulomatous episcleritis: Common proliferative lesion mistaken for neoplasia.
• Uveitis: Inflammation of the uvea, classified into anterior (iritis) and posterior (choroiditis) types; can cause complications like synechiae and glaucoma.
• Equine recurrent uveitis: Major blindness cause in horses, proposed to be immune-mediated.

Lens and Neoplastic Diseases

• Cataracts: Most significant lens disorder, can occur secondary to various conditions; characterized by denatured lens protein deposits.
• Uveal melanoma: Most common intraocular neoplasm, benign in dogs, potentially malignant in cats.
• Lymphoma and ciliary body tumors can affect intraocular structures.

Other Eye Terminology

• Aqueous flare: Increased protein in aqueous humor; Chemosis: conjunctival swelling; Hypopyon: neutrophil accumulation in the anterior chamber; Hyphema: anterior chamber hemorrhage; Iris bombe: anterior iris bowing; Phthisis bulbi: end-stage globe.

Anatomy and Physiology of the Ear

• The ear consists of three parts: external, middle, and inner ear, each with specific roles in hearing and balance.
• The external ear includes the auricle and auditory canal lined with sebaceous and ceruminous glands.

Diseases of the Ear

• Otitis externa: Common in dogs and cats, involving moisture and microbial infection; results in complex pathogenesis including chronic changes.
• Otitis media: Bacterial inflammation of the tympanic cavity; can lead to severe complications if untreated.

Diseases of the Middle Ear

• Bacterial organisms, such as Mycoplasma bovis and Pasteurella multocida, often cause otitis media in cattle with potential for systemic involvement.### Otitis Media in Animals
• Calves: Escherichia coli can cause sporadic otitis media cases.
• Dogs and Cats: Often linked to chronic otitis externa; in Cavalier King Charles spaniels, primary secretory otitis media is common.
• Inflammatory Polyps: These may develop in the middle ear, particularly with otitis media in dogs.
• Cats with Cryptococcosis: Otitis media associated with vestibular disease may be a primary clinical complaint.

Otitis Media in Specific Animals

• Pigs: Unilateral otitis media can occur, associated with pathogens like haemolytic Streptococcus, Actinobacillus pleuropneumoniae, and Mycoplasma hyorhinis.
• Horses: Often not related to otitis externa; can lead to temporohyoid osteoarthropathy and has instances of mycotic otitis media.

Diseases of the Inner Ear

• Otitis Interna: Frequently arises from middle ear inflammation; leads to vestibular dysfunction; may progress to suppurative meningitis and encephalitis.
• Vestibular Dysfunction Signs: Includes head tilt, falling toward the affected side, ataxia, and nystagmus.
• Causes: Can be due to otitis media, trauma, drugs, and neoplasia. Idiopathic vestibular disease noted in older dogs and cats, characterized by sudden onset and rapid remission without identifiable lesions.

Neoplasia of the Ear

• General: Ear neoplasms resemble skin tumors; primary tumours of ceruminous glands and auditory epithelium are rare.
• Cutaneous Histiocytoma: Commonly found in the pinna of dogs.
• Squamous Cell Carcinoma: Frequently seen in the pinna of white cats and sheep exposed to UV light.
• Ceruminous Adenoma/Adenocarcinoma: Common in dogs (mostly benign) and in both benign and malignant forms in cats; adenomas characterized by well-differentiated tubular cystic mass.
• Nasopharyngeal Polyps: Predominantly in cats; consist of loose connective tissue with small blood vessels, covered by ciliated columnar epithelium; origin is unknown, typically from the middle ear or Eustachian tube.
• Inflammatory Polyps: Observed in dogs with chronic otitis externa; histologically reveal hyperplastic surface epithelium and leukocytic inflammation.

Test your knowledge about the various structures and functions of the eye in this quiz. Explore topics such as the cornea, retina, and uvea, and understand how they contribute to vision. This quiz will help solidify your understanding of ocular anatomy and their respective functions.