Diagnostic Key Points of Canine Keratoconjunctivitis Sicca (Dry Eye)

Canine keratoconjunctivitis sicca (KCS), commonly known as dry eye, is a debilitating condition affecting dogs of all breeds and ages. While it can manifest at any point in a dog’s life, it’s particularly prevalent in older animals. The underlying cause is a deficiency in tear production, leading to a dry, irritated, and potentially ulcerated cornea and conjunctiva. Prompt diagnosis and treatment are crucial to prevent vision impairment and potential blindness. This article outlines the key diagnostic steps for identifying KCS in canine patients.

Etiology: Unraveling the Causes of Dry Eye

Understanding the etiology of KCS is paramount for effective diagnosis and management. Multiple factors can contribute to the development of this condition, often interacting in complex ways. These factors can be broadly categorized as:

Immune-Mediated Disease: A significant portion of KCS cases are attributed to autoimmune disorders. The body’s immune system mistakenly attacks the lacrimal glands (tear glands), hindering tear production. This autoimmune destruction can be linked to other systemic autoimmune diseases such as lupus or hypothyroidism. Therefore, a thorough medical history and potentially further diagnostic testing are necessary to rule out these concurrent conditions.

Infections: Certain viral infections, particularly canine distemper, can severely damage the lacrimal glands, resulting in persistent KCS. Bacterial infections can also exacerbate the condition, leading to secondary complications such as corneal ulcers.

Drug-Induced KCS: Certain medications, notably sulfonamides, have been implicated in the development of KCS. This highlights the importance of a thorough review of the patient’s medication history.

Surgical Intervention: Procedures like third eyelid gland removal (often performed to address “cherry eye”) can lead to KCS in the subsequent years.

Neurological Factors: Damage to the nerves that innervate the lacrimal glands (cranial nerves V and VII) can disrupt tear production, often resulting in unilateral (one-sided) KCS. This neurological involvement can be associated with other neurological signs.

Age-Related Changes: As dogs age, the lacrimal glands can undergo natural atrophy, reducing their tear production capacity. This age-related decline is a common factor, particularly in older canine patients.

Environmental Factors: Though less frequently cited, environmental irritants and trauma to the eye region can contribute to dry eye, exacerbating existing issues or initiating the condition.

Genetic Predisposition: Some breeds have a higher predisposition to KCS, possibly linked to genetic factors influencing immune function or lacrimal gland development. Breeds frequently reported to be affected include Maltese, Yorkshire Terriers, Shih Tzus, Poodles, and Bulldogs, among others. However, the condition is not exclusive to these breeds.

Diagnostic Procedures: A Multifaceted Approach

Diagnosing KCS involves a combination of history taking, clinical examination, and specific diagnostic tests:

History Taking: A comprehensive history, including the duration and progression of symptoms, previous illnesses, medication history, and any potential eye trauma, is essential. The owner’s description of the onset and nature of ocular discharge (mucoid, mucopurulent, or absent) is crucial.

Clinical Examination: The veterinarian will examine the eyes for signs of dryness, redness (conjunctival hyperemia), corneal cloudiness, ulceration, and the presence of any ocular discharge. Note should be made of any asymmetry in findings between the two eyes.

Schirmer Tear Test (STT): This is the gold standard test for assessing tear production. A small strip of filter paper is placed in the conjunctival sac, and the amount of wetting after a specific time is measured. A result below 15 mm/minute generally indicates KCS.

Fluorescein Staining: This test identifies corneal abrasions or ulcers. Fluorescein dye is applied to the eye; if any damage is present, the dye will adhere to the affected area, revealing it under a Wood’s lamp.

Tear Break-Up Time (BUT): This assesses the stability of the tear film. After instilling fluorescein, the time it takes for the tear film to break up is measured. A shortened BUT indicates an unstable tear film.

Bengal Rose Staining: This stain assesses the integrity of the corneal epithelium, revealing superficial damage not always visible with fluorescein staining.

Cytology: A cytological examination of conjunctival secretions can identify the presence of bacteria, inflammatory cells, or other abnormal cells, assisting in identifying secondary infections or determining disease severity.

Complete Blood Count (CBC) and Serum Biochemistry Profile: These tests help rule out underlying systemic diseases that might contribute to or exacerbate KCS, such as diabetes mellitus, hypothyroidism, or autoimmune disorders. Specific serological tests for autoimmune diseases might be warranted based on clinical suspicion.

Differential Diagnoses:

It is crucial to differentiate KCS from other conditions that can mimic its symptoms, including:

Conjunctivitis: Inflammation of the conjunctiva, often caused by allergies, irritants, or infections.

Corneal Ulcers: Open sores on the cornea, often painful and potentially vision-threatening.

Entropion (Inward eyelid rolling): Can cause irritation and corneal damage due to eyelash rubbing.

Exposure Keratopathy: Corneal damage due to inadequate eyelid closure.

Conclusion:

Diagnosing canine KCS requires a comprehensive approach combining a detailed history, careful clinical examination, and targeted diagnostic tests. The STT is the cornerstone of KCS diagnosis, while other tests help identify the severity and presence of secondary complications. Early detection and appropriate treatment are critical to managing the condition and preserving the dog’s vision. Early intervention is crucial for optimizing treatment outcomes and improving the dog’s quality of life.