The nasolacrimal system is a thin walled tube that drains tears from the surface of the eye into the nasal passages and oral cavity. The nasolacrimal duct begins at two small openings in the conjunctiva at the eyelid margins on the side of the eye nearest the nose. About half of the tears are lost from the cornea and conjunctiva by evaporation and the remaining half drain from these ducts. Most tear drainage into and through the nasolacrimal duct is moved passively by external forces like gravity. Some drainage is due to capillary action and the force of eyelid closure. Blockage of the nasolacrimal duct may occur in dogs, cats, rabbits, and horses. Symptoms include but are not limited to:
A bright green dye called ﬂuorescein is applied to the eye to identify normal passage of ﬂuid through the nasolacrimal system. In addition, the openings into the duct are examined at high magnification using a slit lamp biomicroscope. The slit lamp enables the ophthalmologist to carefully evaluate the patient’s cornea, conjunctiva, and nasolacrimal ducts. If a nasolacrimal duct blockage is diagnosed or suspected, a small blunt cannula may be inserted into the duct openings in the eyelid. Once in the opening, irrigation of the duct with saline is performed. This is done using a local anesthetic applied to the eye in most cases, but occasionally injectable sedation is needed. Flushing the duct with saline may eliminate blockage or it may provide evidence of ductal scarring that can cause permanent occlusion of the duct. Other diagnostics tests that may be recommended include:
Nasolacrimal blockage can occur due to anatomical defects present at birth, breed related eyelid conformational abnormalities, inﬂammation of or foreign bodies lodged in the ducts, and occasionally tumors pressing on the ducts. Most blocks in dogs occur due to the shape of the eye, nose and face as well as mild inﬂammation of the pink tissues in the eye. In cats, prior infection with the feline herpesvirus can lead to narrowing or scarring down of the ducts.
Many nasolacrimal blockages can be treated medically with anti-inﬂammatory and antimicrobial therapy. Some blockages improve but may not completely resolve. Treatment may be used for comfort if resolution is not achievable. For some cases, surgical intervention may be necessary, particularly for conjunctival or scar tissue obstructing the duct openings, foreign bodies in the ducts, or for tumors) Surgery may involve exploration, resection of tissue or reconstruction of normal tissues.
Complications are rare and including anesthetic risks, only account for less than 5% of cases. Complications that may occur include: