Merck Manual

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Chronic dacryocystitis with fistula formation, Shetland Sheepdog
Chronic dacryocystitis with fistula formation, Shetland Sheepdog

Nasolacrimal flush with the needle within the upper punctum often forces exudates and the saline flush through the fistula as well as the distal nasolacrimal duct. Patency of the nasolacrimal system is essential for the fistula to heal closed.

Courtesy of K. Gelatt.