The lacrimal drainage system includes the upper and lower puncti and canaliculi, lacrimal (tear) sac and the nasolacrimal duct. The upper canaliculi merge, just before the lacrimal sac, to form the common canaliculus. A valve at the entry point of the common canaliculus into the sac prevents backflow of tears towards the eye. The lacrimal sac, situated in the lacrimal fossa in the anterior part of the medial orbit, is compressed during blinking. Tears are drawn into the lacrimal apparatus as the sac re-expands = “lacrimal pump”.
The nasolacrimal duct connects the lower end of the lacrimal sac with the inferior meatus (space below the inferior turbinate) of the nose. Another valve guards the opening and prevents air from entering the sac during nose blowing. A narrowing (stenosis) or obstruction anywhere in the lacrimal drainage apparatus usually results in the eye constantly watering (epiphora).
Lacrimal syringing is chiefly a diagnostic test to assess the patency of the lacrimal apparatus, although occasionally debris is flushed from the system which can result in an improvement in epiphora.
The lacrimal punctum is widened with a dilator which allows insertion of a blunt-tipped lacrimal cannula attached to a 2ml syringe filled with saline. Access of the cannula is easier and less traumatic if the lower lid is tractioned laterally. Saline is gently irrigated via the lower canaliculus and in a normal patent system the patient is immediately aware of a salty taste in the back of their mouth. Any narrowing or obstruction results in increased resistance to flow into the nasal cavity and the saline tends to pass retrogradely back towards the eye via the upper canaliculus.