Evaporation accounts for approximately 10% of tear elimination in the young and for 20% or more in elderly persons. Most of the tear flow is actively pumped from the tear lake by the actions of the orbicularis oculi muscle. Blinking pushes the tears from lateral to nasal on the eyelid margin. When the eyelids open, negative pressure pulls the tears into the sac; when the eyelids close, the action of the orbicularis muscle creates positive pressure that forces those tears through the NLD (Fig 14-4, Video 14-1). A weakened blink interferes with the normal pumping mechanism and contributes to epiphora in patients with eyelid laxity or CN VII palsy.
Figure 14-4 Lacrimal pump mechanism. A, In the relaxed state, the puncta lie in the tear lake, and the lacrimal sac is filled with tears. B, With eyelid closure, contraction of the pretarsal orbicularis closes the puncta and canaliculi. The preseptal orbicularis fibers, which insert onto the sac, also compress the sac, creating positive pressure that propels tears through the duct. C, With eyelid opening, the orbicularis relaxes, and the puncta and sac open, creating negative pressure that draws tears into the canaliculi and lacrimal sac. As the eyelids close, the cycle repeats.
(Original illustration by Christine Gralapp; revision based on an illustration by Cat N. Burkat, MD.)
Lacrimal pump mechanism. Courtesy of Cat N. Burkat, MD; illustration by Christine Gralapp.
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Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.