Glaucoma drainage tubes are placed in the eyes of patients as an alternative to trabeculectomy. Tubes and shunts both facilitate aqueous humor outflow, but over time some of these implants can shift positions in the eye and damage the cornea or even the visual axis, according to researchers from the Department of Ophthalmology and Vision Sciences at the University of Toronto.

The investigators reviewed 11 eyes from nine patients that were fitted with Ahmed glaucoma valves for approximately five years. They assessed tube positioning at baseline (one to two months post-op) and final follow-up with anterior segment optical coherence tomography. They noted numerous significant long-term changes, including an increase in intracameral tube length (from approximately 1.58mm to 1.74mm), a decrease in tube-cornea distance (from approximately 0.36mm to 0.29mm) and a decrease in cornea-tube angle (from 27.76° to 24.82°). Overall, 73% of the tubes moved towards the cornea and experienced an increase in intracameral tube length.

“Tubes tend to move toward the cornea over time,” the researchers determined. To avoid corneal damage and involvement of the visual axis, they recommend surgeons insert tubes tangentially, preferably in the posterior 1/3 of the anterior chamber, and that it’s preferable if the tubes themselves are short.