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Ideal glaucoma drainage device placement varies by model, eye
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The best location for placement of a glaucoma drainage device differs according to the device being used as well as eye-dependent factors, such as axial length and the quadrant involved, a study in cadaver eyes found. Successfully implanted devices should be situated far enough posteriorly to avoid anterior exposure, yet maintain a safe distance from the optic nerve, the authors said.
Malik Y. Kahook, MD, and colleagues at the University of Pittsburgh School of Medicine tested five glaucoma drainage device models on five human cadaver eyes with axial lengths ranging from 22.5 mm to 26 mm. The devices tested included the Ahmed S2 and FP7 (New World Medical Inc.), the Molteno D1 (Molteno Ophthalmic Ltd.) and the Baerveldt BG 103 250 mm² and BG 101 350 mm² (Advanced Medical Optics).
In each quadrant of each eye, the researchers positioned the five devices 2 mm from the optic nerve and measured the distance from the anterior plate edge to the limbus.
The Molteno device, the smallest of the devices tested, could be placed furthest posterior to the limbus for all quadrants and eye sizes. It was also the only device that could be placed into the inferotemporal quadrant without contacting the inferior oblique muscle insertion site, the researchers said. Because of its small size, the Molteno was the only device the authors recommended for use in an anatomically "crowded" superonasal quadrant.
The two Ahmed devices had to be placed the most anteriorly to avoid optic nerve encroachment due to their "longer anterior-posterior dimension," the authors said.
The Baerveldt models could be placed further posteriorly than the Ahmed models. But their lateral lengths — the longest of the five models evaluated — usually encroached upon each quadrant's adjacent muscles, which could cause double vision or discomfort, according to the study authors.
The findings are especially important for eyes with smaller axial lengths because their crowded anatomy increases the risk of exposure of the device, the authors noted.
"It may behoove surgeons to not always place the [device] a standardized distance back from the limbus or 'as posteriorly as possible' in patients with smaller eyes or nanophthalmos," the authors said. "Furthermore, consideration should be given to performing axial length measurements preoperatively, to assess the relative risk of impingement on extraocular structures."
The study is published in the August issue of the British Journal of Ophthalmology.