Evaluation of the position and function of aqueous drainage implants with magnetic resonance imaging

To evaluate position and function of antiglaucomatous Ahmed and Molteno shunts using magnetic resonance imaging with head and surface coils.

Eight patients (5 males) with shunt implants were included, 4 with Ahmed (FP-7) and 4 with Molteno (s1, single plated). All patients were operated at least 6 months before imaging. In 3 cases (2 with Molteno and 1 with Ahmed shunt), the intraocular pressure (IOP) was above 21 mm Hg, despite maximal medical treatment. The shunt endplate, tube and filtering blebs were identified in T1-weighted and T2-weighted images with both head and surface coils. Volumetric measurements of the orbits, eyeball, and filtering bleb and calculation of the endplate position along sagittal, transverse, and vertical axes were performed in T1-weighted and T2-weighted images using head coils.

The shunt endplate was identified in T1-weighted and T2-weighted images (head coils) as a low intensity (dark) circumlinear band at the superotemporal aspect of the eyeball, surrounded by a pocket of water density, corresponding to the filtering bleb. The anterior position of the endplate, and smaller volume of the orbital cavity (less available orbital space) were associated with higher IOP. Filtering bleb volume was inversely correlated with IOP. In the unsuccessful cases, filtering bleb was absent.

Magnetic resonance imaging provides insights into the mechanism of aqueous outflow and causes of failure of shunts. A lower orbital volume is associated with anterior position of the shunt endplate and poor shunt performance.