Ex-PRESS glaucoma shunt safe with magnetic resonance imaging

The Ex-PRESS shunt is used for the surgical management of glaucoma. Since the itis made of stainless steel, the question of its safety with magnetic resonance imaging (MRI) is a recurring one faced by the surgeons using it.  Based on in-vitro tests exposing the Ex-PRESS to magnetic fields of 2 Tesla MR, the manufacturer has stated that the device can be exposed to MRI testing. The present study tested the effect on the Ex-PRESS of a higher magnetic field level (3 Tesla) in four environments: a wet dish filled with water; a dry dish; floating freely in the anterior chamber of a human cadaver eye; and implanted through the sclera of a human cadaver eye.

When able to move freely, as in the wet dish, the device moved when exposed to the MRI, indicating that it will respond to a magnetic force. But the study showed that even a small amount of friction between the device and its surroundings prevents itfrom moving even when exposed to high levels of MRI  This was evidenced by the three other tests, in which no movements by the Ex-PRESS were noted.

Since the present method of implantation is under a scleral flap, creating a great deal of friction, this study's findings  confirm the safety of the Ex-PRESS with MRI testing.The findings also  point to the safety of the device if placed directly under the conjunctiva, the originally proposed method of implantation. Since a second device may be placed in one eye, the effect of MRI on two Ex-PRESS shunts may also need to beexplored.