Intraocular pressure measurement at the choroid surface: a feasibility study with implications for implantable microsystems

Aims-To demonstrate that a sensor, which is inserted through the sclera and placed in intimate contact with the choroid, can reliably detect changes i n the intraocular pressure (POP). Methods-A manometer was used to control the IOP of three cadaver eyes in st eps of 7 mm Hg. A piezoresistive pressure sensor was used to measure the pr essure at the choroid through a 2.5 mm diameter hole that was surgically re moved from the sclera. Data were collected for two configurations; with the sensor: (i) rigidly attached to a miniature positioning stage, and (ii) su tured to the sclera. Results-Both configurations accurately tracked the manometer pressure from 10 mm Hg to 47 mm Hg. For the fixed sensor cases, the average difference be tween the pressure measured at the choroid and in the anterior chamber was 0.8 mm Hg for the three eyes. For the sutured sensor case, the average diff erence was 2.1 mm Hg-although a significant portion of this was attributed to an initial offset. The standard deviations at each pressure level for al l of the choroid measurements were under 1.0 mm Hg. Conclusions-Small changes in IOP can be accurately measured by a sensor in contact with the surface of the choroid, for both a fixed sensor configurat ion and for a sensor sutured to the sclera. These results are the first ste p in the realisation of a surgically implantable microsensor to monitor IOP for patients suffering from low tension and other difficult to manage form s of glaucoma.