Faculty Bibliography

Glaucoma is an optic neuropathy in which irreversible loss of ganglion cells may precede the development of visual field defects and visible optic nerve changes. The diagnosis and management of glaucoma depends on accurate and reliable intraocular pressure (IOP) measurements, visual field analysis, and optic nerve head (ONH) evaluations. The ONH appearance has been used by clinicians since the time of Helmholtz and von Graefe to assess the status of glaucoma; however, the interpretation of the ONH is subjective, and there is wide variation among observers and even among examinations by the same observer.52,63,73 Stereoscopic ONH photography is one of the simplest technologies that can be used by the clinician, and its utility is extremely high. The cost is relatively low, and stereoscopic ONH photographs permit objective recording of the ONH appearance.63 These photographs allow the clinician to compare the appearance of the ONH in three dimensions between clinic visits. Despite the low cost and high utility, the Baltimore Eye Study74 illustrated the potential problem of photography in that high quality stereoscopic photographs could not be obtained in 47% of the glaucoma patients and 22% of normal subjects. Furthermore, stereoscopic ONH photography does not provide a truly objective system for interpretation of ONH appearance and change over time. ONH analyzers preceded the development of the confocal scanning laser ophthalmoscope (CSLO). The ONH analyzers were a necessary first step, but variability and lack of resolution as well as high cost eliminated nearly every device of this type including the Rodenstock Optic Nerve Head Analyzer (G. Rodenstock Instrument GmbH, Postfuch, Germany), Humphrey Retinal Analyzer, and the PAR IS 2000/Topcon IMAGEnet. Even the Glaucoma-Scope, which produced better reproducibility than its predecessors at a relatively low cost, is no longer commercially available. In an era of skepticism but great promise, most still consider visual field testing to be the reference standard, but the earliest visual field defect may indicate as much as 40% ganglion cell death.59 In addition, automated perimetry is a subjective psychophysical test that demands a high degree of performance for a long period and may not suit those individuals who may have advanced visual, mental, or physical deficits. Therefore, much attention has been placed on developing technology to assist in the early detection and progression of glaucoma. The purpose of this review is to give a brief yet comprehensive look at the technology and literature available to image the optic nerve head and the nerve fiber layer in glaucoma. Much of the current literature concerns the Heidelberg Retina Tomograph ([HRT], Heidelberg Engineering GmbH, Heidelberg, Germany), the Nerve Fiber Analyzer or GDx, and Optical Coherence Tomography (Humphrey Systems Inc, Dublin, CA). Consistent with the past, many new methods such as retinal thickness mapping, multifocal electroretinography (ERG), and pattern ERG have appeared trying to prove themselves, whereas the existing devices are constantly being updated with new knowledge and attempts to increase utility

PURPOSE/OBJECTIVE:To develop a protocol for testing glaucoma implant devices and to use this protocol to characterize devices currently available. METHODS:The following devices were obtained: Ahmed Glaucoma Valve Implant, Baerveldt Implant, Krupin Eye Valve, Joseph Valve, and OptiMed Glaucoma Pressure Regulator. Pressure per unit time was measured in real-time during ramped pressure perfusion of the implant devices with an open-manometer system. Each device was measured during a 10 minute interval while a constant flow of fluid was pumped into the system. The open manometer allowed the resistance of the devices to generate a pressure head which was monitored by computer. Four independent runs were averaged for each device. RESULTS:The Ahmed devices demonstrated reliable valve performance with a mean opening pressure of 13.65 mm Hg, a facility flow of 1.2 microL/min/mm Hg and a closing pressure of 6.1 mm Hg. The Baerveldt devices had a mean facility flow of 7.56 microL/min/mm Hg. The Joseph devices had the most variable performance, opening from 2.05 to 6.21 mm Hg. Two of 4 Joseph devices had high facility of flow (5 microL/min/mm Hg). The Krupin devices had a mean opening pressure of 6.25 mm Hg in 3 of 4 devices. The remaining Krupin device did not exhibit valve behavior. The OptiMed device did not reveal an opening pressure, and had a mean facility of flow of 7.08 microL/min/mm Hg for high flow and 6.20 microL/min/mm Hg for low flow. CONCLUSION/CONCLUSIONS:An objective test protocol for glaucoma implant devices was demonstrated. All implant devices had high facility of flow compared to the normal eye and can be grouped into very high facility (Baerveldt and OptiMed), and high facility (Ahmed, Joseph and Krupin). Only the Ahmed device had consistent valve behavior. None of the devices created enough resistance to explain long-term clinical failure of glaucoma drainage device surgery.