IMAGING OF THE OPTIC NERVE HEAD AND NERVE FIBER LAYER IN GLAUCOMA

Abstract

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 Study 74 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.