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laucoma is a chronic disease that begins as damage to the retinal ganglion cells and their respective axons.1,2 The gold standard for the diagnosis and treatment of glaucoma is based on the visual observation of the optic nerve head (ONH) and the evaluation of visual function using standard automated perimetry. The Ocular Hypertension Treatment Study (OHTS) showed that optic disc changes were present in more than half of the patients who progressed to a diagnosis of glaucoma, prior to detectable visual field loss. Significant ganglion cell loss has been shown to occur before standard automated perimetry can detect functional defects.3 Gold standards are slow to change, but newer imaging techniques can more accurately show and measure structural changes in the retinal nerve fiber layer (RNFL) and the ONH.
Current imaging techniques include confocal scanning laser ophthalmoscopy, such as on the HRT II (Heidelberg Engineering GmbH, Heidelberg, Germany). This device images the ONH and peripapillary RNFL using transaxial laser scanning. Another technique, scanning laser polarimetry, is employed by the GDx VCC (Carl Zeiss Meditec, Inc., Dublin, CA). This unit scans the RNFL using the birefringence of a laser beam. Finally, optical coherence tomography (OCT) is used by machines from Optovue Inc. (Freemont, CA), Heidelberg Engineering GmbH, and Carl Zeiss Meditec, Inc. This technique produces cross-sectional imaging of the retina and optic nerve using low-coherence near- infrared light.
All of these devices image and measure the ONH and/or the RNFL, although in different manners and slightly different areas. They have all evolved in their means of data acquisition, the way they process the data into images, and the normative databases they use to provide comparative values as they relate to glaucomatous damage. The results obtained with one device are not interchangeable with those from a different device. Each type of device has great diagnostic capabilities in terms of repeatability, sensitivity, and specificity. Most of the published studies regarding how well they identify structural damage, however, are based on patients with documented visual field loss. The ability of these products to diagnose glaucoma on their own has not yet been proven.
Two major studies that compared the aforementioned imaging modalities came to similar conclusions.4,5 All were excellent for confirming RNFL and ONH damage in known glaucoma patients or in those at high risk for the disease. The studies, however, suggested no significant differences in their ability to distinguish glaucomatous eyes from controls. Caprioli et al found that OCT may identify glaucomatous damage earlier than other imaging techniques in the perimetrically unaffected eyes of patients with primary open- angle glaucoma.6
The mentioned studies compared confocal scanning laser ophthalmoscopy and scanning laser polarimetry to early time-domain OCT (TD-OCT) devices, which were limited in their ability to measure the inner RNFL due to poor resolution and slow acquisition of the image. TD-OCT uses a mechanically scanning reference arm, which requires longer time to acquire scans and diminishes the image's resolution. Spectral-domain or Fourier-domain OCT (FD-OCT) provides faster imaging by employing a stationary reference arm to obtain an interference spectrum, which then undergoes Fourier transformation to allow for the simultaneous measurement of all of the echo time delay of light. The faster imaging speed of FD-OCT decreases motion artifacts and allows better image resolution. More recent studies have shown that FD-OCT can discriminate between normal eyes and those with glaucoma.7
AIDING GLAUCOMA DIAGNOSIS
Measuring macular thickness with FD-OCT can facilitate the diagnosis of glaucoma. Although earlier studies with TD-OCT showed that total macular thickness measurements are not diagnostic for the disease,8 diagnostic accuracy can be improved if macular thickness measurements focus on the inner retinal layers of the macula using FD-OCT.9-11 These retinal layers consist of the nerve fiber, ganglion cell, and inner plexiform and are referred to as the ganglion cell complex. It contains the axons, cell bodies, and dendrites of the ganglion cells, which are the cells most affected by glaucoma (Figure 1). Mapping the ganglion cell complex is on par with and complementary to RNFL imaging.
It is an exciting time in the development of retinal imaging. Today's devices can measure structural changes in the inner retinal layers with micron precision. Combined with improved software algorithms, it is only a matter of time until eye care specialists routinely use OCT measurements to detect glaucomatous damage. Eventually, OCT imaging will not only enhance standard automated perimetry's value in the diagnosis and management of glaucoma, but it may replace visual field testing altogether.
Robert Brass, MD, is an associate clinical professor of ophthalmology at Albany Medical College, and he is in private practice at Brass Eye Center in Albany, New York. He is a speaker for and is involved in clinical research with Optovue, Inc., but he acknowledged no other financial interest in the company or its products. Dr. Brass may be reached at (518) 782-7827; firstname.lastname@example.org.