Thursday, January 15, 2009
The structure of the optic nerve indicates its function. This fits with the definition of glaucoma: a progressive optic neuropathy with functional vision loss. EMGT and OHTS studies both show that early detection and treatment can preserve vision.
Stereo photography is the gold standard for characterizing the optic nerve head, but only 50% of ophthalmologists routinely use this method. Visual field evaluation is of less importance since this occurs late in the disease process.
Imaging technologies that can actually automate the diagnosis for you are the best. GDx-VCC, HRT, OCT. Recently Fourier Domain (FD-OCT) has gained a lot of attention. These systems have about 65 times the speed at twice the resolution when compared to time-domain OCT. This greatly enhances the ability to visualize and identify anomalies in the structure of the optic nerve and the nerve fiber layer.
FD-OCT is particularly good at identifying all of the retinal layers allowing retinal layer segmentation. The three innermost layers of the retina (ganglion cell layer, nerve fiber layer, and the inner plexiform layer) can be visualized and changes identified.
The Advanced Imaging Glaucoma Study is the first to review all of the newest technologies vs. the time-tested methods of fundus photographs and visual field testing. AIGS is a multicenter, 5 yr, prospective, longitudinal trial, looking at glaucomatous changes at 6 month intervals.
Early results show that macular degradation is measurable in glaucoma as evidenced by thinner inner retinal layers. This corresponds to a visual field defect and is this predictive of functional loss? The correlation is good, but not as good as the circumpapillary retinal nerve fiber layer since this measurement takes data from more of the retina. Combining these data improves the diagnostic ability of these tests.
Monitoring these patients over time will help discern whether these changes reflect these early stage glaucomatous progression.
FD-OCT allows for Doppler measurement of retinal blood flow. This may afford the ability to detect NTG pts. Several studies have shown that ocular perfusion, particularly in patients with low diastolic pressures, are at increased risk of developing glaucoma. So Doppler OCT around the optic nerve can be of use in determining retinal blood flow. Adaptive Optics with OCT should enable the visualization structures in even greater detail by compensating for aberrations.