Optic Nerve Head and Retinal Nerve Fiber Layer Imaging in Glaucoma Diagnosis

Glaucoma is a chronic, progressive neurodegenerative disease. It is defined as chronic progressive optic neuropathy with typical optic disk and retinal nerve fiber layer (RNFL) changes with corresponding visual field defect. Intraocular pressure (IOP) is considered only a major risk factor and not a diagnostic criterion. At present, clinically visible RNFL defects are considered as the sensitive indicator for early diagnosis of glaucoma. Experimental studies have shown, that localized RNFL defects can ophthalmoscopically be detected if more than 50% of the thickness of the retinal nerve fiber layer is lost.1,2 The gold standard for detection of the functional damage in glaucoma is automated perimetry. It has been reported that locations with a 5 dB decrease in sensitivity on WWP (white-on-white perimetry) already had a 20% loss of retinal ganglion cells; this was 40% at locations with a 10 dB decrease in sensitivity. As the optic nerve and RNFL damage is irreversible, the early diagnosis of glaucoma is important. Retinal ganglion cell loss is detected as: a. Thinning of the RNFL (RNFL defect) b. Changes in the shape of the ONH (neuroretinal rim loss). Currently, optic disk photography is considered gold standard to detect changes to the optic nerve head. Photographs should be taken at baseline and at follow-ups. However, for routine clinical use optic disk photography has several limitations. With the advent of high-resolution new imaging technologies like HRT (confocal laser ophthalmoscope), GDx VCC (scanning laser polarimetry), OCT (optical coherence tomography), it has become possible to objectively quantify RNFL thinning, which may appear before manifest field defects on WWP.3,4 The objective quantification with photography is possible but tedious, time consuming and requires training.