This allowed for measurement of the optic disc and surrounding retina, including peri-papillary retinal nerve fiber layer (RNFL) thickness and optic disc rim and cup sizes. One of the early applications of OCT was assessment for glaucoma by optic nerve head (ONH) imaging. OCT imaging has assisted significantly in the screening, early diagnosis, and management of glaucoma. SS-OCT sweeps across a narrow band of wavelengths with each scan, providing the highest axial depth, fastest image capture and best signal-to-noise ratios. TD analyzes interference patterns over time to generate an image while SD incorporates a spectrometer to conduct analysis over multiple wavelengths, with improved axial resolution and scan speed resulting in faster and denser sampling. The three primary FDA approved modalities of OCT are time domain (TD), spectral domain (SD) and swept source (SS). SD has the advantage of being able to acquire images more quickly than TD, allowing for more detailed imaging. TD analyzes interference patterns over time to generate an image, while SD incorporates a spectrometer to increase scanning time. The two primary modalities of OCT are time domain (TD) and spectral domain (SD). OCT quickly found applications in ocular imaging, particularly in the assessment of retinal disease and optic nerve pathology. This allows for rapid imaging of living tissue with micron-level resolution to depths of several millimeters without any ionizing radiation. It uses light beams and their pattern of back-scattering to build high resolution cross-sectional images of the retina and optic nerve. Optical coherence tomography (OCT) is an imaging modality first developed in the early 1990s. However, visible changes in appearance on exam are not always apparent or are non-specific. Defects in the retinal nerve fiber layer can be an early sign of glaucoma before other changes are visible. Alterations in the appearance of the retina and optic disc may be visible on exam, including changes to the neuroretinal rim and increased optic cup-to-disk ratio. However, visual field loss is not always apparent until later stages of the disease and IOP is not adequately sensitive or specific for screening purposes. Historically, screening for and diagnosis of glaucoma has relied on measurement of IOP, assessment of visual fields, and fundus biomicroscopy. Risk factors for the development of primary open-angle glaucoma include elevated IOP, age, family history, African race, myopia, thin central corneal thickness, and systemic perfusion pressure. The three main types of secondary glaucoma include neovascular glaucoma, pseudoexfoliation syndrome, and pigmentary glaucoma. Secondary glaucomas result from secondary factors that decrease aqueous drainage and can present with an open or closed angle. Secondary glaucomas account for approximately 10% of all glaucoma sub-types. The three major types of adult glaucoma are primary open angle glaucoma, normal tension glaucoma and angle closure glaucoma. Vision loss in glaucoma is characteristically insidious and begins with peripheral visual field involvement. It is a multifactorial disease, with several distinct pathophysiologies resulting in the same clinical syndrome of progressive optic neuropathy. Glaucoma is an irreversible progressive optic neuropathy involving damage to retinal ganglion cells resulting in gradual visual field loss.
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