الفهرس 
IntroductionOnly 14 pages are availabe for public view
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Abstract
Optical coherence tomography (OCT) became one of the most effective techniques that has appeared in ophthalmology in recent years for study of retina and optic nerve. Time domain OCT (TD-OCT) was the first generation of this technology and was based on the interferometer, which sequentially acquires the information of one A-scan after the other. Spectral domain OCT (SD-OCT) is an improvement from the original TD-OCT. SD-OCT uses a fixed mirror and a spectral tonometer as a detector. With this new method all A scans are acquired almost at the same time. This reduces the test time and increases substantially the number of A scans acquired, allowing for improved coverage of the retina and creation of three-dimensional datasets. In addition, SD-OCT obtains images of higher resolution than TD-OCT with a significant reduction of artifact from ocular movements. Recent development of an ultrahigh-resolution (UHR) OCT in high-speed OCT devices not only enables intraretinal imaging comparable to conventional histopathology but also functional retinal blood flow similar to Doppler ultrasound. In optic nerve disorders four OCT scanning techniques are helpful. The first technique is Peripapillary RNFL scan which provides cross-sectional images of peripapillary retina and a RNFL thickness analysis. It is useful for evaluation of glaucoma, optic atrophy or optic nerve edema. The second technique is Optic nerve head scan which provides a cross-sectional image of ONH. It is useful for the evaluation of glaucoma, papilledema or congenital ON anomalies. The third technique is Macular scan which provides a cross sectional image through fovea and a macular thickness analysis. It can define macular disease that may mimic optic neuropathy. It can diagnose glaucoma by detecting loss of ganglion cells. While the fourth technique is Papillomacular Scan which provides cross-sectional visualization of the full thickness of retina and ON. It is useful for evaluation of the optic nerve-retinal junction where subretinal fluid may accumulate such as in optic nerve pits. In Glaucoma, OCT can detect RNFL and ONH changes years before visual field loss. It is useful for monitoring and treatment of glaucoma patients. In addition to the classic structural changes of the RNFL, SD-OCT can image exposure of second-order blood vessels associated with RNFL thinning. It can also image changes in ONH blood vessels such as double-angulation and baring of the circumlinear ONH blood vessels. The higher speed and resolution of SD-OCT improved the repeatability of macular imaging compared with standard TD-OCT. The improved diagnostic power of macular ganglion cell complex imaging is on par with, and complementary to, peripapillary NFL imaging. Macular imaging with SD-OCT is a useful method for glaucoma diagnosis and has potential for tracking glaucoma progression.
In papilledema, OCT play an important role in following the evolution of the edema and monitoring the effects of treatment. OCT can demonstrate Macular abnormalities that may affect vision in patients with papilledema such as macular edema. It can also distinguish mild papilledema from pseudopapilledema.
Also, OCT is beneficial in optic neuropathies. In anterior ischemic optic neuropathy, OCT reveals increased RNFL thickness consistent with optic disc and parapapillary edema in acute phase, followed by marked thinning of RNFL in chronic phase. OCT can show subretinal fluid in patients with AION. In patients with optic neuritis, OCT can demonstrate mild RNFL thickening or optic disc swelling in acute stage, even when swelling is not seen clinically. A significant reduction in RNFL thickness and macular volume can be seen by OCT 3-6 months later. OCT can serve as a biomarker for the progression of Multiple Sclerosis.
OCT showed that RNFL is thickened in acute Leber hereditary optic neuropathy, whereas it is markedly thinned in atrophic cases. In patients with toxic/nutritional neuropathy, RNFL thickness measured by OCT may be useful to predict the visual prognosis. The severity of RNFL loss is proportional to the visual prognosis. Also, OCT can identify compressive optic neuropathy when there is RNFL loss, and this may be the first clue that there is an intrinsic or extrinsic ON tumors. While in traumatic Optic Neuropathy OCT can demonstrate RNFL thinning. The location and amount of RNFL thinning depend on severity of trauma and extent of visual loss.
In congenital alterations of the optic nerve, OCT has great use in the qualitative as well as quantitative analysis. In optic disc drusen, OCT offers a major contribution in the evaluation, measurement, and follow-up of the RNFL that eventually can be affected. While in optic disc pits, OCT confirmed the bilaminar nature of the associated maculopathy due to primary inner-layer schisis and secondary outer-layer detachment. Also, OCT has a role in diagnosis of Morning Glory Disc Anomaly and detection of associated retinal breaks. It provides good guidance in subsequent surgery and in confirming closure of the break. OCT is also valuable in Superior Segmental Optic Nerve Hypoplasia (SSOH) as it can differentiate SSOH from glaucomatous optic neuropathy.
In Melanocytoma of the optic nerve, OCT is useful for obtaining the exact measurement of the size of the lesion and may be used as a tool to follow the tumor’s growth, local infiltration, and occasional neurosensorial retinal detachment. In patients with Optic nerve sheath meningiomas, OCT shows thinning of RNFL layer.