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Glaucoma is the second most common cause of blindness in the world and affects more than 4.5 million people according to the World Health Organization. While the most common cause of vision loss remains because of cataract, glaucoma is an irreversible process and the visual impairment as a result of this disease is permanent unlike in cataract.
Although glaucoma is commonly understood as a process in which the pressure inside the eye is excessively high, the final common site of damage in this disease remains the optic nerve. There are two major categorizations in glaucoma, open angle and closed angle. The angle refers to the recess where the cornea and iris come together in the eye, tucking away the trabecular meshwork and the canal of Schlemm, both structures vital to the drainage outflow of fluid out of the eye, regulating the eye pressure. In open angle glaucoma, there is no obvious macroscopic obstruction to the drainage structures, but at the microscopic level, there is increased resistance to fluid exiting the eye leading to increased eye pressure ultimately damaging the optic nerve. In closed angle glaucoma, the iris and cornea are in such close anatomical proximity that it is a physical barrier that fluid must overcome to exit the eye. In extreme cases, the angle can be completely barricaded and the eye pressure rise suddenly and cause painful acute permanent vision loss. In the case of closed angle glaucoma, periods of very high eye pressure are equally as deleterious to the optic nerve.
In order to better aid in our understanding as ophthalmologists of disease processes and the optimal treatments for each individual, new technology and focus has been directed at advanced imaging of the optic nerve and of the angle of the eye. Several modalities are designed to objectively measure the optic nerve and the retinal nerve fiber layer: Heidelberg retinal tomography (HRT), optical coherence tomography (OCT), and scanning laser polarimetry (GDx). These diagnostic images are much like CT or MRI scans done on other parts of the body and allow the ophthalmologist to obtain 3-dimensional and topographical images, retinal nerve fiber layer thickness, neuroretinal rim, disc area, and many other parameters of the optic nerve by confocal scanning technology. These tests are especially powerful when done in a serial manner to allow the ophthalmologist to closely follow and monitor the optic nerve for any subtle changes or minute damage that maybe detected before revelation by clinical examination. Most importantly, these tests are non-contact, non-invasive, risk-free and do not involve any radiation like a CT scan would.
The anterior segment optical coherence tomography (AS-OCT) uses similar technology as the OCT to image the optic nerve, but with light at a different wavelength to image the front of the eye more effectively. This non-contact image allows the ophthalmologist to measure to the thousandth of a millimeter the width of the angle opening and make an accurate assessment of the risk of closed angle glaucoma. More importantly, the image obtained may greatly assist in determining the most effective and appropriate treatment for each individual patient whether it be medication, laser, or surgery. In addition, this device provides high resolution imaging of the cornea, the lens, artifical lenses implanted after cataract surgery, and any disease process present in the anterior chamber of the eye.
While clinical acumen is still of paramount importance and it is certain that no technology will replace a detailed and expert eye examination, it remains true that earlier diagnosis and detailed imaging is a great help to any physician. With glaucoma still a major worldwide affliction in the 21st century, this new and emerging imaging technology has added a new tool in the arsenal for the diagnosis and management of this debilitating disease providing patients and ophthalmologists alike with a bright hope for the future.
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