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During a comprehensive eye examination your doctor should check your pressure
(above 21mmHg becomes suspicious), examine the front and backside of your eyes. Your doctor will be looking particularly at your optic nerve for signs of damage. Damage to the optic nerve takes on a certain “look” or appearance. The optic nerve is really the collection of the living nerve fibers from the retina as they exit from the eye and travel to the brain.
The optic nerve represents the entire collection. There is an exit hole from the eye for these and it is at this point where the damage can be found. It is called the optic nerve head. The optic nerve head has a contour much like a doughnut. The tissue fibers from the retina collect around the edges to create the “rim”. When damage occurs from high pressure the amount of tissue that makes up the rim diminishes and the central portion of the optic nerve, which is called the “cup”, becomes larger. The cup represents the leftover space of the exit hole from the eye that has no nerve tissue. So when the nerve degenerates and thins this “leftover” space increases. It is this phenomenon that your doctor is looking for.
Notice in the above samples the cup to disk ratio of the nerve on the left is a smaller ratio of about 0.5 while the nerve on the right has a higher ratio (of about 0.7) and is more suggestive of glaucoma.
Optic nerve head imaging to both document and measure the thickness of the tissues that make up the optic nerve and the tissues surrounding it are an ever more important part of the detection and surveillance of glaucoma. Photographs represent the “gold standard” of glaucoma monitoring. All subsequent technologies are continuously compared to “stereoscopic optic nerve photographs”. With stereoscopic photographs (much like a 3D movie), 2 images of the nerve are captured at the same time and this results in the ability of the doctor to visualize your optic nerve with 3 dimensions. This 3 dimensional view allows characterization of the surface undulations of the optic nerve that can take place over time to help in decision making regarding the therapy for glaucoma.
Optical Coherence Tomography (OCT), Confocal Scanning Laser Ophthalmoscopy (CSLO), and Scanning Laser Polarimetry (SLP) are 3 of the more common forms of computerized devices used to quantify the optic nerve head neuroretinal rim and retinal nerve fiber layer (RNFL).
The image above is an example of the type of information obtained with the OCT. The retinal nerve fiber layer (RNFL) is the layer of the retina as it leads into the optic nerve that specifically undergoes degeneration and thinning as glaucoma damage progresses. In this image the top of the RNFL is delineated by a “white line” (Red Arrow) and the bottom of the RNFL ends at the “black line” (Yellow Arrow). This information is then compared against a collection of normal versus glaucomatous eyes and against any historical patient about an individual patient to help the doctor make decisions.
As demonstrated in the figure above, in addition to the Retinal Nerve Fiber Layer Analysis most of our computerized measurements contain the ability to directly analyze the shape and make-up of the optic nerve head itself. This is another direct benefit in detection and progression and augments the more subjective stereoscopic optic nerve photographs mentioned earlier.
Another of the most specific examinations performed by your doctor for glaucoma is called gonioscopy. Gonioscopy is the examination of drainage angle of the eye. The darins of the eye, known as the trabecular meshwork are situated in the very front corner and cannot be seen without special lenses or mirrors. The act of using one of these special devices to the drains is called gonioscopy.