Section 1-A: Introduction
The eye is a fascinating organ which allows us to make visual observations about the world around us (Figure 1-1). The complex images that we see are transmitted to the brain for processing by way of the optic nerve. Approximately 1.2 million nerve fibers, or axons, make up each human optic nerve (Figure 1-2). The optic nerve travels from the back surface of each eye and joins together to form the optic chiasm. The nerve fibers that entered the chiasm as part of the optic nerves, travel from the chiasm to the lateral geniculate body but are now called optic tracts. Finally, visual input is transmitted from the lateral geniculate body and travel as optic radiations to an area of the brain called the occipital lobe. Visual images from the retina (the “film” of the eye) travel through the optic nerve, optic tract, and eventually to the visual part of the brain. There the images are processed and interpreted by the brain. Any disease process which affects the optic nerve could disrupt this input, leading to visual loss.
Figure 1-1. The eye
Figure 1-2. This is a healthy optic nerve. Notice the healthy rim of nerve tissue with a central cup.
The optic nerve can be damaged by a number of disease entities. The general term for any condition that damages the optic nerve is “optic neuropathy.” Optic neuropathies may be caused by vascular (blood flow) problems, inflammation/infections, metabolic disorders, trauma, tumors, or hereditary diseases. In each case, the result of the disease process is visual loss (temporary or permanent), which can affect central and/or peripheral vision. Each type of optic neuropathy has characteristic signs and symptoms of visual loss. Some cases are temporary, while others are permanent. Some are less noticeable by patients, especially when the peripheral vision is affected rather than central vision.
Ophthalmologists (M.D.s specializing in medical and surgical care of the eye) are trained in differentiating various optic neuropathies based on signs and symptoms. Neuro-ophthalmolgists are ophthalmolgists with additional training in various diseases of the optic nerve. Similarly, glaucoma specialists are ophthalmologists with additional training in diagnosis and treatment of glaucoma (Figure 1-3).
Figure 1-3. Examination of the eye with a goniolens to evaluate glaucoma
Figure 1-4. Aqueous Fluid Flow in the Eye. The aqueous fluid that fills the eye is produced by the ciliary body and flows between the iris and lens, through the pupil and to the drainage angle at the junction of the iris and the cornea. Aqueous fluid exits the eye through a tissue called the trabecular meshwork in the drainage angle.
Glaucoma is one type of optic neuropathy. It is a disease of the optic nerve, often caused by high intraocular pressure (IOP) resulting from poor drainage of fluid from the eye. There are various types of glaucoma but all share the common feature of optic nerve damage which causes irreversible vision loss. Glaucoma differs from other optic neuropathies by a few key features. The main distinguishing feature of glaucoma is “cupping of the optic nerve,” which describes the appearance of the optic nerve when it is damaged by glaucoma.
Another distinguishing feature of glaucoma is that the treatment is aimed at lowering the intraocular pressure (IOP). The eye has a certain pressure determined by the production and drainage of fluid (called aqueous humor) within the eye. The eye fluid is continuously produced and drained from the eye (Figure 1-4). In patients with glaucoma, normal drainage is impaired, often leading to elevation of IOP.
Section 1-B: The Meaning of Cupping
The term cupping of the optic nerve describes the appearance of the optic nerve to the examining eye doctor. When the nerve is viewed through the pupil, it looks like a cup seen from above. The cup is really an empty space in the middle of the optic nerve surrounded by optic nerve fibers. With the loss of nerve fibers from glaucoma, the cup becomes progressively larger because there is less space occupied by the remaining nerve fibers. A healthy optic nerve has many nerve fibers traveling through it (approximately 1.2 million fibers), so there is usually a small cup. As progressively more nerve fibers are damaged, less nerve fibers remain. Consequently, the cup enlarges. The amount of cupping is often described by the eye doctor as the “cup-to-disc ratio” (C/D ratio, Figure 1-5).
Figure 1-5. . The optic nerve is divided into tenths and the cup is compared to the entire optic nerve (optic disc) to obtain the cup-to-disc ratio. This C/D ratio here is 0.4.
Figure 1-6. Normal optic nerve - 0.3 C/D ratio
Figure 1-7. Glaucomatous optic nerve – 0.9 C/D ratio
The C/D ratio attempts to quantify the extent of axonal (nerve fiber) loss. It compares the diameter of the cup to the entire diameter of the optic nerve head (disc). The normal C/D ratio is typically less than 0.5 (meaning that the diameter of the cup is 50% of the diameter of the optic nerve head, Figure 1-6). This number is not a uniform number, because eyes with glaucoma tend to have more cupping in the vertical orientation than the horizontal orientation due to a difference in axonal support structures. Therefore, a measurement in just one orientation may not fully describe the extent of cupping. Eye doctors will often draw pictures and take photographs to accurately document the appearance of a glaucomatous optic nerve (Figure 1-7). Increases in cupping or nerve fiber loss indicate poorly controlled glaucoma.
In glaucoma the position of the blood vessels within the optic nerve can shift with the progressive cupping, and this can be an important clue that the glaucoma is continuing to cause optic nerve damage. Other exam findings suggestive of glaucoma include hemorrhages (bleeding) on or near the optic nerve. This is commonly seen in poorly controlled glaucoma and is indicative of ongoing optic nerve damage or unstable glaucoma.
If glaucoma affects only one eye, there may be asymmetry of the optic nerve cupping between the two eyes. The unaffected optic nerve will look normal while the affected eye will look cupped. Asymmetry of the optic nerves is another clue that can aid in the diagnosis of glaucoma (Figures 1-8, 1-9).
Figure 1-8. There is a slight asymmetry of the cup between these eyes. The right eye (pictured at left) has a slightly larger cup
Figure 1-9. There is a slight asymmetry of the cup between these eyes. The left eye (pictured at right) has a slightly smaller cup
In end stage disease, the nerve may be completely cupped, with no nerve fibers left. In this case, the nerve may appear pale and vision would be poor. While most patients that receive treatment will not progress to end-stage glaucoma, those that do may eventually become totally blind.
It is rare that other optic neuropathies result in cupping of the optic nerve. Most of the time, optic nerve head cupping is caused by glaucoma, and the amount of vision loss corresponds to the extent of cupping.
Section 1-C: Understanding Vision Loss from Glaucoma
Central vision is the fine vision people use to read and recognize faces, while peripheral vision is the side vision that is used for navigating obstacles in the environment (like doorways and coffee tables) and for detecting oncoming vehicles from a side street. The diagnosis of glaucoma is often made late in the disease course, because early stages of glaucoma are usually characterized by loss of peripheral vision and sparing of central vision. Patients often fail to notice peripheral vision loss until it has progressed towards the center of vision. Unless the patient happens to be examined by an eye doctor, they could be unaware that they have glaucoma. The only way to be diagnosed with glaucoma in early stages is to be examined by an eye doctor and undergo an eye exam and visual field testing (Figure 1-10), which measures the amount of vision loss from optic nerve damage (Figures 1-11 to 1-14).
Figure 1-10. Visual field testing with a Humphrey Field Analyzer device shown here is a standard method for monitoring glaucoma.
Figure 1-11. Normal Humphrey visual field test. This is a side vision test which is performed to diagnosis glaucoma. There are no dense black spots indicating vision loss.
Figure 1-12. Superior arcuate visual field loss from glaucoma – as tested by Humphrey visual field device. The black areas represent abnormal blind spots corresponding to areas of vision loss.
Figure 1-13. Superior and inferior visual field loss from glaucoma causing tunnel vision. Despite the peripheral vision loss, central vision is still intact and the vision may be 20/20.
Figure 1-14. End stage glaucoma with dense superior and inferior visual field defects – as tested by Humphrey visual field device. Central vision may be affected and there is severe visual impairment.
The nerve fibers of the optic nerve spread from the nerve head and arch superiorly and inferiorly, meeting at an anatomic line called the horizontal raphe (Figure 1-15). Glaucoma tends to affect the superior and inferior parts of the optic nerve first, thereby producing arching or arcuate visual field defects. This explains the characteristic pattern of visual field loss in glaucoma. Because of the anatomy of the eye, when the superior part of the nerve is damaged, patients will develop inferior visual field loss. Conversely, when the inferior nerve is damaged, patients will develop superior visual field loss. Since the center vision is usually spared, visual acuity may be 20/20 until the center vision is finally affected late in the disease course. In certain forms of glaucoma the central vision can be affected early in the disease process. When the central vision is affected, patients will often notice the vision loss and seek medical help. In this case, patients may notice a black (blind) spot in their vision.
Figure 1-15. A diagram of optic nerve fibers in the retina existing at the optic disc in the retina (right eye)
Section 1-D: Implications of a Glaucoma Diagnosis
Once a diagnosis of glaucoma is made, there is often fear of blindness and uncertainty about what will happen in the future. Glaucoma is a disease which cannot be cured, but it can be effectively treated to prevent further vision loss. People often fear that they will go blind or that they will have a visual disability. While there are certainly patients who are visually impaired due to glaucoma, this is generally not the rule. If glaucoma is diagnosed and treated early, visual impairment may be limited or minimal.
Once optic nerve tissue is damaged, it is not possible to regenerate the damaged nerve fibers. The existing visual field defect or decline in vision from glaucoma is permanent. The goal of treatment, therefore, is to preserve the existing optic nerve. Further optic nerve damage is prevented by lowering the eye pressure. This is done with eye drops, laser treatments, or surgery.
The importance of regular examination schedule with an eye doctor knowledgeable in the treatment of glaucoma is a key factor in preserving sight in glaucoma (Figure 1-16). Not only is continuous and periodic monitoring important, but also adherence to prescribed medical or surgical treatment is critical in preserving the sight. Careful adherence to a schedule for taking glaucoma medications and other treatment regimens is often in the hands of the patient (or patient caretaker) and is the key to treatment success.
Figure 1-16. Eye exam
Chapter 1. References
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