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Exam |
One of the most important tests for determining if a patient has glaucoma is the evaluation of the optic nerve head. Studies have shown that careful evaluation of the optic nerve head has high specificity and the best precision for glaucoma diagnosis of any single test.(2,3) The critical evaluation of the optic nerve head must be performed with equipment that allows stereoscopic viewing of the magnified optic nerve head with high contrast. The binocular indirect ophthalmoscope, while allowing stereopsis, does not offer high enough magnification to provide the details necessary for complete evaluation. The best method readily available to the clinician for performing this examination is high plus lens fundus biomicroscopy. During this examination the patient's pupils must be maximally dilated with a combination of mydriatic agents such as 1% Tropicamide and 2.5% Phenylephrine.
Optimal magnification can be achieved by using a +60D lens which provides 1.5 times the magnification of a 90D lens. Although a yellow lens may be helpful in increasing patient comfort, it may mask some early color changes found in the glaucomatous optic nerve head. For this reason, the high plus lens should be clear. To ensure that the maximum benefit is achieved from the use of the high plus lens, it is crucial that the biomicroscope is appropriately set for the examiner. Even a slight variation from the correct interpupillary distance setting can affect the examiner's stereopsis. Additionally, the angle between the illumination system and microscope system should be no more than 10 degrees to insure stereopsis. It is important to remember that the image seen through a high plus lens is a virtual image, and will be inverted with the right on the left, and the top on the bottom.
Being familiar with the normal optic nerve head is essential in order to critically examine the nerve for the changes typical of glaucoma. Although there is some variability, the normal optic nerve contains approximately one million axons which leave the eye in multiple bundles through the lamina cribrosa. These axons, entering the optic nerve, are made up of the nerve fiber layer of the retina. The convergence of the fibers creates a circular depression in the optic nerve head which is known as the cup. The size of the cup is compared to the size of the disc and is dependent both on the number of nerve fibers leaving the eye, and the size of the disc. Patients with a decrease in the number of nerve fibers leaving the eye, as occurs in the generalized loss of axons in moderate to severe glaucoma; or a larger sized optic disc with all of the axons intact, will both have cup to disc (C/D) ratios larger than normal. Equally true, a small disc will have very little cupping even if a loss of axons has occurred.(4) Because of the effect of the nerve size on the cup-to-disc ratio, it is very important to evaluate the optic nerve head size before determining the C/D ratio.
The normal optic nerve diameter varies in size from 1.2 mm to 2.5 mm with the average being 1.88 mm vertically and 1.77 mm horizontally. The area of the disc varies in normal patients from 0.92 to 5.54 square millimeters with African-Americans having a significantly larger disc area than Caucasians.(4,5) This variation in normal optic disc sizes can affect the cup/disc ratio in two ways. First in a larger disc, there is more area for the nerve fibers to fill, allowing for a larger cup. Secondly, the size of the disc is used as the denominator of the ratio. It has been suggested, that patients with larger optic disc size may have an increased susceptibility to glaucoma. However, a recent study performed by Jonas, et al comparing the visual field loss to the optic disc size for each eye in Caucasian patients with less than 8D of myopia found no correlation between the size of the optic nerve and the likelihood of visual field loss.(6) Nevertheless, the importance of a large or small C/D can only be determined when considered in the context of the size of the optic nerve head. Therefore, it is essential to evaluate the size of the optic disc in all patients who are glaucoma suspects. An easy way to do this clinically, is to use the direct ophthalmoscope. A relatively normal sized optic nerve head will be approximately equal to the spot size projected onto the retina through the small aperture of the scope.
Knowing that each optic nerve head is normal in size also allows the examiner to be sure that any asymmetry in C/D ratios between the two eyes of a patient is due to a difference in the number of axons. Asymmetry in C/D size between the two eyes of a normal patient has been shown to be rare.(7,8) Cup-to-disc ratios differ by 0.2 or less in 96% of normal eyes, so asymmetry of more than 0.2 in a patient with symmetrical disc sizes and no anisometropia, greatly increases the suspicion of glaucoma.(7)
A study of cup to disc ratios performed in the 1960's indicated that only 7% of the normal population had C/D ratios of 0.5 or greater. This study also indicated that 86% of normal C/D ratios were below 0.4.(9) Because of this study, anyone with a C/D greater than 0.4 was automatically considered a glaucoma suspect. Recent studies have changed our views. Evaluation of the contour of the cup with stereoscopic viewing and image analyzers has shown that the average C/D ratio is quite a bit larger than previously thought.(8) In a recent study of 4877 normal individuals, the average horizontal C/D ratio was found to be 0.47 in Caucasians and 0.57 in African-Americans. The average vertical meridians found in this study, were 0.49 for Caucasians and 0.56 for African-Americans.(8) Another recent study supports these results with averages of 0.51 in the horizontal and 0.43 in the vertical meridian. In contrast to the study from the Sixties, this investigation indicates that in order to include 84% of the population, C/D ratios less than 0.74 horizontally and 0.64 vertically should be considered normal. Also, in this study, patients with steeply bordered cups were found to have even larger average C/D's, with a horizontal average of 0.65 and a vertical average of 0.57.(7)
These studies also show that when determining the amount of cupping, it is very important to evaluate the contour and not the pallor of the cup. This is because the optic nerve head damaged by glaucoma typically has cupping which is larger than the pallor, whereas the normal eye has cupping equal to the area of pallor.(10)
Because it is recognized that the average size of a normal C/D ratio is larger than previously accepted and that a large cup/disc size is not definitive as a diagnosis of glaucoma, less attention is being placed on the size of the cup, and more is being focused on the appearance and configuration of the neural rim tissue found between the cup and the edge of the disc. The rim tissue is often the first area to show changes in glaucoma, and must be examined very critically during an optic nerve head evaluation. The normal neuroretinal rim tissue is uniformly pink in color indicating good vascular perfusion. Because there is a round cup located in a vertically elongated oval optic disc, the width of the neural rim tissue varies by quadrant. In the normal eye, the inferior quadrant has the widest rim tissue with the superior portion second in width. The nasal tissue is slightly thinner than the superior tissue and the tissue in the temporal quadrant is the thinnest.(7) This variation in rim sizes will cause large physiologic cups to appear elongated horizontally.(10) A disc with the normal configuration of rim tissue despite a large cup/disc ratio can be seen in Figure 1. As mentioned previously, it is very important to remember that when examining the optic nerve head with a high plus lens , the superior rim tissue will appear inferior and the inferior tissue will appear on top in your view. The reversal will also affect the nasal and temporal rim, which will be switched in your view.
The rim tissue will thin as nerve fibers atrophy. This results in pallor in the area of atrophy and a decrease in the size of the rim tissue over time. If the nerve fiber loss is generalized, the atrophy of nerve fibers will cause an overall decrease in the width of rim tissue and an increase in the size of the cup. This generalized atrophy can be seen in Figure 2, and is typical in moderate to advanced glaucoma, with corresponding visual field loss. Because these changes are obvious only in the later stages of the condition, the increase in cup size is not very helpful in making a diagnosis of glaucoma early in the disease process.
The focal nerve fiber loss in early glaucoma is more subtle and requires close observation to detect. In early glaucoma, the inferior rim is usually affected first, with the superior rim a close second. The next tissue to be damaged is typically the temporal rim, with the nasal rim the last affected.(11) Thinning in one focal area of the disc can cause a "notch" to develop in the rim tissue over time. Since the inferior and superior rim tissues are affected first, notching is typically seen in one of these quadrants. When evaluating the optic nerve, it is helpful to have the results of a visual field test performed on the same day readily available. This allows the comparison of areas of potential visual field defects to the nerve fiber responsible for that area of the field. It is estimated that 20% of the nerve fibers must be atrophied to cause a visual field defect of 5 dB and 40% to cause a 10 dB loss. Because of this, visual field results, are best when used in conjunction with the optic nerve head and nerve fiber layer evaluation.(12)
Figure 3 shows obvious notching of the inferior optic rim tissue. Figure 4 shows a much more subtle area of notching in the 11 o'clock position. This notching would be very easy to overlook without the aid of the visual field. Figure 5 shows the visual field for the patient in figure 4. Although the patient performed poorly on this visual field, making the results of questionable value, it is interesting to note that there is a corresponding change in the patient's inferior nasal visual field, as would be expected from the optic nerve head appearance. When using the visual field as a tool to help evaluate the optic nerve rim tissue, it is essential to remember that the superior rim tissue consists of those fibers responsible for the inferior visual field. Because this visual field reversal matches the reversal in the view of the high plus lens, the area of rim tissue affected in the high plus lens view will be in the same quadrant as the visual field loss.
When both the inferior and superior rim tissues are damaged in glaucoma, vertical elongation of the optic cup occurs. This can be seen in Figure 6. This common vertical elongation of an optic nerve head with glaucomatous damage will have corresponding inferior and superior visual field defects.
Another optic nerve change which has significant diagnostic and therapeutic importance is hemorrhaging. A small disc hemorrhage, known as a splinter or Drance hemorrhage, is commonly associated with normal tension glaucoma. These hemorrhages typically appear blot-like when located on the disc, and more flame shaped if they are in close proximity to the disc. The occurrence of a disc hemorrhage such as the one seen in Figure 7, should make you suspicious of glaucoma. Although hemorrhages such as these can also be found in patients with a history of recent posterior vitreous detachment, branch retinal vein occlusion, or diabetic retinopathy, it is very rare for them to occur in the normal population.(13) The examiner must be particularly vigilant in cases where a splinter hemorrhage is noted in a patient with a history of branch retinal vein occlusion or diabetes as these patients are already at an increased risk of developing glaucoma. If other risk factors are present, the appearance of a disk hemorrhage is a strong indicator for the initiation of glaucoma treatment. They are more likely to occur on the temporal side of the disc, and can be found either superior temporal or inferior temporal with equal frequency.(13) Splinter hemorrhages have been shown to precede nerve fiber layer and visual field changes in some patients.(14,15) Although the hemorrhages can resolve in as short as 2 weeks or as long as 35 weeks, the average time to resolution is 10 weeks. It is quite common for an area of notching to develop after the resolution of one of these hemorrhages, and it is also common for the hemorrhages to recur in the same area, or within 30 degrees of the original location. When a Drance hemorrhage is found in a patient who has already been diagnosed, and is being treated for glaucoma, it indicates an unfavorable prognosis, and the need for more aggressive therapy.(14,15)
Peripapillary atrophy is also an important diagnostic consideration in glaucoma. Peripapillary atrophy appears as a zone of chorioretinal atrophy with large choroidal vessels and sclera visible around the optic nerve head. Further from the optic nerve head, a less pronounced area of irregular hyper- and hypopigmentation can be seen. As demonstrated in Figure 8, it is most common for this to occur on the temporal side of the disc. The area with sclera and large choroidal vessels visible as is seen in Figure 6, is called the central zone or "zone Beta" and has been shown to be more frequent and more extensive in eyes with high tension and normal tension glaucoma. In addition, the location and extent of peripapillary atrophy has been shown to correlate to visual field loss in both types of glaucoma. A recent study by Park et al showed that in low tension glaucoma, the appearance of the zone Beta peripapillary atrophy was actually correlated more closely with visual field loss than the appearance of the optic nerve itself.(16) It is believed that peripapillary atrophy is not caused by glaucomatous damage, but instead indicates that a patient is at an increased risk to develop glaucoma. Although its etiology is uncertain, peripapillary atrophy seems to be an indication that this area of the retina has a poor blood supply. To understand this, it is important to realize that studies have shown a thinned or absent choroid with no choroidal filling in zone Beta during the choroidal filling phase of fluorescein angiography. Since the prelaminar portion of the optic nerve head also relies on the choroid for its blood supply, a compromised choroid could cause ischemia of the optic nerve head in this area, making the axons more susceptible to damage.(16) In eyes with small cup to disc ratios, the appearance of peripapillary atrophy may be a more sensitive indicator of glaucomatous optic nerve damage than cup-to-disc ratios.(4) The appearance of peripapillary atrophy should raise the suspicion of glaucoma, and be used in conjunction with other test results when making clinical decisions on the diagnosis and management of glaucoma.(17)
The examination of the optic nerve head should also include an evaluation for the presence of acquired pits of the optic nerve. Acquired pits of the optic nerve (APON) appear as sharply localized depressions of the lamina cribrosa with a loss of laminar architecture. They are usually found in areas of pallor, and extend to the outer edge of the disc. They are more likely to be located in the inferior temporal quadrant, with the second most common location the superior quadrant. Although, it is not known whether these pits are related to intraocular pressure, a study by Javitt, et al in 1990, indicates they are more common in patients with low tension glaucoma than normals.(18) Like peripapillary atrophy, APON may represent an area of the optic nerve head that is more susceptible to axonal damage.
Although a single evaluation of the optic nerve head can be very useful in the detection of glaucoma, most glaucoma diagnoses require an observed change in the optic nerve head over time. Examples include rim tissue changes, the appearance of lamina cribrosa which was not previously visible, the shifting of a retinal blood vessel on the optic nerve head, or the development of baring of a circumlinear vessel. Because these changes typically occur gradually over several years, and because they can be quite subtle, no optic nerve head evaluation can be considered complete without documentation of the nerve head appearance with high magnification stereo photographs, detailed drawings of specific areas of concern, and good written descriptions.
Another method of monitoring and documenting the configuration of the optic nerve head is by using computer assisted imaging. These instruments measure the vertical and horizontal extent, depth, volume and contour of the cup. Although data is stored and easily recalled for evaluation of changes over time, these instruments have not achieved widespread use, possibly due to the large financial investment required.
Since the definitive diagnosis of glaucoma requires that the optic nerve be affected, the evaluation of the optic nerve head is probably one of the most important tests to perform when evaluating a patient for glaucoma. When carefully performed, the stereoscopic evaluation of the optic nerve has high sensitivity and specificity for the correct diagnosis of glaucoma, but like all other tests for glaucoma, it is best when used in conjunction with the results of many other tests.
Evaluating the Nerve Fiber Layer
Evaluation of the nerve fiber layer is another useful tool to aid in the early diagnosis of glaucoma. This is because nerve fiber layer defects can occur before visual field changes are found. The evaluation of the nerve fiber layer can be done with a bright light source like the binocular indirect ophthalmoscope, or at the biomicroscope with a high plus lens. A clear condensing lens should be used in either case. Red free light, which is absorbed by the pigment of the retinal pigment epithelium and the choroid, is used to provide a dark background. The normal nerve fiber layer reflects light and appears as a whitish haze over the darker underlying retinal structures. There will be a striated appearance to the nerve fibers, with thicker nerve fiber layers appearing brighter. Because the nerve fiber layer is thickest in the superior and inferior arcades closest to the disc, this area should be the brightest portion of the view. There will be less brightness in the thinner papillomacular region and the nasal side of the disc. Symmetry between the reflections in the superior and inferior arcades and between each of the patient's eyes is expected. A normal appearing nerve fiber layer can be seen in Figure 9. This is the same eye as is pictured in Figure 1.
When a patient has suffered nerve damage from glaucoma, darker areas or streaks will appear in the nerve fiber layer.(19) Dark areas which are slightly larger than arterioles and reach the disc following the normal course of the nerve fiber layer are called slit defects. They represent retrograde degeneration of the axons due to focal damage of the optic nerve at the lamina. These can occur in approximately 10% of normal patients.(19) Wedge defects are caused by atrophy of many ganglion cells in the same area of the optic nerve. These defects start at the disc as narrow lines and expand as they get further from the disc. A wedge defect can be seen between 4 and 6 o'clock in Figure 10. Notching of the neural rim tissue, as well as a visual field defect are often associated with wedge defects. The most common type of defect, diffuse atrophy, typically occurs in the superior and inferior arcades. The nerve fiber layer in these areas loses its consistency and looks like it has been combed or raked with darker and lighter areas. In severe cases nerve fiber layer reversal can occur in which the normal pattern of superior and inferior brightness with increasing dimness towards the papillomacular bundle is lost and the papillomacular area becomes the brightest structure. Nerve fiber layer reversal is associated with thinning of the neural rim and a diffuse depression or constriction of the visual field. An example of nerve fiber layer reversal is seen in Figure 11.
Although it can be difficult to see the changes in the nerve fiber layer, especially in lightly pigmented individuals, the technique provides additional early information for determining if a patient has glaucoma. Black and white photography of the nerve fiber layer provides more contrast of the tissue and improves the ability to compare the health of the nerve fiber layer over time.