GLAUCOMATOUS CHANGES IN THE VISUAL FIELD
Damage in glaucoma can be conveniently divided into two types: structural and functional. Structural damage to the eye is seen as a characteristic abnormality in the nerve fiber layer or optic nerve, representing deterioration following ganglion cell loss. Functional loss is determined by a variety of tests that assess visual function, including visual field examinations.
ANATOMY OF VISUAL FIELD DEFECTS
Visual field defects reflect visual pathway abnormalities; their appearance should correlate well with the anatomic arrangement of neurons in that pathway ( Fig. 10-1 ). Glaucomatous field damage results from damage to the intraocular portion of the optic nerve extending from the retinal ganglion cells to just posterior to the lamina cribrosa.
TYPES OF VISUAL FIELD LOSS
Generalized, or diffuse, visual field loss is thought to be caused by a diffuse loss of axons, whereas localized defects result from loss or damage to a contiguous group of axons. The early visual field investigators recognized that generalized constriction, enlargement of the blind spot, and diminished night vision were all seen in early glaucoma. Unfortunately, these same findings occur with age and with other non-specific forms of visual field loss. Previously it was impossible to quantify these changes precisely enough to define normal limits and recognize variations from those limits. This was because isopter plotting with manual Goldmann kinetic perimetry has inherent variability that makes it difficult to distinguish or quantitate mild generalized loss. The quantitative measurements made by static automated perimetry, however, are ideally suited to comparisons between a patient and his or her age-matched normal. Thus we are better able to recognize and quantify diffuse visual field loss ( Fig. 10-2 ).
Localized defects (scotomata)
Scotomata, or localized depressions of the visual field, are more easily recognized than are generalized depressions because the normal neighboring field makes the defect stand out. The margins or walls of the defect may be steep or sloping. Scotomata are also described as absolute or relative. In an absolute scotoma, the brightest stimulus of the machine is not perceived. In a relative scotoma, the brightest stimulus is visible, but dimmer stimuli are not.
GLAUCOMATOUS VISUAL FIELD DEFECTS
Functional loss as determined by visual field testing has long been a diagnostic criterion for glaucoma. A variety of field defects are seen in early- and mid-stage glaucoma, all progressing to the dense defects of late-stage glaucoma. In a retrospective study of 102 glaucoma patients followed for at least 15 years, Eid and colleagues found that 29% of their patients had paracentral scotomas, 20% had nasal steps, and 18% had simple arcuate defects as the predominant diagnostic field abnormality.
Generalized depression can be an early sign of glaucoma, but it can also occur with aging, miosis, or hazy media. In kinetic perimetry, generalized depression is seen as a generalized constriction of the peripheral and central isopters. Unfortunately, this too is a rather non-specific finding. Kinetic perimetry, at least by manual methods, lacks the precision necessary to differentiate generalized depression from normal aging unless there is an obvious difference between the patient’s two eyes or the depression is substantial.
Generalized depression can increase the physician’s suspicion that glaucomatous damage has occurred, especially if it is unilateral or more pronounced in the eye with the higher pressure or larger cup:disc ratio. Interestingly, both the Humphrey and Octopus field machines use MD to represent the amount of generalized loss found in the field. On the Octopus machine, this stands for ‘mean defect’ ( Figs 10-3 and 10-4 ). If the patient has loss (i.e., a defect), then the MD has a positive sign indicating the presence of a defect. If the patient sees better than expected, the MD has a negative sign; a negative defect indicates above-normal sensitivity. On the Humphrey machine, MD stands for ‘mean deviation’ and measures the difference between the patient’s response and normal. If the patient has field loss, the MD has a negative sign; if the patient sees better than expected, the MD is positive – just the reverse of the Octopus nomenclature. Luckily it is easy to tell which system is being used clinically, and in common parlance an abnormal MD means that the patient has some component of generalized loss.
Irregularity of the visual field
There may be a lack of uniformity in the visual field. With computerized perimetry, this ‘roughness’ appears as a variation of decibel level among contiguous points that is greater than that anticipated in normal patients of the same age. These areas of loss appear non-uniformly throughout the field. This variation is expressed statistically as the standard deviation of the deviations found in the field (Humphrey) or the variance (square of standard deviation) of the mean of all points tested (Octopus). Humphrey uses the term pattern standard deviation , whereas Octopus uses the term loss variance . These functions are sensitive to localized loss but are relatively unaffected by generalized loss (see Figs. 10-3 and 10-4 ).
Nasal step or depression
The nasal portion of the visual field is often affected early in glaucoma, and defects may persist until the last stages of the disease. The nasal area is the most important region of the midperipheral and peripheral field to test. Depression may be evidenced by hesitancy in patient response when testing this area, as an inward turning of the isopter in manual perimetry, or by reduced sensitivity on static testing. If a true step that respects the horizontal raphe develops, a defect is present. Such defects may occur centrally ( Fig. 10-5 ), peripherally, or both ( Fig. 10-6 ) and may be isolated or associated with other Bjerrum area defects.
Temporal step or depression
A temporal depression or step may develop as an isolated finding or in conjunction with other glaucomatous defects. They may be detected at any stage of glaucoma but are more commonly found as a component of late-stage disease. Drance and co-workers suggest careful testing of the temporal area to recognize the occasional patients who may develop this condition as their only defect ( Fig. 10-7 ).
Enlargement of the blind spot
Enlargement and baring of the blind spot are considered non-specific changes that can occur in normal patients ( Fig. 10-8 ). If the blind spot enlarges in an arcuate manner, it is called a Seidel’s scotoma and may be seen in early glaucoma ( Fig. 10-9 ).
Isolated paracentral scotomata
Careful manual perimetry using combined static and kinetic techniques may demonstrate small paracentral scotomata. In a classic study, Aulhorn and Harms found similar small defects that did not connect to the blind spot in 20% of glaucomatous visual fields. Early glaucomatous defects may have a small, dense center. If the glaucoma is progressive, these defects enlarge, deepen, and coalesce over time to form arcuate scotomata. Inconsistency of responses in the paracentral area may be an early sign of glaucomatous change.
Static testing through these scotomata may confirm that they are true defects. The most commonly used computerized perimeters use the equivalent of the 30-2 spacing of test spots which are 6° apart; scotomata smaller than 6° may be missed. This is particularly critical in the paracentral region where even very small scotomata can be visually symptomatic. Spacing the test spots closer than 6° (for example 3° apart) increases the chances of identifying such scotomata but also increases the test time to an impractical level. If one is concerned about identifying or monitoring a paracentral scotoma, both the Octopus and the Humphrey have programs that increase the density of tested spots within the central 10° of the visual field – the G-1 or the 10-2 respectively ( Fig. 10-10 ).
Arcuate defects (nerve fiber bundle defects)
The arcuate scotoma represents a complete nerve fiber bundle defect. It begins at the blind spot, arcs around fixation, and ends at the horizontal nasal raphe. The defect may break through into the periphery nasally and then expand further to ultimately become an altitudinal defect ( Fig. 10-11 ). The arcuate defect as described by Bjerrum is a classic finding in middle- to late-stage glaucoma.
Central and temporal islands
In the later stages of glaucoma, most of the axons at the superior and inferior poles of the disc are destroyed, leaving only the papillomacular bundle and some nasal fibers. This destruction produces the characteristic end-stage field, with a small central island and a larger temporal crescent remaining. The central island may split fixation so that only fibers from half of the papillomacular bundle remain ( Fig. 10-12 ). Kolker found that patients with split fixation are more susceptible to central vision loss at surgery, although this is still a very rare outcome. These patients may need to have their pressures controlled in the mid teens or below to slow further progression.
Reversal of visual field defects
Fluctuation and increasing familiarity with the test or random chance may cause subsequent visual field examinations to appear improved. Nevertheless, at least slight reversibility of visual field defects seems to be a real phenomenon in occasional patients following therapy for glaucoma. The rule, unfortunately, is that glaucoma patients do not regain visual function under treatment, but rather they continue to lose field even when controlled. The rate of loss varies, and about 1 in 5 patients are stable over 20 years, but in general some degree of loss is usual. The rate and degree of loss in treated eyes are less than that reported in rare studies of untreated glaucoma.
ANALYSIS OF VISUAL FIELD LOSS
CHRONIC OPEN-ANGLE GLAUCOMA
Any of the preceding types of visual field loss may be seen in chronic open-angle glaucoma. In the early stages there may be a generalized depression that progresses gradually or sometimes in steps from paracentral scotomata to arcuate to altitudinal to end-stage defects. Defects usually become denser and then increase in area in one hemifield before progressing to the next hemifield ( Fig. 10-13 ). Scotomata may show episodic (stepwise), linear, or curvilinear progression.
Many recent investigations have suggested that the two forms of glaucomatous visual field loss, diffuse and localized, may have different pathogenic origins. It has been speculated that increased intraocular pressure (IOP) may cause diffuse loss but have less influence on the development of localized defects. Observer bias may have some influence on these findings, however, because patients with mild diffuse loss and normal pressure are often not identified as abnormal. Conversely, patients with elevated IOPs are examined closely because of the pressure, and, because suspicion is high, mild diffuse defects are recognized. Patients with dense localized defects tend to have localized optic nerve changes and may have visual field studies based on the appearance of the optic nerve. If the IOP is normal, a diagnosis of glaucoma is more likely when the field defect is local and dense rather than diffuse or non-specific. Drance, however, found that patients with increased IOP with localized defects in one hemifield had nearly double the amount of generalized reduction in sensitivity in the other hemifield compared with a similar group of patients with normal-tension glaucoma. Many others have investigated this issue, and there is general agreement that early glaucomatous field loss may appear in different forms.
A study by Gazzard and colleagues found that the pressure level at diagnosis correlated with the amount of visual field loss measured by Advanced Glaucoma Intervention Study (AGIS) score and by mean deviation (MD), but not by pattern standard deviation (PSD) or corrected pattern standard deviation (CPSD). In other words, higher presenting pressures were associated with the degree of diffuse damage but not with the degree of localized damage. The association was stronger for patients with primary angle-closure glaucoma (PACG) than those with primary open-angle glaucoma (POAG). This supports the concept that increased IOP is the proximal cause of damage in PACG, but that other factors may predominate in at least some patients with POAG. In both circumstances, the amount of field loss correlated well with the amount of optic nerve damage.
During the acute phase of angle-closure glaucoma in patients with high IOP, corneal edema and retinal ischemia can produce bizarre field defects that have little clinical value for following disease progression. After the pressure has been normalized, field defects may remain and may sometimes be extensive if ischemic atrophy of the nerve has occurred. In such cases, pallor of the nerve may be more severe than cupping. This is one situation in which glaucomatous field defects may not correspond well to the amount of cupping of the nerve head.
Other diseases may cause arcuate nerve fiber bundle visual field defects ( Box 10-1 ) that may be confused with glaucomatous damage. Generally, if excavation of the optic nerve does not correspond with the appearance of the field, other causes must be sought to explain the defect. If visual field defects occur or progress with normal pressures, normal-tension glaucoma may be the cause (see Ch. 17 ), but the examiner must be sure that other retinal or visual pathway lesions are not present, especially if the process is occurring unilaterally . Glaucoma is a jigsaw puzzle in which all the ‘pieces’ of the disease should fit. If a piece does not fit properly, the physician should be suspicious that it may belong to some other puzzle (disease). Generally, the configuration of the optic nerve and the appearance of the visual field correspond. Superior visual field defects are accompanied by erosion of the inferior portion of the optic disc and vice versa. The nerve in a patient with a temporal visual field defect should have a thinned nasal rim. Although normal-tension glaucoma may account for 10% or more of glaucoma patients, depending on definitions and the patient population being studied, IOP is elevated at some time in most glaucoma patients. If these factors do not occur in appropriate patterns, the possibility of glaucoma is not excluded, but the physician’s suspicions should be heightened and a thorough evaluation should be undertaken to exclude other possible diseases.
Myopic retinal degeneration
Retinal laser damage glaucoma
Optic nerve ischemia
Optic nerve compressive lesions
Drusen of optic nerve head
ESTERMAN DISABILITY RATING
Assessment of disability resulting from visual field loss is often needed, although it can be difficult to quantitate. The American Medical Association (AMA) adopted the Esterman disability rating. This binocular assessment used by government and industry is described more fully in the AMA Guides to the Evaluation of Impairment and the Physicians’ Desk Reference for Ophthalmology.
ANALYSIS OF COMPUTERIZED STATIC PERIMETRY
Computerized static perimetry provides numbers that represent the patient’s responses to stimuli in various areas of the retina. These numbers can be manipulated mathematically and statistically to provide information about the reliability of patient responses and test results. Although not identical, Goldmann visual field plots and computer-generated grey-scale visual field patterns usually are similar ( Fig. 10-14 ).
False-positive and false-negative responses
Reliability indexes usually include false-positive and false-negative responses and some analysis of fixation. False-positive responses occur when the patient indicates that he or she has seen a stimulus when one was not presented. This is usually a reaction to random noise generated by the perimeter. False-negative responses occur when the patient fails to respond to a stimulus that is at least as bright or brighter than one that he or she had previously recognized in that position. This indicates that the response was erroneous at least one of the two times that the position was tested. The lower the percentage of false-positive or false-negative responses, the more reliable is the test. False-positive or false-negative scores in excess of 20–30% indicate a test of questionable reliability.
Fixation reliability can be monitored in a number of ways. The technician can offer a subjective assessment of the patient’s fixation reliability; the computer may stop the test if a video or infrared fixation monitor indicates that the eye has shifted; or the blind spot may be stimulated periodically (Heijl-Krakau technique) with a bright stimulus, anticipating that the properly fixing patient will not see it.
All of these techniques have flaws. It is difficult for technicians to see tiny fixation shifts, and only with considerable experience are they able to judge the shifts’ effects on the test. In addition, it is practically impossible for a technician in a darkened room to maintain concentration on patients’ eye movements all day long. Automatic fixation monitors that interrupt the test can be quite precise; however, most patients cannot fixate ‘perfectly.’ Even the most attentive patient will have minor head and eye movements associated with breathing, heartbeat, etc. If the monitor is set to be very sensitive, the test will be prolonged by frequent interruptions. If the monitor is too insensitive, it has little value. Although constant monitoring is desirable, it is probably not necessary in most patients.
The blind spot is not constant. Only one of eight to ten presentations is directed at the blind spot, so the computer has no way of knowing about the patient’s fixation between those checks. If the computer incorrectly located the blind spot at the beginning of the test, subsequent checks might fall outside the real blind spot and give a false impression of bad fixation. If there is a large scotoma adjacent to the blind spot or a hemianopic field defect, fixation may be poor but the blind spot check will fall into the scotoma and falsely indicate good fixation.
Most patients either fixate well or poorly. Fixation behavior can be improved by encouragement from the technician, but the improvement may be small and inconsistent from test to test. Generally, the clinician needs to know the quality of fixation to help judge the accuracy of the field, and this can be provided by any of the preceding methods. Fixation losses exceeding 20% are considered poor in most circumstances, although the exact effect of such losses on the usefulness of the test is unclear and may vary substantially from patient to patient. Some studies have suggested that fixation losses up to 33% may still produce repeatable and reliable visual fields especially in an urban poor population.
Short-term fluctuation (SF) is measured by most computerized perimeters. This statistical analysis is the result of checking several loci in the visual field twice. The Octopus G-1 program tests each point in the central field twice. The variability that is noted between each of the double tests is reported as its root mean square and defined as SF. For most normal young subjects, overall SF is between 1.5 and 2.5 dB. Short-term fluctuation is affected by age and eccentricity from fixation. Although the overall SF printed on the field chart may be as high as 2.5 dB in normals, a fluctuation of 2.5 dB a few degrees from fixation in a young patient with clear media is unusual whereas fluctuation at 30° eccentricity in a normal 70-year-old individual may be 8 or 10 dB.
Short-term fluctuation is increased in glaucoma suspects, patients who cannot cooperate well for the test, and patients who have decreased sensitivity in areas of the visual field. It is increased dramatically in patients with significant field loss. Heijl and colleagues found SF in glaucoma patients to range between 8 and 18 dB. This indicates that points within the central field can lose and regain as much as half of their sensitivity between examinations due to SF alone.
Short-term fluctuation also provides a guideline for the amount of deviation required to indicate that the amount of depression exceeds the variability inherent in the test. At the 95% confidence level, this approximates two times the SF (roughly equivalent to two times the baseline noise). Thus, as a general rule, a deviation should exceed about 5 dB to be considered abnormal. This rule has several important exceptions, however. Because normal variation in the parafoveal region is much less than that in the midperiphery, deviations smaller than 5 dB can represent significant reproducible pathology when they occur near fixation. Conversely, as mentioned above, a deviation of 10 dB or more may occur at 30° in a normal middle-aged patient. In addition to the age of the patient and the location of the test point, the status of surrounding points can help determine whether a small deviation is significant. A mildly depressed point has a greater likelihood of being pathologic if its neighboring points are also depressed.
Fluctuation also increases locally in areas of reduced sensitivity. There are several possible explanations for this. It is well known that an area of inconsistency often precedes a permanent depression with Goldmann perimetry. Inconsistent responses on Goldmann perimetry appear as increased fluctuation on computerized automated perimetry. Another explanation is that the nature of the test for fluctuation – repeating the test twice during the examination – means that different areas of the field may be tested because of a small fixation shift. In an area of pathology, the second test can examine a slightly different area that legitimately has different sensitivity. The machine compares the first and second tests and indicates the difference between them as SF.
Long-term fluctuation is that which occurs between two separate visual field tests. This is discussed further in the section on recognition of change, p. 122–125.
Global indexes, which reflect the results of the visual field examination, are mathematic summaries of the actual sensitivity data produced by the examination ( Figs. 10-15 and 10-16 ).