Generalized 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 ).

(A) Computerized perimeters have their greatest value in having a normal database against which individual patients’ results can be compared. In this figure, the grey scale indicates a superior visual field loss. The total deviation plot (lower left), however, indicates central visual field depression as compared with normal eyes. The pattern deviation (lower right) indicates only a single spot adjacent to fixation that is reduced below normal. The pattern deviation has subtracted the general depression that is present to expose any scotomatous defect that may be deeper than the general depression. This patient actually does not have glaucoma but rather a cataract is causing this central depression. The superior field slopes more precipitously than does the inferior field, causing the grey scale to appear more depressed in that area. This does not necessarily represent pathology. The deviation plots indicate decreasing probability that a spot may be normal by increasing the density of the symbol. A totally black square has a probability of being normal of less than 0.5%. (B) This graph, taken from a Humphrey STATPAC printout, indicates how a patient’s visual fields compare with normal data. The horizontal line at the zero point represents a normal mean sensitivity level. Negative numbers extending down from that point indicate a mean decibel shift below normal. As can be seen, a mean deficit of slightly more than 3 dB occurs in less than 5% of normal eyes, whereas a mean deficit of slightly more than 5 dB occurs in less than 1% of normal eyes. This patient has had five visual fields. The first two were done with standard strategy. The third was full from prior strategy, which is not calibrated for STATPAC. The fourth had reduced reliability. The fifth was done with standard strategy. It appears that the patient started with a mean deviation of 28 dB, which is distinctly pathologic, and the field has worsened over time.

From the Humphrey STATPAC program.

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.

Generalized depression

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.

Humphrey field showing moderate to severe generalized depression. The mean deviation (MD; lower right) is –13.21.

Octopus field showing slight generalized depression. The mean defect (MD) is +4.1. MS, mean sensitivity; MS + MD, normal sensitivity for the patient’s age.

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.

Central 10° field from the right eye of a patient with advanced glaucoma. The nasal horizontal step (left side in this figure) runs all the way to fixation.

Note the inferior nasal step present on the peripheral and central isopters in this patient. There is an inferior arcuate scotoma present also.

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 ).

Visual field temporal defect. (A, B) Note the temporal wedge that occurred in this patient with erosion of the nasal aspect of the optic nerve.

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 ).

Visual field chart from the right eye of a 56-year-old woman with early glaucoma. Generalized blind spot enlargement is not necessarily a glaucomatous defect. This blind spot is roughly 14°×14°, with sloping margins. The normal blind spot is about 7° vertically and 5.5° horizontally, with sharp borders.

Chart from the right eye of a patient with normal-tension glaucoma showing a Seidel’s scotoma extending from the blind spot. There is also a small peripheral superior nasal step defect.

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 ).

Test grid pattern from the Octopus G-1 program. Twenty-one test points are placed in the central 12° rather than the 16 test points in the more standard Octopus 31 or 32 program. Humphrey has a similar program, the 10-2, which is very useful for identifying and monitoring paracentral scotomata.

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.

Field from the right eye of patient with advanced glaucoma. The superior arcuate defect has expanded to include the entire superonasal quadrant and includes half of the superotemporal quadrant as well. The inferior half of the field is much less severely affected.

End-stage defects

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.

Central island of a patient’s left eye with field defects encroaching on fixation. The temporal field is partially spared.

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.

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

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

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

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.