Ординатура / Офтальмология / Английские материалы / Visual Fields Examination and Interpretation_Walsh_2011

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In general, high false-positive responses are not physiologic and greatly affect the reliability of the field. Fortunately, this problem can often be mitigated with instruction and encouragement of the patient by the perimetrist.

4-4-1-4 False-Negative Responses. In most respects, the patient with high falsenegative responses is the opposite of the patient with high false-positive responses. Instead of being “trigger-happy,” the former patients do not respond to stimuli that are much brighter than already measured thresholds. This situation correlates with technician comments such as “patient tired” or “patient falling asleep.” These patients often become increasingly fatigued during the test, with a tendency toward clusters of reduction in sensitivity toward the edge of the field, the final area tested. The total deviation plots become falsely abnormal, while the pattern deviation plot may be artifactually improved. MD is shifted more negatively. GHT may produce the message outside normal limits or on the borderline or with a general reduction in sensitivity. An extreme example is the cloverleaf field, in which the patient responds to the four primary seed location determinations and then becomes progressively less responsive (Figure 4-6A and B).

A major interpretation problem can occur because bona fide field loss secondary to disease tends to increase the remaining field fatigability. High false-negative responses can thus be physiologic and the field results reliable. Long programs can cause fatigue and reduce reliability. Critics of a fatigue-reducing program like FASTPAC or SITA Fast claim that it underestimates true defects, although perhaps it is the full threshold strategy that is overestimating defects by producing a perimetric “stress test.”

Some clues may help the clinician distinguish between a normal field that looks abnormal because of patient fatigue or frustration and a truly abnormal field, with high false-negative responses as one of its features. True defects are more reproducible and tend to respect anatomic distributions (e.g., arcuate, paracentral), while pseudodefects tend to occur variably in the periphery. Either defect type can produce dense constriction with only a central island of vision remaining. Performance of a 10° program on a subsequent day or after the patient has had a break often disproves or confirms the defect’s presence (Figure 4-7A and 7B). Similarly, the patient can be retested with a less fatiguing test such SITA Fast or with manual kinetic perimetry. Finally, correlation with the clinical examination of the optic nerve head, nerve fiber layer, and nerve fiber layer analysis cannot be overemphasized.

4-4-1-5 STF. STF on the full threshold algorithm, or intratest fluctuation as measured on the full threshold algorithm, can be caused by an inconsistent patient or by true visual field loss that causes the patient to tire easily. If the STF value in the global indices is flagged with a probability symbol, a careful inspection of the actual threshold data may reveal the cause (see Figure 4-8A and B)

4-4-1-6 Stimuli Number. The number of stimuli needed to complete an examination can be used to estimate patient consistency. An abnormal field may require an

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B

Figure 4-6. (Continued)

increased number of questions to complete the examination. If the patient is inconsistent, the testing strategy requires the presentation of more stimuli to arrive at a final threshold value. Even with markedly abnormal fields, it is unusual for a reliable patient to complete a full threshold central 30° (24°) examination with more than a total of 550 (450) questions.

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B

Figure 4-7. (Continued)

4-4-2 Criteria for Abnormality. Once armed with a solid understanding of the methods by which the automated perimeter tests and analyzes the field, as well as familiarity with artifacts and other confounding factors, the clinician is much better equipped to interpret the results. Often, interpretation is relatively easy, as in an ocular hypertensive patient with mildly elevated intraocular pressure, a normal-appearing

A

Figure 4-8. Learning effect recognized. (A) In this series of fields, significant learning occurs between the first and second fields. (B) This improvement is seen as an upward shift in box location on the change analysis printout. The mean deviation also improves greatly from −10.69 dB to −5.07 dB between the first and second tests. (C) The glaucoma change probability function of Auto STATPAC averages the second and third field for a baseline. Because the first field is deleted, the mean deviation (MD) slope is reported as a modified MD slope.

optic disc, normal retinal nerve fiber layer analysis, and a completely unremarkable, reliably performed test. Interpreting moderately abnormal fields is also fairly simple, especially when they conform to classic patterns of loss, such as a sharply demarcated arcuate scotoma ending in a nasal step.

More difficult is the patient who exhibits an abnormal disc and an abnormal field, with evidence of reduced reliability and other confounding factors (e.g., cataract,

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B

C

Figure 4-8. (Continued)

retinovascular abnormality, prior retinal laser treatment). Although recognizing that a field is not normal may be reasonably straightforward, classifying the amount of damage attributable to a specific disorder may pose considerable difficulty. Equally difficult can be determining a subtle abnormality in a borderline clinical case. When attempting to classify precisely the degree of abnormality, clinicians should

TABLE 4-1. Minimal Criteria for Grading Abnormality (Central 30°)

Strict

≥4 adjacent points of ≥ 5 dB loss* each

≥3 adjacent points of ≥ 10 dB loss* each

Difference of ≥ 10 dB across nasal horizontal meridian at ≥ 3 adjacent points Exclusions: physiologic blind spot; superior and inferior rows

Moderate

≥3 adjacent points of ≥ 5 dB loss* each

≥2 adjacent points of ≥ 10 dB loss* each

Difference of ≥ 10 dB across nasal horizontal meridian at ≥ 2 adjacent points Exclusions: physiologic blind spot; superior and inferior rows

Liberal

≥2 adjacent points of ≥ 5 dB loss* each

≥1 adjacent points of ≥ 10 dB loss* each

Difference of ≥ 5 dB across nasal horizontal meridian at ≥ 2 adjacent points Exclusions: physiologic blind spot; superior and inferior rows

*Loss is relative to normal values or to values of surrounding points. For probability maps that compare measured thresholds to normal values. Substitute P <.05 for 5 dB loss, and P <.01 for 10 dB loss.

Reprinted with permission from Caprioli J: Automated perimetry in glaucoma. Am J Ophthalmol 1991;111:235–239.

bear in mind that the majority of true glaucomatous field losses are not reversible. Therefore, it is often more valuable and more possible to simply establish a baseline for the purpose of measuring stability versus change over time.

Despite these caveats, several published guides exist that define diagnostic features for minimal localized abnormalities. Table 4-1 provides strict, moderate, and liberal criteria, which can be modified according to the clinician’s index of suspicion.21 Application of these criteria should take into account the learning effect in a patient new to perimetry (Figure 4-8C). In addition, the importance of confirming a newly recognized defect with a second test cannot be overstated.

Anderson22 published a similar scheme, which defines a minimally significant defect as three or more non-edge points in the pattern deviation plot with sensitivities below the 5th percentile and one of these points below the 1st percentile in a reliably performed field. In general, this cluster should conform to anatomic expectations. A CPSD value below the 5th percentile or a GHT result of abnormal may also be used to define typical minimal criteria (Table 4-2). It is critically important to consider the entire clinical picture in these determinations and not interpret the field in a vacuum. Figures 4-9 through 4-11 demonstrate subtle scotomas that exactly correspond to nerve fiber layer wedge defects seen on fundus examination. Although these fields might be classified as normal by isolated criteria, the patients all have early, recognizable glaucomatous visual field loss when clinical correlation is used.

4-4-3 Staging of Field Loss. Some staging guidelines for glaucomatous visual field loss measured with the Humphrey perimeter have been published. It is reasonable to

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TABLE 4-2. Minimal Critreria for Diagnoising Acquired Glaucomatous Damage

A glaucoma hemifield test (GHT) outside normal limits on at least 2 fields

OR

A cluster of 3 or more nonedge points in a location typical for glaucoma, all of which are depressed on the pattern deviation plot at a

P <5% level and 1 of which is depressed at a P <1% level on 2 consecutive fields

OR

A corrected pattern standard deviation (CPSD) that occurs in less than 5% of normal fields on 2 consecutive fields

Adapted with permission from Anderson DR: Automated Static Perimetry. St Louis: Mosby –Year Book;1992:123.

use scotoma location as well as density and size to define stage, because paracentral defects may have greater functional impact. Although, in general, advanced visual field loss is treated more aggressively, it is important to individualize each case and not rely on a cookbook approach (Table 4-3).23

4-4-4 Test Selection. Standard practice in many offices is to evaluate most glaucoma patients and glaucoma suspects with SITA Standard 24-2 initially. Patients who test poorly often produce reliable results after a second or even third test session with close feedback from the perimetrist. If, after several sessions, the patient is still unable to perform a reliable field, SITA Fast can be used. If automated static perimetry efforts fail, Goldmann perimetry can be performed.

4-4-5 Follow-up of Advanced Field Loss. If a dense defect is found within the central 10°, the defect may be defined with Program 10-2. When widespread absolute defects occur, retesting the patient with stimulus size V is helpful. Areas of the visual field with absolute loss determined with stimulus size III are often found to have some remaining nonabsolute loci (when tested with size V), which can be followed over time (Figure 4-12A and B).

The amount of spatial summation is largely a function of the initial sensitivity. Thresholds near zero with stimulus size III can increase by 20 dB when tested with stimulus size V.24 In particular, tiny central islands of vision, with threatened or early loss of fixation, are often amenable to testing with stimulus size V using Program 10-2.25

4-5 EVALUATION OF A SERIES OF TESTS

The primary reason for evaluating a series of tests is to detect the progression of a disease. Because comparisons over time will be made, it is critical to establish an adequate baseline. Recognizing typical LTF and maintaining constant vigilance for evolving artifacts and other confounding factors will help ensure high specificity to detect subtle glaucoma progression.