EL LUGAR EN EL ESPACIO FENOMENOLOGÍA Y ARQUITECTURA

Subjects underwent Humphrey Field Analyser 24-2 automated static perimetry (Humphrey Instruments Inc, Palo Alto, CA, USA) in both eyes at each visit. All testing was by research technicians dedicated to the Glaucoma Research Unit.

Global indices. Mean deviation (MD) and corrected pattern standard deviation (CPSD) indices given by Humphrey software were used as general measures of visual field status.

Visual field conversion. Subjects’ visual fields were evaluated for change at each visit, but only field progression in the form of conversion in glaucoma suspects was

Contour line

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Reference plane

FIGURE 5.3. Diagram illustrating the position o f the standard reference plane 50pm below the average o f height values between 350-356°on the contour line. The shaded region represents the neuroretinal rim.

Contour line

320|im _{Reference ring}

Rim area Reference plane

FIGURE 5.4. Diagram illustrating the position of the 320pm reference plane 320pm below the mean topographical height of the reference ring. The

specifically noted for further analysis in this investigation (see also Section 5.4). Conversion was defined as reproducible glaucomatous field defects developing in previously normal fields. AGIS criteria (The AGIS investigators, 1994a) were used to score visual fields, wherein a score of greater or equal to one indicated abnormality. Field abnormality had to be repeatable in the same field locations on at least three consecutive reliable tests and independently verified by a glaucoma expert before conversion was accepted. Possible causes of field defects other than glaucoma were excluded.

AGIS visual field scoring criteria. Scoring by AGIS visual field criteria is based on the number of test locations with total sensitivity deviations exceeding specified levels in Humphrey 24-2 grid clusters, as shown in Figure 5.5. The grid is divided into upper and lower hemifields, each with nasal and temporal regions. Deviation of sensitivity from normal is relative to the standard age-matched normal database in the Humphrey perimeter. Fifty-two test locations are analysed after excluding points above and below the physiological blind spot.

A nasal defect comprises a cluster of three or more depressed nasal locations that may cross the horizontal midline. A nasal step comprises at least one depressed nasal location either above or below the horizontal midline. A hemifield defect requires a cluster of three or more depressed locations in a hemifield.

The scoring system is as follows:

• For nasal defects or steps, a score of 1 is given. If at least 4/6 nasal locations are depressed by 12 dB or more, a further score of 1 is added.

• In hemifields, clusters of 3-5 depressed locations are scored 1, 6-12 with a 2, 13-20 with a 3, more than 20 with a 4.

• If half or more of depressed locations are depressed by at least 28 dB, a score of 5 is added; for at least: 24 dB, 4 is added; 20 dB, 3 is added; 16dB, 2 is added; 12dB, 1 is added.

• For hemifields without clusters of three depressed locations, a score of 1 is added if there is a pair of depressed locations in which one location is depressed by 12 dB or more.

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Figure 5.5, AGIS visual field scoring template

Scores are added over the whole field and have a range of 0-20. A score of 0 represents no significant defect, 1-5 early glaucoma, and 20 represents end-stage glaucoma in which all test locations are deeply depressed.

For a visual field to be reliable, it should have: 1. No more than 20% fixation losses

2. No more than 33% false-positive or false-negative responses

A subject initially meeting the criterion of having developed a field defect score of one or more from a baseline score of zero (no defect) was invited back for repeat perimetry within one month. If the field defect was reproduced, a third confirmatory test was needed within four months of the initial visual field with defect. If the defect was reproduced on two consecutive tests (three in a row), the subject’s entire field series was given to a glaucoma expert to be independently assessed to confirm or refute conversion (Miss WA Franks). lOP-lowering treatment was started in all subjects with confirmed conversion. Subjects in whom conversion was not confirmed or visual fields were unreliable were invited back for retesting within one month. If defects were still unconfirmed on the second or third attempt, a routine follow up visit was organised for four months time.

5.4 Reference dataset for evaluating change and validating methods

for identifying progression

Longitudinal image series o f ocular hypertension converter and normal control eyes were combined as a single reference dataset to test, refine and validate the analytical methods we devised for identifying progression. Ocular hypertension converter eyes were assumed to have unambiguous glaucoma progression in the period leading to the development of reproducible visual field defects (conversion). Normal control eyes were taken to be unchanged over the period o f monitoring. Ocular hypertension converters and normal controls were matched for age and duration o f follow up by HRT imaging. Each subject contributed one eye for analysis.

The testing of image series of converter eyes was considered appropriate and rigorous enough for validating glaucomatous change because of the following. 1)

Identifying and confirming progression in conversion fields is simple and unambiguous compared with trying to detect progression in glaucomatous fields. In conversion, it is clear that change has occurred once reproducible perimetric abnormality develops where there previously had been no abnormality. In glaucomatous fields, however, interpreting progression on a background of considerable variability can make the identification of true glaucomatous change difficult. 2) Visual field abnormality was defined objectively by the National Institutes of Health-devised AGIS criteria (The AGIS Investigators, 1994a; 1994b). Abnormality had to meet strict requirements for being reproducible, needing to be repeated in the same test locations on three consecutive tests, then be independently confirmed by a glaucoma expert who was well-versed with AGIS criteria. 3) All subjects had extensive experience with visual field testing. 4) It was then reasonable to assume that eyes undergoing visual field conversion by these criteria had underlying glaucomatous progression with concurrent optic nerve change (Pederson and Anderson, 1980; Sommer et al, 1980; Tuulonen and Airaksinen, 1991; Airaksinen et al, 1992; Zeyen and Caprioli, 1993; Odberg and Riise, 1985; Funk,

1991, 1991b; Motolko and Drance, 1981; Yablonski et al, 1980).