The standard method for threshold measurement used by the HFA is currently the Swedish Interactive Threshold Algorithm (SITA). SITA is a Bayesian test strategy that uses information from a database of healthy individuals and persons with disease to generate a probability distribution function (PDF) representing the probabilities that the visual field sensitivity will be of a particular value at a particular visual field location. As the test progresses, the distribution is then adjusted according to how the person being tested responds to prior stimulus presentations. This continues until the PDF distribution is within a small range, at which point the mean of the distribution is selected as the threshold sensitivity estimate. The PDFs are adjusted for the age of the individual, the visual field location tested, the sensitivity values of neighboring test locations, and the results of previous stimulus presentations. Compared with conventional staircase algorithms employed in the older full-threshold strategy, SITA has been demonstrated to have equal or lower test–retest variability, and testing can often be done in half the time.
The SITA strategy is available in the Humphrey perimeters as SITA Standard and SITA Fast. As the name implies, SITA Fast is a faster testing method than SITA Standard and may have similar accuracy and reliability; however, it may be a more difficult test for the patient because the test stimuli tend to be closer to the patient’s threshold, thus offering less positive feedback to the patient. Patients who are perceived to have difficulties with SITA Standard should not be shifted to SITA Fast; they should continue with SITA Standard. These patients may benefit from careful instruction by the perimetrist, closer surveillance, and positive feedback.
SITA Faster is a newer strategy; its testing times are approximately 30% shorter than SITA Fast and 50% shorter than SITA Standard. The reduction in testing time is achieved by several modifications, including changes in the starting stimulus intensity and in the number of reversals needed to confirm the threshold, as well as elimination of false-negative and fixation-loss catch trials (see the section Interpretation of a Single Visual Field later in this chapter). Although initial studies indicate that SITA Faster may offer results comparable to SITA Standard, its accuracy for longitudinal monitoring of visual field loss has not been established.
Similar to the SITA testing strategy for the HFA, the tendency-oriented perimeter (TOP) algorithm was developed for the Octopus perimeter (Haag-Streit) as an alternative to the lengthy staircase threshold procedures. TOP differs from SITA in that only 1 stimulus is shown at a single location of the visual field. Therefore, in order to estimate the threshold sensitivity at a particular location, TOP supplements this single data point per test location with information obtained at adjoining test locations.
Patterns of Testing Points
The most common patterns used for testing visual function for glaucoma diagnosis and management test the central 24° to 30° of the visual field. The Octopus perimeter implements these in the 32 and G1 patterns, and the Humphrey uses the 24-2 and 30-2 patterns (Fig 1-6). The 24-2 and 30-2 patterns test the central field using a 6° grid. They test points 3° above and 3° below the horizontal midline, thereby facilitating diagnosis of defects that respect this line. A 24-2 pattern test performed with the SITA Standard strategy for obtaining threshold estimates is then usually referred to as SITA Standard 24-2. For patients with advanced visual field loss or with paracentral defects, testing of the central field with the 10-2 or C8 pattern is appropriate (Fig 2-6). These patterns concentrate on the central 8°–10° of the visual field and test points every 1°–2°, enabling the ophthalmologist to follow many more test points within the central island, improving detection of disease progression. Alternatively, a larger stimulus (Size V) can be used in patients with advanced disease or reduced visual acuity.
Figure 6-1 Central 30-2 threshold test pattern, right eye.
(Illustration by Mark Miller.)
Although a strategy that tests the visual field between 30° and 60° is available on most static threshold perimeters, it is rarely used because the threshold variability is very high in these more peripheral regions.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.