Figure 3-17 Commonly used instruments for optic disc and nerve fiber layer imaging in glaucoma. A, Optic nerve head analysis with Heidelberg Retina Tomograph (HRT) shows thinning of the inferior neuroretinal rim using Moorfields regression analysis (green check mark = within normal limits; yellow exclamation mark = borderline; red x = abnormal). B, Retinal nerve fiber layer analysis with scanning laser polarimetry. Top, Deviation map. Bottom, Generalized thinning or diffuse loss of the nerve fiber layer in the right eye. C, Retinal nerve fiber layer analysis with optical coherence tomography. Top, Thinning of the inferior bundle (blunted peak) in the right eye. Middle, Thinning of the inferior and nasal nerve fiber layer in the left eye.
Bottom, Comparison of both eyes. (Reproduced with permission from Salinas-Van Orman E, Bashford KP, Craven ER. Nerve fiber layer, macula, and optic disc imaging in glaucoma. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 2006, module 8.)
Techniques such as scanning laser polarimetry and optical coherence tomography have been used to acquire images of the retinal nerve fiber layer. The scanning laser polarimeter (Fig 3-17B) is basically a scanning laser ophthalmoscope outfitted with a polarization modulator and detector to take advantage of the birefringent properties of the retinal nerve fiber layer arising from the predominantly parallel nature of its microtubule substructure. As light passes through the nerve fiber layer, the polarization state changes. The deeper layers of retinal tissue reflect the light back to the detector, where the degree to which the polarization has been changed is recorded. The acquired data can then be stored, displayed, and manipulated by computer programs, just as with the unmodified confocal scanning laser ophthalmoscope. The fundamental parameter being measured with this instrumentation is relative (not absolute) retinal nerve fiber layer thickness. The addition of a variable corneal compensator (VCC) to include analysis of potential anterior segment birefringence has improved the quality of the information available with this technique.
Optical coherence tomography (OCT) (Fig 3-17C) uses interferometry and low-coherence light to obtain a high-resolution cross section of biological structures. Time-domain OCT instruments have an axial resolution of about 10 µm in the human eye and are able to yield an absolute measurement of nerve fiber layer thickness. In vivo OCT measurements appear to correlate with histologic measurements of the same tissues. With a resolution of 3–5 µm, spectral domain OCT (SD-OCT) provides greatly increased resolution of images and is now clinically available.
Quantitative measurement of the optic disc and retinal nerve fiber layer is a promising nascent science. The instrumentation and techniques used to acquire quantitative imaging and analysis of nerve head and nerve fiber layer anatomical parameters are rapidly evolving. Both for single measurements directed at detecting the presence of glaucoma and, especially, for serial measurements necessary to determine clinical progression of glaucoma, these technologies have great potential. Still, the clinician must remember that no system of measurement and observation is currently more useful or has proven more reliable than good-quality stereophotographs combined with detailed and careful clinical examination.
Chen YY, Chen PP, Xu L, Ernst PK, Wang L, Mills RP. Correlation of peripapillary nerve fiber layer thickness by scanning laser polarimetry with visual field defects in patients with glaucoma. J Glaucoma. 1998;7(5):312–316.
Wollstein G, Garway-Heath DF, Hitchings RA. Identification of early glaucoma cases with the scanning laser ophthalmoscope. Ophthalmology. 1998;105(8):1557–1563.
The Visual Field
For many years, the standard method of measuring the visual dysfunction seen with glaucomatous injury has been assessment of the visual field with clinical perimetry, which measures differential light sensitivity, or the ability of the subject to distinguish a stimulus from a uniform background. The classic description of the visual field, given by Traquair (1875–1954), is “an island hill of vision in a sea of darkness.” The island of vision is usually described as a 3-dimensional graphic representation
of differential light sensitivity at different positions in space (Fig 3-18).
Perimetry has traditionally served 2 major purposes in the management of glaucoma:
1.identification and quantification of abnormal fields
2.longitudinal assessment to detect glaucomatous progression
Figure 3-18 A, Isopter (kinetic) perimetry. Test object of fixed intensity is moved along several meridians toward fixation. Points where the object is first perceived are plotted in a circle. B, Static perimetry. Stationary test object is increased in intensity from below threshold until perceived by the patient. Threshold values yield a graphic profile section. (Reproduced with
permission from Kolker AE, Hetherington J, eds. Becker-Shaffer’s Diagnosis and Therapy of the Glaucomas. 5th ed. St Louis: Mosby; 1983. Modified from Aulhorn E, Harms H. In: Leydhecker W. Glaucoma. Tutzing Symposium. Basel: S Karger; 1967.)
Quantification of visual field sensitivity enables detection of initial loss by comparison with normative data. Regular visual field testing in known cases of disease provides valuable information for helping to differentiate between stability and progressive loss.
Over the past 2 decades, automated static perimetry has become the standard for assessing visual function in glaucoma. With this method, threshold sensitivity measurements are usually performed at a number of test locations using white stimuli on a white background; this is known as standard automated perimetry (SAP), or achromatic automated perimetry. Other technologies have become available that may be useful in evaluating the visual field. According to evidence from detailed investigations using these newer perimetric tests, it is likely that, in individual patients, these different
tests show abnormalities at different times in the course of glaucoma. Some methods may be better for identifying defects than for following their progression, and vice versa.
Clinical Perimetry
Two major types of perimetry are in general use today:
automated static perimetry using a bowl perimeter or video monitor manual kinetic and static perimetry using a Goldmann-type bowl perimeter
The following are brief definitions of some of the major perimetric terms:
Threshold: The differential light sensitivity at which a stimulus of a given size and duration of presentation is seen 50% of the time—in practice, the dimmest spot detected during testing. Suprathreshold: Above the threshold; generally used to mean brighter than the threshold stimulus. A stimulus may also be made suprathreshold by increasing the size or duration of presentation. This is generally used for screening paradigms.
Kinetic testing: Perimetry in which a target is moved from an area where it is not seen toward an area where it is just seen. This is usually performed manually by a perimetrist, who chooses the target, moves it, and records the results.
Static testing: A stationary stimulus is presented at various locations. In theory, the brightness, size, and duration of the stimulus can be varied at each location to determine the threshold. In practice, in a given automated test session, only the brightness is varied. Although static perimetry may be done manually—and is often combined with manual kinetic perimetry—in current practice the term usually refers to automated perimetry.
Isopter: A line on a visual field representation—usually on a 2-dimensional sheet of paper— connecting points with the same threshold.
Depression: A decrease in retinal sensitivity.
Scotoma: An area of decreased retinal sensitivity within the visual field surrounded by an area of greater sensitivity.
Decibel (dB): A 0.1 log unit. This is a relative term used in both kinetic and static perimetry that has no absolute value. Its value depends on the maximum illumination of the perimeter. As usually used, it refers to log units of attenuation of the maximum light intensity available in the perimeter being used.
Variables in Perimetry
Whether automated or manual, perimetry is subject to many variables, including the patient’s level of attentiveness, the perimetrist’s administration of the test, and the size of the stimulus.
Patient
People vary in their attentiveness and response time from moment to moment and from day to day. Longer tests are more likely to produce fatigue and diminish the ability of the patient to maintain peak performance.
Perimetrist
The individual performing manual perimetry can administer the test slightly differently each time. Different technicians or physicians also vary from one another in how they administer the test.
Perimetrist bias is markedly diminished with automated testing. However, the perimetrist can have an effect on test outcome even in automated testing, by monitoring or not monitoring the patient for proper performance and positioning. Most automated instruments can be paused during the test by perimetrist intervention, thereby allowing repositioning or other adjustments to enhance test reliability.
Other variables
Other variables of importance include the following:
Fixation: If the eye is slightly cyclotorted relative to the test bowl, or if the patient’s point of fixation is off center, defects may shift locations. Especially in automated static tests (because the test logic does not change), a defect may thus appear and disappear.
Background luminance: The luminance of the surface onto which the perimetric stimulus is projected affects retinal sensitivity and thus the hill of vision. Clinical perimetry is usually performed with a background luminance of 4.0–31.5 apostilbs. Retinal sensitivity is greatest at fixation and falls steadily toward the periphery.
Stimulus luminance: For a given stimulus size and presentation time, the brighter the stimulus, the more visible it is.
Size of stimulus: The larger the stimulus, the more likely it is to be perceived. The sizes of standard stimuli are: 0 = 1/16 mm2, I = 1/4 mm2, II = 1 mm2, III = 4 mm2, IV = 16 mm2, and V = 64 mm2. The most common stimuli for automated perimetry are size III.
Presentation time: Up to about 0.5 second, temporal summation occurs. In other words, the longer the presentation time, the more visible a given stimulus. Commercially available static perimeters generally employ a fixed stimulus duration of 0.2 second or less. Comparison of perimetric thresholds between instruments is difficult because different manufacturers use different stimulus durations and background luminances.
Patient refraction: Uncorrected refractive errors cause blurring on the retina and decrease the visibility of stimuli. Thus, proper neutralization of refractive errors is essential for accurate perimetry. In addition, presbyopic patients must have a refractive compensation that focuses fixation at the depth of the perimeter bowl. Care needs to be taken to center the patient close to the correcting lens to avoid a lens rim artifact (see Fig 3-22).
Pupil size: Pupil size affects the amount of light entering the eye, and it should be recorded on each visual field test. Testing with pupils smaller than 2.5 mm in diameter may induce artifacts. Wavelength of background and stimulus: Color perimetry may yield different results than white- on-white perimetry.
Speed of stimulus movement: Because temporal summation occurs over a period as long as 0.5 second, the area of retina stimulated by a test object is affected by the speed of the stimulus movement. If a kinetic target is moved quickly, by the time the patient responds, the target may have gone well beyond the location at which it was first seen. This period between visualization and response is termed the latency period or visual reaction time.
Automated Static Perimetry
A computerized perimeter must be able to determine threshold sensitivity at multiple points in the retina, to perform an adequate test in a reasonable amount of time, and to present results in a comprehensible form. The intensity of the stimulus is varied by a system of filters that attenuate the stimulus, usually allowing measurement to approximately 1 dB.
Automated static perimeters have traditionally used staircase algorithms that employ a bracketing approach. These produce more reliable and efficient threshold estimates compared with previous psychophysical test strategies. Any staircase strategy yields threshold estimates that are a compromise between reliability (accuracy and precision) and efficiency (test duration). Threshold estimates from strategies that cross the threshold (reversal) more often or that use smaller staircase intervals are more reliable, but at the expense of requiring a longer test time. The “standard” staircase strategy used by both the Octopus perimeters (Haag-Streit, Mason, OH) and the Humphrey Field Analyzers (HFAs; Carl Zeiss Meditec, Dublin, CA) employs an initial 4-dB step size that decreases to 2 dB on first reversal and continues until a second reversal occurs (Fig 3-19).
Figure 3-19 Full-threshold strategy determines retinal sensitivity at each tested point by altering the stimulus intensity in 4-dB steps until the threshold is crossed. It then recrosses the threshold, moving in 2-dB steps, in order to check and refine the
accuracy of the measurement. (Reproduced with permission from The Field Analyzer Primer. San Leandro, CA: Allergan Humphrey; 1989.)
General testing categories in common use are as follows:
1.Threshold: Threshold testing is the current standard for automated perimetry in glaucoma management. As described earlier, threshold may be determined by a variety of bracketing and statistical strategies.
2.Efficient threshold strategies: Full-threshold testing algorithms suffer from patient fatigue, high variability, and generally poor patient acceptance. In an attempt to achieve shorter threshold testing with good accuracy and reproducibility, the Swedish interactive thresholding algorithm (SITA) was developed. Unlike staircase strategies, which use discrete intervals to step toward threshold, SITA employs a logical best-guess, or forecasting, approach to threshold estimation. Briefly, the best-guess intensity of SITA’s initial stimulus presentation at each test location corresponds to the intensity associated with the highest probability of being seen by an agematched individual. Depending on the patient’s response to this first stimulus, the intensity of each subsequent presentation is modified. This iterative procedure is repeated until the likely threshold measurement error is reduced to below a predetermined level, with at least 1 reversal occurring at every test location. SITA also uses neighborhood comparisons to optimize the bestguess procedure: if adjacent test locations show lower or higher sensitivity than expected, the initial stimulus intensity is altered. SITA monitors the timing of patient responses in order to interactively pace the test. Similar to the SITA test strategy for the HFA, the tendency-oriented perimeter (TOP) algorithm was developed for the Octopus perimeter as an alternative to the lengthy staircase threshold procedures. The intent of both of these strategies was to provide a faster, more efficient test procedure that maintained the same degree of accuracy and reliability as the staircase procedures.
Comparisons between SITA testing algorithms and older thresholding algorithms have suggested that the SITA Standard yields visual field results comparable to, though not exactly the same as, fullthreshold testing. Both SITA (Standard and Fast) strategies yield marginally higher values for differential light sensitivity compared with other algorithms. The average test time with SITA Standard is approximately 50% of the full-thresholding strategy time, and SITA Fast results in an additional reduction of approximately 30% compared with SITA Standard. The considerably reduced test time with SITA Standard appears to be achieved without a significant reduction in accuracy or an increase in variability or noise levels within the test. SITA Fast should not be used in the routine evaluation of glaucoma suspects or patients with glaucoma and should be reserved only for patients who are unable to perform SITA Standard because of mental or physical limitations.
Screening tests
These tests may or may not be threshold-related, and they cover varying areas of the visual field. Suprathreshold tests are not recommended for glaucoma suspects because they do not provide a good reference for future comparison. They may be useful with other conditions causing visual field loss, however.
Threshold tests
The most common programs for glaucoma testing are the central 24° and 30° programs, such as the Octopus 32 and G1 and the Humphrey 24-2 and 30-2 (Fig 3-20). These programs test the central field using a 6° grid. They test points 3° above and 3° below the horizontal midline and facilitate diagnosis of defects that respect this line. For patients with advanced visual field loss that threatens fixation, serial 10-2 or C8 visual fields should be used. These visual fields 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 and improve the detection of progression.
Although a 30°–60° program is available on most static threshold perimeters, it is rarely performed because the threshold variability is very high in these more peripheral regions.