Cataract surgery is among the most common and most successful surgical procedures performed today. Among the cataract preoperative screening tests, microperimetry is not usually performed; visual acuity, refractions, and fundus examinations generally provide sufficient clinical information about retinal status. In addition, macular optical coherence tomography (OCT) is rapidly becoming the standard of care for preoperative screening, particularly when a multifocal intraocular lens (IOL) is being considered. These tests are obtained during routine screening along with special biometry to provide axial length, corneal curvature, and anterior chamber depth.
After cataract surgery, expectations are high for improved visual acuity and absence or significant reduction of common cataract symptoms. These are numerous and can include cloudy, blurred, or dim vision, increased difficulty with night vision and driving at night, sensitivity to light and glare, seeing “halos” around bright lights, frequent changes in eyeglass or contact lens prescription, alteration of color perception, and monocular diplopia. Patients who have opted for premium channel IOLs have even higher expectations1; those who have chosen multifocal IOLs may expect improved uncorrected distance, intermediate, and near visual acuity. Therefore, patients who do not achieve a measureable improvement in their visual status after cataract surgery are understandably frustrated. However, although postoperative surprises can occur after cataract surgery in the best of hands, these are not always owing to unforeseen issues in IOL calculations. Many older patients may have underlying eye disorders, such as age-related macular degeneration (AMD) or other abnormal retinal conditions that are not diagnosed prior to surgery. AMD can be missed in patients with cataract because a clear view of the retina may not be possible during routine dilated examination.2 The lesions of AMD can be detected with macular OCT, but some patients have lesions on OCT that are not functionally significant; in these cases, patients may be steered away from having a multifocal IOL that they may otherwise have enjoyed for many years. Even if the patient has a monofocal IOL inserted, if the diagnosis of functionally significant AMD is made after cataract surgery, when visual function does not improve as expected, the patient may be led to suspect that the surgery caused the retinal dysfunction.3
It is important to emphasize that the standard diagnostics for abnormal retinal conditions (fundus photography and angiography, OCT, ultrasound examination, and ultrasound biomicroscopy) are sometimes not useful in a patient with cataract, because they may detect visually insignificant lesions.4 Approximately 4% of preoperative patients with cataract have normal fundus examinations but still have the lesions of AMD on OCT, and yet these lesions have no significant impact on retinal function.3 Once again, based on OCT or angiography alone, these patients might be guided by their surgeon to choose a monofocal lens when they could have selected, and enjoyed, the benefits of a multifocal lens. The standard tests produce images of retinal structure that are used to infer macular dysfunction, whereas microperimetry can evaluate macular function directly. It is just as important to document normal retinal function in the presence of retinal pathology that will have no significant impact on postoperative visual acuity as it is to detect preexisting macular dysfunction that will have a significant impact on postoperative visual acuity.5
Several functional retinal tests can be used to assess visual function before cataract surgery. However, simple tests such as the Amsler grid chart, color and two-point discrimination, the Maddox rod, and potential acuity meter and laser interferometer instruments do not have a direct “function to anatomy” correlation and, therefore, cannot provide adequate information about macular function.6
Microperimetry instruments, also known as fundus perimetry, are designed to provide a spatial mapping of retinal sensitivity to light stimuli on the central retina while directly observing the fundus. With the convenience of real-time compensation for eye movements, fundus perimetry is the only reliable method of visual field testing in patients with unstable or eccentric fixation caused by macular pathologies.7,8 In addition, scanning laser ophthalmoscope-enhanced microperimetry can be useful in patients with up to 3+ cataracts (as in the current study).
The purpose of the current study is to test the hypothesis that ocular pathology determined with structural tests (OCT) may not be indicative of macular dysfunction. Microperimetry can be used to determine macular function prior to cataract surgery. This test specifically measures spatial variation in retinal light sensitivity and unstable fixation stability, which are hallmarks of visual disability.8 This information will help set expectations for visual acuity after surgery and help in the selection of IOL type.
Patients and Methods
Patients were enrolled from the patient population of the Ophthalmology Consultants of Long Island, Rockville Centre, New York. Approval from the institutional review board of Biomed, San Diego, CA, was obtained and eligible patients completed an informed consent form. Inclusion criteria were mild (1+) to moderate (3+) cataract, absence of vitreous opacity, cornea opacity, or other cause of opaque optical media apart from the cataract itself, and absence of retinal detachment. No exclusions were made based on race, age, gender, or ethnicity.
This study consisted of three scheduled patient visits (preoperative screening, surgery, and postoperative week 1) over a 4-month period. At the preoperative screening, uncorrected distance visual acuity, corrected distance visual acuity (CDVA), and intraocular pressure were recorded and a slit-lamp microscopy examination was performed to evaluate cataract classification. Prior to the scheduled cataract phacoemulsification surgery, a MAIA microperimeter (Centervue S.p.A, Padova, Italy) test was performed to assess baseline macular function. At postoperative week 1 following standard phacoemulsification cataract surgery, uncorrected distance visual acuity, CDVA, intraocular pressure, and slit-lamp microscopy were repeated and a MAIA microperimeter follow-up test was performed to assess postoperative macular function. Patients exited the study after the postoperative 1-week follow-up visit.
The MAIA microperimeter combines a scanning laser ophthalmoscope to image the retina with an automated stimuli projection test similar to visual field analyzers. A 25-Hz retinal eye tracker ensures accurate positional stability and evaluates patient fixation.8 The automatic central 10° MAIA microperimetry test “expert test” covering the macular area was performed before and 1 week after cataract IOL surgery. The MAIA test parameters were set to: grid = standard; number of stimuli = 37; range of stimuli = 0 to 36 dB; macular coverage = 10°; stimulus size = Goldmann III; threshold strategy = 4 to 2, full threshold staircase; background luminance = 4 asb; maximum stimulus luminance = 1,000 asb; and nominal test duration = 5 minutes. The primary outcome measures included the retinal sensitivity metrics of average threshold sensitivity (ATS), percent reduced threshold (%RT), fixation metrics of fixation stability (FS), and fixation drift (FD) of all comparisons of changes in MAIA microperimetry data from patients with cataract. In patients without cataract, ATS and FS have been determined previously in normal patients and in patients with AMD.7
Retinal Sensitivity. Retinal sensitivity was determined from the ATS, calculated from the simple mean of the thresholds for the 37 stimuli in the MAIA examiantion. An index of macular dysfunction was represented by the %RT, which is the percentage of stimuli lower than 25 dB. For reference, the threshold of each stimulus that is detected in normal visual acuity is greater than 26 dB.7
Fixation Metrics. FS and FD were determined from the characteristics of fixation nystagmus, which was tracked and recorded at 25 Hz by the MAIA. Stability was deduced from the specific characteristics of the cloud of fixation points within the retinal area used for fixation during the examination. The preferred retinal locus (PRL) of the cloud is the barycenter or mean position (centroid) of all of the fixation points within the cloud.9 P1 is defined as the percentage of fixation points within the central 1° from the barycenter. P2 is defined as the percentage of fixation points within 2° of the barycenter.
Classification of FS has been established as stable if P1 is greater than 75%, relatively unstable if P1 is less than 75% but if P2 is greater than 75%, and unstable if P2 is less than 75%.10 PRLinitial is the barycenter of the fixation area in the initial 10 seconds of the test. PRLfinal is the barycenter of the fixation area over the full examination period.11
FD is often related to instability of fixation in patients with macular dysfunction12 and is calculated from PRLinitial to PRLfinal, the length of the vector between these two barycenters. FD is used to determine an unstable fixation from normal variation. Patients with FD greater than 0.5° are scored as not stable.
Statistical Methods. The data from the MAIA were exported directly into an Excel spreadsheet (Microsoft Corporation, Redmond, WA). Basic statistics (averages, error estimates, number of observations, etc.) were calculated directly in Excel, whereas t tests and normality of distributions were determined with SigmaStat (Systat Software, Inc., San Jose, CA) software. Snellen acuity values were converted to logMAR values for statistical purposes.
Of the 10 individuals (4 females and 6 males) entered into the study, there were equal numbers of right and left eyes. Cataract grades ranged from 1+ to 3+ (Figure 1). Ages ranged from 49 to 83 years old. Preoperative intraocular pressures were all within normal limits ranging from 10 to 20 mm Hg. CDVA before and after surgery is shown in Figure 2. All of the patients either retained their preoperative acuity level, or gained up to five lines of visual acuity.
Cataract grades in patient population.
Corrected distance visual acuity for study patients preoperatively (pre-op) and 1 week postoperatively (post-op). All eyes worse than 20/20 (logMAR = 0.0) improved after surgery.
The nominal 5-minute examination time for the MAIA test ranged from 279 to 380 seconds. The first visit average was 341 ± 35 seconds compared to 311 ± 18 seconds at the 1-week follow-up visit. This reduction in examination time was statistically significant (two-tailed t test; P = .037); whether this 30-second difference in average examination time was due to a learning effect or to improved clarity of vision after cataract removal is not known.
The MAIA microperimeter was able to confirm normal macular functional conditions in patients with cataract. Figures 3A–3B show normal MAIA macular sensitivity for patient 5 who underwent surgery for a 3+ cataract. In contrast, the microperimeter indicates reduced macular sensitivity in patient 6, also a patient with 3+ cataract, but with a ring scotoma (Figures 4A–4B). Preoperative and postoperative abnormal FS and FD are shown in the microperimeter output of patient 2 (Figures 5A–5B).
(A) MAIA microperimeter (Centervue S.p.A, Padova, Italy) output detail of patient 5 before surgery showing normal values of macular sensitivity (average threshold sensitivity = 27.4 dB, percent reduced threshold = 2.7) and fixation stability (P1 = 100%, fixation drift = 0.1). The barycenters for PRLinitial (red point) and PRLfinal (aqua point) essentially overlap, indicating insignificant fixation drift during the examination. (B) MAIA microperimeter output detail of patient 5 after surgery showing normal values of macular sensitivity (average threshold sensitivity = 28.6 dB, percent reduced threshold = 0) and fixation stability (P1 = 99%, fixation drift = 0.1).
(A) MAIA microperimeter (Centervue S.p.A, Padova, Italy) output of patient 6 with 20/80 corrected distance visual acuity before cataract surgery: abnormal average threshold sensitivity = 10.8 dB; abnormal percent reduced threshold = 94.6; fixation stability = normal. Note the black area representing characteristics of ring scotoma. (B) MAIA microperimeter output of patient 6 with 20/25 corrected distance visual acuity after cataract surgery: low average threshold sensitivity = 11.0 dB; abnormal percent reduced threshold = 83.8; fixation stability = normal. Note the black area representing characteristics of ring scotoma that was identified preoperatively.
(A) MAIA microperimeter (Centervue S.p.A, Padova, Italy) output of patient 2 demonstrating fixation characteristics before surgery: eccentric preferred retinal locus (PRL), relatively unstable fixation and PRL fixation drift between PRLinitial (red point) and PRLfinal (aqua point). Corrected distance visual acuity = 20/100. (B) MAIA microperimeter output of same patient still demonstrating abnormal fixation characteristics after surgery: eccentric PRL, relatively unstable fixation and PRL fixation drift between PRLinitial (red point) and PRLfinal (aqua point). Corrected distance visual acuity = 20/70.
With respect to the MAIA examinations, patient 10 was unable to complete the examination in a satisfactory manner at either of the two follow-up visits due to small pupil size. The follow-up examination mode was not used for patients 2, 7, and 9 due to the inability of the microperimeter to match preoperative and postoperative fundus images. These patients were tested in the new examination mode at the postoperative visit. Hence, the follow-up data analysis of ATS and %RT could not be evaluated for patients 2, 7, 9, and 10. Of the remaining 6 patients, patient 6 was the only one described as having abnormal retinal sensitivity before surgery from the ATS data (Figure 6) and from the %RT data (Figure 7). Fixation data were included for all 10 patients. FS was abnormal in patient 2 before and after surgery (Figure 8). FD was also evident in patient 2 before and after surgery, with a slight improvement in patient 6 after surgery (Figure 9). The study site (Ophthalmology Consultants of Long Island) described patient 6 to have a known diagnosis of macular degeneration before surgery, whereas there was no documentation of retinal dystrophy for patient 2.
Average threshold sensitivity of all patients. Note that patient 6 shows a significantly low average threshold sensitivity both before (pre-op) and after (post-op) surgery. Note that patients 2, 7, 9, and 10 did not have the follow-up examination; therefore, the change in average threshold sensitivity is not shown.
Percent reduced threshold (%RT) of all patients. In the MAIA microperimeter system (Centervue S.p.A, Padova, Italy), lower %RT numbers are associated with normal retinal sensitivity. Note that patient 6 shows abnormal %RT both before (pre-op) and after (post-op) surgery and is a statistical outlier. Note that patients 2, 7, 9 and 10 did not have the follow-up examination; therefore, the change in %RT is not shown.
Fixation stability of all patients. Note that patient 2 had an abnormally low fixation stability both before (pre-op) and after (post-op) surgery (P1 < 75). In this group of patients, 2 was a statistical outlier before and after surgery.
Fixation drift of all patients. Patient 2 had abnormally high fixation drift before (pre-op) and after (post-op) surgery, whereas patient 6 improved after surgery. Subject 2 is a statistical outlier after surgery.
Although few studies have reported the use of microperimetry in patients with cataract, they have only included patients with normal fundi.13 The current study is the first of its type to evaluate macular function prior to cataract surgery using microperimetry in patients with or without macular pathologies. Among the current tests normally used to detect retinal disease, the microperimeter is the only test that measures macular sensitivity directly rather than infer this from structural examinations such as retinal photography, OCT, and ultrasound. Cataracts impact visual acuity and, therefore, although poor acuity is used as an indication for surgery, CDVA cannot be used to assess macular functional potential. Being able to detect macular dysfunction prior to cataract surgery is important to balance the expectations of the patient with the potential for suboptimal improvement in visual acuity, and is crucial for well-informed selection of the ideal IOL (monofocal vs multifocal) for each patient.
CDVA before surgery does not offer discriminatory evidence of retinal dysfunction as shown in Figure 2. This is not unexpected, because light scatter and obscuration of the visual field varies with the nature of the cataractous lens. On the other hand, several variables from the MAIA microperimeter assessing retinal sensitivity (ATS and %RT) and fixation characteristics (FS and FD) were useful to detect macular dysfunction in the presence of mild to moderate cataract. Normal macular functional conditions can be confirmed with the microperimeter as shown in Figures 3A–3B. One of the most effective parameters in detecting macular dysfunction is the %RT, which measures the loss of localized retinal sensitivity caused by defects that may be masked by the averaging process when calculating the mean value of a wider retinal area. As shown in Figure 7, patient 6 had preoperative and postoperative values worse than normal for patients with cataract. The reduced ATS for patient 6 is also evident in Figure 6. Despite the fact that this patient’s CDVA improved from 20/80 to 20/25 after surgery, the MAIA output demonstrates clear indications of ring scotoma before (Figure 4A) and after (Figure 4B) surgery. Hence, it is clear this eye had retinal dysfunction preoperatively that was not related to a possible complication of the cataract surgery.
The fixation characteristics of stability and drift are also related to degradation of retinal function. Patient 2 had abnormal variables at both preoperative and postoperative time periods (Figures 8–9). FS and FD are clear indicators of retinal dysfunction. It is interesting that patient 2 showed drift of his final PRL to a different retinal area (compare preoperative Figure 5A and postoperative Figure 5B). This could be due to IOL tilt or surgically induced corneal aberrations, but more likely this was due to improved visual clarity after removal of the cataract. It should be noted that patient 6, who was abnormal in %RT, was essentially normal in fixation variables; therefore, the characteristics of fixation and sensitivity may not be correlated and should be considered as independent factors in the evaluation of macular functionality.
This study strengthens the proposition that microperimetry can be useful to reveal undetected macular dysfunction prior to cataract surgery and normal macular function in the presence of abnormal retinal structural findings. In the case of the latter, microperimetry can identify preoperative patients with cataract who may benefit from multifocal IOLs despite macular lesions. It seems clear that %RT and FS and FD are strong predictors of macular dysfunction. This study should be expanded to confirm the potential utility of microperimetry in screening patients with cataract comparing those results to examination with structural methods such as OCT.
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