Regional Macular Light Sensitivity Changes in Myopic Chinese Adults: An MP1 Study

Abstract

Purpose.: To investigate the variation of macular light sensitivity (MLS) in myopic Chinese adults by using microperimetry.

Methods.: MLS was recorded with the MP1 microperimeter (Nidek Technologies, Padova, Italy) in eyes affected by various degrees of myopia. Subjects were divided into group A (18–30 years) and group B (31–60 years). Subjects in both age groups were further divided based on refractive status: (1) high myopia (spherical equivalent, SE < −6.00 D); (2) low to moderate myopia (−6.00 D ≤ SE ≤ −1.00 D); and (3) no myopia (−1.00 D < SE ≤ +0.50). All patients had corrected visual acuity greater than 20/20. The macular area covered by the MP1 grid contains nine areas. MLS was quantified in each part and correlated with the refractive data.

Results.: MLS correlated significantly with SE or axial length (AL). Average MLS of the outer ring, total macula, and superior quadrant significantly decreased in the high and the low to moderate myopia eyes (both P < 0.05) in both age groups. MLS in the temporal, inferior, and nasal quadrants decreased in the high myopia eyes, but not in the low to moderate myopia eyes, except in the temporal quadrant in group A and in the nasal quadrant in group B.

Conclusions.: Axial myopia was associated with reduced total and quadrant-specific MLS, particularly in the superior quadrant. These findings emphasize functional differences in the macula between high, low to moderate, and no myopia. Any evaluation of MLS by MP1 microperimetry in the macula should be interpreted in the context of the degree of refractive error and the region of measurement.

Myopia is prevalent in most populations, particularly in developing countries such as China. ¹ Complications related to myopia, such as glaucoma, cataract, retinal detachment, and myopic macular degeneration, are major causes of visual dysfunction. Although pathologic myopia is recognized as a leading cause of visual dysfunction, the effect on vision of myopia without evidence of clinically overt retinal disease has received less attention. 

Previous histopathologic studies have shown that increased scleral and retinal thinning are associated with myopia, which may be caused by increased axial length (AL) and relative enlargement of the globe beyond normal dimensions. ^2,3 With the availability of modern imaging technologies such as optical coherence tomography (OCT), in vivo measurements of retinal thickness have been made possible. ^4–10 Studies have used OCT to identify subtle anatomic macular changes in children and adults with myopia. ^9,10 Regional variation in the macular thickness measurements of myopic eyes correlate with AL and refractive error. ^9,10 Electroretinogram (ERG) testing, including multifocal (mf)ERG, has shown decreased b-wave amplitudes with increased AL in eyes affected by nonpathologic myopia, which indicates early injury of the cones. ^11–14  

Although myopia has been associated with changes in the central retinal area of the posterior pole of the eye, quadrant-specific changes in macular function have not been examined. The MP1 microperimeter is commonly used for clinical assessment of macular function. ^15–20 The exact correlation between fundus disease and corresponding functional defects can be determined by integrating fundus imaging and computerized threshold perimetry. Previous studies have shown the usefulness of this method as a component of the follow-up examinations of patients with progressive macular diseases. ^21,22 The purpose of this study was to use microperimetry to investigate the correlation of macular light sensitivity (MLS) with refractive errors and AL in Chinese adults with myopia. 

Methods

Subjects

Two hundred sixty-one Chinese patients who met the inclusion criteria were examined during the period from June 2009 to August 2009 in the Eye Institute of the EENT Hospital (Fudan University, Shanghai, China). All subjects underwent a full ophthalmic examination, including visual acuity, refraction (noncycloplegic), gonioscopy, and intraocular pressure measurement with Goldmann tonometry, dilated fundus examination with stereoscopic biomicroscopy of the optic nerve head under slit lamp, indirect ophthalmoscopy, and A-scan ultrasound biometry. All eyes included in this study had corrected visual acuity of at least 20/20. Because age has a definite effect on myopic macular function and MP1 microperimetry results, ^23,24 the subjects were divided into group A (18–30 years) and group B (31–60 years). ¹⁴ Subjects in both age groups were further divided based on refractive status: (1) high myopia (spherical equivalent, SE < −6.00 D); (2) low to moderate myopia (−6.00 D ≤ SE ≤ −1.00 D); and (3) no myopia (−1.00 D < SE ≤ +0.50). SE was defined as spherical power plus half of the negative cylinder power. Eyes with negative cylinder power < −1.00 D were excluded. 

Subjects with clinical evidence of concurrent disease other than refractive error, such as glaucoma, media opacity, uveitis, retinal disease, or history of intraocular surgery, refractive surgery, neurologic diseases, and diabetes, were excluded. Subjects with both eyes meeting the inclusion criteria had one eye randomly selected for microperimetry. The study was conducted in accordance with the ethical standards stated in the Declaration of Helsinki and approved by the local clinical research ethics committee, with informed consent obtained from all patients. 

Microperimeter Examination

Microperimetry (ver. SW1.4.1 SP1; Nidek Technologies) was performed in a dim room without cycloplegia. The MP1 device projects stimuli on a liquid crystal display, and the software includes an automated tracking system to correct eye movements. At the beginning of the examination, an infrared camera (resolution of 1 pixel, equivalent to 0.1°) tracked a reference frame, and an area of interest was defined. During examination, eye movement was detected by image acquisition at a rate of 25 frames per second. The computer then calculated the shift between the reference image and the real-time fundus images with the stimulus position on the display corrected according to the actual location of the fundus. At the end of the examination, the perimetric findings were overlaid onto the color fundus images. The stimuli were projected on a white background with illumination set to 4 asb. The stimulus decibel range was from 20 dB (equal to 20 asb) to 0 dB (equal to 400 asb). Stimulus duration was 100 ms, the starting stimulus value used in the study was 16 dB, and the software applied a staircase algorithm to the stimulus presentation. All subjects were exposed to at least five test spots before beginning the examination, to familiarize them with the instrument and minimize the effects of learning on their performance. The test commenced after the subject was comfortable with the procedure. A superthreshold stimulus was projected onto the blind spot by the machine to monitor false-positive responses. This retinal locale was manually identified before the examination as the region over the optic nerve. Any test that produced a false positive was excluded. The SE of the patient was added to the focus to adjust the clearance of the image. 

Goldmann III stimuli and a 4-2-1 staircase strategy were used. Threshold fundus perimetry was performed on the central 10° (diameter) of the retinal circular area (1° = 300 μm, thus 10° = 3000 μm, which encompasses the macular area) by using a 40-point strategy. Eight points were tested at 1° from the center of the fovea, and 16 points each were tested at 3° and 5° from the center (Fig. 1). The 10° macular area was divided into nine parts: superior middle/outer, inferior middle/outer, temporal middle/outer, nasal middle/outer, and foveal areas. The macular area division pattern covered by the MP1 grid is similar to the nine-part division of the (OCT) fields in the Fast Macular Thickness program (ETDRS-type; Carl Zeiss Meditec, Oberkochen, Germany). Sixteen parameters of MP1 examination were analyzed, including the total and nine parts of the 10° macular area, two circular areas (outer and middle rings), and four quadrants (superior, inferior, temporal, and nasal). 

Figure 1.

 

Nine-part division of the macula and the overlay of perimetric findings onto the fundus image. The test procedure included 40 stimulus locations in 10° of the macular area; these were divided into three circles from the inner macula to the outer periphery, with 8, 16, and 16 stimulus locations (left). The 10° macular area is divided into nine parts (1–9, right): the superior middle/outer, inferior middle/outer, temporal middle/outer, and nasal middle/outer quadrants and the fovea.

The fixation target used for all subjects was a single cross (1°). The software automatically calculated the percentage of fixations that were maintained within a 2° and 4° diameter. The total examination time and the total effective time (defined as the time it took for the MP1 to track the fundus and project the point, which excludes the time the machine paused due to patient mobility or blinking) were noted for each patient. 

Statistical Analysis

Data were analyzed and the mean and SD are reported (Stata, ver. 9.0; Stata, College Station, TX). Kruskal-Wallis test and Mann-Whitney U test were applied to statistically compare continuous variables. In addition, the Bonferroni-corrected post hoc test was applied to adjust the observed significance level for multiple comparisons if the null hypothesis was rejected. Statistical analysis of categorical variables was performed using the χ² test. Associations between clinical variables were examined by Spearman's rank correlation test and expressed as the Spearman correlation coefficient or Spearman partial correlation coefficient, with adjustments for the selected covariates. The statistical significance level was set at 0.05. 

Results

A total of 261 eyes were analyzed: 149 in group A, with SE ranging from +0.5 to −15 D (mean ± SD, −4.16±1.34) and AL ranging from 22.15 to 29.30 mm (mean ± SD, 24.71 ± 1.09) and 112 in group B, with SE ranging from +0.5 to −14.25 D (mean ± SD, −4.59 ± 1.46) and AL ranging from 22.50 to 31 mm (mean ± SD, 24.79 ± 0.69). Close correlation was found between AL and SE in both groups (A: r = 0.884, P < 0.001; B: r = 0.878, P < 0.001). The refractive error and AL distribution in each diagnostic group is displayed in Table 1. No significant difference was found in sex and age distribution between the groups (P = 0.788; P = 0.179). 

Table 1.

 

Baseline Data of Three Diagnostic Groups

	Age	High Myopia	Low to Moderate Myopia	No Myopia	P
Eyes, n	<30 y	83	40	26
	>30 y	56	38	18
SE, D	<30 y	−8.60 ± 1.89 (−15.0 ≤SD <−6.0)	−3.77 ± 1.69 (−6.0 ≤SD ≤−1.0)	−0.25 ± 0.30 (+0.5 ≤SD <−1.0)	0.0001*
	>30 y	−9.07 ± 2.20 (−14.5 ≤SD <−6.0)	−4.44 ± 1.80 (−6.0 ≤SD ≤−1.0)	−0.25 ± 0.39 (+0.5 ≤SD <−1.0)	0.0001*
AL, mm	<30 y	26.88 ± 1.30 (24.53–29.30)	24.62 ± 1.21 (23.00–25.28)	22.62 ± 0.78 (22.15–23.52)	0.0001*
	>30 y	27.14 ± 1.01 (25.41–31.00)	24.51 ± 0.77 (23.51–25.55)	22.73 ± 0.28 (22.50–23.37)	0.0001*
Sex, male/female	<30 y	47/41	18/22	10/16	0.176†
	>30 y	26/30	20/18	8/10	0.788†
Age, y	<30 y	24.66 ± 2.94 (18–30)	25.03 ± 2.79 (20–30)	24.92 ± 3.74 (18–30)	0.8974*
	>30 y	39.25 ± 7.77 (31–60)	41.89 ± 9.91 (31–60)	42.77 ± 7.47 (31–60)	0.1794*

 Data in parentheses represent the range. SE, spherical equivalent; AL, axial length.

*  Kruskal-Wallis rank test.

†  χ² test.

The relationships between MLS and SE or AL were analyzed by using Spearman's rank correlation test after adjustment for age and sex. Significant correlations were observed in all 16 parameters in the unadjusted data. The effects of SE or AL were represented as a covariate and used to calculate the adjusted data for AL or SE (only the eight main parameters were shown in Table 2). The effect of SE on the measurements was negligible in the adjusted data for both age groups, indicating that the significant relationship between SE and the measurements in the unadjusted data were strongly induced by AL. In group A, after adjustment, a significant relationship still existed between AL and the measurements in the foveal area, total macular, superior, and temporal quadrants. No relationships between MLS and age or sex were detected in each group (data not shown). 

Table 2.

 

Correlation between MLS and SE and AL*

	AL				SE
	Age < 30 y		Age > 30 y		Age < 30 y		Age > 30 y
	r	P	r	P	r	P	r	P
Outer macula
Unadjusted	0.330	<0.0001	0.589	<0.0001	−0.295	0.0003	−0.602	<0.0001
Adjusted	0.154	0.0632	0.153	0.1105	−0.003	0.9707	−0.114	0.253
Middle macula
Unadjusted	0.337	<0.0001	0.303	0.0013	−0.305	0.0002	−0.296	0.0017
Adjusted	0.150	0.0707	0.094	0.3291	−0.012	0.8851	−0.061	0.5240
Foveal macula
Unadjusted	0.381	<0.0001	0.379	<0.0001	−0.330	<0.0001	−0.338	0.0003
Adjusted	0.204	0.0138	0.183	0.0561	0.023	0.7836	−0.006	0.9488
Total macula
Unadjusted	0.364	<0.0001	0.467	<0.0001	−0.319	<0.0001	−0.476	<0.0001
Adjusted	0.185	0.0251	0.110	0.2504	0.013	0.8812	−0.155	0.1084
Superior quadrant
Unadjusted	0.343	<0.0001	0.543	<0.0001	−0.303	0.0002	−0.546	<0.0001
Adjusted	0.167	0.0437	0.153	0.1113	0.004	0.9591	−0.168	0.0792
Temporal quadrant
Unadjusted	0.396	<0.0001	0.510	<0.0001	−0.362	<0.0001	−0.529	<0.0001
Adjusted	0.173	0.0372	0.105	0.2734	−0.023	0.7821	−0.195	0.0514
Inferior quadrant
Unadjusted	0.249	0.0024	0.367	<0.0001	−0.217	0.0084	−0.422	<0.0001
Adjusted	0.126	0.1286	−0.015	0.8694	0.011	0.8895	−0.126	0.1286
Nasal quadrant
Unadjusted	0.287	0.0004	0.385	<0.0001	−0.290	0.0004	−0.409	<0.0001
Adjusted	0.065	0.4335	0.054	0.5738	−0.079	0.3403	−0.159	0.0967

*  Spearman's rank correlation test.

Total and regional MLS in each group are summarized in Tables 3 and 4, and significant P values are displayed in Figure 2 (P1: high myopia group versus no myopia group, P2: high myopia group versus low to moderate myopia group, P3: low to moderate myopia group versus no myopia group, P4: age (>30) vs. age (<30) in high myopia group). When MLS in the high myopia group was compared with that in the no myopia group, significant decreases were observed in all regions except the middle ring in older subjects. When MLS in the high myopia group was compared with that in the low to moderate myopia group, although no significant difference in foveal MLS was found, the average outer ring, superior quadrant, and total MLS were significantly lower in the high myopia eyes (P < 0.05) in both age groups. MLS only significantly decreased in the nasal quadrant in group A (P = 0.0257) and in the temporal quadrant in group B (P = 0.0010) of the high myopia group. When the low to moderate myopia group was compared with the no myopia group, only MLS of the foveal macula and temporal outer quadrant was significantly lower in the low to moderate myopia eyes (P < 0.05) in both age groups. When MLS was compared between the two age groups, only those of the outer ring and the superior outer region were significantly lower in the high myopia older subjects. The MLS difference in low to moderate myopia and no myopia eyes between the two age groups were not significantly different (data not shown). Relationships between fixation stability within 2° and 4° and SE or AL after controlling for age and sex were also nonsignificant (data not shown). 

Table 3.

 

Macular Light Sensitivity Measurements (dB) in Three Diagnostic Groups

	Age	High Myopia	Low to Moderate Myopia	No Myopia
Outer average	<30 y	19.05 ± 1.38 (13.50–20.00; 83)	19.40 ± 0.76 (17.20–20.00; 40)	19.83 ± 0.28 (19.00–20.00; 26)
	>30 y	18.87 ± 1.19 (14.75–20.00; 56)	19.49 ± 0.78 (16.75–20.00; 38)	19.95 ± 0.09 (19.75–20.00; 18)
Middle average	<30 y	19.28 ± 1.27 (14.75–20.00; 83)	19.83 ± 0.24 (19.25–20.00; 40)	19.96 ± 0.08 (19.75–20.00; 26)
	>30 y	19.31 ± 1.15 (15.13–20.00; 56)	19.71 ± 0.69 (17.00–20.00; 38)	19.85 ± 0.24 (19.25–20.00; 18)
Foveal average	<30 y	18.83 ± 1.81 (13.00–20.00; 83)	19.74 ± 0.49 (18.00–20.00; 40)	19.92 ± 0.21 (19.25–20.00; 26)
	>30 y	18.69 ± 1.92 (11.25–20.00; 56)	19.64 ± 0.66 (17.00–20.00; 38)	19.79 ± 0.39 (19.00–20.00; 18)
Total average	<30 y	19.09 ± 1.39 (13.90–20.00; 83)	19.67 ± 0.47 (18.15–20.00; 40)	19.90 ± 0.14 (19.60–20.00; 26)
	>30 y	19.01 ± 1.27 (14.40–20.00; 56)	19.39 ± 1.40 (11.95–20.00; 38)	19.88 ± 0.15 (19.6–20.00; 18)
Superior outer	<30 y	18.47 ± 1.78 (12.40–20.00; 83)	18.98 ± 1.39 (14.40–20.00; 40)	19.72 ± 0.61 (18.00–20.00; 26)
	>30 y	18.02 ± 1.72 (12.40–20.00; 56)	19.07 ± 0.95 (16.80–20.00; 38)	19.89 ± 0.25 (19.00–20.00; 18)
Superior middle	<30 y	19.12 ± 1.52 (13.20–20.00; 83)	19.76 ± 0.45 (18.40–20.00; 40)	19.93 ± 0.17 (19.50–20.00; 26)
	>30 y	19.09 ± 1.39 (14.40–20.00; 56)	19.61 ± 0.74 (16.80–20.00; 38)	19.93 ± 0.15 (19.60–20.00; 18)
Superior quadrant	<30 y	18.79 ± 1.59 (12.80–20.00; 83)	19.37 ± 0.90 (16.40–20.00; 40)	19.83 ± 0.33 (18.75–20.00; 26)
	>30 y	18.56 ± 1.49 (13.40–20.00; 56)	19.34 ± 0.79 (16.80–20.00; 38)	19.91 ± 0.19 (19.30–20.00; 18)
Temporal outer	<30 y	19.24 ± 1.36 (13.20–20.00; 83)	19.78 ± 0.58 (17.60–20.00; 40)	19.74 ± 0.16 (19.50–20.00; 26)
	>30 y	18.96 ± 1.32 (14.00–20.00; 56)	19.56 ± 0.93 (16.80–20.00; 38)	19.80 ± 0.10 (19.50–20.00; 18)
Temporal middle	<30 y	19.32 ± 1.29 (14.80–20.00; 83)	19.88 ± 0.24 (19.20–20.00; 40)	19.96 ± 0.14 (19.50–20.00; 26)
	>30 y	19.43 ± 1.06 (15.20–20.00; 56)	19.63 ± 0.61 (18.00–20.00; 38)	19.25 ± 0.07 (19.00–20.00; 18)
Temporal quadrant	<30 y	19.28 ± 1.29 (14.00–20.00; 83)	19.83 ± 0.39 (18.40–20.00; 40)	19.94 ± 0.15 (19.50–20.00; 26)
	>30 y	19.19 ± 1.12 (14.60–20.00; 56)	19.59 ± 0.72 (17.40–20.00; 38)	19.85 ± 0.06 (19.20–20.00; 18)
Inferior outer	<30 y	19.40 ± 1.29 (13.25–20.00; 83)	19.81 ± 0.37 (18.80–20.00; 40)	19.97 ± 0.11 (19.60–20.00; 26)
	>30 y	19.44 ± 0.94 (15.20–20.00; 56)	19.59 ± 1.20 (15.60–20.00; 38)	19.90 ± 0.26 (19.20–20.00; 18)
Inferior middle	<30 y	19.40 ± 1.23 (14.80–20.00; 83)	19.90 ± 0.27 (19.20–20.00; 40)	19.98 ± 0.09 (19.50–20.00; 26)
	>30 y	19.46 ± 1.14 (15.20–20.00; 56)	19.63 ± 0.74 (18.00–20.00; 38)	19.95 ± 0.03 (19.90–20.00; 18)
Inferior quadrant	<30 y	19.40 ± 1.23 (14.60–20.00; 83)	19.86 ± 0.22 (19.40–20.00; 40)	19.96 ± 0.12 (19.50–20.00; 26)
	>30 y	19.45 ± 0.98 (15.40–20.00; 56)	19.61 ± 0.88 (16.80–20.00; 38)	19.95 ± 0.13 (19.60–20.00; 18)
Nasal outer	<30 y	19.13 ± 1.40 (14.40–20.00; 83)	19.51 ± 0.78 (17.20–20.00; 40)	19.96 ± 0.14 (19.50–20.00; 26)
	>30 y	19.08 ± 1.21 (14.40–20.00; 56)	19.56 ± 0.84 (17.20–20.00; 38)	19.84 ± 0.28 (19.20–20.00; 18)
Nasal middle	<30 y	19.32 ± 1.29 (15.20–20.00; 83)	19.72 ± 0.44 (18.80–20.00; 40)	19.92 ± 0.23 (19.20–20.00; 26)
	>30 y	19.26 ± 1.49 (13.60–20.00; 56)	19.76 ± 0.66 (17.60–20.00; 38)	19.91 ± 0.26 (19.20–20.00; 18)
Nasal quadrant	<30 y	19.23 ± 1.31 (15.20–20.00; 83)	19.62 ± 0.58 (18.20–20.00; 40)	19.95 ± 0.13 (19.60–20.00; 26)
	>30 y	19.17 ± 1.30 (14.60–20.00; 56)	19.66 ± 0.72 (17.40–20.00; 38)	19.88 ± 0.17 (19.60–20.00; 18)

 Data on the first line represent the mean ± SD, and data in parentheses represent the range and the number of patients.

Table 4.

 

P MLS Measurements in Three Diagnostic Groups

	Age	P	P1	P2	P3	P4
Outer average	<30 y	0.0037	0.0010	0.0151	0.3153
	>30 y	0.0001	0.0000	0.0004	0.0014	0.0214
Middle average	<30 y	0.0042	0.0017	0.0212	0.1688
	>30 y	0.1214	0.0701	0.4778	0.1457	0.5253
Foveal average	<30 y	0.0011	0.0010	0.0704	0.0237
	>30 y	0.0033	0.0063	0.4073	0.0084	0.2787
Total Average	<30 y	0.0014	0.0006	0.0171	0.1179
	>30 y	0.0001	0.0001	0.0185	0.0155	0.0614
Superior outer	<30 y	0.0007	0.0002	0.0117	0.1693
	>30 y	0.0001	0.0000	0.0000	0.0026	0.0313
Superior middle	<30 y	0.0082	0.0052	0.1421	0.0664
	>30 y	0.0070	0.0034	0.0304	0.1166	0.4181
Superior quadrant	<30 y	0.0017	0.0006	0.0224	0.1282
	>30 y	0.0001	0.0000	0.0000	0.0108	0.0632
Temporal outer	<30 y	0.0030	0.0046	0.3735	0.0172
	>30 y	0.0001	0.0001	0.0180	0.0040	0.0540
Temporal middle	<30 y	0.0159	0.0096	0.1343	0.0962
	>30 y	0.0109	0.0027	0.0055	0.7030	0.7669
Temporal quadrant	<30 y	0.0019	0.0026	0.2598	0.0152
	>30 y	0.0001	0.0000	0.0010	0.0581	0.1144
Inferior outer	<30 y	0.0764	0.0295	0.0854	0.3993
	>30 y	0.0133	0.0262	0.7826	0.0133	0.1470
Inferior middle	<30 y	0.0027	0.0047	0.2110	0.0213
	>30 y	0.0342	0.0089	0.0260	0.5283	0.8894
Inferior quadrant	<30 y	0.0930	0.0378	0.0722	0.5028
	>30 y	0.0238	0.0088	0.1697	0.1370	0.2101
Nasal outer	<30 y	0.0017	0.0004	0.0032	0.3859
	>30 y	0.0018	0.0028	0.5103	0.0076	0.1387
Nasal middle	<30 y	0.0845	0.0327	0.0433	0.7067
	>30 y	0.0145	0.0258	0.5100	0.0234	0.4924
Nasal quadrant	<30 y	0.0127	0.0038	0.0257	0.3797
	>30 y	0.0052	0.0104	0.8003	0.0084	0.2101

  P: multiple comparison of three groups.

  P1: high myopia group vs. nonmyopia group.

  P2: high myopia group vs. low to moderate myopia group.

  P3: low to moderate myopia group vs. nonmyopia group.

  P4: age >30 y vs. <30 y in the high myopia group. No significant difference was detected between age groups in low to moderate myopia and nonmyopia groups (data not shown).

Figure 2.

 

Significant P values on each of the nine parts of the 10° macular area in group (A) and group B (B). The nine parts correspond with the divisions shown in Figure 1. Significance (P, P1, P2, P3, and P4) was assigned to the corresponding part. The significance of the results for the entire 10° macular is shown on the bottom of the circle. P: multiple-comparison of three groups; P1: high myopia group versus no myopia group; P2: high myopia group versus low to moderate myopia group; P3: low to moderate myopia group versus no myopia group; P4: age (>30) vs. age (<30) in high myopia group.

Discussion

In the present study in Chinese adults with corrected visual acuity better than 20/20, axial myopia was associated with lower total MLS, especially in subjects with high myopia. A key finding was that changes in MLS in all nine macular divisions correlated significantly with both SE and AL. To our knowledge, the differences in MLS measured by the MP1 (Nidek Technologies) between myopic and nonmyopic subjects have not been reported. The correlation between MLS and both SE and AL emphasizes the existence of functional differences in the macula between adults with high, low to moderate, and no myopia. The effect on MLS was significant in all quadrants in the high myopia subjects, with the magnitude of the reduction in MLS being highest in the superior outer quadrant (see examples in Fig. 3). 

Figure 3.

 

Digital results and photographs obtained by the MP1 microperimeter (Nidek, Padova, Italy) in myopic and nonmyopic maculas. (A, C) Images and perimetry results from a 55-year-old emmetropic man, with light sensitivity at 20 dB in all 40 local points. (B, D) Images and MP1 results from a 35-year-old man with high myopia (best corrected vision of 20/20), in whom the MLS in the superior quadrant significantly decreased.

The MP1 microperimeter quantifies macular sensitivity and fixation in an exact, fundus-related method, thus providing detailed information regarding the degree and pattern of macular function alteration; it has been successfully used in the diagnosis and monitoring of different macular disorders, including age-related macular degeneration, macular dystrophies, and diabetic macular edema. ^15–20 The parameters in normal subjects of different age, sex, and race, as well as inter- and intraexaminer reliability have been studied to determine the effects of these extraneous variables on measurements. ^22,24 In these reports, MP1 measurements exhibited high inter- and intraexaminer reliability. Macular function measured by MP1 in normal subjects decreased with age, but had no apparent relationship with sex and race, ^22,24 providing support for further usage of MP1 in clinical applications. 

In high myopia eyes, many studies have reported declines in parameters of visual function such as corrected visual acuity, ²⁵ visual field, ²⁶ color vision, ²⁷ light sense, ²⁷ and contrast sensitivity. ²⁷ Previous studies have reported generally decreased macular ERG response in patients with myopia and normal corrected visual acuity. ^11–14 In contrast, Thorn et al. ²⁸ reported that contrast sensitivity in patients with high myopia and normal corrected visual acuity was not significantly different from that in normal patients. Consequently, they hypothesized that this parameter would not be affected by high myopia until the photoreceptor cells were injured. The findings of the present study provide information to clinicians regarding the pattern of regional variations in macular function in patients affected by myopia. 

The observations in the present study also suggest that refractive error should be considered in the interpretation of MLS normograms. Since the current normative database in the MP1 (Nidek) database does not take refractive error into account, clinicians should be aware of the effect of this parameter when evaluating macular function in the diagnosis and monitoring of diseases such as diabetic macular edema, glaucoma, or after-cataract maculopathy. 

In myopic eyes, the elongation of the globe leads to mechanical stretching and thinning of the retinal area. Previous studies have used OCT to identify a correlation between AL or SE and retinal thickness and regional variations in the relationship between myopia and macular thickness. ^9,10 In those studies, significantly decreased macular thickness was detected in the outer ring, defined as the circular area 1.5 to 3 mm outside of the fovea, whereas macular thickness in the fovea increased. The results of the present study were compared with those in previous studies in which macular thickness was measured by OCT. To correlate retinal thickness data accurately with retinal sensitivity data, we compared the more central OCT fields 1 to 5 (ETDRS-type) and excluded OCT fields 6 to 9, as in previous studies because the 10° MP1 grid pattern covered only a limited area of these outer fields. ¹⁷ Although the macular thickness in the fovea increased in high myopia eyes with normal corrected visual acuity, the foveal MLS decreased. In contrast, the results of both OCT and MP1 examinations of the foveal macular region in low to moderate myopia patients were comparable to those of normal patients. These results may indicate the existence of a relationship between foveal macular functional impairment and anatomic changes in high myopia. The outer ring MLS decreased significantly in both myopia groups. It was predicted that the most severe retinal elongation would occur at the outer ring in the OCT field. There were minor changes in inner ring thickness, and MLS in this region decreased, possibly indicating that MP1 is more sensitive than OCT to macular changes. Although it remains uncertain whether there are any direct links between decreased macular thickness and the subsequent onset of clinically significant macular disease, MLS has been identified as a relevant variable in the quality of visual function. ^9,22 A 20° MP1 grid pattern, which covers a larger macular area including the outer ring of the OCT field, should be selected to detect changes in myopic macular function; both MP1 and OCT examinations should be performed on the same patient to optimize comparisons in future studies. 

Maculopathy develops in myopic eyes with age, and the thinning of the macula could be due to the stretching of a normal volume of retina over a larger area or a decreased number of photoreceptors. Luo et al. ⁹ noted that axial myopia was associated with reduced quadrant-specific macular thickness, except in the inferior quadrants. Although they examined a group of nonpathologic myopic children, their findings provide support for the results of the present study. The results are further supported by the association of longer AL and increased retinal thinning observed in the posterior pole of high myopia eyes affected by early chorioretinal atrophy. ^27,29,30 Macular light sensitivity was decreased in all four quadrants in both age groups affected by high myopia, whereas only the superior quadrant MLS was decreased in both age groups affected by low to moderate myopia. Only the inferior quadrant MLS did not change in both age groups affected by moderate to low myopia, indicating that the superior quadrant may be more vulnerable to myopia than other quadrants, and the inferior quadrants may be more resistant. 

The similar pattern of associations of MP1 measurements with SE and AL observed in the present study confirms the correlation between these variables. Although no differences in MP1 measurements attributable to age and sex were found between the groups, the unadjusted data in Table 2 were analyzed after adjustment for these variables. Significant correlations between MLS and AL or SE were found in all 16 parameters. When the effect of AL was standardized as a covariate, the effect of SE on the measurements disappeared (Table 2). These results indicate that the significant relationship between SE and the measurements was strongly induced by AL. 

The degree of myopia is proportional to AL, which is largely due to increased vitreous chamber depth. ^31,32 Thus, AL possibly has a greater effect on variation in total macular volume than does SE. The stretch effect from the elongation of AL in myopic progression may partially explain the reduction of macular retinal thickness as a component of the overall stretching of ocular structures in both high and low to moderate myopic human subjects and animal models. The quadrant-specific stretch effect may also explain the functional abnormality in our study. Increased stretching of the macular area may lead to an overall decrease in MLS in high myopia patients, as in group B. This finding could be explained mechanistically by a change in the ocular structure due to increased AL, which may cause a coarser distribution of cones in the posterior retina. ³³ In group A, after adjustment, the relationship between AL and MP1 measurements was still significant in the fovea, total macula, and superior and temporal quadrants; AL and SE may have independent effects on the results of the measurements in these parts. These independent effects may be attributable to stronger accommodation in younger patients, so that there may be lower correlation between AL and SE in younger adults undergoing microperimetry. 

The MLS in the outer ring and superior outer region decreased with age in the high myopia group but not in the low to moderate myopia and no myopia groups. This difference may be due to decreased photoreceptor cell density in older patients. However, spot light sensitivity in the eccentric retinal area of myopic eyes may not be solely due to the functional changes in the retina, such as the effects of retinal stretching or thinning. During fixation by the fovea of the relatively prolate myopic eye, a spot of light projected onto the peripheral retina would be diffused due to the relative hyperopia. ³⁴ The synergistic combination of this effect and the induced oblique astigmatism due to the eccentric viewing causes the spotlight to spread out even further and elevates the threshold at the periphery in elongated, prolate eyes. This peripheral defocusing is exaggerated in eyes with macular areas affected by posterior staphyloma. It remains unclear whether this peripheral defocusing effect achieves statistical significance for the 10° eccentric testing in this study. 

For MP1 measurements in normal subjects, there was a decline in fixation stability with age, but no statistically significant differences between the median fixation stability values for groups of different sex or race. ²⁴ In the present study, we suggest that the fixation stability within 2° and 4° in myopic subjects with normal corrected visual acuity did not need to be taken into consideration when analyzing the results. 

In summary, MLS detected by the MP1 (Nidek Technologies) decreased in the myopic adults with corrected visual acuity better than 20/20, especially in high myopia eyes. The quadrant-specific abnormality correlated with AL suggests that the MLS changes are intimately related with axial myopia. We suggest that any evaluation by MP1 of MLS in eyes affected by macular diseases and glaucoma should always be interpreted in the context of the degree of refractive error and the region of measurement. 

Footnotes

 Disclosure: Y. Qin, None; M. Zhu, None; X. Qu, None; G. Xu, None; Y. Yu, None; R.E. Witt, None; W. Wang, None