Electrophysiological and Psychophysical Differences between Early- and Late-Onset Strabismic Amblyopia

Abstract

purpose. To compare visual evoked potentials (VEPs) and contrast sensitivity in adults with early- or late-onset strabismic amblyopia.

methods. Twelve adults with early- and 12 with late-onset strabismic amblyopia with similar ranges of visual acuity were studied. Pattern-onset VEPs to 30-minute checks were recorded at a range of contrast levels. Contrast sensitivity (CS) was measured at 3.2 cyc/deg using a two-alternative, forced-choice staircase method.

results. There was no significant difference in VEP CII latency or amplitude between amblyopic and fellow eyes across all contrast levels for the early-onset group, but in the late-onset group, CII latencies were significantly longer and amplitudes smaller in the amblyopic eye. CII responses in both amblyopic and fellow eyes of the early-onset amblyopes were of significantly shorter latency and smaller amplitude than normal. In the late-onset group the CII responses from the amblyopic eye were of significantly increased latency and reduced amplitude compared with normal, whereas latency and amplitude of fellow eye responses did not differ significantly from normal. Late-onset amblyopes showed reduced CS across the central field for the amblyopic eye, but increased CS for the fellow eye compared with normal. In the early-onset group, central CS did not differ between amblyopic and fellow eyes or from normal.

conclusions. There are significant differences in the electrophysiological and psychophysical characteristics of adults with early- and late-onset strabismic amblyopia.

Previous electrophysiological studies of amblyopia have described both increased latency and reduced amplitude of responses from the amblyopic eye, ^{1 2 3 4 5} but have not considered the age of onset of amblyopia. However, data from studies in nonhuman primates indicate that there are two different periods of developmental sensitivity, before and after 3 months of age, during which the patterns of change in cell size in the lateral geniculate nucleus (LGN) produced by abnormal visual experience are distinctly different and during which the interactions between the pathways from the two eyes differ. ⁶ It has been suggested from anatomic evidence that, if there are two comparable sensitive periods in humans, then the corresponding ages are probably before or after 18 months of age. ⁶ This is based on the generally acknowledged premise that 1 week of age in the monkey is equivalent to 1 month in the human, together with an addition to allow for the anatomic evidence that the human visual system is substantially less mature at birth than that of the monkey. ⁶  

In the present study, we examined visual evoked potentials (VEPs) and contrast sensitivity (CS) in two groups of adults with strabismic amblyopia who had a clear history of onset either before or after 18 months of age. Pattern-onset stimulation was used because it was of particular interest to obtain responses across a wide range of contrast levels, particularly at low contrast levels for comparison with psychophysically determined CS thresholds. Whereas both pattern-onset and reversal stimulation elicited good responses at high contrast, reliable low-contrast responses were obtained only with pattern-onset stimulation. 

Preliminary data from this study have been presented in abstract form. ⁷  

Subjects and Methods

Twelve adults with early- and 12 with late-onset strabismic amblyopia were recruited after attendance at a strabismus clinic. Patients were assigned to early- or late-onset groups on the basis of an unequivocal history of onset of amblyopia before or after 18 months of age. Parental confirmation was obtained when necessary. The age of onset of squint was clearly remembered as, in most cases, was the age at which patching was started. All subjects underwent full orthoptic and ophthalmic assessment before testing. Fifteen normal young adults formed the control group for the VEPs and 12 for the CS measurements. 

Visual Evoked Potentials

Monocular VEPs were recorded to a checkerboard pattern appearance stimulus at 100%, 80%, 40%, 20%, 10%, and 5% contrast levels, while the other eye was patched. Space-averaged mean luminance was 90 cd/m² for both the stimulus checkerboard and interstimulus display. Field size was 20° × 16°, with a check size of 30 minutes and a viewing distance of 1 m. The stimulus presentation was pattern on for 40 ms and pattern off for 500 ms. Fixation was maintained using a central red spot. Responses were recorded from an electrode at Oz (midline) referenced to Fz (midfrontal). A ground electrode was placed on the forehead. The surface electrode impedance was less than 5 KΩ. The number of responses averaged for each trial was at least 64, with a minimum of two replications being recorded for each contrast level for each eye. The analysis time was 300 ms. The filter bandwidth was from 1 to 100 Hz. The patient’s normal refractive correction was used. Peak latency and peak-to-peak amplitudes of the CII component were measured for each contrast level. 

Contrast Sensitivity

A horizontally oriented 3.2-cyc/deg sinusoidal grating pattern with spatial (σ_x = σ_y = 0.5°) and temporal (σ_t = 125 ms) Gaussian windows was computer generated with 14-bit VSG card and commercial software (Matlab; The MathWorks, Natick, MA). This gave a stimulus subtending approximately 2° of visual angle at the viewing distance of 0.93 m. Monocular detection thresholds were measured for the stimuli with a temporal, two-alternative, forced-choice (2AFC) technique. A staircase procedure driven by the subject’s responses and controlled by computer determined the detection threshold. Each trial consisted of two presentations (cued by sounds) one of which contained the stimulus, whereas the other was a blank field of the same space-averaged luminance. Thresholds were determined by a one-up, one-down staircase procedure in which the contrast was divided by 1.15 after a correct response and multiplied by 1.15 after an incorrect response. Until the first error the divisor was 1.25. Every time an incorrect response was followed by a correct response, a reversal was noted. The session ended after 10 reversals. Threshold was defined as the average contrast over the last five reversals. Stimuli were presented in the center of a computer monitor (MultiSync; NEC, Tokyo, Japan) at a mean luminance of 90 cd/m². The stimuli were surrounded by a luminance-matched field (10° × 8°). The room was darkened. The display screen’s contrast linearity was measured and found to hold up to 98% contrast. For central measurements a fixation target was computer generated within the luminance-matched field. For eccentric measurements a small fixation spot was fixed to the monitor screen at 5° and 10° along the horizontal meridian, and the subject was observed to ensure that fixation was maintained. The patients’ normal refractive correction was used during testing. 

Statistical Analysis

Visual acuities between groups were compared by unpaired t-test after conversion to log of the minimum angle of resolution (logMAR) equivalents. Latencies and amplitudes of the CII VEP response were compared using analysis of variance (ANOVA). CS measurements were log transformed. Comparisons of central CS between amblyopic and fellow eyes were made with paired t-tests and between groups, with unpaired t-tests. CS across the central field was analyzed by ANOVA. 

The research followed the tenets of the Declaration of Helsinki. The subjects gave informed consent after explanation of the nature and possible consequences of the study. The research was approved by the Ethics Committee of Moorfields Eye Hospital. 

Results

Eleven early- and 10 late-onset amblyopes completed the electrophysiological testing. Data for one early-onset amblyope were subsequently excluded from the ANOVA because there was no detectable response at the lowest contrast level. Eleven early and 11 late-onset amblyopes completed the CS measurements (Table 1) . 

Most of the strabismic amblyopes studied had initially been esotropic as children, although a number had now become consecutively exotropic (Table 1) . All patients had a manifest squint after reaching adulthood. Most had no, or only occasional, diplopia, indicating suppression of the deviating eye. Suppression was confirmed on testing with Bagolini glasses in nine subjects in the early-onset group and eight in the late-onset group. Patient 21 had a manifest squint with troublesome diplopia. Patient 19 had an accommodative squint and amblyopia that was treated during childhood and had experienced an increase in squint angle several years previously. She recovered to a stereoacuity of 120 seconds of arc (TNO Stereoacuity Test; Richmond Products, Boca Raton, FL) after treatment with Botulinum toxin. 

Most subjects in each group had low hypermetropic refractive errors in both amblyopic and fellow eyes, with no significant differences between early- and late-onset groups (mean spherical equivalents: early amblyopic eye, 0.85 D; early fellow eye, 0.18 D; late amblyopic eye, 1.84 D; late fellow eye, 0.77 D). Four subjects in each group had anisometropia of more than 1.5 D of spherical equivalent (Table 1) . No control subject had significant anisometropia. Patients in both early- and late-onset groups showed a similar spread of acuities in the amblyopic eyes (logMAR equivalent means: early onset 0.63, Snellen equivalent means ≈ 6/26, 20/85; late onset 0.83, Snellen equivalent means ≈ 6/40, 20/135; P = 0.25). There was no significant difference in fellow eye acuity between the groups (logMAR equivalent means: early onset = −0.04 Snellen equivalent means ≈ 6/5, 20/18; late onset −0.08, Snellen equivalent ≈ 6/5, 20/17; P = 0.34) nor any differences from the normal group. All normal subjects had a visual acuity of at least 6/6 in each eye with binocular single vision. 

Visual Evoked Potentials

Comparison of Amblyopic with Fellow Eyes.

There was no significant difference in CII latency or amplitude between amblyopic and fellow eyes in the early-onset amblyopes, across all contrast levels. In the late-onset amblyopes the latency in the amblyopic eye was longer and the amplitude markedly smaller than that of the fellow eye (Table 2 ; Figs. 1 2A 2B ). 

Comparison between Early- and Late-Onset Amblyopes.

The CII latencies of the amblyopic eyes of the early-onset group were markedly shorter and the amplitudes larger than those of the amblyopic eyes of the late-onset group, across all contrast levels. The latencies of the fellow eyes of the early-onset group were similarly shorter than those in the late-onset group, but the amplitudes of the late-onset fellow eyes were larger than in the early-onset group (Table 3 ; Figs. 2A 2B ). 

Comparison of Early- and Late-Onset Amblyopes with Normal Subjects.

The CII latencies of both the amblyopic and fellow eyes of the early-onset amblyopes were shorter than those in the normal group and the amplitudes smaller (Table 4 ; Figs. 2C 2D ). In the late-onset group, the CII latency in the amblyopic eye was longer than normal and the amplitude markedly smaller, whereas the latency and amplitude of the fellow eye did not differ significantly from normal (Table 4 ; Figs. 2E 2F ). 

Analysis of Combined Early- and Late-Onset VEP Data.

To allow comparison with previous studies, data from the early- and late-onset amblyopes were combined and analyzed together. The mean CII latency in the amblyopic eye of the combined group was significantly shorter than in the fellow eye, across all contrast levels, and the mean amplitude was smaller (Table 2 ; Figs. 3A 3B ). 

Data from the first seven subjects in the early- and late-onset groups were combined and analyzed as a single group and compared with the data from 14 normal subjects. The mean CII amplitude for the amblyopic eyes of the combined group was significantly smaller than normal, but the mean latency in the amblyopic eyes and latency and amplitude of the fellow eyes did not differ significantly from normal (Table 4) . 

Relationship of VEPs to Visual Acuity.

The CII VEP latency to 100% contrast for early- and late-onset amblyopic eyes is plotted against logMAR equivalent acuity in Fig. 4A . There was no relationship between CII latency and acuity, with the latency difference between early- and late-onset groups being present across the whole acuity range. Similarly, there was no relationship between CII latency and acuity of the fellow eyes (Fig. 4B) . 

Contrast Sensitivity

Central CS showed no difference between amblyopic and fellow eyes in the early-onset group (Table 5) or between amblyopic or fellow eyes and normal eyes (Table 6 ; Figs. 5 ). In the late-onset amblyopes the CS of the amblyopic eyes was significantly less than that of the fellow eyes (Table 5) . The CS of the amblyopic eyes was also significantly less than normal, whereas the fellow eyes had a significantly greater CS than normal (Table 6 ; Figs. 5 ). 

In the late-onset amblyopes and normal subjects CS measurements were also taken at 5° and 10° along the nasal and temporal horizontal meridians. This showed that the differences in CS extended across the central field (Table 7 ; Fig. 6 ). Comparison with normal subjects showed both that the CS across the central field of the late-onset amblyopic eyes was reduced and the CS of the fellow eyes was increased compared with normal (Table 7 ; Figs. 6 ). 

Combined CS Analysis

Data for central CS in early- and late-onset groups were combined and analyzed as a single group. The CS of the combined amblyopic eyes was significantly less than that of the combined fellow eyes (Table 5) . Neither the CS of the combined amblyopic eyes nor of the combined fellow eyes differed significantly from normal (Table 6) . 

Discussion

The electrophysiological findings in human strabismic amblyopes with an onset before 18 months of age differ significantly from those in amblyopes of later onset. In the early-onset amblyopes CII responses from both eyes were of shorter latency and smaller amplitude than normal, with no difference between amblyopic and fellow eyes. In contrast, the CII response from the amblyopic eye of the late-onset amblyopes was of increased latency and markedly reduced amplitude compared with normal eyes, whereas both latency and amplitude of the response from the fellow eye showed no such differences. These differences are not attributable to differences in visual acuity, which was similar in the two groups, but rather suggest differences in cortical pathophysiology. 

The overall degree of amblyopia, as judged by visual acuity, was similar in the early- and late-onset groups. Thus, the greater difference between the CII response from amblyopic and fellow eyes found in the later onset amblyopes does not indicate a greater general sensitivity to abnormal visual experience at a later age, but rather that a particular aspect of visual processing had been more affected at a later age. It is known that different visual functions develop at different rates, ^{8 9} that there is more than one sensitive period in visual development, ^{6 10} and that the sensitivity of the human visual system to abnormal visual experience declines with age. ¹¹ However, no previous human studies appear to have investigated the possibility that abnormal visual experience starting at different ages within the sensitive period may result in amblyopia with different characteristics. Previous studies presumably combined data from both early- and late-onset amblyopes and reported reduced amplitude pattern onset VEPs. ^{4 5} In addition, Shawkat et al. ⁴ demonstrated a reduced amplitude from the fellow eye compared with normal. These findings could be replicated in the present study by combining data from early- and late-onset amblyopes, when the other differences between the groups canceled out. 

Although the present data indicate clear differences in pathophysiology between early- and late-onset amblyopes, they do not indicate the nature or location of the changes in central visual pathway function. The CII component of the pattern-onset VEP was evaluated quantitatively, because it is the most consistent component and thus most commonly measured. The exact generators of the different VEP components have not been fully ascertained. CII may arise from extrastriate visual areas, ¹² but nevertheless, changes in CII can occur consequent on changes at earlier stages of visual processing. Qualitative differences are apparent in the CI component of the group mean waveforms. This component is thought to originate in the striate cortex. ¹³  

It is unusual to find a shortening of VEP latency in a pathologic condition. One possibility is that the shortened latency is caused by an enhancement of magnocellular in relation to parvocellular responses in strabismic amblyopia. Findings in nonhuman primates show an increase in the ratio of magnocellular to parvocellular cell size in both undeprived and deprived laminae of the LGN after monocular deprivation. ⁶ Also, there is evidence of relative sparing of the motion system in human amblyopia as judged by motion VEPs. ¹⁴ Preliminary data indicate a shortening of motion VEP latencies from both eyes in strabismic amblyopes. ¹⁵  

Despite the use of only one spatial frequency, the CS changes also show clear differences between early- and late-onset amblyopes in keeping with differences in underlying pathophysiology. They also emphasize the importance of making comparison with normal subjects as well as with the fellow eye. ¹⁶  

The present results depend critically on the ability of the patients to identify the time of onset of their amblyopia to before or after 18 months of age. Patients were only recruited to the study when a clear history was available, which was in approximately half of potential patients. The age of onset of squint was clearly remembered, as, in most cases, was the age at which patching was started. Although amblyopia may not develop in children with early-onset strabismus who cross fixate, ¹⁷ there is evidence that those who become amblyopes do so soon after the onset of squint. ^{18 19 20} Conversely, most of the late-onset amblyopes started to squint well after 18 months of age. Although it is possible for anisometropic amblyopia to predate a squint, it is relatively uncommon, and most of these patients were not anisometropic. It is thus unlikely that amblyopia was present before 18 months of age in this group. It is difficult to explain the striking differences found on any basis other than the age of onset of amblyopia. 

If similar patterns of visual development with two distinct sensitive periods occur in monkeys and humans, then there are several implications. First, it may explain the apparent discrepancy between the lengths of the critical period in monkeys and humans. ²¹ Second, most of the published data regarding cortical changes after visual deprivation in primates apply only to early-onset amblyopia in humans. In particular, changes in the ocular dominance columns in layer IV of the primate visual cortex occur only during the early sensitive period. ^{6 22 23} This would explain why no ocular dominance column changes were found in a human strabismic amblyope with onset at the age of 2 years. ²⁴ There are very few physiological data on late-onset amblyopia in the monkey. Finally, the demonstration of differences between early- and late-onset strabismic amblyopes has potential clinical implications. In the early sensitive period in monkeys, reverse suture is necessary to equalize the sizes of LGN cells and reverse the changes in ocular dominance columns. ^{6 22 25 26} In the late sensitive period, substantial recovery in both the deprived and undeprived LGN laminae occurs after simply reopening the deprived eye, although a small difference in cell size between deprived and undeprived cells remains. ²⁶ Simply removing the cause of deprivation in late-onset human amblyopes may allow a substantial degree of recovery and could explain the recovery of acuity demonstrated by correcting only a refractive error in amblyopic children. ^{27 28} However, greater improvement was obtained by those who also wore a patch, either simultaneously ²⁸ or subsequently. ²⁷  

This study has demonstrated marked electrophysiological and psychophysical differences between early- and late-onset amblyopia in humans, which are in keeping with the evidence for two sensitive periods in nonhuman primates. Understanding the differences between patients with early- and late-onset amblyopia may lead to better strategies for treatment. 

 Supported by the Special Trustees, Moorfields Eye Hospital; the Ann Allerton Fund, Royal College of Ophthalmologists; and by a Wellcome Trust Basic Science Training Fellowship (ARD).

 Submitted for publication March 8, 2002; revised July 31, 2002; accepted August 22, 2002.

 Commercial relationships policy: N.

 The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked “advertisement” in accordance with 18 U.S.C. §1734 solely to indicate this fact.

 

Table 1.

 

Summary of Amblyopic Subjects

Subject	Age (y)	Diagnosis	Anisometropia	Snellen Acuity			VEPs	Contrast Sensitivity
										Amblyopic Eye	Fellow Eye
Early onset
1	18	Residual ET	N	6/36	6/9	+	+
2	33	Residual ET	N	6/24	6/6	+	+
3	24	Residual XT	Y	6/18	6/5	+	+
4	40	Residual ET	Y	6/9	6/6	+	+
5	50	Consecutive XT	N	6/24	6/5	+	+
6	35	Consecutive XT	N	6/60	6/5	+	+
7	26	Consecutive XT	N	6/12	6/4	+	+
8	24	Consecutive XT	N	6/9	6/5	+	+
9	24	Residual ET	N	6/24	6/5	+	+
10	23	Residual ET	Y	6/24	6/9	+	−
11	21	Consecutive XT	N	6/18	6/4	+	+
12	17	Residual XT	Y	1/60	6/5	−	+
Mean	28.9
Late onset
13	29	Consecutive XT	Y	6/24	6/5	+	+
14	31	Consecutive XT	Y	2/60	6/4	+	+
15	30	Consecutive XT	N	6/24	6/5	+	+
16	27	Residual ET	N	6/24	6/5	+	+
17	36	Primary ET	N	6/9	6/5	+	−
18	30	Consecutive XT	Y	1/60	6/4	+	+
19	21	Primary ET	N	6/24	6/6	+	+
20	36	Consecutive XT	N	6/60	6/5	+	+
21	28	Primary ET	Y	6/18	6/5	+	+
22	34	Consecutive XT	N	6/36	6/6	+	+
23	37	Residual ET	N	6/60	6/6	−	+
24	49	Consecutive XT	N	6/60	6/5	−	+
Mean	32.3

Table 2.

 

Comparison of VEP CII Parameters for Amblyopic and Fellow Eyes

	Latency					Amplitude
		Mean % Difference Amblyopic vs. Fellow Eye	ANOVA			Mean % Difference Amblyopic vs. Fellow Eye	ANOVA
				F*	P			F*	P
Early onset	0	<0.01	0.93	−8.8	0.03	0.86
Late onset	+12.1	10.69	0.0014	−38.8	21.70	<0.0001
Combined	+6.1	5.54	0.019	−25.9	13.45	0.0003

Figure 1.

 

Group mean pattern-onset VEPs for early and late amblyopes and normal control subjects. The CII responses from both amblyopic and fellow eyes of the early-onset amblyopes are of shorter latency and smaller amplitude than normal. The CII response from the amblyopic eye of the late-onset amblyopes is of increased latency and reduced amplitude whereas that from the fellow eye does not differ significantly from normal.

Table 3.

 

Comparison between Early- and Late-Onset Amblyopes of CII VEP Parameters from Amblyopic and Fellow Eyes

	Latency					Amplitude
		Mean % Difference Late vs. Early Group	ANOVA			Mean % Difference Late vs. Early Group	ANOVA
				F(1,108)	P			F(1,108)	P
Amblyopic eye	+24.4	31.52	<0.0001	−15.7	4.23	0.042
Fellow eye	+11.0	7.25	0.0082	+22.2	8.56	0.0042

Figure 2.

 

Latencies and amplitudes of the CII pattern onset VEP responses at different contrast levels for early- and late-onset amblyopes and normal subjects. (A) Latencies and (B) amplitudes of early- and late-onset amblyopic and fellow eyes compared. (C) Latencies and (D) amplitudes of early-onset amblyopic and fellow eyes compared with normal eyes. (E) Latencies and (F) amplitudes of late-onset amblyopic and fellow eyes compared with normal eyes. Error bars, SE.

Table 4.

 

Comparison of CII VEP Parameters of Amblyopic and Fellow eyes with Normal Eyes

	Latency					Amplitude
		Mean % Difference from Normal	ANOVA			Mean % Difference from Normal	ANOVA
				F*	P			F*	P
Early amblyopic	−8.1	7.15	0.0087	−22.7	6.73	0.0108
Early fellow	−8.1	6.37	0.0131	−15.4	5.40	0.0220
Late amblyopic	+14.1	18.11	<0.0001	−36.1	17.59	<0.0001
Late fellow	+1.9	0.50	0.4808	+3.2	0.59	0.4426
Combined amblyopic	+2.5	1.10	0.2955	−29.2	11.62	0.0008
Combined fellow	−3.3	1.55	0.2150	−4.3	0.29	0.5926

Figure 3.

 

(A) Latencies and (B) amplitudes of the CII pattern onset VEP responses at different contrast levels for combined data from early- and late-onset amblyopes and normal subjects. The amplitude of the combined responses of the amblyopic eyes was reduced, but other differences largely canceled out. Error bars, SE.

Figure 4.

 

CII pattern-onset VEP latencies at 100% contrast plotted against logMAR equivalent acuity for (A) amblyopic and (B) fellow eyes. The latency differences between early- and late-onset amblyopes were present across the whole range of acuities.

Table 5.

 

Comparison of Log Central CS for Amblyopic and Fellow Eyes

	Mean % Difference Amblyopic vs. Fellow Eye	t	P
Early onset	+1.9	0.59	0.57
Late onset	−21.6	3.97	0.0026
Combined	−10.3	2.51	0.021

Table 6.

 

Comparison of Log Central CS of Amblyopic and Fellow Eyes with Normal Eyes

	Mean % Difference from Normal	t	P
Early amblyopic	−0.5	0.13	0.9
Early fellow	−2.4	0.85	0.41
Late amblyopic	−16.6	2.93	0.0081
Late fellow	+6.4	2.21	0.039
Combined amblyopic	−8.5	1.64	0.11
Combined fellow	+2.0	0.7	0.49

Figure 5.

 

Box-and-whisker plots for log central CS at 3.2 cyc/deg for early- and late-onset amblyopes and normal control subjects. Amblyopic and fellow eyes of early-onset amblyopes do not differ from normal. In late-onset amblyopes the CS of the amblyopic eye was reduced and that of the fellow eye increased compared with normal. Upper and lower box limits represent 75th and 25th percentiles, respectively.

Table 7.

 

Log CS across the Central Field for Late-Onset Amblyopes and Normal Subjects

	Mean % Difference	ANOVA
				F(1,100)	P
Late amblyopic vs. fellow	−21.0	36.2	<0.0001
Late amblyopic vs. normal	−16.1	18.61	<0.0001
Late fellow vs. normal	+7.0	8.5	0.0046

Figure 6.

 

Log CS across the central visual field of late-onset amblyopes and normal control subjects. CS of the amblyopic eye was reduced and that of the fellow eye increased compared with normal. Error bars, SE.