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Chromatic visual evoked potentials in young patients with demyelinating disease Manca Tekavčič Pompe,* Jelka Brecelj, and Branka Stirn Kranjc Eye Clinic, University Medical Centre, Ljubljana, Slovenia *Corresponding author: manca.tekavcic‑[email protected] Received September 19, 2013; revised November 30, 2013; accepted December 8, 2013; posted December 17, 2013 (Doc. ID 197965); published January 21, 2014 The purpose of this study was to evaluate color vision in young patients with demyelinating disease both clinically and electrophysiologically. Thirty young patients (8–28 years, mean age 19 years) with demyelinating disease with or without a history of optic neuritis (ON) were investigated. Color vision was evaluated clinically with the Ishihara test and the Farnsworth–Munsell 100 hue (FM 100 hue) test and electrophysiologically with chromatic visual evoked potentials (cVEPs). Color deficiency axis and error score (ES) obtained with the FM 100 hue test were analyzed. cVEPs to isoluminant red–green (R-G) and blue–yellow (B-Y) stimuli were recorded. The stimulus was a 7 deg circle composed of horizontal sinusoidal gratings with a spatial frequency of 2 cycles∕deg and 90% chromatic contrast. Onset–offset mode of stimulation (ON : OFF  300∶700 ms) was used. Since the majority of the patients were adults (>18 years), the negative wave (N wave) of the cVEP respones is the prominent part and therefore was analyzed. Sixty eyes were studied—22 with at least one episode of ON (ON group) and 38 without any clinically evident episode of ON (nON group). The average ES in the ON group was 179.18  171.8, whereas in the nON group it was 87.60  65.34. The average N-wave latency in the ON group was 144  44 ms for the R-G stimulus and 146  56 ms for the B-Y stimulus, whereas in the nON group, it was 117  13 ms for the R-G stimulus and 121  22 ms for the B-Y one. The average N-wave amplitude in the ON group was 9.3  7.1 μV for the R-G stimulus and 5.1  3.9 μV for the B-Y one, whereas in the nON group, it was 10.8  8.3 μV for the R-G stimulus and 6.4  4.3 μV for the B-Y one. A significant difference between the ON and the nON group was found: in the ON group, ES was higher (p  0.01) and N-wave latency was longer (p  0.01) compared with those in the nON group. The study showed that color vision is expectedly more affected in the ON group, but also often in the nON group, which may indicate increased parvocellular visual pathway vulnerability in demyelinating diseases. © 2014 Optical Society of America OCIS codes: (330.1720) Color vision; (330.4270) Vision system neurophysiology. http://dx.doi.org/10.1364/JOSAA.31.000A82

1. INTRODUCTION Color vision is often affected in patients with demyelinating disease, especially after one or more episodes of optic neuritis (ON) [1–4]. However, the pathophysiology of acquired color vision deficiency in patients with demyelinating disease is still not so well understood [5]. In young patients with an episode of ON, visual acuity often recovers completely, but residual color vision deficiency persists much longer, sometimes even permanently [1,6]. Demyelinating disease can cause damage to all parts of the visual system (retina, optic nerve, chiasm, optic tract, lateral geniculate body, visual radiation, and visual cortex) and the parvocellular visual pathway seems to be affected more often [7]. Color vision deficiency in patients with demyelinating disease also seems to be more a consequence of injury to the anterior visual pathway and less due to damage of the cortical areas [4,6]. Acquired dyschromatopsia in patients with optic neuropathies, among which ON is the prevalent one, was described already at the beginning of the twentieth century when the red–green (R-G) color vision axis was believed to be affected far more often than the blue–yellow (B-Y) one [8]. Later studies have also confirmed that the R-G color vision axis is predominantly affected in ON [9–11]; however, there are some studies that found the B-Y axis being more affected [6,12] and some that found diffuse color vision deficiency [4,13]. 1084-7529/14/040A82-05$15.00/0

The Optic Neuritis Treatment Trial, where a large, homogenous sample of demyelinating ON patients was followed longitudinally, showed that 6 months after an acute attack of ON, the R-G color vision axis was affected more often than the B-Y one, whereas at the acute phase of ON, vice versa was shown [1]. However, color vision is often affected also in patients with demyelinating disease without a history of ON [5]. A recent study of patients with different subtypes of multiple sclerosis, with or without a history of ON, has shown that color vision is affected in both groups, with no specific color vision axis that would be predominantly affected [4]. Color vision can be assessed clinically with different tests; some of them serve mostly for screening purposes (Ishihara pseudo-isochromatic plates), whereas others can be converted from a simple arrangement test to a score that gives both the type and the degree of color vision deficiency. Clinically, the most widely used arrangement test is the Farnsworth–Munsell hue 100 (FM 100 hue) test. The idea of electrophysiological assessment of the parvocellular visual pathway by using chromatic visual evoked potentials (cVEPs) is not new [14–23] and may serve as an important additional test in electrophysiological evaluation of the visual system in daily clinical practice, especially in a pediatric population [24–27]. In our previous studies, cVEPs have been used to test children with congenital color deficiency [28], as well as school-age children (7–18 years) with © 2014 Optical Society of America

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normal color vision to obtain normative data [26]. cVEP response in young adults with normal color vision consists of a major negative wave (N wave), whereas in younger children, the predominant part of the response consists of a major positive wave (P wave). Conversion between the two takes place around 10 years of age [24,26]. The purpose of this study was to evaluate color vision in young patients with demyelinating disease both clinically by using standard screening and arrangement tests and electrophysiologically by using cVEPs. The results of the color vision tests were compared among two groups of patients with demyelinating disease, those with a history of ON and those without it.

2. METHODS Thirty young patients (8–28 years, mean age 19 years) with demyelinating disease with or without a history of ON were investigated. The characteristics of patients are described in Table 1. Best corrected visual acuity in all eyes except in two was 1.0 (20/20). A control group of 11 age- and sexmatched healthy subjects with normal vision and normal color vision was also included in the study. Color vision was monocularly evaluated clinically with the Ishihara test and the FM 100 hue test and electrophysiologically with cVEPs. Color deficiency axis and error score (ES) obtained with the FM 100 hue test were analyzed. The webbased FM 100 hue test calculation (http://www.torok.info/) using age-matched normative data was utilized [29]. VEPs to R-G and B-Y chromatic stimuli were performed in all patients to monocular stimulation. Stimuli were chosen to selectively activate the opponent L–M (R-G stimulus) and SL  M (B-Y stimulus) pathways. Visual stimuli were generated with an Espion system (Diagnosys LLC, Lowell, Massachusetts, USA) and displayed on a color Multi-Sync XM 29 CRT monitor (NEC Corporation of America, Irving, Texas, USA). The CIE (1931) coordinates for the colors of the stimuli were as follows: red (x  0.59, y  0.35), green (x  0.36, y  0.55), blue (x  0.20, y  0.13), and yellow (x  0.37, y  0.35); mean luminance was 14 cd∕m2 for both R-G and B-Y stimuli. This resulted in a nominal cone contrast

Table 1. Patient Demographics and Clinical Characteristicsa Parameter n Sex (M/F) Age Age at onset Disease duration (average) Disease subtype CIS RR SP ADEM NMO DMDs (yes/no) Clinical event of ON (yes/no)

Demyelinating Disease 30 (60 eyes) 8∕22 19  4 years 13.7  2.7 years 5 years 8 14 2 5 1 24∕6 22∕38 eyes

a CIS, clinically isolated syndrome; RR, relapsing–remitting; SP, secondary progressive; ADEM, acute disseminated encephalomyelitis; NMO, neuromyelitisoptica; DMDs, diseasemodifying drugs; ON, optic neuritis.

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of S  73% along the S axis and L  11% and M  28% along the L–M axis. In a previous study, the isoluminant point was assessed individually for every participant with normal color vision [26]. The study showed that more than 95% of them (n  60) had the isoluminant point at an r ratio of 0.5 [r ratio being the luminance of red to total luminance; red∕red  green] for the R-G stimulus and at a b ratio of 0.5 [b ratio being the luminance of blue to total luminance; blue∕blue  red  green] for the B-Y stimulus. In this study, isoluminance was therefore set at r  0.5 and b  0.5. The stimulus size was 7 deg. The stimulus was a circle composed of horizontal sinusoidal gratings, with a spatial frequency of 2 cycles∕degree and 90% gratings chromatic contrast. The patterns appeared for 300 ms and were replaced by a homogenous field of the same mean luminance and chromaticity for 700 ms. VEPs were recorded from Oz, O1, and O2 (active), and Fz (reference) positions, which were determined according to the International 10–20 system [30]. Only the results from the Oz position were analyzed. Silver–silver chloride electrodes were applied to the scalp. Electrode impedance was kept below 5 kΩ. Signals were filtered (0–100 Hz) and averaged (50–100 responses). Trials were carried out at least twice to assess the reproducibility of the response. The latency and amplitude of the N wave were measured. Amplitude was measured from the baseline. Statistical analysis was performed with the GraphPad Prism 5 software. Results from both eyes were included in the analysis. The D’Agostino & Pearson omnibus normality test did not show normal distribution of variables; therefore, Spearman correlation was utilized. The study was approved by the Ethical Commission of the Medical Faculty of University of Ljubljana and followed the tenets of the Declaration of Helsinki.

3. RESULTS A total of 22∕60 (37%) eyes with a history of ON formed the optic neuritis group (ON group), whereas the other 38∕60 (63%) eyes without a clinical history of ON formed the nooptic neuritis group (nON group). The average time from the last episode of ON was 13 months; however, there was a young lady who had an acute episode of unilateral ON at the time of our investigations. The Ishihara test (15 plates) showed an average score (SD) of 11.45  5.2 for the ON group and 13.97  2.05 for the nON group. The FM 100 hue test showed color vision affected in 60% of the eyes in the ON group and in 45% of the eyes in the nON group. The majority of eyes with color vision affected in both groups showed nonselective (diffuse) color vision loss, 40% in the ON group and 42% in the nON group. Another 15% of the affected eyes in the ON group showed predominantly R-G and 5% B-Y color vision deficiency, whereas in the nON group, only 3% of the eyes with color vision affected showed R-G color vision deficiency. The FM 100 hue test total ES average for the ON group was 179.2  171.8, whereas for the nON group it was 87.6  65.3. The difference between both groups was significant (p  0.013). The mid-point was at 51.5  13.9 for the ON group and at 45.2  15.7 for the nON group. Details are shown in Fig. 1. Reliable cVEP recordings were obtained from 20∕22 eyes in the ON group and from all 38∕38 eyes in the nON group for the

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Fig. 1. FM 100 hue test ES for eyes with ON, without ON (nON), and in the control group (Control).

R-G stimulus, whereas reliable recordings for the B-Y stimulus were obtained from 16∕22 eyes in the ON group and 38∕38 eyes in the nON group. The two eyes of the same patient with unrecordable cVEP to the R-G (and B-Y) stimulus had poor visual acuity (0.1 in decimal representation) and also, a very prolonged and attenuated P100 wave of pattern-reversal VEP, which indicated very poor function of both optic nerves after at least three episodes of acute ON bilaterally over the past 5 years. Another four eyes of three patients in the ON group had a much attenuated cVEP response to the B-Y stimuli; therefore, the response could not be distinguished from the noise. However, the cVEP response to the R-G stimulus in these eyes was recordable reliably, but was markedly attenuated.

Fig. 2. Average of all cVEP responses in the ON group and the nON group compared with a control group.

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All cVEP responses in both groups and in the control group of aged-matched healthy subjects with normal color vision are shown in Fig. 2. The negative (N) wave is indicated. The N-wave latency and amplitude for the R-G and B-Y cVEP stimuli in both groups and in the control group are shown in Fig. 3. The difference between the N-wave latency in the ON and nON groups is significant (p  0.010) for the R-G stimulus and of borderline significance for the B-Y one (p  0.051). The difference between the N-wave amplitude for the ON and nON groups is not significant (p  0.227) for the R-G stimulus and also not for the B-Y one (p  0.318). The correlation between ES and electrophysiological parameters (N-wave latency and amplitude for both R-G and B-Y stimuli) in eyes with (ON group) and without (nON group) a history of ON is shown in Fig. 4. The best correlation was found between ES and N-wave amplitude for B-Y stimulation (r  0.55; p  0.05) and between ES and N-wave latency for R-G stimulation (r  0.40; p  0.08), both in eyes with a history of ON.

4. DISCUSSION The study has shown that cVEP responses to both R-G and B-Y stimuli differ in young patients with demyelinating disease compared with a control group, especially if a positive history of at least one clinical episode of ON is present. A significant difference in the clinical parameter of color vision evaluation (FM 100 hue total error score, ES) and also in the electrophysiological parameter of parvocellular visual pathway evaluation (N-wave latency) was found between eyes with ON compared with those without it. Clinical assessment of color vision was performed with Ishihara plates and with the clinically most widely used arrangement test for testing color vision (the FM 100 hue test). Since Ishihara plates can only detect R-G color vision deficiency, another screening test (for example, Hardy Rand and Rittler test) that is able to also detect B-Y color vision deficiency would give us a wider insight into the color vision defects of young patients with demyelinating disease. The FM 100 hue test showed diffuse color vision deficiency in most eyes in both groups. Other studies have also shown no specific color vision axis to be predominantly affected after demyelinating ON [4,13]. However, in this study, there was a young lady who experienced an acute episode of ON during our investigations (within a week) and her color vision was at that time predominantly affected in the B-Y axis in the affected eye. The other eye of the same lady (with a past episode of ON 5 months previously) showed diffuse color vision deficiency,

Fig. 3. cVEP N-wave (N) latency (in milliseconds, ms) and amplitude (in microvolts, μV) for both groups and for the control group for the R-G and B-Y chromatic stimuli. Average  SD are plotted for every parameter shown. ON, ON group; nON, no-optic neuritis group; Control, control group. The upper normal limit (unl) for N-wave latency and the lower normal limit (lnl) for N-wave amplitude (from the control group data) are indicated.

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Fig. 4. Correlation between total ES and N-wave latency and amplitude for both R-G and B-Y stimuli in the ON and no-optic neuritis (nON) groups.

which is in accordance with our knowledge from the Optic Neuritis Treatment Trial [1]. The FM 100 hue test showed a high percentage of eyes with affected color vision in the nON group (45%) in this study. It may indicate some subclinical ON episodes, which were not noticed by some patients or that affected color vision can be a result of demyelinating injury to the posterior and accessory visual pathways rather than the macular area and the optic nerve. When responses to R-G and B-Y stimuli are compared within each group (control, ON, and nON), they show similar characteristics: responses to the R-G stimulus have shorter latency and higher amplitude compared with the B-Y ones, which is in accordance with our previous knowledge that the koniocellular visual pathway response is weaker than the parvocellular response [31] and therefore, if affected by demyelinating ON, the damage can be more devastating. The average ON group cVEP response to the B-Y stimulus in this study was so small that it was within noise and therefore, not reliably detectable. When an average cVEP response from a control group and from eyes with ON is compared, a longer latency and smaller amplitude of the response to both R-G and B-Y stimuli can be observed in the ON group. This is in accordance with adult studies of longer P100 latency and smaller P100 amplitude in eyes after one or more episodes of ON when utilizing classic pattern-reversal VEP stimulation [32], whereas in children, P100 latency and amplitude often normalize completely within 6 months after an acute episode of ON [33]. The average cVEP response in this study (presented in Fig. 2) has shown that eyes with ON have a longer latency and smaller amplitude compared with eyes without ON. Eyes without ON showed similar characteristics of response latency and amplitude to those in the control group, which may confirm that there were not many “silent” (clinically undetected) episodes of ON in the eyes in the nON group.

However, the nON eyes showed greater variability of the cVEP response compared with those in the control group. The isoluminant point in this study was not determined individually, since our previous study [26] has shown that in less than 5% of children tested, their isoluminant point falls out of r  0.5 for the R-G and b  0.5 for the B-Y stimulus, which was then used for this study. In people with demyelinating disease in whom we expect the parvocellular visual pathway being predominantly affected, individually determined isoluminant point would probably change the average group response slightly. This also might be the reason why the average nON response to the B-Y stimulus shows different waveform characteristics than the ON and the control group responses. The average nON group response is composed of a negative–positive complex, whereas a predominantly negative response is typical for the control and ON group. Correlation between the clinical parameter (total ES in the FM 100 hue test) and the electrophysiological parameters (N-wave latency and amplitude) showed that both parameters correlate only in eyes with ON (ON group). The best correlation was observed between ES and N-wave amplitude for the B-Y stimulus and between ES and N-wave latency for the R-G stimulus. We believe that an even stronger correlation would be observed if more patients were included in the study. In this study, total ES was used for comparison between clinical and electrophysiological parameters. The results might be slightly different if partial ESs for the R-G and B-Y axes are utilized and compared with the cVEP stimulus of the same axis. This study has shown that cVEPs to isoluminant R-G and B-Y stimuli can be recorded reliably in most young patients with demyelinating disease and can, therefore, represent a useful additional tool to evaluate the parvocellular visual pathway integrity in everyday electrophysiological evaluation of these patients. However, the comparison of cVEP responses to classic VEP responses in these patients will be addressed.

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ACKNOWLEDGMENTS The authors thank Ignac Zidar for help with the statistical analysis and Maja Šuštar, Ph.D., for technical support, and Marija Jesenšek, Ana Jeršin, and Helena Lindič for help in recording the young patients. This study was supported by the Slovenian Research Agency (P3-0333).

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Chromatic visual evoked potentials in young patients with demyelinating disease.

The purpose of this study was to evaluate color vision in young patients with demyelinating disease both clinically and electrophysiologically. Thirty...
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