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(Investigative Ophthalmology and Visual Science. 2007;48:5773-5781.)
© 2007 by The Association for Research in Vision and Ophthalmology, Inc.
DOI:  10.1167/iovs.07-0834
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Is Optical Coherence Tomography Really a New Biomarker Candidate in Multiple Sclerosis?—A Structural and Functional Evaluation

Fatih C. Gundogan,1 Seref Demirkaya,2 and Gungor Sobaci1

1From the Departments of Ophthalmology and 2Neurology, Gulhane Military Medical Academy, Ankara, Turkey.


   Abstract
Top
 Abstract
Methods
 Results
 Discussion
 References
 
PURPOSE. To assess the structural and functional status of patients with multiple sclerosis (MS) without a history of optic neuritis.

METHODS. Thirty-nine patients with MS who had reported no visual symptoms before and after the time of MS diagnosis were included. Thirty-eight healthy subjects were included as a control group. Retinal nerve fiber layer (RNFL) thickness was determined by optical coherence tomography. Pattern visual evoked potentials (PVEP), full-field electroretinogram (ERG), and multifocal electroretinogram (mfERG) were performed.

RESULTS. There was a significant reduction (P = 0.011) only in temporal RNFL thickness in patients with MS. P100 latency was significantly delayed with both 60-min arc checks (P < 0.001) and 15-min arc checks (P < 0.001); however, P100 amplitude was significantly reduced only in 60-min arc checks (P = 0.026). Rod response b-wave implicit time and standard combined response a- and b-wave implicit times were significantly delayed in patients with MS. Patients with MS with a delayed P100 latency (21/39; 53.8%) had significantly reduced cone response b-wave amplitude and significantly delayed cone response a- and b-wave implicit times in ERG. mfERG results did not differ between MS and control subjects and between patients with a delayed and a normal P100 latency. Pearson correlations between RNFL thickness and P100 amplitude and latency in patients with MS were not significant (P > 0.05).

CONCLUSIONS. There is no correlation between RNFL thickness and P100 response in patients with MS. PVEP seems to be a more reliable biomarker in determining visual pathway involvement in patients with no history of optic neuritis.

Multiple sclerosis (MS) is an autoimmune-mediated neurodegenerative disease with characteristic inflammatory demyelination in the central nervous system.1 2 In the early stages, the disease is primarily subclinical, but most patients enter a stage of progressive clinical deterioration within 10 to 20 years of diagnosis.3 Most patients present with a relapsing–remitting pattern of acute neurologic dysfunction, with variable periods of remission punctuated by new exacerbations.

MS is often associated with involvement of the visual pathway that can lead to clinically evident manifestations, such as optic neuritis, nystagmus, and diplopia, and to more frequent subclinical manifestations.4 Visual dysfunction occurs in 80% of patients with MS during the course of the disease and is a presenting feature in 50%.5 6 7 Acute idiopathic demyelinating optic neuritis is frequently the initial clinical manifestation.7 In some cases, the patient reports blurred vision, even if visual acuity is normal. In other cases, no ocular symptoms are reported, but specific examinations can reveal subclinical abnormalities. Most patients with MS presenting with optic neuritis have relapsing–remitting disease wherein visual acuity recovers after resolution of acute inflammation. Some permanent visual symptoms can persist, however, and repeat episodes of optic neuritis often lead to optic atrophy and loss of vision.8

Because the retina contains unmyelinated nerve fibers, it is a unique model of neurodegeneration and neuroprotection. The myelinated part of the ganglion cell axons begins behind the eye, at the level of the lamina cribrosa. For this reason, changes in the structure of the retinal nerve fiber layer (RNFL) principally represent axonal damage. Hence, the retina can be used to focus on the neuronal and axonal components of the pathologic changes in MS.9 Our knowledge about the retinal functional status assessed by full-field ERG in patients with MS is limited to a few studies that were performed years ago.10 11 12 As the years passed, new functional (multifocal [mf]ERG) and structural (optical coherence tomography; OCT) modalities allowed ophthalmologists to investigate the retina and optic nerve more thoroughly. Recent studies13 14 15 showed RNFL thickness reductions in eyes of patients with MS, with and without a history of optic neuritis. RNFL thickness reduction in MS eyes without a history of optic neuritis suggests subclinical optic nerve involvement. For this reason, a detailed structural and functional evaluation including assessment of correlations between the functional and structural changes of the retina and optic nerve are still warranted.

The purpose of this study was to determine the association of retinal neurodegeneration and the correlation of structural and functional tests in patients with MS who had no history of clinically evident optic neuritis.


   Methods
Top
 Abstract
 Methods
Results
 Discussion
 References
 
Participants
Thirty-nine patients with a diagnosis of definite MS were enrolled. All the patients were under routine observation by a single neurologist (SD). Only the patients with MS who did not have any subjective visual problem were included. The research followed the tenets of Declaration of Helsinki, and informed consent was obtained from the subjects after explanation of the possible consequences of the study. The research was approved by the institutional review board. Only the patients who had best corrected Snellen visual acuity (VA) of at least 1.0 in both eyes and no ocular history of optic nerve involvement were included. They were specifically asked about visual complaints including vision blur, visual loss, diplopia, periorbital pain, and color vision disturbances (change in seeing traffic lights or in the brightness of colors in one or both eyes) throughout their lifetime. An ophthalmic examination, including anterior segment biomicroscopy, applanation tonometry, and indirect ophthalmoscopy including the examinations of the peripheral retina, was performed. Thirty-eight age-matched (mean age: MS patients, 36.4 ± 8.7 years; control subjects, 33.9 ± 6.0, P = 0.14; independent-samples t-test), and sex-matched (male/female ratio: 16/23 in MS group, 17/21 in control group. P = 0.820, {chi}2 test) healthy subjects without any known ophthalmic and systemic disease (including diabetes and systemic hypertension) were included in the control group. Mean disease duration was 5.4 ± 5.0 years. Thirty-five (89.7%) patients had relapsing–remitting disease, and the remaining had secondary progressive disease. Individual demographic data of the patients are shown in Table 1 .


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  		TABLE 1. Individual Demographic Data, P100 Latency, and RNFLtemporal Thicknesses in Patients with MS

 
Structural Examination: OCT
OCT is a noninvasive, noncontact technique for imaging of the layered structure of the retina.16 Measurements were performed by means of near infrared low-coherence illumination (84 nm) with a resolution of approximately 10 to 17 µm. After dilation with 1% tropicamide (Tropamid; Bilim, Ístanbul, Turkey), the Stratus OCT device (Carl Zeiss Meditec, Inc., Oberkochen, Germany) and software were used to acquire three 3.4-mm diameter circular scans centered on the optic disc for each eye. RNFL thickness was automatically assessed by computer, assuming the correlation with the red, highly reflective layer at the vitreoretinal interface. Throughout scanning, the patient kept the eyes constantly fixed on an internal target provided by the equipment. Scans were performed by an experienced OCT technician who was masked to the clinical status of each subject.

The mean of the data was used to express RNFL thickness as a single average value for the whole 360° scan (RNFLaverage) and also according to RNFL quadrant (superior, nasal, inferior, and temporal). The data obtained in the temporal quadrant only was identified as RNFLtemporal. The data were taken to evaluate the temporal fiber, in which the papillomacular bundle fibers are included.

Functional Evaluation: Electrophysiological Examinations
Functional aspects of the eye were assessed by electrophysiological recordings. The recordings were performed using the Roland-Consult RetiScan system (Wiesbaden, Germany) on the basis of ISCEV (International Society for Clinical Electrophysiology of Vision) standards.

Pattern Visual Evoked Potentials.
Monocular pattern visual evoked potentials (PVEPs) were recorded with gold disc surface electrodes. Active electrodes were placed on the scalp over the visual cortex at Oz, with the reference electrode at Fz. The ground electrode was placed on the forehead. Refractions of the subjects were corrected with trial lenses before recordings were made. Each subject sat in a moderately lighted room 1 m in front of a 20 x 30-cm black-and-white video display monitor. The checkerboard stimulus subtended a visual angle of 5.7° vertically and 8.5° horizontally on either side of the fixation. Luminance was <1 cd/m2 for the black hexagons and 115 cd/m2 for the white hexagons (contrast, 99%). The responses to a large (60 min arc) and a small check (15 min arc) were recorded (Fig. 1A) . Background light was dimmed (20 cd/m2). The reversal rate was one per second. The responses to 100 stimuli were averaged. Subjects were instructed to fixate on a red marker at the center of the screen. If the cooperation of the subject was poor, the PVEP recording was repeated. Fixation stability, eye movements, and prolonged closing of the eye were monitored closely by an experienced electrophysiology technician throughout the entire testing period. Individual P100 latency to 60-min arc check and RNFLtemporal thicknesses are shown in Table 1 .


Figure 1
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  		FIGURE 1. PVEP responses to two check sizes (A), 61 hexagons in the mfERG stimulation pattern (B), concentric ring analysis (C), and an example for a single mfERG waveform showing P1 and N1 peaks (D).

 
Multifocal ERG.
The stimulus matrix, consisting of 61 scaled hexagonal elements (Fig. 1B) covering a visual field of 30° (center-to-end, both horizontally and vertically), was presented on a 20 x 30-cm monitor. The frame rate was 60 Hz, and the eye–screen distance was 26 cm. The stimulus hexagons were modulated between white and black according to a pseudorandom m-sequence. Luminance of the black hexagons was <1 cd/m2 and of the white hexagons was 115 cd/m2 (contrast, 99%). The surrounding background light was dimmed (20 cd/m2). A red fixation cross was used to maintain fixation stability. The recording period comprised eight intervals of 47 seconds providing a total recording time of 7 minutes and 26 seconds. Subjects rested for 10 seconds between segments.

Pupils were dilated with 0.5% tropicamide (Tropamid; Bilim). mfERG recordings were not started until the pupils dilated to at least 7 mm in diameter. A corneal jet electrode was used as the active electrode. Signals were bandpass filtered (10–100 Hz) and amplified (gain 100,000, SN 9902S-308; Roland-Consult). Any segments associated with blinks or eye movements were rejected and repeated immediately. The measurement was real-time monitored. Incoming raw data signal were observed to detect the fixation losses. Fixation was directly observed by an experienced electrophysiology technician.

For the purpose of data analysis, the mfERG responses were averaged over five retinal regions (Fig. 1B 1C ; i.e., the central hexagon [CH] 0.0°–2.3°), and four concentric rings (ring 1 [R1: 2.3°–7.4°], ring 2 [R2: 7.4°–12.0°], ring 3 [R3: 12.0°–19.4°], and ring 4 [R4: 19.4°–30.0°]). Amplitudes and latencies of N1 and P1 were evaluated. We defined the first negative and positive deflections of the mfERG as N1 and P1, respectively (Fig. 1D) . The amplitude of N1 was measured from the baseline to the first negative peak. The amplitude of P1 was measured from the first negative peak to the first positive peak. The latencies of N1 and P1 were defined as the elapsed time from the stimulus to the peak of N1 and P1 responses, respectively.

Full-Field ERGs.
Pupil dilation was performed by instilling one drop of tropicamide 1%. To record the rod ERG, standard combined response (SCR), and oscillatory potentials, all subjects were dark adapted for 30 minutes. DTL fiber electrodes were then positioned before recording under dim red light. A white flash of 0.0095 cd/m2 was used with an interval of 2 seconds to obtain rod ERG. To record SCR and oscillatory potentials, a standard white flash (3.0 cd/m2) was used at intervals of 10 and 2 seconds, respectively. Subjects were then light adapted to a white rod–saturating background (25 cd/m2) for 10 minutes before cone and 30-Hz flicker responses were recorded with a standard white flash. The interval between the stimuli in recording single-flash cone ERG was 2 seconds. The responses were amplified with a gain of 10,000 and recorded over a bandwidth of 100 to 500 Hz for oscillatory potentials and 1 to 300 Hz for the remaining recordings. Figure 2 shows a representative ERG recorded from one of the patients.


Figure 2
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  		FIGURE 2. An ERG specimen belonging of a patient with MS. All ERGs were recorded on the basis of ISCEV (International Society for Clinical Electrophysiology of Vision) recommendations.

 
Statistics
The data are reported as the mean ± SD. The differences between control subjects and patients with MS were statistically evaluated by independent-samples t-test. The differences between the subgroups in patients with MS were investigated by Mann-Whitney U test. To assess whether a correlation exists between OCT and electrophysiological results, linear regression analysis (Pearson’s test) was used. P < 0.05 was considered significant.


   Results
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 Abstract
 Methods
 Results
Discussion
 References
 
Optical Coherence Tomography
Mean RNFL thicknesses varied significantly across retinal quadrants, with greater mean thicknesses in the superior and inferior quadrants. There was a significant reduction in RNFL analysis only in the temporal quadrant of the optic nerve (Table 2) .


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  		TABLE 2. RNFL Thicknesses in Patients with MS and Control Subjects

 
Pattern Visual Evoked Potentials
P100 amplitude was reduced in 60-min arc check in patients with MS. P100 amplitude response to the 15-min arc check was reduced, but the difference was nonsignificant. P100 latency was delayed significantly in patients with MS for both 60- and 15-min arc checks (Table 3) .


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  		TABLE 3. P100 Amplitude and Latency Results

 
Electroretinogram
Rod response b-wave implicit time and SCR a- and b-wave implicit times were significantly delayed in patients with MS (Table 4) .


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  		TABLE 4. Full-Field ERG Results

 
Multifocal ERG
There was no significant difference in P1 and N1 amplitude and implicit times in the CH and in the rings, between patients with MS and control subjects (Table 5) .


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  		TABLE 5. mfERG Results

 
Grouping the Patients Based on P100 Latency Changes
PVEP results showed subclinical optic nerve involvement in some patients with MS although there were no symptoms of optic neuritis (ON) in those patients. We decided to divide patients with MS into two subgroups according to the PVEP latency response to the 60-min arc check. The 95% CI of the P100 latency of the control group for the 60-min arc check was assessed and the result was 109.4 ms (Table 3) . Patients with MS with a lower P100 latency than that value were labeled MS-P100 normal and patients with MS with a longer P100 latency were termed MS-P100 delay. There were 18 MS-P100 normal (46.2%) and 21 MS-P100 delay (53.8%) patients (Table 1) . Age and gender difference between MS-P100 normal and MS-P100 delay patients was not significant (Table 6) .


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  		TABLE 6. Characteristics of the Subgroups Based on P100 Latency and RNFLtemporal Thickness

 
There were significant delays in cone response a- and b-wave implicit times and significant reduction in cone response b-wave amplitude in MS-P100 delay patients compared to MS-P100 normal patients. Thirty-Hz flicker response P1 amplitude was insignificantly reduced in MS-P100 delay patients; however, a tendency toward significance was observed (P = 0.054; Table 7 ).


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  		TABLE 7. Full-Field ERG Responses in Patients with MS, with and without P100 Latency Delay

 
mfERG P1/N1 amplitude/implicit time did not differ significantly between MS-P100 normal and MS-P100 delay patients. Similarly RNFLaverage and RNFLtemporal thickness differences between these two subgroups were not statistically significant (P > 0.05 for all quadrant and average thickness comparisons). Correlations between RNFL thickness (both temporal and average) and P100 latency and amplitude in both MS-P100 normal patients and MS-P100 delay patients were not significant (P > 0.05).

Grouping the Patients Based on RNFLtemporal Thickness
A similar analysis was performed in RNFLtemporal measurements. We decided to divide patients with MS according to the 5% CI (58.0 µm; Table 2 ) by using normative data for RNFLtemporal in the control eyes. The patients with MS who had a thicker RNFLtemporal than the 5% CI value of the normative data (that is 58.0 µm) were deemed MS normal RNFLtemporal and patients with MS with a thinner RNFLtemporal than the 5% CI value was determined as MS thin RNFLtemporal. There were 27 (69.2%) MS normal RNFLtemporal and 12 (30.8%) MS thin RNFLtemporal patients (Table 1) . Age and gender difference between MS-normal RNFLtemporal and MS-thin RNFLtemporal was not significant (Table 6) .

None of the full-field ERG and mfERG amplitude–implicit time differences between MS-normal RNFLtemporal and MS-thin RNFLtemporal patients were significant (P > 0.05, not shown in the table). The P100 amplitude/latency (for both 60' and 15' checks) differences were also not significant between these two subgroups (Table 6) . The ratios of patients with normal to those with thinner temporal RNFL thickness among patients with normal and delayed P100 latency was not significantly different (Table 8) .


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  		TABLE 8. Distribution of Patients According to P100 Latency and RNFLtemporal Thickness

 
PVEP-RNFL and PVEP-ERG Correlations
After assessing significant differences in RNFL thickness, PVEP, and ERG results, we determined to investigate the correlations between PVEP latency–RNFL thickness and PVEP latency–ERG results.

The correlations between RNFL thickness and PVEP latency in patients with MS were not significant (r = 0.01, P = 0.950 for RNFLtemporal; r = –0.150, P = 0.361 for RNFLaverage). However, regarding all the participants including healthy controls, P100 latency was significantly correlated to RNFLaverage and RNFLtemporal thicknesses (r = –0.314, P = 0.034 for RNFLtemporal; r = –0.431, P = 0.015 for RNFLaverage). Similarly, P100 latency was significantly correlated to SCR b-wave implicit time when all participants were included (r = 0.295, P = 0.009; Fig. 3 ). In patients with MS PVEP latency positively, but insignificantly, correlated to SCR b-wave implicit time (r = 0.163, P = 0.322; Fig. 3 ).


Figure 3
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  		FIGURE 3. Relation of P100 latency to RNFL thickness and standard combined response b-wave implicit time in patients with MS and healthy control subjects.

 
In contrast to the PVEP–RNFL thickness correlation, PVEP latency positively correlated to some of the ERG implicit times, but negatively to some of the ERG amplitudes in patients with MS (Table 9) .


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  		TABLE 9. Significant Correlations between PVEP Latency and ERG Results in MS Patients

 
None of the correlations between PVEP latency and mfERG results (P1/N1 amplitude and implicit times in central hexagon and in rings) were significant (P > 0.05).


   Discussion
Top
 Abstract
 Methods
 Results
 Discussion
References
 
The purpose of this study was to explore the electrophysiological and functional status of patients with MS who did not have symptoms of optic neuritis. One of the breakthroughs in establishing the clinical value of PVEP occurred when Halliday et al.17 first described that, in carefully examined patients with MS who had never had optic neuritis, more than 90% of the subjects had abnormal, delayed PVEPs. Supporting the findings in the literature, 53.8% (21/39) of patients with MS included in this study had a P100 latency delay with respect to the 95% CI of the control subjects. It is quite possible that the real ratio of patients with a past subclinical optic nerve involvement is above this value, because delayed P100 latency may recover to normal with time.18 The patients in this study did not report any visual symptoms. This finding emphasizes the importance of routine PVEP recordings in patients with MS to diagnose subclinical optic nerve involvement. Since the P100 latency responses to a large check size (60 min arc) were more consistent with the clinical findings, this check size seems more applicable for this purpose.

In 1974, Frisen and Hoyt19 described qualitative changes in the RNFL in patients with MS. In a subsequent study of 14 eyes with optic atrophy due to various causes, Frisen and Quigley20 reported that visual acuity was associated with the amount of surviving axons within the temporal quadrant of the optic nerve head. The first use of OCT in MS was reported in 1999 by Parisi et al.13 This group assessed 14 patients with definite MS with a history of optic neuritis (MSON) associated with good visual recovery. Despite this recovery, the researchers reported a 46% reduction in the RNFLaverage in the MSON eyes compared with healthy control eyes, and a 28% reduction in the unaffected contralateral eyes (MSCE). In 2005, Trip et al.15 provided a more detailed and systematic characterization of OCT changes associated with inflammatory demyelinating optic neuritis and visual dysfunction. They studied 25 patients (14 with a clinically isolated syndrome and 11 with clinically definite MS). These patients had experienced a single episode of acute unilateral optic neuritis without recurrence. Furthermore, in contrast to those described in Parisi et al.,13 these patients had incomplete recovery of visual function. When compared with healthy people, the RNFL was reduced by 33% (P < 0.001) in patients with a history of optic neuritis. A comparison of the affected eye with the unaffected eye showed that RNFL thickness was reduced by 27% (P < 0.001). The RNFL changes associated with optic neuritis predicted low P100 amplitudes, but not P100 latency. However, the results of this study did not confirm this finding, because P100 latencies in MS-P100 delay and MS-P100 normal did not correlate with RNFL thicknesses.

Parisi et al.13 reported a significant reduction in RNFLaverage thickness (but not in RNFLtemporal) in MSCE eyes without a history of optic neuritis. In MSCE eyes, they reported a nonsignificant delay in P50 latency and a nonsignificant reduction in P50 and N95 amplitudes of the pattern ERG when compared with those of control subjects. They stated that their findings suggest that some degree of axonal involvement may develop at the retinal level in patients with MS, even in the absence of clinical symptoms and electrophysiological abnormalities. These investigators also found reduced P100 amplitudes and delayed P100 latencies for both 60-min arc checks and 15-min arc checks in MSON eyes compared with MSCE and control eyes. We, in the present study, found delayed P100 latency for both checks, but reduced amplitude only in response to 60-min arc checks. P100 amplitude responses to 15-min arc checks was reduced in patients with MS, but the difference was not significant (P = 0.075). Similar to their findings, we found no significant correlation between RNFLtemporal and RNFLaverage thicknesses and P100 amplitude and latency in patients with MS. They suggested that the absence of such a correlation could be explained by considering that the abnormal PVEP response observed in patients with MS may result from both an impaired retinal function and a delayed neural conduction in the postretinal visual pathways. In addition to their PERG findings indicating retinal dysfunction, our full-field ERG results confirmed their suggestion.

A recent study by Fisher et al.14 showed RNFLaverage thickness reductions in MSON and MSCE eyes when compared with healthy control eyes. Using normative data included in the OCT 4.0 processing software for OCT-3 (Carl Zeiss Meditec), they reported that only 40 of 180 eyes (25%, including MSON and MSCE eyes) had RNFLaverage thicknesses that were abnormal in one or both eyes. In our study, RNFLtemporal thicknesses of 12 (30.7%) patients with MS (Table 8) were thinner than the 5% confidence interval of the RNFLtemporal thickness in the control subjects. The authors compared RNFLaverage thicknesses in eyes of patients with MS without a history of acute optic neuritis (ON) in either eye (MS non–ON eyes) versus fellow eyes of patients with MS with a history of acute ON in one eye (MS ON fellow eye). In contrast to Parisi et al.,13 Fisher et al.14 reported no significant difference in RNFLaverage thickness between MS ON fellow eyes and MS non-ON eyes. They reported that eyes with a history of ON had significantly reduced RNFL thickness compared with both groups of non-ON eyes.

The ERG measures the response of the entire retina to a flash stimulus and is characterized by a negative waveform (a-wave) that represents the response of the photoreceptors, followed by a positive waveform (b-wave) generated by a combination of cells in the Müller and bipolar cell layer. Previous ERG studies of patients with MS have found diminished ERG responses and abnormalities of the b-wave overall.10 11 12 One study12 included 105 patients with MS in four groups. The first group had no history or clinical evidence of optic nerve dysfunction, the second and third groups had either right or left optic nerve disease, and the fourth group had historical or clinical evidence of optic nerve disease. The investigators reported no significant difference for b-wave implicit times in the first group but significant delay in the other three groups and greater interocular latency differences in four groups compared with the control subjects. In addition, a recent study conducted by Forooghian et al.21 showed significantly delayed rod-cone (SCR) b-wave response, cone b-wave response, and rod a-wave response in patients with MS. To our knowledge, a delayed rod response a-wave in patients with MS was first reported by that group. Supporting their findings, we found a b-wave implicit time delay in rod response and a- and b-wave implicit time delays in SCR. In addition, we found b-wave amplitude reduction, and a- and b-wave implicit time delay in cone response in patients with a P100 latency delay compared with patients with a normal P100 latency. These ERG results provide neurophysiological evidence that retinal damage is not only a consequence of myelin loss in the optic nerve but is an early feature of MS.

Because a-wave is generated by photoreceptors, the ERG results in the present study and in Forooghian et al.21 show early photoreceptor cell involvement in patients with MS. The retina is embryologically derived from the central nervous system (CNS). MS has been associated with pars planitis,22 23 suggesting an immunologic link between the uvea and central nervous system (CNS). Pars planitis and MS are both associated with the HLA-DR15 allele.24 These findings suggest a common immunogenetic predisposition between these two conditions. In addition, animal models have demonstrated that antigens coexpressed in the CNS and uvea/retina may be pathogenically relevant in MS.25 26 Similarly, a recent report27 showed that some patients with MS with autoantibodies against the retinal protein {alpha}-enolase have reduced ERGs. One other possible explanation for delayed a wave in ERG is retrograde transsynaptic degeneration of retinal layers secondary to optic nerve involvement. However, we think that transsynaptic degeneration as distal as the outermost layer of the retina (photoreceptor cell layer) is unlikely. Moreover, pathologic changes in the retina after transection of the optic nerve were shown to be restricted to the innermost layers by light and electron microscopic examinations.28 For this reason, autoimmunity is the plausible explanation for diminished ERG results in patients with MS.

Of note, Pierelli et al.11 found a pathologic b-wave amplitude increase mainly with red flash stimuli in patients with a clinical history indicating involvement of visual pathways. They explained this finding to an involvement of centrifugal optic nerve fibers having inhibitory functions on retinal cells.

The first significant comparison of mfERG with the standard full-field ERG has been performed by Hood et al.29 By slowing the stimulus down, they showed that there is good correspondence between the full-field ERG a-wave and the multifocal ERG N1 component and between the full-field ERG b-wave and the mfERG P1 component. It is generally accepted that little of the mfERG response is generated by the cone photoreceptors per se. Rather it is dominated by the responses of the on and off bipolar cells.30 31 We found no significant difference in mfERG results between the patients with MS and control subjects. The differences in mfERG results between patients with a normal P100 latency and those with a delayed latency were also not significant. Since cone responses in ERG are different in the groups, we think that longitudinal and larger series are warranted to investigate the mfERG changes in patients with MS.

In this study, we included only the patients with MS who did not report any visual complaint. Almost 54% of the patients had a delayed PVEP latency. However, only 30.7% of the patients had a thinner RNFL thickness when compared with the control subjects. Moreover, P100 latency changes to 60-minute check size had significant correlations to single cone responses in ERG.

In conclusion, we showed that electrical potential abnormalities in the retina may be indicative of neurodegeneration and that PVEP seems to be a more valuable biomarker than OCT-assessed RNFL thickness in the diagnosis of subclinical optic nerve involvement in patients with MS.


   Footnotes
 
Submitted for publication July 5, 2007; revised July 30, 2007; accepted September 21, 2007.

Disclosure: F.C. Gundogan, None; S. Demirkaya, None; G. Sobaci, None

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.

Corresponding author: Fatih C. Gundogan, Gulhane Askeri Tip Akademisi, Goz Hastaliklari Anabilim Dali, 06018 Ankara, Turkey; fgundogan{at}yahoo.com.


   References
Top
 Abstract
 Methods
 Results
 Discussion
 References
 

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