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To employ a novel stimulation paradigm in order to elicit multifocal electroretinography (mfERG)–induced optic nerve head component (ONHC) responses, believed to be contingent upon the transformation in electrical transmission properties of retinal ganglion cell axons from membrane to saltatory conduction mechanisms, as they traverse the lamina cribrosa and obtain oligodendrocyte myelin. We further sought to characterize abnormalities in ONHC responses in eyes from patients with multiple sclerosis (MS).
In 10 normal subjects and 7 patients with MS (including eyes with and without a history of acute optic neuritis), we utilized a novel mfERG stimulation paradigm that included interleaved global flashes in order to elicit the ONHC responses from 103 retinal patches of pattern-reversal stimulation.
The number of abnormal or absent ONHC responses was significantly increased in MS patient eyes compared to normal subject eyes (p < 0.001, by general estimating equation modeling, and accounting for age and within-subject, intereye correlations).
Studying the relationship between ONHC abnormalities and alterations in validated structural and functional measures of the visual system may facilitate the ability to dissect and characterize the pathobiological mechanisms that contribute to tissue damage in MS, and may have utility to detect and monitor neuroprotective or restorative effects of novel therapies.
Electroretinography (ERG) is a physiologic technique used to study intraretinal electrical responses to stimuli with well-defined characteristics.1–4 The development of multifocal ERG (mfERG) has facilitated the transition from analysis of a consolidated global retinal response to a topographical mapping of normal and pathologic patterns of retinal activity. However, unlike multifocal visual evoked potential (mfVEP) responses, those derived from mfERG studies are highly stereotyped, both within and across normal subjects.1–5
Recognizing that the retinal ganglion cell (RGC) contribution to the mfERG is small, and overlaps with signals generated from other retinal sources (e.g., bipolar neurons), Sutter and colleagues1,2 developed a modified high-precision mfERG stimulus paradigm to include global flash stimuli that are interleaved at specific intervals, in order to elucidate a discrete neurophysiologic response signature that corresponds to the normal electrical transmission mechanisms of RGC axons across the topographical landscape of the retinal nerve fiber layer. This induced component of the mfERG is referred to as the optic nerve head component (ONHC) response, and its presence signifies the normal electrical transformation from membrane to saltatory transmission properties, as unmyelinated.
RGC axons traverse the lamina cribrosa, beyond which they are myelinated (figure 1).1,2 We employed the global flash mfERG stimulation paradigm to demonstrate definitive abnormalities of ONHC responses in patients with multiple sclerosis (MS).
Our objective for this pilot investigation was to characterize the abnormalities of mfERG-generated ONHC responses in patients with MS and a history of acute optic neuritis (AON), when compared to the fellow eye, and with respect to eyes from normal subjects.
We examined 10 normal subjects (mean age 29.4 years, n = 20 eyes) and 7 patients with definite MS (mean age 41.9 years, n = 14 eyes) as confirmed using the McDonald modified criteria6 and a history of AON (table). The patients with MS were recruited consecutively in the Clinical Center for MS at UT Southwestern Medical Center, and were excluded if they had any other ophthalmologic condition (e.g., glaucoma, macular degeneration), high myopia (>−5.0 D), or any major medical condition with impact upon the visual system other than MS. Further, we only included patients whose episode of AON was ≥6 months from the onset of visual symptoms.
For mfERG assessments, a scaled hexagonal array with a pattern-reversal stimulus was utilized to provoke responses that can be collected as corneal signals by a Burian-Allen bipolar contact lens electrode, as previously described (figure 1).1,2
Briefly, subjects fixated on a centralized 2-mm red-cross marker within the stimulator. Fixation was ensured by continual fundus monitoring (VERIS; EDI, Redwood City, CA). A novel stimulus paradigm (the ONHC 103-hexagon global-flash mfERG VERIS protocol) with 5 frames per m-step was used.1,2 This paradigm enhances the inner retinal responses, and hence, the generation of the ONHC response. The first frame contained focal flashes (128 cd/m2) controlled by the VERIS pseudorandom m-sequence; the second and fourth frames contained global flashes (128 cd/m2); and the third and fifth frames were dark (1 cd/m2) (figure 2). No value of impedance greater than a 2-Hz threshold was considered acceptable. Upon completion, the Burian-Allen electrode was removed, and a slit-lamp examination was performed. None of our subjects sustained any corneal injuries.
The mfERG responses were analyzed using VERIS software version 6.3.3d7. The response traces were organized as concentric rings around the fovea, and were then plotted in vertical columns (figure 2). The tracings are mathematical extractions of signals that are correlated with time. For the analysis of mfERG retinal patch stimulation sequences, 2 principal waveforms were identified—the direct component, which is dominated by the retinal component appearing early, and the induced component, which is dominated by the ONHC response waveform that appears later. We scored ONHC waveforms as being abnormal (waveform disorganization or absent) utilizing a colorized map (pink or red filled hexagons designate the abnormal retinal patches, whereas white unfilled hexagons designate normal responses) (figures 3 and 4).
Statistical analyses were performed using Stata 12.0 software. The total number of waveforms with abnormal ONHC responses in MS eyes with AON was compared to MS eyes without a history of AON and with respect to healthy control eyes using generalized estimating equation (GEE) modeling.
All participants provided informed and written consent prior to the beginning of study procedures. Consent was obtained according to the Declaration of Helsinki. The protocol was approved by the Investigational Review Board of UT Southwestern Medical Center.
In 16/20 normal eyes, we did not identify any ONHC waveform abnormalities (table and figure 2), whereas in 4/20 normal eyes, there were occasional ONHC response abnormalities (range of 3–9 abnormal waveforms per eye out of 103 hexagonal patches of stimulation) that tended to be localized to the outermost ring of stimulation (ring 5) where the density of RGC axons is sparse (table).
The number of abnormal or absent ONHC responses was significantly associated with MS eyes vs those from control subjects (table, figures 3 and 4). On average, we observed 34 more abnormal or absent ONHC responses from MS eyes when compared to eyes from healthy individuals (p < 0.001 by GEE and accounting for age and within-subject, intereye correlations). Alternately, among MS eyes, and irrespective of positive or negative history of AON, the loss of ONHC responses was not significantly different (p = 0.34). If corroborated in larger future studies, this observation may represent one of the most interesting and conspicuous aspects of our investigation. In particular, the magnitude of the severity of intraretinal pathology that ultimately compromises the fidelity in the transition from membrane to saltatory axonal conduction mechanisms at the lamina cribrosa may be affected similarly by manifest episodes of AON vs those mechanisms that contribute to the occult subclinical damage sustained by tissue elements that culminate in abnormal or abolished mfERG-induced ONHC responses.
In this pilot investigation, we underscore the application of a novel mfERG interleaved global flash stimulation paradigm to demonstrate loss or abnormality of ONHC responses in MS eyes. These findings are in keeping with a cardinal pathophysiologic principle in MS-associated optic neuropathy: translaminar demyelination (either secondary to AON or as a derivative of occult optic neuropathy) and the loss of the normal transformation of membrane to saltatory electrical transmission properties of RGC axons as they traverse the lamina cribrosa.7 Notwithstanding this hypothesis, the mechanisms responsible for abnormalities in ONHC responses are likely manifold. For instance, persistently abolished ONHC responses may also occur in the context of fixed damage to RGCs or their axons (e.g., as in glaucoma).8 Alternately, ONHC may also be reversibly disorganized or absent in the context of AON, under circumstances of transient inflammation, edema, and ion channel perturbations, and with subsequent reconstitution of normal RGC axonal physiology. The limitations of a pilot investigation such as ours include the small sample size, lack of age matching, and the variability in the epoch of time from symptom onset to the time of the experimental assessments. Moving forward, the careful, systematic, and longitudinal investigation of the mfERG-induced ONHC responses in MS, and the relationship to validated structural (e.g., optical coherence tomography) and functional measures (e.g., contrast acuity, visual field analysis, mfVEP, and pupillometry) of the visual system, will ultimately determine the validity (both face and construct) and the utility of the ONHC response to detect and monitor neuroprotective or restorative effects of novel therapies.
The authors thank Jason Thean Kit Ooi for collaboration and artistic design work that created figure 1.
Editorial, page 518
Teresa Frohman is the Director of the Eye Testing Laboratory at the University of Texas Southwestern MS Program and Neuro-Ophthalmology Research Manager. She contributed to all aspects of the study, and prepared the manuscript. Shin Beh was involved in the formulation of the study, execution of the studies on our patients and control subjects, and was involved in the data analysis and preparation of the manuscript. Zane Schnurman was involved in the formulation, design, and execution of the study. He participated in the analysis of the data, preparing the manuscript, and its final revision. Amy and Darrel Conger contributed to the study through data collection and analysis and with respect to assistance with the editing and revision of the manuscript. Shiv Saidha contributed to all aspects of the data analysis and with respect to assistance with the editing and revision of the manuscript. John Ratchford contributed to all aspects of the data analysis and with respect to assistance with the editing and revision of the manuscript. Carmen Lopez contributed to the acquisition of the data, coordinating patient enrollment, and assisted in all aspects of the experimentation on all MS and normal subjects at the Center. Steven Galetta contributed to the analysis of the data and formulation and editing of the manuscript. Peter Calabresi contributed to all aspects of the study. Laura Balcer contributed to all aspects of the study. Ari Green contributed to the analysis of the data as well as the formulation and editing of the manuscript. Elliot Frohman is the senior author and contributed to all aspects of the study.
Supported by National Multiple Sclerosis Society RG 3780a3/3 to E.M.F.; National Multiple Sclerosis Society RG 4212-A-4 to L.J.B. and subcontracted to P.A.C. and E.M.F.; National Eye Institute (R01 EY 014993 and R01 EY 019473 to L.J.B. and subcontracted to P.A.C. and E.M.F.); and Braxton Debbie Angela Dillon and Skip (DADS) Donor Advisor Fund (to E.M.F. and subcontracted to P.A.C., L.J.B.).
T. Frohman has received speaker and consultant fees from Biogen Idec, Novartis, and Acorda. S.C. Beh, Z. Schnurman, D. Conger, A. Conger, and S. Saidha report no disclosures. J. Ratchford received a speaking honorarium from Biogen-Idec, Novartis, and Sun Pharmaceuticals. Carmen Lopez reports no disclosures. S. Galetta has received consulting honorarium from Biogen-Idec and Teva. P. Calabresi has provided consultation services to Novartis, EMD-Serono, Teva, and Biogen-Idec and has received grant support from EMD-Serono, Teva, Biogen-Idec, Genentech, Bayer, Abbott, and Vertex. L. Balcer has received honoraria for consulting on development of visual outcomes for MS trials from Biogen-Idec, Novartis, Acorda, Vaccinex, and Bayer. She is on a clinical trial advisory board for Biogen-Idec. A. Green has provided consulting services for Prana Pharmaceuticals, Novartis, Biogen, Roche, and Acorda Pharmaceuticals. He has served on an end point adjudication committee for a Biogen-sponsored trial and provided expert legal advice for Mylan Pharmaceuticals. E. Frohman has received speaking and consulting fees from Biogen Idee, TEVA Neuroscience, Acorda, Bayer, and Novartis. He has received consulting fees from Biogen Idee, TEVA Neuroscience, Acorda, Novartis, and Abbott Laboratories. Go to Neurology.org for full disclosures.