- Open Access
Altered birefringence of peripapillary retinal nerve fiber layer in multiple sclerosis measured by polarization sensitive optical coherence tomography
© The Author(s). 2018
- Received: 11 February 2018
- Accepted: 2 June 2018
- Published: 17 June 2018
The retina has been used to study the pathophysiology of multiple sclerosis (MS). Peripapillary retinal nerve fiber layer (pRNFL) thinning has been suggested as an ocular biomarker of neurodegeneration in MS. The goal of this project was to determine the birefringence of the pRNFL by measuring the fiber birefringence using polarization sensitive optical coherence tomography (PS-OCT).
Sixty-six MS patients without history of optic neuritis (age: 39.9 ± 11.0 yrs. old, 53 females and 13 males) and 66 age- and gender-matched normal controls (age: 40.7 ± 11.4 yrs. old) were recruited. Custom built PS-OCT was used to measure phase retardation per unit depth (PR/UD, proportional to the birefringence) and pRNFL thickness in each quadrant of the pRNFL. In addition, clinical manifestation was used to correlate with the pRNFL birefringence.
The pRNFL was thinner in the temporal and inferior quadrants in MS patients compared with normal controls (P < 0.05). The PR/UD of the pRNFL was significantly decreased in MS patients (P < 0.05) in all quadrants except for the nasal quadrant. In both groups, the PR/UD from all four quadrants was not related to the averaged pRNFL thickness (P > 0.05). In MS patients, the PR/UD was not related to the expanded disability status scale (EDSS) nor disease duration (r ranged from − 0.17 to 0.02, P > 0.05).
This is the first study using PS-OCT to study the pRNFL birefringence in MS patients. Decreased birefringence of the pRNFL may indicate microtubule abnormality, and could be a potential biomarker for detecting early neurodegeneration in MS.
- Remitting relapsing multiple sclerosis (RRMS)
- Peripapillary retinal nerve Fiber layer (pRNFL)
- Microtubule dysfunction
- Polarization sensitive optical coherence tomography (PS-OCT)
Multiple sclerosis (MS) is an inflammatory demyelinating disorder in which axonal degeneration is known as the main pathological substrate underlying the progressive disability in patients with MS . Visible MS lesions (focal demyelination associated with inflammation) shown on conventional magnetic resonance imaging (MRI) represent only a small fraction of MS pathology, whereas diffuse mild inflammatory degenerative alterations exist in normal appearing white and grey matters (NAWG) [2–4]. The non-conventional MRI, such as the diffusion tensor imaging (DTI) MRI, could quantify demyelization and axonal loss that is not visible on conventional MRI . Similarly, the magnetization transfer (MTR) MRI can detect microstructural damage that is strongly correlated with the percentage of residual axons and the microscopic tissue damage in NAWG . However, the DTI and MTR MRIs cannot detect axonal microstructural changes.
MS often involves the optic nerve presenting as acute optic neuritis or subclinical optic neuropathy and/or retrograde degeneration of the MS lesions within the posterior optic pathways . The peripapillary retinal nerve fiber layer (pRNFL), composed of unmyelinated axons, has been suggested as a biomarker representing the axonal degeneration in the brain of MS patients. pRNFL can be noninvasively imaged by optical coherence tomography (OCT) . However, monitoring the thickness of the pRNFL for detecting early microstructural changes of the axon may not be as sensitive. Instead, imaging its microstructural integrity may further improve the ability in detecting early axonal degeneration and monitoring the disease progression and treatment efficacy.
The pRNFL is known to exhibit birefringence , which is related to the structure of dominant axonal filaments, such as neurofilaments, axoplasmic membranes and microtubules . Changes in the axonal cytoskeleton, such as neurofilament compactness and phosphorylation, resulting in related birefringence changes, usually precede axonal loss [9, 10]. Birefringence is a unitless difference of refractive indices and is expressed as delta n. Delta n is related to phase retardation/unit depth by the following equation: delta n = (λ/360) × (PR/UD), where λ is the central wavelength used in the measurement system, PR is the phase retardation, and UD is the unit depth . We hypothesize that the pRNFL birefringence in MS patients is altered, which could be another indicator of axonal pathology in patients with MS. The pRNFL birefringence can be quantitatively measured using PS-OCT, which can assess the depth-resolved polarization properties of the tissue to provide additional information about tissue integrity [12, 13]. The goal of the present study was to determine the birefringence of the pRNFL in patients with relapsing-remitting MS (RRMS).
Demographics and clinical manifestations of patients and normal subjects
39.9 ± 11.0
40.7 ± 11.4
13 M 53F
13 M 53F
1.8 ± 1.9
6.4 ± 7.0
Descriptive statistics were used to summarize patient demographic and clinical information. All data were expressed as the mean ± SD. The thickness and the birefringence of the pRNFL between the two groups were analyzed by two-tail t-tests with two sample equal variance. Pearson’s correlation analysis was used to evaluate the strength of association between the thicknesses and the birefringence of the pRNFL. P values less than 0.05 were considered significant.
The averaged PR/UD was not correlated to averaged pRNFL thickness in both MS (r = − 0.10, P < 0.05) and control (r = − 0.02, P < 0.05) groups as well as in all subjects (r = − 0.001, P < 0.05). In MS, the average PR/UD was not related to EDSS and disease duration (r ranged from − 0.17 to 0.02, P > 0.05).
In this study, MS patients were mildly disabled with relatively short disease duration. The eyes of RRMS patients in the remitting stage without a history of optic neuritis were studied, and the decreased birefringence found in the present study indicates the alteration of the microtubule structure abnormality in these patients. Furthermore, the alteration of birefringence appeared to exist in MS patients regardless of the pRNFL thickness. Birefringence alterations were not only found in MS patients with profound neurodegeneration as evident by pRNFL thinning, but also observed in MS patients whose pRNFL thickness were within normal ranges. Hence, pRNFL birefringence loss may precede pRNFL thinning, which could be an independent marker of early neurodegeneration. However, whether the change of birefringence occurs prior to pRNFL thinning may not be addressed in this cross-sectional study, and should be further investigated in future longitudinal studies.
In contrast to the PS-OCT with the depth resolved measurement of pRNFL birefringence, the scanning laser polarimetry (GDx) applies the polarized light on the retina to provide an integrated polarization measurement, so that the thickness of the retinal nerve fiber layer (RNFL) can be determined in MS patients [21, 22]. Many GDx studies have demonstrated that the pRNFL is thinner in MS patients as compared to healthy controls [21, 22]. However, the thinning of the pRNFL measured by GDx was not correlated with EDSS or other clinical parameters [23, 24]. Furthermore, GDx could not provide any in-depth information. Therefore, the birefringence (i.e. retardation per unit depth) cannot be measured because it measures the integrated phase retardation and then uses a fixed RNFL birefringence to convert phase retardation to RNFL thickness .
Unlike GDx, PS-OCT detects the polarization properties of the light collected from birefringent samples, such as the pRNFL [14, 26], which has been suggested to reflect the integrity of the pRNFL microstructure, such as microtubules . It provides cross-sectional images of phase retardation, birefringence and optical axis information in the sample with depth information, using polarization-modulated light. Therefore, both structural information (traditional OCT structural image) and birefringence information (potentially connected to the integrity of retinal ganglion cell axon microtubules and neurofilaments) can be acquired simultaneously in two- and three-dimensions with a high resolution . In other words, PS-OCT has the capacity to not only corroborate OCT findings of pRNFL thinning, but also provide insight into the microstructural damage that may precede or occur in the absence of pRNFL thinning identified by OCT, as shown in this study.
Our PR/UD measurements are slightly lower than the values of previous reports [27, 28]. Cense et al. reported that PR/UD in the healthy human retina ranged between 10 and 37 degrees/100 μm [13, 28]. The averaged PR/UD measurement in the present study was 8.2 degrees/100 μm in normal healthy subjects with a range between 2 and 17 degrees /100 μm. A similar range was found in the MS group although the average PR/UD was lower than controls. The discrepancy of the measurements in normal subjects may be due to different cohorts among studies or possible discrepancy of system calibration for scan depths. In addition, the difference of the numbers of A-scans used to plot the retardation as a function of tissue depth may also contribute to the slight difference of the measurements. We used 512 A-scans for the measurements and Cense et al. used fewer A-scans [13, 28]. While the measurement discrepancy warrants further investigation, this may not influence the comparison of the PR/UD between groups measured using the same device. There was a difference in the pRNFL thickness measurement between our custom PS-OCT and other OCT systems, such as the commercial Cirrus OCT. Our PS-OCT system was calibrated for its scan depth in air and converted from the optical distance to the geometric distance (by dividing tissue refractive index, 1.38 for the retina) and the slight difference we demonstrated in this study may be due to hardware configuration, segmentation algorithms and possible different scan location around the optic nerve head [18, 19]. However, a good correlation was found between our system and the commonly used Cirrus OCT.
Although a significant alteration of the pRNFL birefringence was found in MS patients for the first time, this study has some limitations. First, this was a cross-sectional study, which may not validate if RNFL thinning happened before the changes of birefringence. Second, the sample size may be a concern. The repeatability is about 13% and the change of the pRNFL birefringence was about 28%. To detect 20% of the change, a sample size of 21 subjects would have a detection power of 0.99, which was determined using a software program (GPower, Ver. 3.0) developed by Faul et al. . Therefore, the sample size of 66 MS patients and matched controls was sufficient to detect the true change in birefringence.
In summary, this is the first study using PS-OCT to study the pRNFL birefringence in MS patients. Decreased birefringence of the RNFL may indicate microtubule abnormality, which may be a potential biomarker for detecting early neurodegeneration in patients with MS.
Grant/financial support: Supported by the National Multiple Sclerosis Society, NIH Center Grant P30 EY014801, and a grant from Research to Prevent Blindness (RPB).
Availability of data and materials
The datasets used and analyzed for the present study are available from the corresponding author.
HJ, WC, YL, LY and JW collected and analyzed the data. HJ, SD and JW interpreted the data. HJ, WC and JW were the major contributors in writing the manuscript. All authors read and approved the final manuscript.
Ethics approval and consent to participate
All research methods are in accordance with the tenets of the Declaration of Helsinki and approved by the ethics committee board of the University of Miami. All subjects were recruited voluntarily and were informed about the purposes, methods, and the potential risks of the study. A signed consent form was obtained from each volunteer.
Consent for publication
All study subjects gave informant consent.
The authors declare that they have no competing interests.
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