Multiple Sclerosis (MS) is a multifaceted autoimmune disorder characterized by axonal damage, gliosis, and inflammatory demyelination within the central nervous system (CNS) [1]. Over 2.5 million individuals are affected by MS worldwide making it a leading cause of atraumatic neurological disability in young adults [2, 3]. The diagnosis of MS has been a diagnosis of exclusion, based on evidence of CNS inflammation that is disseminated over space and time [4, 5]. An initial acute episode, known as a clinically isolated syndrome, occurs in 80–85% of patients who go on to develop MS [6, 7]. Acute optic neuritis (ON), inflammation of the optic nerve, is the clinically isolated syndrome in a substantial subset of MS patients, occurring in approximately 21–45% of newly diagnosed cases, where it typically presents with a range of visual disturbances, including blurred vision, reduced color perception, and retrobulbar pain [7–10]. Furthermore, impaired contrast sensitivity in patients with MS correlates with severity of CNS lesion size demonstrating the critical role of the visual pathway as an indicator of disease progression in MS, even in the absence of ON [11, 12].
Declines in the visual system due to MS extend beyond clinical symptoms and include structural and functional alterations detectable via non-invasive methods including pattern electroretinography (pERG) recordings and optical coherence tomography (OCT) imaging. pERG records the electrical signal produced by the retina as a function of time in response to a stimuli of a contrast-reversing checkerboard pattern and is typically recorded noninvasively from the lower eyelid or cornea [13, 14]. MS disease progression has been associated with reductions in pERG amplitude [15, 16]. OCT has emerged as a valuable non-invasive tool for assessing retinal structures in individuals with MS [17]. More precisely, OCT enables high-resolution, cross-sectional imaging of the retina, allowing for precise measurements of the retinal nerve fiber layer (RNFL) and the inner plexiform layer (IPL), collectively representing retinal ganglion cell (RGC) / IPL complex thickness [18, 19]. The RNFL represents a unique CNS structure in that it is not normally myelinated [20]. Therefore, changes visualized in the RNFL are the result of retrograde degeneration from lesions in the optic nerve, chiasm, or visual tract [21]. Thinning of the RNFL and RGC/IPL complex has been associated with MS, particularly when evaluated over time [22–25]. OCT has demonstrated that even MS patients with no history of ON show evidence of RGC loss [26, 27]. This quantitative approach to retinal assessment provides insights into structural changes associated with MS [18, 23]. Furthermore, OCT offers the opportunity to detect subtle retinal changes associated with MS, making it an essential tool for studying the relationship between CNS involvement and peripheral manifestations such as visual deficits [28]. Petzold et al (2017) conducted a meta-analysis of 5,776 MS eyes and found significant thinning was present in multiple retinal layers, thereby demonstrating that neuroaxonal injury is measurable by OCT [26]. They compared 1,667 MS ON eyes and 4,109 MS non-ON eyes to 1,697 eyes from healthy control subjects. Peripapillary RNFL values were found to be thinner in both MS ON eyes and MS non-ON eyes relative to healthy control eyes. Also, RGC and IPL thinning was pronounced in both MS ON eyes and MS non-ON eyes when compared to control eyes. These findings demonstrate the value of OCT as a robust and reproducible tool for monitoring MS progression, and that the visual system is a potential surrogate marker for disease progression in MS.
Experimental autoimmune encephalomyelitis (EAE) is a well-established animal model of MS [29, 30]. EAE closely mimics the pathological features of MS, including immune-mediated demyelination, axonal injury, and inflammatory responses within the CNS as well as ON [31, 32]. Thus, EAE is a valuable tool for exploring connections between CNS dysfunction and peripheral manifestations such as visual impairments. There are multiple variants of the EAE model; induction via myelin oligodendrocyte glycoprotein fragment 35–55 (MOG35 − 55) in C57BL/6J mice is particularly advantageous for visual impairment studies as the rate of ON is ~ 100% [32, 33]. Current primary outcomes largely depend on the EAE grading scale and are ultimately related to inflammation of the spinal cord. Ideally, outcomes would be less subjective and more directly confirmatory of inflammation of the optic nerve.
Optic nerve histopathology offers an excellent method for discovering structural alterations in the visual system of the EAE model. Specifically, the ability to detect and quantify demyelination of the axon and immune cell infiltration using readily available lab techniques, such as Luxol fast blue (LFB) and hematoxylin and eosin (H&E) staining [34]. LFB directly stains the myelin sheath, and thus an observation of whitening of the optic nerve directly indicates demyelination [35]. H&E staining allows for direct observation of immune cells’ infiltration into the optic nerve tissue [35]. The principal downside to this otherwise advantageous method is the necessarily posthumous nature of tissue collection. In all other aspects, this method provides an independent standard of optic nerve condition against which other methods can be measured. The optic nerve, as a key component of the marvelously complex afferent visual system, is unique in that it is directly and only related to visual structure and function, whereas other CNS tissues such as the spinal cord, are affected by inputs from multiple body systems [36–38]. Alterations in sectioning the spinal cord may cause key inflammation events to go unnoticed; however, due to the specialized nature of the optic nerve, any visual disfunction present can be expected to cause alterations in the optic nerve.
This study aims to bridge the gap in our understanding of the relationships between structural / functional changes in the visual system and the wider spectrum of motor-sensory deficits observed in EAE. By inducing EAE in a murine model and closely monitoring clinical progression, we intend to determine whether alterations in the visual system, including pERG amplitude, and RGC/IPL complex thinning as measured by OCT, correlate with motor-sensory impairments and structural alterations of the optic nerve. These investigations not only hold the potential to enhance our understanding of the interrelation between structural, functional, and imaging parameters in the context of EAE and MS but also offer insights into the feasibility of employing non-invasive visual assessments, particularly OCT, as potential indicators of CNS involvement. OCT is particularly suitable for evaluating the potential treatment effects of new drugs in preclinical settings, as it has been previously used as a primary and secondary clinical outcome [39–42].
Here we demonstrate significant correlations between overall motor-sensory impairment, physical alterations of the optic nerve, visual function, and structural changes in the retina. These correlations underscore the promise of OCT as a critical tool for understanding the relationship between the visual system and motor-sensory deficits in the context of EAE and its potential application in MS research and clinical practice.