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Acute optic neuritis (ON) causes axonal degeneration, which can be quantified from the blood by neurofilament protein (Nf) levels. Within about three months, atrophy of the retinal nerve fibre layer (RNFL) follows. However, it remains challenging to explain why there is also transient thickening of the macular RNFL in acute ON, not related to optic disc swelling, in some patients in the studies reviewed. The authors of this paper hypothesised that because of the association of axonal diameter with Nf concentration, this transient phenomenon could be related to a change of retinal tissue Nf levels. The concentration of retinal tissue Nf levels following ON is not known. This paper presents an experimental model to investigate this theory further. The myelin-oligodendrocyte-glycoprotein (MOG) induced experimental autoimmune encephalomyelitis (EAE) model was used to investigate the retinas of Brown Norway rats with (i) visual evoked potentials (VEP) confirmed ON, (ii) VEP confirmed absence of ON and (iii) control animals. Twenty retinas were collected from MOG-EAE and control rats 27 days after immunisation. Retinal tissue Nf concentrations per total protein (μg/mg) were significantly higher in MOG-EAE rats with ON (median 4.29, interquartile range [IQR] 3.41-5.97) compared with MOG-EAE rats without ON (1.14, IQR 1.10-1.67) or control rats (0.93, IQR 0.45-4.00). The data suggest that up-regulation of Nf expression in the retinal ganglion cells precedes development of RNFL atrophy and plausibly explains the transient increase of axonal diameter and RNFL thickening. In conclusion, this paper presents a concept which helps to integrate structural and biomarker observations in ON. The proposed sequence of pathology in ON is that, following the inflammatory damage to optic nerve axons, retinal ganglion cells react in a compensatory way, which includes up-regulate expression of Nf proteins. This then leads to a transient increase of the axonal diameter explaining the OCT observation of RNFL thickening. This phase is followed by one of two options: preservation of axonal integrity with normalisation of the RNFL or progression of retrograde axonal degeneration with atrophy of the RNFL. During this latter phase, which lasts for about three months, the intraretinal up-regulation of Nf isoforms combined with anterograde axonal transport sustains the increase of Nf isoforms levels measured in the patients’ blood.

Upregulated retinal neurofilament expression in experimental optic neuritis.
Weissert R, Hugosson T, Petzold A.
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Claire Howard

Salford Royal NHS Foundation Trust, Salford, UK.

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