The main goal of the current study was to determine whether RNFL loss occurs in the presence or absence of ON in MS patients. This was achieved by detecting changes in RNFL thickness among Omani MS patients both with and without ON over a period of four years in comparison to healthy controls. Our findings indicated that there was a significant reduction in RNFL thickness in the affected eyes of MS patients with ON when comparing baseline and final OCT imaging measurements over a prolonged period of time.
These results are consistent with those from previous reports. Costello et al.11 observed that 74% of MS patients demonstrated RNFL thinning within 3–6 months of an ON diagnosis, with a significant reduction in RNFL thickness in ON-affected compared to non-affected eyes. A prior study by Garcia-Martin et al7 concluded that MS patients with ON experienced a significant reduction in RNFL thickness over a period of five years. Balk et al12 similarly reported that the loss of RNFL thickness in eyes with MS-associated ON occurs over time. According to Feng et al6, disease duration was inversely related to average RNFL thickness among MS patients. In contrast, Henderson et al9 reported that there was no relationship between disease duration and RNFL thinning in ON-affected eyes. Likewise, both Eyre et al13 and Balk et al12 found no significant relationship between number of ON episodes or relapses and mean RNFL thickness.
In the present study, a comparison of baseline and follow-up OCT measurements indicated that changes in RNFL thickness over time among MS patients without ON were not statistically significant. Similarly, a study performed by Henderson et al9 found a non-significant association between mean RNFL thickness and disease duration in selected eyes with no known history of ON. Khanifar et al14 stated that RNFL thinning was correlated with an increased risk of ON, and that such thinning may represent a predictive cutoff point for the presence or absence of ON. However, other studies have reported conflicting findings. A study conducted by Gelfand et al15 in the United States demonstrated that RNFL reduction begins in the early stages of MS, independently of ON. Similarly, Balk et al16 found that the eyes of MS patients without a history of ON also demonstrated a significant reduction in RNFL thickness. In a two-year follow-up study, Garcia-Martin et al17 found that RNFL changes occurred at a similar rate in non-affected eyes with no evidence of ON. Eslami et al18 found a significant inverse correlation between MS duration and RNFL thickness, regardless of the presence or absence of ON.
No significant difference in RNFL thickness were observed between ON-affected and non-affected eyes at a four-year follow-up in the current study. However, there was a significantly greater loss of RNFL thickness in ON-affected eyes compared to non-affected eyes. In line with these findings, Gelfand et al15 reported that even though eyes affected by ON demonstrated greater RNFL thinning compared to those without ON, the difference between the two groups was not statistically significant. Another study also revealed non-significant differences in RNFL thinning between affected and unaffected eyes of unilateral ON patients.19 These assumptions are also supported by findings from other research.20,21 Nevertheless, a study by Feng et al6 conducted in China concluded that there was a significant difference in mean RNFL thickness between the eyes of MS patients with and those without a history of ON. Other studies have also found reduction in RNFL thickness to be significantly greater in eyes with a history of ON compared to unaffected eyes and the eyes of healthy controls.22,23
In our study, we demonstrated a significant reduction in mean RNFL thickness among MS patients compared to healthy controls. In line with these results, a study performed in Germany by Bock et al8 indicated that average RNFL loss was significantly greater among MS patients compared to healthy controls. Another study conducted in China confirmed significant RNFL thinning in the MS group compared to the control group.6 These findings can be explained by the fact that the optic nerve has a high density of axons, which makes it vulnerable to atrophy. However, Saxena et al19 reported a non-significant difference in RNFL thickness in the nasal and superior quadrants of the eyes of MS patients when compared to the healthy group. Garcia-Martin et al7 also reported non-significant differences in RNFL changes over a prolonged follow-up period of five years when comparing the eyes of MS patients with those of a healthy control group.
Finally, there was significant RNFL thinning in the eyes of MS patients both with and without ON in the present study when compared separately with the control group. These significant correlations can be understood in light of previous studies. A study from Spain noted progressive thinning over five years in the eyes of MS patients both with and without a history of ON when compared to healthy controls.24 Garcia-Martin et al7 also found that MS patients exhibited a greater reduction in RNFL thickness than healthy controls, regardless of ON history. However, a prior study conducted in the United Kingdom found that there was no significant difference in RNFL thickness between MS eyes with no history of ON compared to a healthy control group.22
The results of this study should be interpreted in the light of certain limitations. Due to its retrospective design and the reliance on chart review as the primary method of data collection, it is possible that some data might be missing which may have resulted in selective bias. Moreover, the sample size was relatively small and the findings may therefore not be representative of all MS patients in Oman. Further studies are therefore recommended to address these limitations.