Authors have attributed the preoperative extent of the VF deficit as the major influence on its recovery13–35. However, the VF examination is burdened by an element of subjectivity and has not always been sufficiently sensitive to identify VF defects10–12. This led us to the idea of evaluating the potential of OCT and VEPs, mainly the M-VEPs, in the management of OC compression.
Earlier studies have affirmed that preoperative average RNFL thickness below 70–85 µm is a negative prognostic factor for both immediate and long-term visual improvement36–38. However, Loo et al. admitted that visual recovery may occur even with average peripapillary RNFL of less than 70 µm38. Based on the negative correlation between preoperative thickness of RNFL, either temporal or average, and improvement in VA/VF (Fig. 5a), our results indicate that lower average preoperative RNFL thickness (<85 µm; to note the absence of a clear-cut) tends to have a bigger functional benefit for postoperative visual functions. In general, the observed dependencies were more obvious when the temporal RNFL was taken into consideration. It reflects better the effort to evaluate the crossing fibers of the OC. The preoperative temporal RNFL thicknesses of our patients were scattered around the lower values (median 65 µm), and we did not observe an RNFL threshold limiting the functional improvement.
More recent studies have accentuated the role of the GCL, or GCC (Ganglion Cell Layer Complex). Some works have highlighted that binasal thinning of the GCC often corresponds with a bitemporal blind spot in the VF12,16–18,20,39,40. Paradoxically, Yoneoka presented a result of a stronger correlation with the RNFL41. An analysis of our results shows a more significant concordance between preoperative nasal GCL and VF defects (Spearman rho = 0.678; p = 0.039⋅10− 5) in comparison to temporal RNFL (Spearman rho = 0.559; p = 0.001). This could be explained by the less systematic projection of the VF into the topic of the nerve fibers than in the GCL cells. The negative correlation between preoperative nasal GCL and VF improvement (Fig. 5b) is stronger in contrast with the temporal RNFL. Nasal GCL thickness less than 40 µm (without a clear-cut) tends to have a bigger functional benefit for postoperative visual functions. Given the complex structure underlying visual perception, RNFL or GCL thickness might not fully correspond to the visual impairment. For an accurate assessment of the impact of the preoperative RNFL or GCL thickness on postoperative visual changes, a group of patients with the same functional impairment should be examined.
A continuous thinning of RNFL and GCL, with interindividual differences, is documented by our data despite a significant improvement in the VF. An ongoing process of nerve fiber degeneration serves as an explanation. Neither should be excluded manipulation of the optic nerves (during tumor resection) as a plausible exogenic determinant for axonal degeneration.
The use of VEPs to monitor visual functions in the case of optic nerve or OC compressions has not been as widespread as for optic neuritis or glaucoma22,24,25,42−45. It is believed that prolongation of peak time (in mf-VEPs) is seen when the pressure acting on the OC is not yet fixed (so-called active). In such cases and also in ischemic optic diseases or glaucoma, patients with severe VF impairment have normal mf-VEP latencies. Two teams showed a statistically significant and strong correlation between blind spots in the temporal half of the VF and mf-VEP parameters22,25. Our results support wider use of VEPs to detect abnormalities of the visual pathway in OC compression. In patients with bitemporal hemianopia, stimulation by hemifields showed a statistically insignificant decrease in amplitude of P-VEPs and M-VEPs for crossed fibers.
Conventional VEPs (e.g., P-VEPs) are able to harvest responses from approximately the central 15˚ of the VF46. This means that those protocols are not able accurately to display the spatial details and peripheral affections of the perimeter. A more objective evaluation of the functional integrity of the visual pathway is achieved with mf-VEPs that encompass 25–32˚ of the VF47,48. M-VEPs, under certain stimulation conditions, offer the possibility for testing even more peripheral parts of the VF (up to 50˚ eccentricity)26. Using low spatial frequency stimulation in the periphery of the VF activates predominantly magnocellular visual input, which helps to achieve such a wide range. Activation of this part of the visual pathway may provide a different sensitivity than the P-VEPs. The receptive fields of the retina for magnocellular input information (parasol ganglion cells) are found mainly in the extrafoveal part of the retina. This suggests that the M-VEPs might be more helpful and accurate in testing patients with minimal peripheral VF abnormalities. The same applies for mf-VEPs. It has not been possible to compare our M-VEPs results with those from other works. To the best of our knowledge, no publication has yet described the use of M-VEPs to assess the functional state of the visual pathway in OC compression. In all three postoperative controls, in most cases M-VEPs showed statistically significant shortening of peak time and increasing of amplitude (Table 1). Such evident results were not obtained in P-VEPs, supporting the fact that M-VEPs have a higher sensitivity to stimulation in the peripheral areas of the retina.
Correlation analysis has demonstrated only few dependencies between preoperative and postoperative parameters of P-VEPs / M-VEPs and VA or MD improvement (Fig. 5c, d). A statistically significant relationship was observed only between improvement of MD and N160 amplitudes (Spearman rho = -0.392; p = 0.038) (Fig. 5d). No statistically significant differences were found in any of the P-VEPs and M-VEPs parameters when comparing patients with or without bitemporal hemianopia. Four of those 8 patients without preoperative subjective visual impairment had a normal VF examination with no evidence of incipient hemianopia or quadrantanopia. In one case, M-VEPs had a diagnostic validity (patient #8). Preoperatively, an increase in the N160 peak time was apparent on the right eye. This was subsequently improved in the postoperative period.
Results from a perimeter or OCT do not present unequivocal prognostic parameters. The little-discussed preoperative variable, the degree of OC compression, has a strong impact on the indication for surgery and the postoperative visual outcome.49 Our results show that the mean thickness of the RNFL (Fig. 1b) as well as of the GCL (Fig. 2b) was statistically significantly greater in grade 0–1 than in grade 2–4. However, it must be emphasized that there were patients with a significant OC compression but who had a satisfactory thickness of RNFL and GCL. This underlines the concept of multifactorial cause of chiasmal syndrome (OC micro-compression or disturbed blood supply leading to local ischemia at the level of the OC)11,50.
When comparing patients with grade of compression 0–1 with those with grade 2–4, the median preoperative latencies were not statistically significantly different for P-VEPs and M-VEPs. In other words, there was no peak time prolongation in patients with greater OC compression. In both groups, there was a shortening of N160 peak time in the first postoperative control. However, for other follow-up controls, the peak time values were close to preoperative values. We attribute the improvement at the first postoperative control mainly to the effect of decompression. Conversely, subsequent progressive mild “deterioration” of the parameters could be explained by slow postoperative changing of traction forces (as a component of scarring) acting on the OC, or by continued antero- and retrograde degeneration51,52. Although less probably, it could be attributed to the manipulation of the optic nerves (during tumor resection). In summary there were no important preoperative pathological VEP changes in the case of the higher grades. Again, it indicates that the etiology of chiasmal syndrome has more components than solely morphology.
Our unique findings with radial motion stimulation in patients with OC compression suggest that M-VEPs are able to detect functional changes resulting from compression of those optic nerve fibers that carry information from the peripheral parts of the retina. Radial movement tends to be the most effective way of stimulation53. This is mainly because the stimulus design respects the cortical magnification factor, and probably also because this type of motion resembles the optic flow, which is present during observer self-motion through an environment.
The study has several limitations. Assuming the greatest impact of a tumor compression is on crossed fibers, as seen in the nasal GCL, we chose the temporal RNFL segment for preferential evaluation. However, the average (global) RNFL appeared to have a strongest link to changes in VF and VA. As already mentioned, probably it is due to the poorer retinotopic mapping of the RNFL. VEP examination generally has a high sensitivity to functional changes in visual perception. However, we did not observe this in our sample. This decreased sensitivity is likely due to the suboptimal stimulation, where stimulation patterns were projected only in a small field (11˚ x 14˚) laterally from the center of the fovea. This weakens the outcome, mainly of P-VEPs. The VEPs examination, which requires systematic attentive cooperation from patients, may contribute to possible bias of the results, because patients may become tired during the examination. While all patients were cooperative in our group, in less cooperative patients, mf-VEPs were considered more reliable than the VF examination54. The group of 32 eyes represents a relatively small cohort. In addition, some patients lacked a complete series of all three follow-up postoperative controls.