Visual field defects after radiosurgery for mesial temporal lobe epilepsy

Authors


To the Editors:

In their article, Hensley-Judge et al. (2013) hypothesize that, in a study of patients with mesial temporal sclerosis (MTS) undergoing radiosurgery (RS), the extent of postoperative visual field defects (VFDs) would either correlate with the Gy dose administered by RS or with improved patient outcome (postoperative seizure freedom).

The study did not reveal a significant difference in the extent of VFDs postoperatively between patients receiving low (20 Gy) and high (24 Gy) dose radiation. However, on the basis of the findings from neuroimaging studies in patients receiving conventional anterior temporal lobe resection or selective amygdalohippocampectomy, this is of no surprise. Numerous studies have shown a significant correlation between postoperative optic radiation (OR) damage and resulting visual field defect, the largest study of which was conducted by Chen et al. (2009). This correlation is tentatively supported by the data generated by Hensley-Judge et al., with the finding that more severe postoperative VFDs were found in patients rendered seizure-free. This phenomenon is assumed a consequence of wider resection margins causing further damage to the OR, but remains untested. Therefore, the extent of postoperative VFDs is more likely a consequence of the overlap of the RS target with the OR, regardless of the Gy dose induced.

In our opinion, the investigators have missed an opportunity to examine the integrity of the OR, comparing preoperative and postoperative diffusion tensor imaging (DTI) and tractography (also known as fiber tracking), in order to measure the injury inflicted by RS. Recent studies have shown the feasibility of conducting DTI measures in patients undergoing RS for glioma (Chang et al., 2013) and RS callosotomy for symptomatic generalized epilepsy (Moreno-Jiménez et al., 2012). It would be interesting to investigate the overlap between the RS target area and the estimated location of the OR, and how this relates to the extent of postoperative VFDs. Of even further interest would be if using DTI methods would allow precise targeting in RS to avoid damage to the OR and thus prevent VFDs in patients undergoing MTS treatment.

In conclusion, we suggest that the postoperative VFD consequences are anatomically dependent rather than dose dependent for both RS and conventional resection methods of treating MTS. Further evidence is required to make conclusions about the relationship between the integrity of the OR and postoperative VFDs in patients undergoing RS for the treatment of MTS. A potential finding that RS may cause less damage to the optic radiation than conventional resection may give merit to its potential as a conventional alternative intervention in MTS.

Disclosure

None of the authors has any conflict of interest to disclose. We confirm that we have read the journal's position on issues involved in ethical publication and affirm that this report is consistent with those guidelines.

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