Abstracts

Rationale:
Anteromesial temporal lobe resection (ATLR) results in long-term seizure freedom in drug-resistant focal mesial temporal lobe epilepsy (MTLE). There is significant anatomical variation in the anterior projection of the optic radiation (OR) between subjects and between hemispheres in the same subjects. Damage to OR results in a contralateral superior temporal quadrantanopsia (CSTQ) that may preclude driving in 20-50% of cases, even when the patient achieves seizure freedom. Tractography of OR has been shown to prevent visual field deficits (VFD) when surgery is performed in an interventional MRI (iMRI) suite. As the access to iMRI is limited in most centres we investigate whether a neuronavigation system with microscope overlay in a conventional theatre is sufficient to prevent significant VFDs during ATLR.
Method:
A single-centre prospective cohort study was performed in 20 patients with drug-resistant MTLE that underwent ATLR (9 right-side and 9 male) over a two year period Tractography of ML was performed from pre-operative 3T multi-shell diffusion data and overlaid onto the surgical field using a conventional neuronavigation system linked to a surgical microscope (see Figures 1 and 2). Phantom testing revealed overlay projection errors of < 1 mm prior to clinical use. Following anterior temporal lobe resections, VFDs were quantified from pre- and 3-12 month post-operatively using both Humprey and Estermann perimetry.
Results:
Perimetry results were available for all patients post-op but only 11/20 pre-operatively. In 5% (1/20) of cases, a significant VFD occurred that would prevent driving in the UK based on Estermann perimetry. The VFD was identified early in the series despite the surgical approach not transgressing the optic radiation tractography and was subsequently found to be due to retraction injury. The tractography was then also used from this point onwards to inform retractor placement and no further significant VFDs occurred.
Conclusion:
OR tractography overlay outside of an iMRI suite, when used during the approach to the temporal horn of the lateral ventricle, results in a 5% risk of VFD that is significant enough to preclude driving post-operatively. It can also be used to inform retractor placement. These results warrant a large prospective comparative study of optic radiation tractography guided mesial temporal resections.
Funding:
:Wellcome Trust (Grant 106882) Wellcome/EPSRC [203145Z/16/Z] GW was supported by an MRC Clinician Scientist Fellowship (MR/M00841X/1)