Introduction: One third of epilepsy patients are refractory to medical treatment. A common complication of the traditional anterior temporal lobectomy (ATL) is a superior homonymous quadrantanopia which can prevent driving in patients who are seizure-free post surgery. The Selective Transsylvian Amygdalohippocampectomy (sTAH) microsurgical approach may produce better visual field outcomes. Diffusion tensor imaging (DTI) tractography can visualise white matter in vivo. This study set out to demonstrate surgical disruption of Meyer’s loop for the first time in patients who have developed visual field deficits (VFDs) following sTAH. The study also explored the significance of both the anterior extent Meyer’s loop and the length of surgical resection, for visual field outcomes.
Methods: Structural and DTI scans were acquired on three subjects who had undergone sTAH and had standardised post-operative visual field assessments. Tractography was carried out to visualise the optic radiations. The distances from the anterior edge of Meyer’s loop to landmarks in the temporal lobe were measured on the non-resected side, and the lengths of resection were estimated.
Results: DTI tractography demonstrated disruption to the antero-lateral edge of Meyer’s loop in two subjects who developed a quadrantanopia. The mean distance from the anterior edge of Meyer’s loop to the temporal pole (ML-TP) was 40.3 mm (range 36-43 mm), which falls within the range reported by previous tractography studies. The mean length of resection was 52% of the temporal pole-occipital pole distance (range 47.4%- 60.3%). A shorter ML-TP distance and longer resection were associated with worse visual outcome.
Conclusions: The sTAH approach may spare many of the fibres that are lost in traditional surgery and result in a lower incidence and severity of post-operative VFDs. Further investigation is needed to determine the significance for visual field outcomes of both the anterior extent of Meyer’s loop and the resection length.
Patient Care: Post-operative visual field deficits are common and most patients cite the ability to drive as one of the most important factors in achieving full rehabilitation. Visual field outcomes need to be considered when choosing the optimal approach to temporal lobe surgery. This research supports the use of a selective technique which could preserve vision in a greater proportion of patients and significantly improve their post-operative quality of life.
Learning Objectives: By the conclusion of this session, participants should be able to: 1) Appreciate the potential of selective transsylvian amygdalohippocampectomy to provide better visual field outcomes for patients, 2) Discuss the clinical significance of white matter imaging and its potential to be integrated into the standard imaging of patients undergoing neurosurgical procedures - notably to identify patients at greater risk of visual problems and to better guide surgical procedures, 3) Identify areas of future research to improve preservation of vision in patients undergoing temporal lobe surgery.
References: Sherbondy A, Dougherty RF, Napel S, Wandell BA. Identifying the human optic radiation using diffusion tensor imaging and fibre tractography. Journal of Vision. 2008; 8(10): 12.1-12.11.