Challenges of the Anatomy and Diffusion Tensor Tractography of the Meyer Loop

Abstract

SUMMARY:This review addresses the complex and often controversial anatomy of the anterior bundle of the OR, also known as the Meyer loop. Before the advent of MR imaging, 2 main types of studies attempted to ascertain the "safe" distance for anterior temporal lobe resection to avoid postsurgical VFDs. There were those based first on postoperative VFD correlation and second on anatomic dissection studies. In the past decade, noninvasive diffusion MR imagingϪbased tractography techniques have been developed in an attempt to elucidate white matter connectivity. Although many of these techniques are still experimental, there are some clinical situations for which they may prove to be very helpful if properly performed and validated. The motivation for this review was to improve the outcome of patients with TLE undergoing temporal lobectomy: Would having anatomic information about the OR available to the neurosurgeon decrease the risk of postsurgical VFDs?ABBREVIATIONS: ATL ϭ anterior temporal lobectomy; CSD ϭ constrained spherical deconvolution; DT-FT ϭ diffusion tensor fiber tractography; DTI ϭ diffusion tensor imaging; FA ϭ fractional anisotropy; LGB ϭ lateral geniculate body; LGN ϭ lateral geniculate nucleus; OR ϭ optic radiations; TLE ϭ temporal lobe epilepsy; TP ϭ temporal pole; VFD ϭ visual field deficit T emporal lobectomy has become a common and successful form of epilepsy surgery, which can stop the occurrence of TLE in most appropriate patients who are refractory to medical treatment. A common complication of ATL is the occurrence of a postoperative VFD due to disruption of the OR, specifically the anterior bundle, the Meyer loop. Unfortunately, the extent of a postoperative VFD cannot be accurately predicted by conventional MR imaging or from the extent of the resection performed. Risk of damage to the OR is substantial because the tracts are not separately visible under the operating microscope. As the Meyer loop transmits visual information from the contralateral superior field of both eyes, damage will cause homonymous superior quadrantanopia.

Tractography of Meyer's Loop asymmetries

“…Asymmetry measurements in the current study support our hypothesis and demonstrate that the mean ML-TP distance is significantly shorter in the left (39.70 mm) compared to the right (45.64 mm) temporal lobe (table). The asymmetry of the anterior extent of the OR has been initially discussed in dissection studies (Ebeling and Reulen, 1988), and representative figures of the OR in dissection and tractography studies often depict leftward asymmetry (Govindan et al, 2008;Hofer et al, 2010;Mandelstam 2012;Sherbondy et al, 2008). Consistent with the significant leftward asymmetry of ML-TP in our study, Burgel et al (1999) report significant leftward asymmetries of the LGN and OR in a dissection study of ten adult human brains.…”

“…However, these measurements are high in contrast to other deterministic tractography studies that report ML-TP distances that are ≤ 37.3 mm (Chen et al, 2009;Taoka et al, 2008;Yamamoto et al, 2005, Yogarajah et al, 2009; for review see Mandelstam 2012). Indeed, streamline algorithms have been criticised due to their high false negative rate (Mandelstam 2012;Sherbondy et al, 2008). Probabilistic tractography algorithms allow for multiple pathways for each DTI sample point and direct comparisons of deterministic and probabilistic tractography report longer ML-TP distances using the former (mean 41mm, range 34-51mm) versus the latter (mean 30mm, 17-42mm) technique (Nilsson et al, 2010).…”

“…The overall distance between ML-TP in this study is 42.9 mm, which is comparable to measurements reported by Nilsson et al (2007Nilsson et al ( , 2010 and Wu et al (2012) (40.2mm) M a n u s c r i p t using deterministic tractography. However, these measurements are high in contrast to other deterministic tractography studies that report ML-TP distances that are ≤ 37.3 mm (Chen et al, 2009;Taoka et al, 2008;Yamamoto et al, 2005, Yogarajah et al, 2009; for review see Mandelstam 2012). Indeed, streamline algorithms have been criticised due to their high false negative rate (Mandelstam 2012;Sherbondy et al, 2008).…”

“…In a recent review Mandelstam (2012) suggests that there may be significant hemispheric asymmetries within individuals and cites studies that have postulated that a leftward asymmetry of the OR may be due to expanded language areas in the left posterior temporal lobe that have displaced the OR on that side (Jeelani et al, 2010;Wang et al, 2008 ).…”

“…Another neurosurgical study showed that left-sided surgery resulted in VFD in 6 out of 11 patients compared to 3 out of 10 with right-sided surgery (Yogarajah et al, 2009). The potential for significant hemispheric asymmetry of ML has also been recently suggested in a thorough review on the challenges of the anatomy and tractography of ML (Mandelstam 2012).…”