Meyer's loop constitutes the most anterior extension of the optic radiation (OR) in the temporal horn.

Sparing during surgery is crucial to preserve the visual function of the patient. Its exact anatomy is still challenging to define and its position is not easy to accurately identify, even with diffusion tensor imaging tractography and the most refined tracking procedure 1).

Anterior temporal lobectomy is often complicated by superior quadrantic visual field deficits (VFDs). In some cases this can be severe enough to prohibit driving, even if a patient is free of seizures. These deficits are caused by damage to Meyer's loop of the optic radiation, which shows considerable heterogeneity in its anterior extent. This structure cannot be distinguished using clinical magnetic resonance imaging sequences.

Optic radiation tractography by DTI could be a useful method to assess an individual patient's risk of postoperative visual deficit 2). 3).

Sarubbo et al., identified two groups of fibres forming the OR. The superior component runs along the lateral wall of the occipital horn, the trigone and the supero-medial wall of the temporal horn. The inferior component covers inferiorly the occipital horn and the trigone, the lateral wall of the temporal horn and arches antero-medially to form the Meyer's Loop 4)

The fiber microdissection technique provides clear evidence that a loop in the anterior temporal region exists, but that this temporal loop is not formed exclusively by the optic radiation. Various projection fibers of the sublentiform portion of the internal capsule (IC-SL), of which the optic radiation is only one of the several components, display this common course. The inherent limitations of the fiber dissection technique preclude accurate differentiation among individual fibers of the temporal loop, such as the optic radiation fibers 5).

LGN Lateral geniculate nucleus

There is considerable variation in the anterior extent of Meyer's loop. In view of this, diffusion tensor tractography of the optic radiation is a potentially useful method to assess an individual patient's risk of postoperative VFDs following anterior temporal lobe resection 6).

Temporopolar tumors led to a posterior shift, always including Meyer's loop; therefore, a pterional transcortical approach is recommended 7).

Bertani et al., made a DTI study of twenty-six patients undergoing resection of a temporo-parieto-occipital lesion; then, we reconstructed the ORs of each patient with two techniques (the first developed by our team, the other taken from the literature), using the same tracking software and parameters. We evaluated the accuracy of each technique measuring three distances, which define Meyer's Loop position. We made five data groups and compared the two techniques with each other. Finally, we compared our results with eight anatomical dissection studies and other tractographic studies.

The study shows that the technique allows a more accurate definition of Meyer's Loop position: they found a statistically significant difference (p<0.05) for all the distances between the two techniques; our results resembled those obtained in dissection studies. Our technique is also easy to perform and repeatable.

The tracking technique could be of marked interest for the evaluation and anatomical definition of Meyer's Loop position, particularly to neurosurgeons approaching the anterior temporal region 8).

1) , 8)
Bertani GA, Bertulli L, Scola E, Cristofori AD, Zavanone M, Triulzi F, Rampini PM, Carrabba GG. Optic Radiation Diffusion Tensor Imaging Tractography: an Alternative and Simple Technique for the Accurate Detection of Meyer's Loop. World Neurosurg. 2018 May 29. pii: S1878-8750(18)31084-2. doi: 10.1016/j.wneu.2018.05.131. [Epub ahead of print] PubMed PMID: 29857218.
Borius PY, Roux FE, Valton L, Sol JC, Lotterie JA, Berry I. Can DTI fiber tracking of the optic radiations predict visual deficit after surgery? Clin Neurol Neurosurg. 2014 Jul;122:87-91. doi: 10.1016/j.clineuro.2014.04.017. Epub 2014 May 5. PubMed PMID: 24908224.
James JS, Radhakrishnan A, Thomas B, Madhusoodanan M, Kesavadas C, Abraham M, Menon R, Rathore C, Vilanilam G. Diffusion tensor imaging tractography of Meyer's loop in planning resective surgery for drug-resistant temporal lobe epilepsy. Epilepsy Res. 2015 Feb;110:95-104. doi: 10.1016/j.eplepsyres.2014.11.020. Epub 2014 Nov 27. PubMed PMID: 25616461.
Sarubbo S, De Benedictis A, Milani P, Paradiso B, Barbareschi M, Rozzanigo U, Colarusso E, Tugnoli V, Farneti M, Granieri E, Duffau H, Chioffi F. The course and the anatomo-functional relationships of the optic radiation: a combined study with 'post mortem' dissections and 'in vivo' direct electrical mapping. J Anat. 2015 Jan;226(1):47-59. doi: 10.1111/joa.12254. Epub 2014 Nov 17. PubMed PMID: 25402811.
Goga C, Türe U. The anatomy of Meyer's loop revisited: changing the anatomical paradigm of the temporal loop based on evidence from fiber microdissection. J Neurosurg. 2015 Jan 30:1-10. [Epub ahead of print] PubMed PMID: 25635481.
Yogarajah M, Focke NK, Bonelli S, Cercignani M, Acheson J, Parker GJ, Alexander DC, McEvoy AW, Symms MR, Koepp MJ, Duncan JS. Defining Meyer's loop-temporal lobe resections, visual field deficits and diffusion tensor tractography. Brain. 2009 Jun;132(Pt 6):1656-68. doi: 10.1093/brain/awp114. Epub 2009 May 21. PubMed PMID: 19460796; PubMed Central PMCID: PMC2685925.
Faust K, Vajkoczy P. Distinct displacements of the optic radiation based on tumor location revealed using preoperative diffusion tensor imaging. J Neurosurg. 2015 Oct 2:1-10. [Epub ahead of print] PubMed PMID: 26430843.
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