Meyer's Loop and the Optic Radiations in the Transsylvian Approach to the Mediobasal Temporal Lobe

OBJECTIVE: In the transsylvian approach to the mediobasal temporal structures, the temporal horn is approached through the floor of the sylvian fissure. The anterior bundle of the optic radiations (Meyer's loop) courses between the floor of the sylvian fissure and roof of the temporal horn and could be damaged in this approach. This study was designed to define the route through the floor of the sylvian fissure least likely to damage the optic pathways. METHODS: Meyer's loop was dissected by applying Klingler's fiber dissection technique in 10 formalin-fixed human hemispheres. Several measurements quantified the relationship of the Meyer's loop to surgically important structures. RESULTS: This study identified a triangular safe area below the floor of the sylvian fissure through which the temporal horn could be accessed in the transsylvian approach with a low risk of damaging the optic radiations. An incision in the floor of the sylvian fissure directed downward at the level of limen insula and the adjacent 5 mm of the inferior insular sulcus would avoid the optic radiations. An incision directed straight downward 10, 15, and 20 mm behind the limen in the inferior insular sulcus would cross Meyer's loop and would need to be directed downward and medially as much as 80 degrees from the sagittal plane to avoid Meyer's loop. CONCLUSION: In the transsylvian approach to the temporal horn, incisions at the level of the limen, or adjacent 5 mm of the inferior insular sulcus, are less likely to damage Meyer's loop and the optic radiations than more posterior incisions along the inferior insular sulcus. Incision at this safe level commonly opens into the amygdala, a portion of which is removed to provide entry into the temporal horn for removal of the mediobasal structures.