Root exit zone of the trochlear nerve on the dorsal brainstem: an MRI study

Purpose To date, no study has explored the root exit zone of the trochlear nerve (TroN) on the dorsal brainstem; therefore, we aimed to characterize the location using magnetic resonance imaging (MRI). Methods A total of 85 patients underwent thin-slice axial T2-weighted MRI. Results TroN was identified in 91% of 85 patients, 60 (71%) on the right side, and 67 (79%) on the left. The distances between the apex of the inferior colliculus and the original site of TroN on the dorsal brainstem were variable, with mean values of 2.4 ± 1.8 mm (range 0–8 mm) on the right and 2.2 ± 1.7 mm (range 0–5 mm) on the left. Most of the root exit zones were distributed within 0–5 mm below the apex of the inferior colliculus. In addition, the distances between the midline and the root exit zones of the TroN were variable, with mean values of 4.4 ± 1.4 mm (range 1.9–7.5 mm) on the right and 4.6 ± 1.6 mm (range 1.1–7.8 mm) on the left. Most of the root exit zones were located within 1–7 mm range lateral to the midline. Conclusions The root exit zone of the TroN may be mostly located in a small square area measuring 8 mm × 8 mm, lying at and below the apex of the inferior colliculus. The TroN may arise from any site in the square area, and significant attention is necessary when performing surgical maneuvers in and around it.