Significance of the tentorial alignment in approaching the trigeminal nerve and the ventral petrous region through the suboccipital retrosigmoid technique

OBJECT: In this study, the authors aimed to identify the factors that would predict the operative distance between the trigeminal nerve (fifth cranial nerve) and the acousticofacial nerve complex (seventh-eighth cranial nerves) preoperatively when approaching the cerebellopontine angle (CPA) through the suboccipital retrosigmoid approach. METHODS: In 40 consecutive patients who underwent microvascular decompression of the trigeminal nerve via a suboccipital retrosigmoid approach for trigeminal neuralgia, the following three parameters were assessed on preoperative magnetic resonance images: 1) the angle between the tentorium and the line drawn from the hard palate (tentorial angle); 2) the angle between the lines drawn along the petrous bones ventral to the internal auditory canals (petrous angle); and 3) the angle between the tentorium and the line connecting the opisthion to the inion (occipital angle). The distance between the trigeminal nerve and the acousticofacial nerve complex (referred to as "distance") was measured intraoperatively. Statistical analysis was performed using the Pearson correlation test. RESULTS: The mean values were 50.9 +/- 11.5 degrees for the tentorial angle, 102.5 +/- 13.1 degrees for the petrous angle, 83.4 +/- 9.7 degrees for the occipital angle, and 3.1 +/- 1.5 mm for distance. There was a strong inverse correlation between the tentorial angle and distance (r = -0.228, p = 0.08). The mean distance was 3.5 +/- 1.9 mm for a tentorial angle less than 51 degrees and 2.7 +/- 1.1 mm for a tentorial angle of at least 51 degrees. No correlation existed between either the petrous or occipital angles and distance. CONCLUSIONS: The distance between the trigeminal nerve and acousticofacial nerve complex decreases in the presence of a steep tentorial angle. This limits the operating field between these cranial nerves when reaching the petroclival or the superior CPA regions through the retrosigmoid approach. Awareness of such anatomical features at the time of preoperative planning is of paramount importance in selecting the optimum surgical approach and minimizing operative complications.