Introduction: We describe a safe entry zone to the ventrolateral pons located just above the point where the trigeminal nerve emerges from the pons. We refer to this approach as “epitrigeminal”. To our knowledge, this safe entry zone has not been described before literature.
Methods: To define the inter-subject variability of the surgically relevant anatomy related to this approach, we performed an anatomical study using the high-definition fiber-tractography (HDFT) maps of 77 normal subjects (154 sides). HDFT dissections were also used to compare the epitrigeminal approach to the other known approaches accessing the ventrolateral pons. We also present an illustrative case of a cavernous malformation spanning the entire pons, which was approached through the epitrigeminal safe entry zone and a subtemporal transtentorial surgical corridor.
Results: The average direct distance of the corticospinal tracts from the trigeminal entry zone was 12.6 mm (8.7 mm to 17mm). The average vertical distance was 3.6 mm (-2.3mm to 8.7mm). The mean distances did not differ significantly from side to side, or across any of the groups studied: right handed, left handed, and ambidextrous (Figure 1). When compared to the other known safe entry zones to the ventrolateral pons, a possible advantage of the epitrigeminal approach is an angle of attack that provides access to lesions that extend both above and below the trigeminal nerve. A possible disadvantage is the need for temporal lobe retraction (Figure 2). The patient in the illustrative case had a complete resection of the lesion, as well as improvement in his symptoms (Figure 3).
Conclusions: The epitrigeminal entry zone to the brainstem appears to be safe, and effective for treating intrinsic ventrolateral pontine pathology. Familiarity with the subtemporal transtentorial approach, and the reliable surgical landmark of the trigeminal entry zone should make this a straightforward approach.
Patient Care: The current study delineates an approach to he ventrolateral brainstem, that has not previously been described in detail in the English literature. Furthermore, the high-definition fiber tractography study elucidates the important surgically relevant anatomic variability of the corticospinal tracts in the ventrolateral pons. In addition, the virtual dissections help understand the specific indications, as well as the advantages and disadvantages of the known approaches and safe entry zones to the ventrolateral pons.
Learning Objectives: By the conclusion of this session the participants should be able to 1) describe the anatomic nuances, as well as the indications of the epitrigeminal approach to the brainstem. 2) Discuss the anatomic variability of the surgically relevant fibertracts in the ventrolateral pons. 3) Discuss the indications, advantages and disadvantages of all the approaches and safe entry zones to the ventrolateral pons.