To Drill or Not to Drill: Prominence of the Suprameatal Tubercle and Its Impact on Microvascular Decompression for Trigeminal Neuralgia—A Proposed Classification System

Introduction: The suprameatal tubercle (ST) is a bony prominence of variable size on the posterior surface of the petrous bone, located above the porus of the internal acoustic meatus. Drilling of the ST has been described as a component of the posterior intradural petrous apicectomy approach (PIPA) to expose tumors of the cerebellopontine angle (CPA) that extend into Meckel’s Cave. An enlarged ST, however, can also pose an impediment to adequate surgical exposure of the trigeminal nerve root from the pontine root entry zone to the porus trigeminus in microvascular decompression (MVD) for trigeminal neuralgia. We present two cases in which we deemed it critical to drill the ST to achieve adequate exposure for trigeminal MVD. We describe the anatomy of the ST to impart to neurosurgeons a greater comfort level with safety, utility and effectiveness of ST reduction. We compared the anatomic relationships of the ST, the cisternal segment of the trigeminal nerve, and the cistern itself in our two index patients compared with 20 control patients to develop a Grading Scale that will allow neurosurgeons to predict on preoperative MRI whether this technique will be either helpful or critical to perform a safe and adequate exposure for trigeminal MVD in a given patient.

Methods: Preoperative thin-cut T2 (FIESTA) MRIs were reviewed in the drilling cohort. Twenty patients operated on within the past 4 years, for which there was a preoperative FIESTA sequence available for review, served as historical controls. A line of sight was projected from the posterior half of the suprameatal tubercle to the trigeminal nerve. A visibility grade (A–D) was provided for each patient, based on how much of the trigeminal nerve (from root entry zone to Meckel’s cave) was contained within the projected line of sight (A = 75–100%, B = 50–75%, C = 25–50%, D = 0–25%). Additionally, the cisternal distance (in mm) from root entry zone to petrous bone was recorded. Each patient was then given an overall alphanumeric score containing both the visibility grade and cisternal distance.

Results: Drilling of the suprameatal tubercle in both index patients was without complication and resulted in adequate visualization to complete the microvascular decompression. Both of these patients had a preoperative visibility grade of “D” and a cisternal distance less than 1 mm (D1). Within the historical controls, visibility grades ranged from “A–D” (A = 10, B = 5, C = 3, D = 2) and cisternal distance ranged from 3 to 7 mm (mean: 4.6 mm). The cisternal distance of both patients with a grade-D visibility in the historical control group was 3 mm.

Conclusion: Exposure during microvascular decompression surgery for trigeminal neuralgia exists on a spectrum, and at times may be limited by a prominent suprameatal tubercle. A composite score, combining the degree of visibility of the trigeminal nerve and distance from nerve to petrous bone, appears to be a reasonable surrogate for anticipating quality of exposure. Furthermore, in our experience, the need for drilling of the tubercle is rare, but when required, can be performed safely.