Safety of brainstem safe entry zones: comparison of microsurgical outcomes associated with superficial, exophytic, and deep brainstem cavernous malformations

Abstract

OBJECTIVE Safe entry zones (SEZs) enable safe tissue transgression to lesions beneath the brainstem surface. However, evidence for the safety of SEZs is scarce and is based on anatomical studies, case reports, and small series. METHODS A cohort of 154 patients who underwent microsurgical brainstem cavernous malformation (BSCM) treatment during a 23-year period and who had preoperative MR images and intraoperative photographs or videos was retrospectively examined. This study assessed the safety of SEZs for access to deep BSCMs, preoperative MRI to predict BSCM surface proximity, and the relationships between BSCM subtype, surgical approach, and SEZs. Lesions were characterized as exophytic, superficial, or deep on the basis of preoperative MRI and intraoperative inspection. Outcomes were scored as good (modified Rankin Scale [mRS] score ≤ 2) or poor (mRS score > 2) and relative outcomes as stable/improved or worse relative to baseline (± 1 point). RESULTS Resections included 34 (22%) in the midbrain, 102 (66%) in the pons, and 18 (12%) in the medulla. Of those, 23 (15%) were exophytic, 57 (37%) were superficial, and 74 (48%) were deep. Established SEZs were used for 97% (n = 72) of deep lesions; the preferred SEZ associated with its subtype was used for 91% (n = 67). MR images accurately depicted exophytic BSCMs that did not require SEZ approaches (sensitivity, 96%) but overestimated the proximity of lesions superficial to brainstem surfaces (specificity, 67%), resulting in unanticipated SEZ use. Final neurological outcomes were good in 80% of patients with follow-up data (119/149), and relative outcomes were stable/improved in 93% (139/149). Outcomes for patients with brainstem transgression through an SEZ did not differ from outcomes for patients with superficial or exophytic lesions that did not require SEZ use (final mRS score ≤ 2 in 72% of all patients with deep lesions vs 82% of all patients with superficial or exophytic lesions [p = 0.10]). Among patients with follow-up, the rates of permanent new cranial nerve deficits in patients with deep BSCMs and superficial or exophytic BSCMs were 21% and 20%, respectively (p = 0.81), with no significant change in overall cranial nerve deficit (0 and −1, p = 0.65). CONCLUSIONS Neurological outcomes for patients with deep BSCMs were equivalent to those for superficial or exophytic BSCMs, validating the safety of SEZs for deep BSCMs. Preoperative T1-weighted MR images overestimated the lesion’s surface proximity, necessitating detailed knowledge of SEZs and readiness to use them in cases of radiological-microsurgical discordance. Most patients achieved favorable outcomes despite the transgression of eloquent brainstem tissue in and around SEZs.