Brainstem Infarction Following Resection of a Cerebellopontine Angle Epidermoid Cyst with Normal Intraoperative Neurophysiological Monitoring: A Case Report

Technical Summary: Brainstem Infarction Following Resection of a Cerebellopontine Angle Epidermoid Cyst with Normal Intraoperative Neurophysiological Monitoring

Problem Statement
Cerebellopontine angle (CPA) epidermoid cysts are rare, slow-growing benign tumors that frequently encase the vertebrobasilar system and adhere tightly to the ventral brainstem. While gross-total resection is the standard treatment, it carries a high risk of morbidity due to the technical difficulty of dissecting the tumor from critical neurovascular structures. Postoperative brainstem infarction is a rare but devastating complication. A critical clinical uncertainty exists regarding the reliability of Intraoperative Neurophysiological Monitoring (IONM), specifically Brainstem Auditory Evoked Potentials (BAEPs) and Somatosensory Evoked Potentials (SSEPs), in predicting ischemic events in these high-risk scenarios. The literature lacks documentation of immediate postoperative brainstem infarction occurring despite completely uneventful IONM readings.

Methodology
This study presents a single case report of a 33-year-old male with untreated hypertension and a 15-day history of dizziness. Preoperative MRI confirmed an epidermoid cyst in the interpeduncular, ambient, suprasellar, and prepontine cisterns, which completely encased the bilateral vertebral and basilar arteries.

• Surgical Intervention: A senior neurosurgeon performed a standard retrosigmoid craniotomy with gross-total resection of the tumor (8 hours and 20 minutes duration).
• Monitoring: Continuous IONM, including BAEPs and SSEPs, was utilized throughout the procedure.
• Postoperative Protocol: The patient underwent immediate neurological examination, emergency head CT, and subsequent MRI/DWI. Management included mechanical ventilation, vasodilator therapy (nimodipine), and cerebral protective measures.
• Follow-up: The patient was monitored for pupillary changes, motor function, and complications (e.g., meningitis) over a 31-day hospital stay. Histopathological analysis confirmed the diagnosis.

Key Results

• Intraoperative Findings: Despite the tumor's complete encasement of the vertebrobasilar system, BAEPs and SSEPs remained entirely stable with no significant changes throughout the 8-hour-20-minute procedure.
• Immediate Postoperative Complication: Immediately upon waking, the patient exhibited a fixed, dilated left pupil (4.5 mm) with absent light reflexes. Emergency CT ruled out hemorrhage.
• Confirmation of Infarction: MRI/DWI on postoperative day 1 confirmed an acute infarction in the ventral brainstem. Clinical examination revealed right-sided hemiparesis (upper limb power 0/5).
• Recovery and Complications: The patient developed fever and meningitis on day 10, which was successfully treated with antibiotics. By discharge (day 31), the patient was awake with reactive pupils and improved motor function (right upper limb 3/5, right lower limb 5/5).
• Pathology: Histopathology confirmed stratified squamous epithelium with laminated keratin debris, consistent with an epidermoid cyst.

Key Contributions
This case report provides the first documented instance of catastrophic ischemic brainstem infarction following CPA epidermoid cyst resection where intraoperative neurophysiological monitoring (BAEPs and SSEPs) remained completely normal. The study highlights three specific contributions:

1. Identification of a Monitoring Blind Spot: It demonstrates that standard IONM modalities (BAEPs and SSEPs) assess specific pathways (auditory and dorsal column-medial lemniscal) that do not fully overlap with the perfusion territory of the ventral brainstem (supplied by small perforating arteries). Consequently, ischemia confined to the ventral brainstem can occur without triggering IONM alarms.
2. Mechanism Elucidation: The authors propose that the infarction likely resulted from injury to small perforating arteries (vasospasm, traction, or local thrombosis) or severe vasospasm induced by mechanical manipulation, rather than damage to the larger, monitored vessels.
3. Clinical Protocol Emphasis: The case underscores the critical value of immediate postoperative pupillary examination as a rapid, equipment-free diagnostic tool that detected the injury before imaging confirmation.

Significance and Claims
The paper claims that this case fills a documentation gap by combining three specific features: (1) immediate postoperative brainstem infarction, (2) tumor encasement of the vertebrobasilar system, and (3) completely uneventful intraoperative neurophysiological monitoring.

The authors assert that a normal IONM run does not guarantee freedom from ischemic complications in high-risk tumor surgeries involving the vertebrobasilar system. They argue that surgeons must recognize that IONM reflects functional integrity of specific tracts rather than global perfusion safety. Consequently, the paper proposes that preoperative counseling must explicitly warn patients of this risk even if monitoring is normal, and postoperative surveillance must include rigorous, frequent pupillary assessments to enable early detection and intervention (vasodilator therapy and perfusion optimization). The study concludes that while IONM is valuable, it has limitations in detecting focal ischemia in the ventral brainstem, necessitating reliance on clinical signs like pupillary abnormalities for early diagnosis.