Delayed identification and treatment of a cervicomedullary spinal vascular lesion following angiogram-negative, high-grade subarachnoid hemorrhage: illustrative case

BACKGROUND: An angiogram-negative subarachnoid hemorrhage (SAH) with a high Hunt-Hess grade and atypical radiographic features warrants evaluation for occult lesions. Cervicomedullary spinal arteriovenous malformations (AVMs) are rare causes frequently missed on initial imaging. OBSERVATIONS: A 54-year-old male presented with a Hunt-Hess grade V SAH, and initial multimodal angiography findings were negative. Neurological status improved following external ventricular drain (EVD) placement. Imaging clues included a focal ventral cervicomedullary spot sign, disproportionate prepontine blood accumulation with inferior tracking, and an intraventricular hemorrhage. Angiography performed on day 10 revealed a 3-mm ruptured anterior spinal artery (ASA) aneurysm with cervicomedullary AVM. Endovascular treatment was high risk given the proximity of the ASA. Resection via far-lateral transcondylar approach was performed. Neuromonitoring demonstrated bilateral motor evoked potential (MEP) decline with preserved D-wave amplitude, prompting reinspection, revealing an inadvertent ASA trunk clipping, and enabling correction and complete MEP recovery. The lesion was obliterated without deficits. LESSONS: In a high-grade, angiogram-negative SAH, ventral blood distribution and spot signs should prompt targeted spinal angiography at 7-14 days. EVD placement can reverse herniation and coma in high-grade presentations. The far-lateral approach and cord rotation provide direct visualization for ventral spinal pathology when endovascular access is restricted. D-wave monitoring with transcranial MEPs enables real-time distinction between reversible ischemic changes and irreversible mechanical injury, guiding intraoperative decision-making. https://thejns.org/doi/10.3171/CASE26229.