Delayed thrombosis of a ruptured anterior spinal artery aneurysm after prolonged morphological stability: illustrative case.

backgroundIsolated anterior spinal artery (ASA) aneurysms are rare vascular lesions with heterogeneous morphologies and clinical behaviors. Spontaneous regression has been reported in approximately half of conservatively managed cases and typically occurs within the early months after diagnosis. However, delayed thrombosis after prolonged morphological stability is rare. OBSERVATIONS: A 75-year-old man presented with subarachnoid hemorrhage caused by a 3-mm ruptured ASA aneurysm at the C3 level. Conservative management was initially selected because of his poor neurological status. Eleven months after diagnosis, selective microcatheter angiography demonstrated unchanged aneurysm morphology without interval growth or thrombosis. No angiographic features suggesting arterial dissection or associated vascular pathology were identified. Four weeks later, at planned embolization, repeat angiography demonstrated complete absence of aneurysmal filling with preserved parent artery continuity, consistent with isolated aneurysm sac thrombosis. No intervention was performed. Follow-up imaging confirmed persistent occlusion without recanalization. LESSONS: Delayed thrombosis can occur in ruptured ASA aneurysms that remain morphologically stable beyond the typical window for spontaneous regression. This case demonstrates a rare delayed thrombosis pattern after prolonged stability in a nondissecting saccular ASA aneurysm. Repeat angiographic evaluation is essential when intervention is delayed, as aneurysm status may change unexpectedly. Continued imaging surveillance remains warranted to confirm durability. https://thejns.org/doi/10.3171/CASE26474.