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Background Free-floating thrombus of the internal carotid artery (ICA) is an uncommon stroke mechanism associated with high risk of embolic events. There are very few reports of a free-floating thrombus ‘jailed’ between two distinct ICA occlusions and supplied entirely by retrograde collateral flow. We report a case of recurrent monocular visual loss from a jailed proximal cavernous ICA thrombus maintained via external carotid–ophthalmic collateral pathways, with angiographic correlation and successful medical management.Case Presentation A 59-year-old man with a history of recurrent venous thromboembolism on lifelong anticoagulation, suspected hypercoagulable state, prior transient ischemic attacks, and patent foramen ovale closure presented with repeated episodes of transient right monocular vision loss. Symptom phenomenology included brief scintillations, scotomas, and transient complete visual loss. Ophthalmologic examination revealed a normal retina without evidence of retinal emboli.Initial computed tomography angiography demonstrated a new complete occlusion of the right cervical ICA, previously documented as patent. Magnetic resonance imaging revealed punctate acute infarcts in the right precentral gyrus. Given symptoms and imaging, digital subtraction angiography (DSA) was performed. DSA revealed a free-floating intraluminal thrombus within the proximal cavernous ICA extending toward the ICA terminus, ‘jailed’ between a proximally occluded cervical ICA and a distally occluded ICA segment. The isolated cavernous ICA segment was perfused by retrograde flow from the external carotid artery via distal ethmoidal, meningeal and ascending pharyngeal branch vessels. Presumably, there was also retrograde perfusion through the ophthalmic artery, likely existing in dynamic equilibrium. Additional occlusion of the left external carotid artery was noted.Management Given the patient’s low NIH Stroke Scale score and the complex vascular anatomy, endovascular intervention was not pursued. Careful review revealed the patient’s intermittent compliance and subtherapeutic dosing of rivaroxaban. Anticoagulation was optimized by increasing rivaroxaban to full therapeutic dosing, and low-dose aspirin was added for additional large artery atherosclerotic prophylaxis. The patient experienced complete resolution of visual symptoms with no further events on follow-up.Conclusion This case illustrates a rare and underrecognized mechanism of ischemic stroke and ocular ischemia: a jailed free-floating cavernous internal carotid artery thrombus sustained by retrograde external carotid–ophthalmic collateral flow. Recognition of this anatomy is critical, as it may guide management decisions and favor optimized antithrombotic therapy over endovascular or surgical intervention. Advanced angiographic evaluation is essential in patients with recurrent monocular visual symptoms and incongruent noninvasive imaging.Disclosures C. Amankwah: None. J. Arbucci: None. A. Bati: None. T. Katner: None. M. Lee: None. S. Boo: None. D. Mandel: None.Abstract E-121 Figure 1Digital subtraction angiography of the right carotid system in lateral projection. (A) On early arterial phase there is complete occlusion of the proximal right cervical internal carotid artery with remnant stump and absence of antegradeflow. (B) On late arterial phase there is filling of the proximal cavernous internal carotid artery via distal ethmoidal meningeal and ascending pharyngeal branch vessels with sluggish antegrade flow across the anterior genu of the cavernous ICA segment towards the ophthalmic artery. There is an intraluminalfilling defect consistent with free-floating thrombus within an isolated cavernous ICA segment (‘jailed’ thrombus) between proximal and distal occlusions