Author’s Disclosure Block: Samira Jafari, none; Jonathan A. Micieli, none; Edward Margolin, none

Abstract Body
Purpose:To determine the structural causes and incidental neuroimaging findings of pharmacologically confirmed Horner syndrome. We also categorized incidental findings based on the required follow-up (none, routine, urgent, emergency). Study Design: Retrospective, observational cohort study of 135 patients (58% female, 42% male; mean age 52.4±17.9 years) who underwent neuroimaging (MRI and CT) for Horner syndrome between July 2008 and June 2024. Methods: Imaging details and reasons were collected from electronic records, and incidental findings unrelated to Horner syndrome were categorized based on follow-up needsinto four follow-up categories (none, routine, urgent,andemergency). The duration of Horner syndrome upon first presentation wasdividedintoacute (<21 days),subacute (21 days to 3 months) and chronic (>3 months). Imaging reports were reviewed independently by an ophthalmologist and a neuro-ophthalmologist. Statistical analyses were conducted to assess the prevalence and significance of incidental findings. Results: Among 135 patients, Horner syndrome was acute in 18.5%, subacute in 10.4%, and chronic in 71.1%. Imaging modalities included MRI/MRA head and neck in 56.3% of patients, CT/CTAhead and neck in 40.7% and others in 3%. A structural cause for Horner syndrome was identified by neuroimaging in 9.6% of patients. This was found in 20% (n=5) of acute, 14.3% (n=2) of subacute and 5.2% (n=5) of chronic cases. The most common structural causes of Horner syndrome overall were ICA dissection (n=4, 3%) and ICA aneurysm (n=3, 2.2%). Dissection was the most common cause of acute Horner syndrome (3, 2.2%). In the subacute group, one patient had bilateral ICA dissection, and another patient had an infiltrative necrotic mass in the brachial plexus. In the chronic subgroup, one patient had an ICA aneurysm, one had a metastatic spinal mass, and another had a thoracic (T1) peripheral nerve sheath tumour. Incidental findings were present in 34.8% of patients (n=47), with the most common being microangiopathic changes and brain atrophy (10%), sinus and mucosal changes (3%), and thyroid nodules (3%). 17.8% of patients(n=24) with incidental findings required routine follow-ups, and 1.5%(n=2) required urgent follow-ups of incidental findings. none required emergency follow-up. Conclusion: Approximately 1 in 10 cases of Horner syndrome had an identifiable cause on neuroimaging, with ICA dissection and aneurysms being the most common causes. Chronic cases were less likely to have an identifiable structural cause.Incidental findings were common in about 1 in 3 cases and may require ongoing follow-up.