Predictive Ability of Orbital Apex Crowding Index for Dysthyroid Optic Neuropathy in Comparison With Barrett's Index

The most widely accepted theory of Dysthyroid optic neuropathy (DON) is direct compression of the optic nerve by enlarged extraocular muscles at orbital apex. Barrett's index (BI)1, measuring muscle crowding at mid-retrobulbar level on coronal orbital Computed Tomography (CT), helps in identifying DON. Recent study defined the orbital apex as the posterior ⅕ of retrobulbar space2. This study aims to evaluate and compare the predictive potential of BI to the new indexes measured at orbital apex, called Orbital Apex Crowding Index (OACI), in distinguishing TED with DON from those without DON.

359 Predictive Ability of Orbital Apex Crowding Index for Dysthyroid Optic Neuropathy in Comparison with Barrett’s Index Siriphat Daocharoenporn 1, Oranan Tritanon 2, Weerawan Chokthaweesak 1 department of ophthalmology, Faculty of medicine, Ramathibodi Hospital, Mahidol University, 2 department of radiology, Faculty of medicine, Ramathibodi Hospital, Mahidol University 1 Introduction: The most widely accepted theory of Dysthyroid optic neuropathy (DON) is direct compression of the optic nerve by enlarged extraocular muscles at orbital apex. Barrett’s index (BI)1, measuring muscle crowding at mid-retrobulbar level on coronal orbital Computed Tomography (CT), helps in identifying DON. Recent study defined the orbital apex as the posterior ⅕ of retrobulbar space2. This study aims to evaluate and compare the predictive potential of BI to the new indexes measured at orbital apex, called Orbital Apex Crowding Index (OACI), in distinguishing TED with DON from those without DON. Methods: Between 2010 and 2022, 101 TED patients (193 eyes) were retrospectively reviewed and categorized into TED with and without DON. A blinded radiologist calculated BI and two new indexes i) OACI-2M (greater value between ratio of horizontal rectus muscles diameters to horizontal orbital apex width and vertical rectus muscles diameters to vertical orbital apex width ii) OACI-4M (ratio of entire four rectus muscles diameters to the sum of horizontal and vertical orbital apex widths). These parameters were assessed for their predictive capability in identifying DON. Results: This study included 71 eyes of TED with DON and 122 eyes without DON. OACI-2M, OACI-4M and BI exhibited significantly higher mean values in the TED with DON groups (p< 0.001), with distinct differences in the area under the ROC curve (p=0.0014): 0.887, 0.923, and 0.8818 (95% CI), respectively. Optimal cut-off values for diagnostic accuracy were as follows: OACI-2M at 53.5% (sensitivity: 95.8%, specificity: 67.2%, accuracy: 81.5%), OACI-4M at 54% (sensitivity: 84.5%, specificity: 84.4%, accuracy: 84.5%), and BI at 51% (sensitivity: 91.5%, specificity: 75.4%, accuracy: 83.5%). All three parameters exhibited a strong correlation with best-corrected visual acuity (BCVA) (p< 0.001). Conclusions: Both OACI demonstrate higher prediction performance for DON compared to BI and are potentially applied in clinic as alternative methods for screening DON. References: 1. Barrett L, Glatt HJ, Burde RM, Gado MH. Optic nerve dysfunction in thyroid eye disease: CT. Radiology. 1988;167(2):503–507. 2. Zurinam O, Safieh C, Redler Y, Orbach A, Lumelsky D, Neeman Z, Briscoe D. CT definition of the surgical apex in the orbit. Sci Rep. 2021 May 26;11(1):11016. Keywords: Neuro-ophth & systemic disease ( eg. MS, MG, thyroid), Graves' disease, Neuroimaging, Diagnostic tests (ERG, VER, OCT, HRT, mfERG, etc)yEyelid & adnexal disease, Orbit Financial Disclosures: The authors had no disclosures. Grant Support: none Contact Information: Siriphat Daocharoenporn, [e-mail] 514 | North American Neuro-Ophthalmology Society