Purpose
Dysthyroid Optic Neuropathy (DON) is a sight threatening and diagnostically challenging complication of Thyroid Eye Disease (TED). We provide a comparative analysis of the MRI features associated between patietnts with and without DON.

Methods
Anonymised retrospective cohort study of patients with TED over eleven years. All patients had Axial T1-weighted and coronal 3 mm MRI STIR images. In a subset, a 3-mm-thick non-echoplanar HASTE DWI sequence was acquired in the coronal plane, and an ADC map was calculated using the diffusion scan raw data. Assessment of apical crowding (AC), extraocular muscle (EOM) enlargement (E), peri-muscular fat (PMF)/muscle signal intensity (SI) and Apparent Diffusion Coefficient (ADC) were analysed on coronal images.

Results
Twenty-six consecutive DON and 516 non-DON cases. In the DON group, elevated EOMSI, PMFSI, EOME and AC were present in 54.6%, 25.9%, 72.7% and 64.6%, respectively, compared to 24.1%, 6.2%, 42.1% and 5.9% in the non-DON (p = 0.001, p = 0.000, p = 0.001 and p = 0.000). The average ADC value in the DON cohort was 1373 ± 319 versus 973 ± 237 in the non-DON (p = 0.000). Likelihood of DON on Univariable Regression Analysis (Odds Ratios): Apical Crowding (29.1 x p = 0.000) and ADC Value ≥ 1200 (7.3 x p = 0.000). On Multivariable Regression Analysis (Odds Ratios): Apical Crowding 22.1 x (p = 0.000) and ADC Value ≥ 1200 3.7 x (p = 0.027).

Conclusion
MRI features associated with a higher diagnostic likelihood of DON include significant AC and elevated ADC values. ADC may show reasonable promise in diagnosing and predicting DON.