Extraocular muscle volume index at the orbital apex with optic neuritis: a combined parameter for diagnosis of dysthyroid optic neuropathy

Objectives To evaluate the diagnostic performance of the extraocular muscle volume index at the orbital apex (AMI) and the signal intensity ratio (SIR) of the optic nerve in dysthyroid optic neuropathy (DON). Methods Clinical data and magnetic resonance imaging were collected retrospectively from 63 Graves’ ophthalmopathy patients, including 24 patients with DON and 39 without DON. The volume of these structures was obtained by reconstructing their orbital fat and extraocular muscles. The SIR of the optic nerve and axial length of eyeball were also measured. The posterior 3/5 of the retrobulbar space volume was used as the orbital apex to compare parameters in patients with or without DON. Area under the receiver operating characteristic curve (AUC) analysis was used to select the morphological and inflammatory parameters with the highest diagnostic value. A logistic regression was performed to identify the risk factors of DON. Results One hundred twenty-six orbits (35 with DON and 91 without DON) were analyzed. Most of the parameters in DON patients were significantly higher than in non-DON patients. However, the SIR 3 mm behind the eyeball of the optic nerve and AMI had the highest diagnostic value in these parameters and are independent risk factors of DON by stepwise multivariate logistic regression analysis. Combining AMI and SIR had a higher diagnostic value than a single index. Conclusions Combining AMI with SIR 3 mm behind the eyeball’s orbital nerve can be a potential parameter for diagnosing DON. Clinical relevance statement The present study provided a quantitative index based on morphological and signal changes to assess the DON, allowing clinicians and radiologists to monitor DON patients timely. Key Points The extraocular muscle volume index at the orbital apex (AMI) has excellent diagnostic performance for dysthyroid optic neuropathy. A signal intensity ratio (SIR) of 3 mm behind the eyeball has a higher AUC compared to other slices. Combining AMI and SIR has a higher diagnostic value than a single index.