Magnetic resonance imaging in thyroid eye disease: Signal intensity ratio to measure disease activity

Thyroid Eye Disease (TED) is characterized by eyelid retraction, motility limitation with or without double vision, and proptosis.[1] Typically, TED follows a course of early activity characterized by soft tissue signs—caruncular edema, conjunctival redness and edema, eyelid redness and edema, chemosis along with symptoms of morning puffiness, and retrobulbar ache.[2] Quiescence in TED sets in overtime.[1,2] TED patients may be left with sequelae necessitating rehabilitative surgery for the orbit, extraocular muscles, and eyelids.[2] Management of TED is guided primarily by the severity and activity of the disease. Conservative management is reserved for mild TED.[2] Invasive interventions are reserved for moderate-severe TED.[1,2] Sight-threatening disease—dysthyroid optic neuropathy and corneal breakdown—seen in 5% of all TED patients, is an emergency requiring prompt intravenous corticosteroids and orbital decompression.[2,3] Diabetes mellitus (DM), a significant risk factor, doubles the risk of sight-threatening TED.[3] Smoking and DM when present together in a TED patient increase the risk of sight-threatening disease manifold.[3]

Eyelid retraction is the most common sign in TED seen in 90% of patients.[4] Several hypotheses have been proposed to explain the mechanism of eyelid retraction in TED—increased sensitivity of Muller’s muscle to adrenergic stimulation, overactivity of levator palpebrae superioris (LPS) and Muller’s muscle, and fibrosis of the LPS muscle or even adhesions between LPS and adjacent tissues.[5,6,7,8] Surgical management of eyelid retraction reserved for inactive TED is challenging, especially for restoration of an esthetic appearance of the eyelid contour and height. Injection of triamcinolone acetonide (TA) has shown promise in the correction of eyelid retraction. Side effects of TA injection reported include elevated intraocular pressure (20%), menstrual irregularities (13.5%), and moon-face (9.5%).[5,9] Xu D et al. postulated an inflammatory etiology for eyelid retraction in TED. In a retrospective interventional study on 56 eyes of 36 patients with clinical activity score (CAS) ≤3, Xu et al. reported TA injections into the LPS muscle had an overall efficacy of correcting upper eyelid retraction in 68% versus 17% in the treatment and control groups, respectively. Further, when the treatment group was split into those treated earlier than 6 months and later, the response improved to 83%.[9] Reports showing the efficacy of local corticosteroid injection to correct upper eyelid retraction brought the need to identify predictive factors for the same. Mourits et al. described CAS as a predictor of response to systemic corticosteroid therapy in active TED.[10] Most researchers today depend on CAS to differentiate active from inactive TED.[2] However, we need to keep in mind the limitations of CAS in predicting response to local corticosteroid injection for eyelid retraction. CAS is examiner-dependent, and patients with low CAS score < 3 are known to respond to corticosteroid therapy.[10] Since many TED patients with upper eyelid retraction do not always have high CAS scores, more objective measures possibly derived from imaging modalities that can fill this gap are needed.

Magnetic resonance imaging (MRI) best differentiates active from inactive TED among imaging techniques.[11,12,13] Short Tau Inversion Recovery (STIR) sequences on MRI suppress signals from fat and show high signal intensity from fluid-filled tissues.[11,12,13] Orbital tissues in active TED tend to generate high signal intensity on STIR because of the inflammatory edema.[11,12,13] Signal intensity ratio (SIR) is the ratio of STIR intensity from orbital muscle (s) and the temporalis muscle on coronal sections and is typically high in active disease.[13] Tortora et al. found that contrast-enhanced T1W images and STIR conformed well with CAS in TED patients, thus obviating the need to inject contrast agents.[11]. Xu et al. have earlier demonstrated that repeated injections of TA 5 mm above the superior border of the tarsus were efficacious in correcting the mean upper eyelid retraction by 2.31 mm.[9] Sagittal MRI imaging showed this result to correlate with a decrease in thickness of the LPS muscle.[9]

A recent study in this issue has reported on the role of SIR in predicting response in TED with upper eyelid retraction.[14] There were 62 patients (77 eyes) with low CAS scores and upper eyelid retraction. The authors found SIR and thickness on T1W with contrast and T2W with fat suppression showed a significant decrease corresponding to the response in upper eyelid retraction after TA injection.[14] Further, the SIR of the group that showed a response showed a significant difference from the non-effector group.[14] Authors concluded that SIR on MRI may have predictive value in treating upper eyelid retraction.[14] There are some inconsistencies between this report’s findings versus existing literature information.[5,9,14] Contrary to earlier evidence, SIR in the non-effector group was significantly higher than the effector group suggesting inflammation was worse in the non-effector group.[5,9,14] Thickness of the LPS muscle was however found to decrease after injection TA in both groups.[14] Earlier studies have used coronal sections for measuring SIR in TED which show both the recti and temporalis muscle.[11,12,13] In contrast, the SR-LPS muscle complex on coronal scans makes precise measurements on LPS difficult. LPS muscle is best differentiated on sagittal and temporalis muscle on coronal MRI, thus making SIR measurements demanding. Nevertheless, SIR may help select patients who benefit from TA injection. Meanwhile, further studies to understand the correlation between inflammation, SIR, and eyelid retraction are warranted in TED.

About the author

Dr Suryasnata Rath

Dr Suryasnata Rath is an ophthalmic plastic surgeon by training and heads this service at LVPEI’s Mithu Tulsi Chanrai Campus at Bhubaneswar. Following his basic medical training in Odisha, Dr Rath underwent long-term fellowships at LVPEI Hyderabad and subsequently in July 2006 joined the Bhubaneswar campus as faculty, and contributed to the expansion of the ophthalmic plastics service at the campus. His international fellowship with Peter Dolman at the University of British Columbia, Canada, helped him build a comprehensive thyroid clinic at the Bhubaneswar Campus. His areas of interest include thyroid eye disease and lacrimal disorders. He is on the executive board of the Asia Pacific chapter of International Thyroid Eye Disease Society and the editor of a textbook titled ‘Surgery in Thyroid Eye Disease: A Conceptual Approach’ that was recently published by Springer.