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In addition to the clinical assessments described in the section “How Do You Decide whether a Patient Has Dysthyroid Optic Neuropathy?”, several ancillary tests can also help to identify DON. These include visual-evoked potentials, contrast sensitivity and imaging.

Abnormalities in both latency and amplitude of visual-evoked potentials can be supportive in the diagnosis of DON; however, several issues affect their value in practice. Firstly they can be affected by thyroid dysfunction, which is frequently present at the time when DON develops [13]. In addition, many laboratories have no normal data for patients over 60 years old, making it hard to interpret findings in those who are already the most difficult to diagnose due to confounding pathology [4]. Hence, they should be interpreted with caution in patients with no other evidence of DON.

Contrast sensitivity, which measures spatial resolution at all levels of contrast, appears to be a sensitive indicator of DON [73]; however, it is less readily available and still subject to confounding pathologies.

Imaging has a very valuable place in supporting the diagnosis of DON. Coronal images on CT or MRI demonstrate apical crowding in the majority of patients. This is defined as the effacement of perineural orbital fat in the posterior orbit. The combination of apical crowding with evidence of fat herniation through the superior orbital fissure seen on axial images is thought to have a specificity of 91% and a sensitivity of 94% for DON [75].

In another study, radiological criteria which were significantly predictive of DON were apical crowding (p < 0.001) and medial rectus volume (p = 0.005) or diameter (p = 0.003), whereas proptosis and orbital angle (p = 0.895) were not [76]. Nugent et al. [77] noted mild or no apical crowding in 17% of DON patients, while 13% had severe apical crowding but no clinical evidence of DON. More recently, the objective quantification of apical orbital crowding based on square area measurements was shown to be a more efficient diagnostic tool than subjective quantification. In this study, the best-performing index was highly efficient at detecting DON, with a sensitivity of 91.7% and a specificity of 90.0% [78]. However, as with evoked potentials and contrast sensitivity, DON should not be diagnosed from imaging alone. Stretching of the optic nerve is less easy to diagnose without both axial and sagittal images: the latter are not generally available with CT.