In a 2008 study, almost half of patients with nonorganic visual loss wore sunglasses inside the eye clinic. From http://www.ncbi.nlm.nih.gov/pubmed/18195266:
During the study period, 59 patients were diagnosed with NOVL (nonorganic visual loss), among whom 27 (45.8%) wore sunglasses. The sensitivity of wearing sunglasses for NOVL was 0.46 (95% CI 0.33 to 0.59). The probability that a patient walking into our clinic had NOVL was 0.043 (95% CI 0.033 to 0.055); it increased to 0.79 (95% CI 0.62 to 0.91) in sunglasses patients. The specificity of sunglasses for the diagnosis of NOVL was 0.995 (95% CI 0.989 to 0.998). At least one of the following characteristics (highly positive review of systems, workers’ compensation claim, disability, and lawsuit) was found in 26 of 27 (96.3%) of NOVL patients wearing sunglasses and in none of the sunglasses patients with organic neuro-ophthalmic disorders. All 7 sunglasses patients with organic neuro-ophthalmic disorders had reasonable ophthalmic explanations for wearing sunglasses.
The “sunglasses sign” in a patient without an obvious ophthalmic reason to wear sunglasses is highly suggestive of nonorganic visual loss.