This is a masked study of 33 patients who had been taking a prostaglandin analogue in one eye only for at least a year, to look for signs of prostaglandin-associated periorbitopathy (PAP). The investigators devised a new grading system to quantify the degree of fat atrophy and superior sulcus deformity, as well as taking exophthalmometry and margin reflex distance measurements. All patients were assessed by three masked observers. There were equal numbers of patients on bimataprost, travaprost and latanaprost. Treated eyes had significantly more superior sulcus deformity, upper lid retraction, lagophthalmos and lid redness than untreated eyes. They also had more enophthalmos, lower lid retraction and higher skin creases but these were not statistically significant. Interestingly very few patients noticed or complained about the changes, and the difference in PAP grade between treated and untreated eyes increased significantly with age but not with duration of prostaglandin analogue use. Some parameters were significantly worse in those on bimataprost compared with latanaprost or travaprost. The authors noted that contrary to most previous reports of PAP, the syndrome induces lid retraction rather than ptosis. Although 15% of the treated eyes did have ptosis, other causes for this were found in every patient including large superior blebs, pre-existing ptosis or cataract surgery-induced ptosis. The mechanism behind PAP remains obscure.