Margaret is a 53 year old receptionist who presents to eye casualty with a painless red right eye. She wears glasses and has not noticed any changes to her vision. She denies any photophobia or discharge from the eye. Past medical history includes Coeliac disease.
On examination, there is a focal area of erythema over the upper half of the right eye. This disappears on compression and is mobile when pressure is applied with a cotton bud. Visual acuity is 7/10. Neurological examination is normal.
What is the most likely diagnosis?