What is the difference between cosmetic and functional Eyelid surgery?

Essentially cosmetic is to enhance your existing eyelid features and so deals with beauty or aesthetic alone. Functional eyelid surgery is to improve the function of the eyelids, to protect the eyes better or to help you see out better, so is medical. Functional should also be aesthetic as all surgeons will want to do the most beautiful surgery they can for the patient, even if for medical reasons. The area where it is more fifficult to say whether it is functional is where the eyelid surgery is being done for strong psychological reasons, to boost confidence such as in a congenital deformity, correct an asymmetry which is noticeable and affects the persons ability to be social or get work, yet is not interfering with sight. In many cases that can also be temed functional if it meets certain criteria in the UK.  The surgeon will follow strict rules in assessing you, ask specific questions and do tests such as visual fields, to see if the eyelid surgery is to improve function, called medical indicaiton. 

The surgeries are very similar, but for insurance to cover the procedure strict criteria must be met.  These can include demonstrable visual field loss by testing, photographs demonstrating partial obstruction of the pupil, and issues with visual activities such as driving interference/concerns due to the overhanging tissues.  Cosmetic eyelid surgery can include additional procedures such as fat sculpting, repositioning, chemical/laser peeling of the skin, brow sutures to avoid the eyebrows being pulled downward etc.