Boston Mayor Thomas M. Menino delivers his major policy speech at the annual meeting of the Boston Municipal Research Bureau in South Boston. (Image credit: Jonathan Wiggs/The Boston Globe/Getty Images)

Boston Mayor Thomas Menino is undergoing surgery today lift his droopy eyelids, his office said.

Menino, 69, was diagnosed with dehiscence ptosis, a condition caused by the slow stretching of tiny tendons that hold up the upper eyelids.

"His ptosis occurred over a long period of time," Dr. Mami Iwamoto, the Boston-based ophthalmologist who will perform the procedure, told the Boston Globe. "It can creep up on people."

The upper eyelids usually hang two millimeters above the pupil, the tiny window through which light enters the eye. But in Menino's case, the window was partially blocked.

"It's like looking out a porthole with a shade down," Iwamoto said, describing how low-hanging can obscure vision. "The shade interferes with the view out the porthole."

Menino will be awake but sedated for the hour-long surgery, called blepharoplasty, to shorten the stretched-out tendons through tiny incisions in his eyelids. Swelling and bruising is expected to keep him out of the public eye for a week or so, the Globe reported.

Because eyelids droop with age, some people opt for blepharoplasty for cosmetic reasons. A February 2012 study found that face and eyelid lifts knocked 7.5 years off a person's estimated age. But for Menino, the surgery was purely functional, according to Iwamoto.

"The need for a correction is definitely driven by a medical need, not aesthetics," she told the Globe.

Menino is no stranger to surgery. Since he was elected in 1993, the Mayor has gone under the knife at for a cataract, a torn tendon and two tumors, his office confirmed.