Why do I have an under eye bag in my left eye only? (Photo)

These are genetic and a consequence of aging, but there
are many ways to treat or give imrovement.  It is important to see a cosmetic
dermatologist who is well trained in skin care, lasers, and fillers, as
the majority of cases require a combination of treatments to give you
the best results. All my patients are on topical lightening creams and
sun protection as well as antioxidants to improve the skin texture and
tone. Many get peels or lasers to help tighten, improve the complexion
and decrease wrinkling. Fillers can replace lost volume and cover up
the vessels below to make the discoloration less noticeable. Laser
genesis, sublative radiofrequency, and nonablative clear and brilliant
lasers are procedures my patients love for this condition. Undereye
wrinkles need to be treated with a combination approach of skin care,
botox, lasers and/or radiofrequency. You can get amazing results, but
you should see a cosmetic dermatologist with experience in treating this
area which is sensitive and requires a meticulous provider to limit any
complications. Your best
next step is to find someone reputable and experienced for a formal in
person evaluation for facial rejuvenation. Best, Dr. Emer.

There are three compartments of fat in the lower eyelid.  The fat in these areas are referred to as "brown fat", not because they are brown in color but because this fat is biochemically the same type of fat that brown bears use to keep alive while hibernating.  The brown fat in both lower eyelids is held in place by a piece of tissue known as the "septum orbitale".  As we age, this piece of tissue gets weaker and weaker, and as it weakens, it allows the fat to pooch forward leading to undereye bags.Some people show undereye bags in their teen years, and others not until their 60's or 70's.  This is because there is a genetic predisposition for this tissue to weaken, but also environmental factors may help the septum orbitale break down sooner rather than later.  Why you are showing the undereye bag on only one side has any number of potential answers.; i.e. possibly because you sleep with your face on the left side and the pressure on the eyeball on that side increases pressure on the septum orbitale causing it to break down sooner than the right side.  Obviously, there are other causes, but this is at least a viable working diagnosis.At 29, you are much too young for surgical intervention.  If you were my patient, I would recommend injectables to build up your orbital rim on the left side which will camouflage the presence of your eyebag. 

The nature of the bag dictates the best treatment.  Generally when you say bag, surgeons think eyelid surgery.  However, you bag is caused by exposure of your orbital rim due to lack of soft tissue coverage in this area.  You can prove this to yourself by putting your finder on the top of the "bag."  The firm structure you feel is your bony inferior orbital rim.  The current state of the art means of treating this is with Restylane filler.  You need expert service for optimal treatment.  For someone your age, this could last easily 1-2 years.

To answer your question completely accurately, we would need to see a picture of your entire face, or at the very least, both of your orbits and both sides of the forehead.  This is because in issues of asymmetry where one side of the face looks different than the other, the entire definition of asymmetry depends upon the relationship between the two sides of the face, not just what is seen on one side or the other.  In your case, it is very likely that you have different shapes to the bony skeleton of the face around the eyeball called the orbit.  In some instances one cheek bone develops more fully than the other, one brow bone develops more or less than the other, the forehead may be flatter on one side and rounder on the other, and so on.  When this happens, the soft tissues follow suit, and they may drape or sit upon their bony foundations differently, giving different appearances like a "bag" under one eye, or more hollowing under one eye than the other.  In a young person such as yourself, who would not be expected to have a lot of changes to the soft tissues themselves due to age or environmental exposures, this is most likely the cause of what you are seeing.  Many of these things are genetically determined or run in families - part of the reason why we look like our parents and other close relatives - and sometimes they just happen for reasons we can't yet explain.  In any event, the options and strategies used to correct these asymmetries depend upon what we are actually dealing with.  We may suggest some injections with fillers or fat transfer to camouflage any volume deficits, or we may actually suggest a facial implant to address skeletal deficiencies.  It will be impossible to give you any sound advice on what options would be best for you at this point.  You will need to make an appointment for consultation with an experienced board certified plastic surgeon or facial plastic surgeon who can properly assess your facial features in their entirety and give you the best recommendations for your individual situation.  Good luck.