Sometimes confused with amblyopia, or “lazy eye”, ptosis is a condition affecting the muscles atop the eye’s orbit that causes drooping of one or both upper eyelids. The drooping can be subtle, but in severe cases, it can cause the entire eye to shut. While the risk of drooping eyelids increases as we age, it can afflict children as well as adults.
Ptosis may be caused by a wide range of conditions. It may develop slowly and present itself in old age, or be caused by a congenital condition and be noticeable from birth. Congenital ptosis is rare: the total prevalence of congenital ptosis in general population is 0.18–1.41%. It can be bilateral or unilateral. 
Injuries can also interrupt the muscles responsible for lifting the eyelid, resulting in drooping eyelids. These include complications from corrective eye surgery, for example, cataract surgery.
Ptosis can also be a symptom of a more severe underlying condition, such as a neurological or neuromotor condition, diabetes, or an eye tumour.
Some cases of drooping eyelids are subtle enough, and develop slowly enough, to evade detection until they become significantly worse. The first step toward a positive diagnosis can be as simple as comparing photographs from years ago with ones taken recently.
When drooping eyelids are especially pronounced, it may affect the sufferer’s vision. Some find themselves arching their eyebrows or even tilting their heads back to see clearly.
Along with amblyopia, ptosis is sometimes confused for dermatochalasis, a disease of the connective tissues. Because the symptoms of ptosis can be so easily confused with those of other conditions, anyone who suspects that they may have ptosis should consult an ophthalmologist for a full examination.
While ptosis can be caused by many different factors, its resolution usually involves surgery. For children, this is especially important: left untreated, childhood ptosis can lead to diminished vision in the affected eye. In any case, children diagnosed with ptosis should see an ophthalmologist regularly, to be sure that the condition’s progression is being tracked and to be sure that any necessary surgery is scheduled optimally.
Of the three muscles responsible for retracting the upper eyelid, the levator muscle is the most important, and damage to the levator is nearly always responsible for ptosis.
When the levator muscle has been weakened but is still functional, it becomes stretched by the weight of the eyelid. Ptosis correction surgery can shorten the muscle, returning its function and giving it a chance to regain strength. When the levator muscle is too damaged for this approach, surgery may sever or remove it, allowing the other two muscles – Müller’s muscle and the frontalis muscle—to take up the responsibility of retracting the eyelid. 
Because surgery for drooping eyelids directly affects the eye’s musculature, it may result in slightly asymmetrical eyelids. When muscles do not respond as hoped, they may not be able to retract the eyelid completely or at all following surgery. Not all eye surgeons have significant experience with ptosis surgery, so patients are advised to ask plenty of questions before settling on a surgeon with an adequate background.