Blepharoptosis, also referred to as ptosis, is defined as an abnormal low-lying upper eyelid margin with the eye in primary gaze. The normal adult upper lid lies 1.5 mm below the superior corneal limbus and is highest just nasal to the pupil.  See the images below.
Blepharoptosis can be classified as congenital, as shown below, or acquired.  This differentiation is based on age. A more comprehensive classification is based on etiology and includes myogenic, aponeurotic, neurogenic, mechanical, traumatic, and pseudoptotic. The most common cause of congenital ptosis is myogenic due to the improper development of the levator muscle. 
Most cases of acquired blepharoptosis are secondary to aponeurotic causes, such as involutional changes, a disinsertion, or a dehiscence. Identification of the underlying pathophysiologic mechanism is paramount to institute proper treatment.
Blepharoptosis is the result of dysfunctioning of one or both upper eyelid elevator muscles. These elevator muscles are the levator palpebrae superioris and the Mueller muscle.
The levator palpebrae superioris is a striated muscle innervated by the superior division of the oculomotor nerve (cranial nerve III). This muscle is about 40 mm long and originates from the lesser wing of the sphenoid. It continues anteriorly, and at the Whitnall ligament, it travels inferiorly as an aponeurosis. The aponeurosis is 14-20 mm long and inserts into the anterior aspect of the tarsal plate. It also sends attachments to the skin, forming the upper eyelid crease. The levator muscle and aponeurosis is the major elevator of the upper eyelid.
The Mueller muscle, a sympathetically innervated smooth muscle, originates from the undersurface of the levator superioris. Approximately 12 mm long, it inserts superiorly on the tarsal border and elevates the upper eyelid by approximately 2 mm.
The associated mortality is usually due to anesthetic complications from surgery. Kearns-Sayre disease, a subtype of chronic progressive external ophthalmoplegia, is a syndrome with associated myogenic ptosis, retinal pigmentary changes, and cardiac conduction abnormalities that can cause death.
Morbidity is associated with blockage of the visual axis in the severely ptotic eyelid. Congenital cases can obstruct vision and lead to amblyopia. Even without visual axis obstruction, the eyelid may induce refractive errors, especially astigmatism resulting in amblyopia.
In adults, the morbidity is associated with constriction of the superior visual fields. Patients may complain that they tire easily when reading and experience frontal headaches as they lift their eyebrows in an effort to keep the eyelids open. Patients may be dissatisfied with their appearance.
No racial predilection has been described.
No sexual predilection has been described.
Acquired ptosis can occur at any age, but it is commonly seen in older adults. Congenital ptosis occurs at birth.
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