Ptosis, commonly known as a drooping upper eyelid, is a frequent symptom ranging from a minor cosmetic concern to a potentially serious health problem. This condition occurs when the upper eyelid falls to a lower position, sometimes partially or completely covering the pupil. Ptosis is often related to nervous system issues, as nerves control the muscles responsible for lifting the eyelid. Determining which nerve is involved is a primary step in finding the appropriate treatment.
The Oculomotor Nerve (CN III) and Eyelid Control
The Oculomotor Nerve (CN III) is most directly responsible for upper eyelid elevation, providing motor innervation to the Levator Palpebrae Superioris muscle. This muscle is the primary structure that pulls the eyelid upward. Damage to CN III typically results in the most severe neurogenic ptosis, often causing the eyelid to droop completely shut.
The nerve fibers controlling the Levator Palpebrae Superioris muscle are in the superior division of CN III. Paralysis of this nerve affects several other muscles that control eye movement. Since CN III innervates four of the six muscles that move the eyeball, its paralysis leaves the eye in an unopposed “down and out” position.
If damage involves the outer, superficial fibers, it may affect the parasympathetic fibers that constrict the pupil. Ptosis alongside a fixed and dilated pupil suggests a compressive lesion, such as an aneurysm.
When nerve damage is caused by small vessel disease (e.g., diabetes), the inner somatic fibers controlling the eyelid are affected, while the outer parasympathetic fibers are spared. An ischemic third nerve palsy presents with ptosis and eye movement issues but a normal pupil.
Other Nerve-Related Causes of Ptosis
Ptosis can also arise from disruption of the sympathetic nervous system, which controls Müller’s muscle (superior tarsal muscle), leading to Horner Syndrome and characteristically mild ptosis. Since Müller’s muscle contributes only a small amount of lift, its paralysis does not cause the severe droop seen in CN III palsy.
Horner Syndrome is identified by a classic triad: mild ptosis, a constricted pupil (miosis), and reduced sweating (anhidrosis). The ptosis occurs because the sympathetic nerve supply to Müller’s muscle is interrupted, which can happen anywhere along the three-neuron sympathetic pathway.
Some ptosis is caused by miswiring or aberrant regeneration of a nerve after injury, known as synkinesis. The Marcus Gunn Jaw-Winking Syndrome is an example, where the Levator Palpebrae Superioris muscle receives an abnormal connection from the Trigeminal Nerve (CN V). Jaw movement causes the misdirected signal to briefly lift the affected eyelid.
Structural and Muscular Reasons for Eyelid Droop
Not all ptosis results from a cranial nerve problem; many cases involve the muscle or surrounding physical structures.
Aponeurotic Ptosis
The most common acquired form in adults is Aponeurotic Ptosis (involutional ptosis). This results from the stretching or detachment of the Levator Aponeurosis, the tendon connecting the Levator Palpebrae Superioris muscle to the eyelid. The muscle receives a normal CN III signal, but the mechanical connection is weak, causing the eyelid to fall.
Myogenic Ptosis
Myogenic Ptosis involves a disorder of the eyelid muscle tissue or the junction where the nerve meets the muscle. Myasthenia Gravis, an autoimmune disease, is a frequent cause where the immune system attacks communication points between nerves and muscles. The resulting weakness often fluctuates, worsening with fatigue and improving after rest.
Mechanical Ptosis
Mechanical Ptosis occurs when the eyelid is physically weighed down by an external factor, such as a mass, tumor, or significant swelling (edema). The nerve signal and muscle are intact, but the excess weight overcomes the muscle’s lifting capacity. These structural causes must be differentiated from neurogenic issues.