Third nerve palsy is a neurological condition affecting the oculomotor nerve. This nerve is responsible for controlling most eye movements and the position of the eyelid. When this nerve is damaged or dysfunctional, it can lead to problems with how the eye moves and how the eyelid stays open. The condition is characterized by weakness or paralysis of the muscles that control eye movement and the eyelid.
Understanding the Oculomotor Nerve
The oculomotor nerve (cranial nerve III) is one of twelve cranial nerves that originate directly from the brain. It plays a major role in controlling the movement of the eyes and the upper eyelid. Specifically, this nerve innervates four of the six muscles responsible for moving the eyeball: the superior rectus, medial rectus, inferior rectus, and inferior oblique muscles. These muscles allow the eye to move upward, downward, and inward towards the nose. Beyond eye movement, the oculomotor nerve also has functions related to the pupil and eyelid. It controls the levator palpebrae superioris muscle, which is responsible for raising the upper eyelid. Additionally, the nerve carries parasympathetic fibers that regulate pupillary constriction, meaning it helps the pupil become smaller in response to light.
Recognizing the Signs
Individuals experiencing third nerve palsy typically present with a distinct set of symptoms related to eye movement and eyelid function. One common sign is a drooping eyelid, medically known as ptosis, which can range from mild to complete closure of the eye, occurring because the levator palpebrae superioris muscle is no longer receiving proper signals from the oculomotor nerve. Another characteristic symptom is the inability to move the affected eye in certain directions, particularly upward, downward, and inward; the eye may appear to turn outward and slightly downward, a position often described as “down and out,” due to the unopposed action of other eye muscles. This misalignment often leads to double vision, or diplopia, as the brain receives two different images. The pupil of the affected eye may also be enlarged or dilated and might not constrict normally when exposed to light, as the nerve fibers controlling pupil constriction are also affected; pupillary involvement can provide clues about the underlying cause of the palsy.
Common Underlying Causes
Third nerve palsy can arise from various underlying conditions.
Microvascular Ischemia
One frequent cause, particularly in adults, is microvascular ischemia, which involves reduced blood supply to the nerve. This is often associated with systemic conditions like diabetes and high blood pressure, which can damage small blood vessels. In such cases, the pupil is often spared because the pupillary fibers are located on the surface of the nerve and have a different blood supply.
Aneurysms and Trauma
Another significant cause is the compression of the nerve by an aneurysm, which is a balloon-like bulge in a blood vessel. Aneurysms, especially those near the posterior communicating artery, can press directly on the oculomotor nerve, often affecting the pupillary fibers and causing a dilated pupil. This type of palsy is considered a medical emergency due to the potential for aneurysm rupture. Trauma to the head can also lead to third nerve palsy by directly damaging the nerve.
Other Causes
Tumors growing near the nerve pathway can compress it, leading to a gradual onset of symptoms. Inflammation, such as that seen in conditions like ophthalmoplegic migraine or Tolosa-Hunt syndrome, can also affect the nerve, sometimes causing fluctuating symptoms. In some instances, third nerve palsy can be congenital, often due to developmental abnormalities or birth trauma.
Diagnosis and Management
Diagnosing third nerve palsy typically begins with a thorough neurological and eye examination. The healthcare provider will assess eye movements, eyelid position, and pupillary responses to light. Observing the characteristic “down and out” position of the eye, along with ptosis and a dilated pupil, helps confirm the palsy. The extent of the palsy, whether partial or complete, is also evaluated. To identify the underlying cause, imaging studies are frequently performed. Magnetic Resonance Imaging (MRI) of the brain is a common choice, providing detailed images of the nerve and surrounding structures to detect tumors, inflammation, or signs of stroke. A Computed Tomography (CT) scan, particularly a CT angiography, may be used if an aneurysm is suspected, as it can visualize blood vessels. Blood tests may also be ordered to check for conditions like diabetes or other vascular risk factors.
Management of third nerve palsy depends on its identified cause. If an underlying medical condition like diabetes or hypertension is responsible, treating and managing that condition is the primary approach. For cases caused by aneurysms or tumors, surgical intervention may be necessary to relieve pressure on the nerve. In some situations, especially with microvascular causes, the palsy may improve spontaneously over several months. Symptomatic relief for double vision can involve using prism glasses to help align images or an eye patch to cover the affected eye. Eye muscle surgery may be considered for persistent misalignment or severe ptosis after the condition has stabilized.