Third nerve palsy is a medical condition affecting the third cranial nerve, also known as the oculomotor nerve. This nerve originates in the brain and controls most muscles that move the eye, the upper eyelid, and pupil constriction. When this nerve experiences dysfunction, it impairs coordinated eye movement and function. The condition can manifest with varying degrees of severity, from partial weakness to complete paralysis.
Symptoms of Third Nerve Palsy
Third nerve palsy presents with distinct visual and ocular signs. A prominent symptom is ptosis, the drooping of the upper eyelid. This occurs because the oculomotor nerve innervates the levator palpebrae superioris muscle, which holds the eyelid open. Drooping can range from mild to complete closure of the eye.
The affected eye often presents in a “down and out” position. This misalignment occurs because the third nerve’s control over most eye muscles is compromised, leaving the lateral rectus and superior oblique muscles unopposed, pulling the eye outward and downward. This ocular deviation results in diplopia, or double vision.
The pupil of the affected eye may also appear larger than the unaffected eye, a condition known as mydriasis, and might not react to light. This pupillary involvement is an important sign, distinguishing between “pupil-involving” and “pupil-sparing” third nerve palsies. Pupillary fibers are located superficially on the oculomotor nerve, making them more susceptible to external compression. A pupil-involving palsy can indicate a more serious underlying cause, while a pupil-sparing palsy is more commonly associated with other etiologies.
Underlying Causes
The causes of third nerve palsy are diverse. One category involves nerve compression, where an external mass presses upon it. This can stem from a brain aneurysm, which can be life-threatening and often causes a pupil-involving palsy. Other compressive causes include brain tumors or swelling from inflammation.
Ischemia, or reduced blood flow to the nerve, is a common cause, especially in older adults with vascular conditions. Diabetes and high blood pressure can damage the small blood vessels supplying the oculomotor nerve, depriving it of oxygen and nutrients. This type of palsy presents as “pupil-sparing” because the pupillary fibers are less vulnerable to internal ischemic damage.
Head trauma can directly damage the third cranial nerve, leading to a palsy. Some individuals are born with third nerve palsy, categorized as congenital. Other causes include inflammatory conditions or infections like meningitis. Ophthalmoplegic migraine or autoimmune disorders can also lead to temporary third nerve palsy.
The Diagnostic Process
When third nerve palsy is suspected, a comprehensive diagnostic approach identifies the underlying cause. The initial step involves a detailed neurological examination. During this, a doctor assesses eye movements, checks pupil reaction to light, and observes eyelid position to confirm the palsy’s presence and extent. They also look for other neurological signs.
Imaging tests provide detailed views of the brain and surrounding structures. Magnetic Resonance Imaging (MRI) is preferred for its high-resolution images, aiding in detecting intracranial anomalies like tumors or inflammatory lesions. A Computed Tomography (CT) scan may also be used, particularly in acute settings where speed is a concern, such as to detect bleeding.
For suspected vascular causes, specialized blood vessel imaging may be performed. Magnetic Resonance Angiography (MRA) or Computed Tomography Angiography (CTA) can visualize brain blood vessels, helping to identify or rule out an aneurysm. Blood tests are also routinely ordered to check for systemic conditions that could contribute to the palsy, such as diabetes or high blood pressure, or markers of inflammation like ESR and CRP.
Treatment and Management Strategies
Treatment for third nerve palsy is determined by its underlying cause. If caused by a compressive lesion like a brain aneurysm or tumor, neurosurgical intervention is required to relieve pressure and prevent further damage. For palsies from ischemia, such as those linked to diabetes or high blood pressure, management focuses on controlling these systemic conditions through medication and lifestyle adjustments to improve blood flow.
While the underlying cause is addressed, strategies manage symptoms and improve quality of life. For double vision, an eye patch can be used over the affected eye to block the misaligned image. Alternatively, prism glasses may be prescribed to bend light and realign images, enabling single vision.
If nerve recovery is incomplete after several months, surgical options may be considered. Strabismus surgery can realign eye muscles, improving eye alignment and reducing double vision. If significant eyelid drooping persists, eyelid surgery might be an option to lift the eyelid and restore a more open appearance.