What is the oculomotor nerve?
The oculomotor nerve is one of 12 sets of cranial nerves. Many of these nerves are part of the autonomic nervous system. The autonomic nervous system supplies (innervates) organs, like your eyes.
The oculomotor nerve is the third cranial nerve (CN III). It allows movement of the eye muscles, constriction of the pupil, focusing the eyes and the position of the upper eyelid.
Cranial nerve III works with other cranial nerves to control eye movements and support sensory functioning.
- Olfactory nerve (CN I) enables sense of smell.
- Optic nerve (CN II) enables vision.
- Trigeminal nerve (CN V) enables sensation in your face.
- Vestibular and cochlear nerves (CN VII) enable balance and hearing.
What is the function of the oculomotor nerve?
It controls four of the six muscles that enable eye movement. CN III makes it possible to:
- Elevate the upper eyelid.
- Focus the eyes.
- Respond to light by making the black center of the eye (pupil) smaller.
- Move your eyes inward, outward, up and down and control torsion.
How does CN III work?
It coordinates eye movement with motions that include:
- Accommodation, focusing on an object that’s moving closer to or further away from you.
- Optokinetic reflex, changing eyes back to their previous position after focusing on an object.
- Saccades, quick motions that shift your gaze from one object to another.
- Smooth pursuit (visual tracking), which enables you to hold your gaze on a moving object.
- Vestibule-ocular reflex, which adjusts eye positioning when your head is moving.
- Visual fixation, gazing at an object that is not moving.
What is the anatomy of cranial nerve III?
CN III starts in the midbrain. It travels through many structures in your head until it reaches the back of your eyes. Its course includes:
- Exiting the front of the midbrain.
- Passing nearby arteries.
- Poking through the brain’s thick outer tissue (dura).
- Entering the cavernous sinus, a hollow space behind the nose.
- Leaving the skull through the orbital fissure, a large round hole behind each eye.
- Connecting to the back of the eye.
- Dividing into superior and inferior branches.
The superior and inferior branches connect to four of the muscles that control eye movements, as well as an upper eyelid muscle and muscles inside the eye that control pupil size and focusing of the lens.
These include the:
- Inferior oblique, which controls eye torsion, upward gaze and outward gaze.
- Inferior rectus, which controls downward gaze.
- Medial rectus, which controls inward gaze.
- Superior rectus, which controls upward gaze.
- Levator palpebrae superioris, which controls your ability to raise your upper eyelid.
Conditions and Disorders
What conditions can affect the oculomotor nerve?
Oculomotor dysfunction is a group of disorders affecting CN III. These disorders are also known as third nerve palsy. They occur when cranial nerve III becomes paralyzed.
Third nerve palsy can be present at birth. It may also occur later in life due to:
- Inadequate blood flow: Causes lack of oxygen that nerves need to function properly.
- Nerve compression: Abnormal pressure on a nerve.
Conditions that may cause oculomotor dysfunction include:
- Brain aneurysm.
- Brain tumor.
- Head injuries.
- Demyelinating disease (multiple sclerosis).
- Microvascular disease (diabetes and high blood pressure).
- Infections, including HIV and Lyme disease.
How can I prevent oculomotor nerve dysfunction?
It might not be possible to prevent certain conditions that affect CN III. For example, complications from neurological disease or tumors may be unavoidable.
Preventive measures that are within your control include:
- Quitting smoking, vaping or other tobacco products.
- Avoiding dangerous activities that could result in a head injury.
- Staying current with treatments for chronic conditions, such as diabetes and high blood pressure.
What can be done to correct a third nerve palsy?
Treatment for third nerve palsy depends on what caused it. There is no treatment to reestablish function of a congenital third nerve palsy. An acquired third nerve palsy may resolve on its own, depending on the cause. If a brain tumor or aneurysm is causing the third nerve palsy, surgery to relieve pressure on the third nerve may help it function.
An ophthalmologist will wait at least 6 months after onset of third nerve palsy to see if it gets better on its own. During this time, an eye patch or prism glasses may help you with double vision. Eye muscle surgery (strabismus surgery) may help to realign the eyes so that the eyes are straight when looking straight ahead. But it can sometimes take numerous procedures. People typically still experience double vision when looking to the sides after strabismus surgery.
Frequently Asked Questions
When should I call a healthcare provider about concerns with cranial nerve III?
Contact your healthcare provider if you experience symptoms of third nerve palsy. These include:
- An abnormally large pupil.
- A pupil that doesn’t shrink in response to light.
- Double vision (diplopia).
- One or both eyes drifting off to the side (strabismus).
- Drooping eyelid (ptosis) that may cause it to cover the pupil.
A note from Cleveland Clinic
The oculomotor nerve is the third cranial nerve. It controls four of the six muscles that enable eye movement. Conditions affecting cranial nerve III include third nerve palsy. It is often a complication of medical issues such as uncontrolled diabetes or a brain aneurysm. Third nerve palsy might impact your vision and the appearance of one or both of your eyes. Many people make a full recovery, although it can take around two months.
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