Oculomotor Nerve (CN III)

What is the oculomotor nerve?

The oculomotor nerve is one of the main nerves you use to control how your eyes move. Its branches connect to muscles on multiple sides of your eyeballs, delivering movement commands from your brain to those muscles.

Despite its name sounding like it’s for one nerve, it’s a paired set, one for each eyeball. The oculomotor (pronounced “OCK-yoo-lo-mo-tor”) nerve pair is the third of the 12 pairs of cranial nerves, named because they directly connect to your brain. Experts also refer to this nerve using the abbreviation and Roman numeral combination “CN III.”

What does the oculomotor nerve do?

The oculomotor nerve is one of three cranial nerve pairs that play a role in eye movement, and the oculomotor nerve does the most out of all three. Seven different muscles in and around each eye rely on the oculomotor nerve for the signals that tell them what to do.

The oculomotor nerve lets you do the following:

• Open your eyes.
• Rotate your eyeballs up and down.
• Rotate your eyeballs inward (toward your nose).
• Rotate an eye upward when that eye is also pointed inward.
• Focus your eyes at different distances using the ciliary muscle to adjust the lens shape.
• Adjust pupil width to let more or less light into your eye.

The only two movements that the oculomotor nerve doesn’t control are:

• Rotating each eyeball downward when that eye is pointed inward. The muscle that controls this movement relies on the trochlear nerve (pronounced “TROCK-lee-er”), also known as CN IV.
• Rotating each eyeball outward (toward the ear on the same side). That muscle relies on the abducens nerve (CN VI).

Oculomotor nerve contributions to other abilities

Because your vision plays a major role in many abilities and processes, your oculomotor nerves have to work seamlessly with several other systems. When they do, you have the following abilities:

• Accommodation. This is your ability to continuously focus on an object while it moves toward or away from you.
• Scanning eye movements. Known as saccades (pronounced “sa-KODZ”), these are quick back-and-forth eye movements that help you scan your field of view.
• Smooth pursuit. This lets your eyes smoothly follow moving objects.
• Vestibulo-ocular reflex. This reflex combines your vestibular system (which controls balance) with head and eye muscle movements to keep your gaze steady even when you experience sharp movements or direction changes.
• Pupillary light reflex. When your eyes adjust to sudden light changes, your oculomotor nerve is the muscle doing the reacting. Signals traveling through your optic nerves (CN II) tell your brain when it’s too bright or dim. Your brain tells your oculomotor nerve to adjust your pupils’ width accordingly.

What conditions can affect the oculomotor nerve?

Your oculomotor nerve is vulnerable to the same conditions as any other nerve. And because it runs through various brain areas, certain brain conditions can affect this nerve, too.

When the oculomotor nerve doesn’t work correctly, it’s known as oculomotor nerve palsy (sometimes “third nerve” replaces “oculomotor”) or dysfunction. Many things can cause oculomotor nerve dysfunction.

Examples include:

• Circulatory conditions. Type 2 diabetes and high blood pressure can disrupt blood flow and cause ischemia (pronounced “iss-KEE-me-uh”), which means the nerve malfunctions and starts to die. That can cause permanent loss of control over the muscles linked to the oculomotor nerve. Hemorrhages (bleeding) can also cause similar circulation disruptions.
• Injuries. Trauma to your head, face and brain can damage the oculomotor nerve, keeping it from carrying signals correctly.
• Infections. Bacteria, viruses and other pathogens can disrupt how nerves work. Examples include HIV and bacteria that cause Lyme disease.
• Brain and nervous system diseases. Conditions that disrupt your brain functions can affect your oculomotor nerve. These include conditions like migraines or seizures or more severe conditions like strokes and transient ischemic attacks (TIAs).
• Inflammation. Swelling from inflamed tissues can press on the oculomotor nerve, preventing it from correctly relaying signals. Inflammatory conditions can happen for many reasons, including autoimmune issues where your own immune system damages nerve tissues. An example would be a condition like multiple sclerosis (MS).
• Tumors (including cancer and benign growths). These can compress the oculomotor nerve, damaging it and disrupting how signals travel through it.
• Congenital conditions. These are conditions you have when you’re born. This usually means the oculomotor nerve or a structure around it didn’t develop correctly, disrupting the nerve’s ability to carry signals. Some of these may be treatable.
• Idiopathic causes. Sometimes, experts can’t find what’s causing oculomotor nerve issues. When the cause is unknown, the term for that is “idiopathic.”

What are the symptoms of conditions that affect the oculomotor nerve?

Oculomotor nerve-related symptoms include:

• Trouble moving one or both eyes.
• Drooping eyelid (ptosis).
• Eye misalignment (strabismus).
• Double vision (diplopia).
• Blurred vision.
• Eye strain and headaches.
• Unusually large pupil of your eye (mydriasis) that responds to light sluggishly or not at all.

How can I prevent oculomotor nerve conditions and issues?

Taking care of your oculomotor nerve health is much like caring for your whole body. Some things you can do include:

• Get annual checkups and regular eye exams, as these can often detect eye and health issues before symptoms appear.
• Quit using nicotine products (including smoking, vaping and smokeless tobacco) or avoid starting them. Ask your provider for helpful resources on quitting.
• Use safety gear like helmets and seatbelts to avoid head injuries.
• Manage chronic conditions like Type 2 diabetes, high blood pressure, etc.