Internuclear ophthalmoplegia (INO) is the inability to move both your eyes together when looking to the side. It can affect only one eye, or both eyes.

When looking to the left, your right eye will not turn as far as it should. Or when looking to the right, your left eye will not turn fully. This condition is different from crossed eyes (strabismus), which occurs when you’re looking straight ahead or to the side.

With INO, you can also have double vision (diplopia) and rapid involuntary motion (nystagmus) in the affected eye.

INO is caused by damage to the medial longitudinal fasciculus, a group of nerve cells leading to the brain. It’s common in young adults and older people. INO is rare in children.

INO is classified into three main types:

  • Unilateral. This condition affects only one eye.
  • Bilateral. This condition affects both eyes
  • Wall-eyed bilateral (WEBINO). This severe, bilateral form of INO occurs when both eyes turn outward.

Historically, specialists have also separated INO into anterior (front) and posterior (back) varieties. It was thought that certain symptoms could indicate where in the brain the nerve damage was located. But this system is becoming less common. MRI scans have shown that the classification is unreliable.

The main symptom of INO is not being able to move your affected eye toward your nose when you want to look to the opposite side.

The medical term for the motion of the eye toward the nose is “adduction.” You may also hear a specialist say you have impaired motion of the adducting eye.

The second main symptom of INO is that your other eye, called the “abducting eye,” will have an involuntary back-and-forth sideways motion. This is called “nystagmus.” This motion usually lasts just a few beats, but it can be more severe. Nystagmus occurs in 90 percent of people with INO.

Although your eyes aren’t moving together, you may still be able to focus both eyes on the object you’re looking at.

Some other possible symptoms of INO include:

  • blurry vision
  • seeing double (diplopia)
  • dizziness
  • seeing two images, one on top of the other (vertical diplopia)

In a mild case, you may feel the symptoms just for a short time. When the adducting eye catches up with your other eye, your vision becomes normal.

About half of the people with INO will experience only these mild symptoms.

In more severe cases, the adducting eye will only be able to turn part of the way toward the nose.

In extreme cases, the affected eye may only reach the midline. That means your affected eye will appear to be looking straight ahead, when you’re trying to look fully to the side.

INO is the result of damage to the medial longitudinal fasciculus. This is a nerve fiber leading to the brain.

The damage may be due to many causes.

About one-third of cases are the result of strokes and other conditions that block the blood supply to the brain.

A stroke can be called ischemia, or an ischemic attack. Strokes most commonly affect older people, and affect only one eye. But a stroke affecting one side of the brain may sometimes cause INO in both eyes.

About another third of cases result from multiple sclerosis (MS). In MS, the INO usually affects both eyes. MS-caused INO is more common in teenagers and young adults.

Keep in mind that MS is a description of a condition, not a cause. In this condition, the immune system attacks the myelin sheath that surrounds and insulates the nerve fibers. This can cause injury to the sheath and to the nerve fibers that it surrounds.

With INO, it’s not always known what’s causing the damage to the myelin sheath, called “demyelination.” Various infections, including Lyme disease, have been associated with it.

Other conditions that can cause INO include:

Tumors such as pontine gliomas or medulloblastomas are important causes of INO in children.

Your doctor will take a medical history and carry out a careful examination of your eye motions. The signs of INO may be so clear that little testing is needed to confirm the diagnosis.

Your doctor will ask you to focus on their nose, and then rapidly shift your gaze to a finger held out to the side. If the eye overshoots when turning to the side, it’s a sign of INO.

You may also be tested for back-and-forth motion of the abducting eye (nystagmus).

Once the diagnosis is made, your doctor may do imaging tests to discover where the damage is located. An MRI and possibly a CT scan may be ordered.

Up to 75 percent of people are likely to show some visible damage to the medial longitudinal fasciculus nerve fiber on an MRI scan.

Proton-density imaging may also be used.

INO may be a sign of a serious underlying condition that must be treated. If you have an acute stroke, hospitalization may be required. Other conditions such as MS, infections, and lupus will need to be managed by your doctor.

When the cause of the internuclear ophthalmoplegia is MS, infection, or trauma, most people show a complete recovery.

Full recovery is less favorable if the cause is a stroke or other cerebrovascular problem. But full recovery is more likely if INO is the only neurological symptom.

If double vision (diplopia) is one of your symptoms, your doctor may recommend a botulinum toxin injection, or a Fresnel prism. A Fresnel prism is a thin plastic film that attaches to the back surface of your eyeglasses to correct double vision.

In case of the more severe variant known as WEBINO, the same surgical correction used for strabismus (crossed eyes) may be used.

New stem cell treatments are available to treat demyelination, such as from MS or other causes.

INO can usually be diagnosed by a simple physical examination. The outlook is good for most cases. It’s important to see your doctor and rule out, or treat, the possible underlying causes.