What Is Internuclear Ophthalmoplegia?
Internuclear ophthalmoplegia (INO) is an eye movement disorder that results from damage to a specific fiber tract in the brainstem called the medial longitudinal fasciculus (MLF). This tract coordinates the movements of the two eyes so that they move together in a synchronized way. When the MLF is damaged on one side, the affected eye cannot move inward (adduct) when the person tries to look toward the opposite side. The condition is often abbreviated as INO and is a key neurological finding that helps doctors localize lesions within the brainstem.
The Role of the Medial Longitudinal Fasciculus
The MLF is a paired bundle of nerve fibers running through the brainstem. It connects the abducens nucleus in the pons, which controls the lateral rectus muscle of the eye, to the oculomotor nucleus in the midbrain, which controls the medial rectus muscle. During horizontal gaze, the abducens nucleus sends a signal to the ipsilateral lateral rectus to abduct one eye, and simultaneously, via the MLF, it sends a signal to the contralateral oculomotor nucleus to adduct the other eye. This pathway ensures that both eyes move horizontally in the same direction at the same speed. A lesion anywhere along the MLF disrupts this communication and produces INO.
Common Causes of INO
The two most frequent causes of INO are multiple sclerosis (MS) and ischemic stroke. In younger adults, particularly between the ages of 20 and 50, MS is the most common underlying condition. The demyelinating plaques of MS often affect the white matter tracts of the brainstem, including the MLF. In older adults, a small lacunar stroke in the brainstem is a frequent cause, especially when the INO is unilateral and the patient has vascular risk factors such as hypertension, diabetes, or high cholesterol.
Other causes include brainstem tumors, infections such as tuberculosis or Lyme disease, traumatic brain injury, Wernicke encephalopathy (often due to thiamine deficiency), and rare inflammatory conditions such as vasculitis. In some cases, INO can also appear as a complication of certain medications or as a feature of congenital eye movement disorders. The cause is identified based on the patient's age, history, and findings on brain imaging.
Recognizing the Signs and Symptoms
The hallmark sign of INO is weakness of adduction in one eye on attempted horizontal gaze to the opposite side. When the patient looks away from the side of the lesion, the affected eye lags behind and may not cross the midline fully. At the same time, the other eye abducts normally but often shows a rapid, back-and-forth jerking movement known as nystagmus. This combination—an adduction deficit on one side and nystagmus in the abducting eye—is called dissociated nystagmus and is strongly suggestive of INO.
Patients may complain of horizontal double vision (diplopia), blurry vision, or difficulty reading, especially when tired. Some people notice that objects seem to jump or oscillate when they shift their gaze. If the lesion is large or affects neighboring structures, there may be additional neurological signs such as vertigo, nausea, problems with balance, or weakness of the face or limbs. In some cases, the eyes are misaligned vertically, a finding known as skew deviation.
Bilateral vs. Unilateral INO
INO can be unilateral or bilateral. Bilateral INO is classically associated with multiple sclerosis, because demyelinating plaques often occur on both sides of the brainstem. Unilateral INO is more often caused by a small stroke affecting one MLF, although either pattern can occur with various diseases. When INO is bilateral, the patient may have difficulty with horizontal gaze in both directions and typically experiences more pronounced double vision.
Typical Clinical Presentations
In everyday neurological practice, INO appears in a few recognizable patterns. A 30-year-old person with relapsing-remitting MS may present with brief episodes of double vision on rightward gaze. Examination reveals limited adduction of the left eye and right-beating nystagmus in the right eye, consistent with a left MLF lesion. This is a classic unilateral INO that may resolve after a course of corticosteroids for an acute MS relapse.
In contrast, an older patient with a history of hypertension and diabetes may suddenly notice blurred vision and imbalance. If a stroke affects the right MLF, the right eye fails to adduct when looking left, and the left eye develops nystagmus. Over the following weeks, the eye movement disturbance often partially improves, and treatment focuses on preventing further vascular events.
Bilateral INO can appear in a patient with progressive MS, producing difficulty looking to either side and noticeable diplopia when reading. In all these scenarios, the physical examination is the key to diagnosis, and neuroimaging confirms the location and nature of the lesion.
Diagnosis and Evaluation
The diagnosis of INO is primarily clinical, based on the observed eye movement abnormalities. A careful cranial nerve examination, including testing of saccades, smooth pursuit, and convergence, helps confirm the pattern. Convergence is usually preserved in INO, which helps distinguish it from a simple medial rectus muscle palsy or third nerve palsy.
Magnetic resonance imaging (MRI) of the brain and brainstem is the most important diagnostic test. Diffusion-weighted imaging can detect an acute stroke in the MLF within hours, while T2-weighted and FLAIR images may reveal demyelinating lesions in patients with MS. MRI can also help exclude tumors, infections, or other structural lesions. In some situations, additional testing such as blood tests, lumbar puncture, or visual evoked potentials may be recommended, depending on the suspected underlying cause.
Treatment and Outlook
There is no specific treatment that directly repairs the MLF itself. Instead, management focuses on the underlying disease and on relieving symptoms. If MS is diagnosed, treatment may include corticosteroids for acute relapses and long-term disease-modifying therapies to reduce future attacks. For stroke, the focus is on secondary prevention with antiplatelet agents, blood pressure control, lipid management, and lifestyle changes. Prompt recognition of a posterior circulation stroke is important because timely treatment can reduce disability.
For diplopia, simple measures such as wearing an eye patch or using prism glasses can help. In some cases, botulinum toxin injections or strabismus surgery may be considered if double vision persists. The prognosis for INO depends on the cause. In patients with a small stroke, the eye movement disorder often improves significantly over weeks to months as the brain recovers. In MS, symptoms may fluctuate, and recovery is more variable. Early diagnosis and treatment of the underlying condition offer the best chance for a good outcome.