Internuclear Ophthalmoplegia (INO) is the medical term for a disorder that specifically impairs coordinated, or conjugate, eye movement, particularly when a person attempts to look horizontally to the side. The word “internuclear” refers to the disruption of communication between the nerve control centers, or nuclei, that govern the eye muscles. This condition manifests as an inability for one eye to turn inward toward the nose when the other eye moves outward. INO is not a primary eye disease but rather a sign of underlying damage to a specific pathway in the brainstem, which is the communication hub for the nerves controlling eye position and movement.
Understanding the Anatomy of Eye Movement
Coordinated horizontal eye movement relies on a finely tuned connection within the brainstem called the Medial Longitudinal Fasciculus, or MLF. This heavily myelinated fiber tract acts as a communication highway between the cranial nerve nuclei responsible for moving the eyes: the abducens nerve (Cranial Nerve VI) and the oculomotor nerve (Cranial Nerve III). When a person decides to look to the right, the signal is sent to the right abducens nucleus (CN VI), which then directly tells the right lateral rectus muscle to pull the eye outward. Simultaneously, interneurons from the right abducens nucleus cross the midline and travel up the left MLF to signal the left oculomotor nucleus (CN III). This signal tells the left medial rectus muscle to pull the left eye inward, allowing both eyes to move in unison to the right.
Damage to the MLF disrupts this precise signal transmission, leading to the characteristic failure of the eye to move inward, a motion known as adduction. If the left MLF is damaged, the left eye cannot receive the signal to adduct when the person looks to the right. The right eye, however, still receives its direct signal from the right abducens nucleus and moves outward, or abducts, but it often exhibits a rapid, involuntary movement called nystagmus. This specific pattern of impaired adduction in one eye and nystagmus in the other is the defining feature of INO. The eye’s ability to turn inward during convergence, such as when focusing on a near object, often remains intact because that function uses a separate neural pathway that bypasses the MLF.
Underlying Causes and Risk Factors
The underlying causes of INO vary significantly depending on the person’s age. In younger adults, typically those under 50, the most frequent cause is Multiple Sclerosis (MS), an autoimmune disorder that attacks the myelin sheath protecting nerve fibers. MS-related damage to the MLF often occurs bilaterally, meaning both eyes are affected, and INO can sometimes be the first noticeable sign of the disease.
For older adults, the primary cause is most often a stroke, or ischemic event, which accounts for approximately 38% of all INO cases. A stroke interrupts the blood supply to the brainstem, leading to tissue damage in the area of the MLF. Stroke-related INO is generally unilateral, affecting only one eye, and is a strong indicator of vascular disease.
Less common causes include brainstem tumors, which can compress the MLF, or traumatic head injuries. Certain infections, such as neurosyphilis or Lyme disease, and inflammatory disorders like lupus, can also lead to INO. Additionally, some nutritional deficiencies, such as Wernicke encephalopathy, or the toxic effects of certain medications, may rarely be implicated.
Clinical Presentation and Treatment Options
A patient with INO experiences double vision, or diplopia, especially when looking to the side where the coordination fails. They may also report blurry vision or a sense of environmental movement, known as oscillopsia, due to the involuntary eye movements (nystagmus). Diagnosis is primarily made through a clinical examination where a healthcare provider observes the characteristic eye movements during horizontal gaze testing.
Once INO is suspected, the next step involves imaging to confirm the lesion and identify the underlying cause. Magnetic Resonance Imaging (MRI) is the preferred method, as it provides detailed visualization of the brainstem and can reveal the specific damage to the MLF, whether it is an MS plaque or a small stroke lesion. Management of INO focuses almost entirely on treating the underlying condition.
For cases caused by MS, treatment involves disease-modifying therapies to manage the autoimmune activity and reduce future relapses. If a stroke is the cause, management centers on controlling vascular risk factors like high blood pressure and cholesterol. Some individuals may find relief for persistent double vision through the use of prism glasses, which help realign the images seen by each eye. In rare, long-standing cases where a significant misalignment persists, eye muscle surgery may be considered to improve the eye position and reduce diplopia.