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Internuclear Ophthalmoplegia

What is Internuclear Ophthalmoplegia?


Internuclear ophthalmoplegia (INO) is a condition that affects the conjugate lateral gaze, resulting in problems in eye adduction. The condition involves the impairment of horizontal eye gaze due to disruptions in the connection between cranial nerve centers within the brain. The affected eye usually cannot turn inwards but can turn outwards during horizontal gaze.

The exact phenomenon involves a minimal adduction of the contralateral eye when a person attempts to see something from the contralateral side. The unaffected eye abducts with associated nystagmus. In simpler terms, when the eyes gaze towards one side, the affected eye does not turn to the side to look at the object, but rather it stays fixed in the middle, while the unaffected eye is able to turn to the side to look at the object but undergoes involuntary rapid movement called nystgamus.

The eyes may also have horizontal diplopia which involves double vision in the affected eye when it looks towards the opposite direction (for example, when the right eye is affected, it suffers double vision when it looks towards the left).

Internuclear Ophthalmoplegia Symptoms

Symptoms of INO often include:

  • Painless onset of disturbance in vision
  • Absence of diplopia in primary gaze
  • Presence of horizontal diplopia in lateral gaze
  • Problems with adduction on the affected eye
  • Nystagmus on the unaffected eye with abduction

Most commonly, INO is unilateral, but some people may experience bilateral INO. Bilateral affectation may involve medial rectus palsy. Patients may also have no problems in convergence in posterior INO and bilateral INO, but some people with anterior INO may experience problems in convergence.


Causes of INO include dysfunction in the medial longitudinal fasciculus (MLF), which is a heavily myelinated tract that connects the oculomotor nucleus to the paramedian pontine reticular formation. This mechanism allows conjugate movements of the eye. INO can also be caused by other factors such as:

Multiple sclerosis

MS causes bilateral INO in young patients due to demyelination of the nerves. INO in young patients is usually bilateral as a result of multiple sclerosis.


Stroke also causes unilateral INO, affecting the ipsilateral side of the brain. Stroke may be in the form of brainstem infarction or other related types. Stroke is the most common cause of INO in older patients.

Myasthenia gravis

Myasthenia gravis (MG) can produce a pseudo-INO.


  • Viral infection
  • Fourth ventricular tumors
  • Syphilis
  • Trauma
  • Lyme disease
  • Subdural hematoma
  • Drug intoxication from tricyclic antidepressants and phenothiazine


The presence of synchronous eye movements is the responsibility of several structures in the eye and brain. The cranial nerves VI, IV, and III communicate through the medial longitudinal fasciculus (MLF). The medical nerve fasciculus is the main pathway for nerve impulses from the cranial nerves to produce normal eye movements. In INO, there is possibly lesion in the pathway, disrupting the communication between the cranial nerve nuclei and the MLF.

For instance, in order to look at the right side, the control center of the left horizontal eye movement must send signals to the left cranial nerve VI nucleus, while that of the right horizontal eye movement must send signals to the right cranial nerve VI nucleus. However, when lesions occur in the left and right pathways, the cranial nerves may not be able to send signals to the medial rectus, thereby causing problems in gaze. Due to this, the unaffected eye assumes a lateral gaze through abduction, while the affected eye will not adduct to follow the gaze. The left eye will also go into abducting nystagmus.

Certain infections may damage the neural pathway, thereby leading to INO.


Diagnosis of INO involves identifying possible causes of the condition in order to come up with the proper treatment regimen. Diagnostic tests include:

  • MRI of the brainstem

MRI of the brain stem may identify lesions in the area that may cause INO.

  • VDRL

Venereal disease research lab (VDROL) testing is a special diagnostic test to identify the presence of syphilis. This involves blood tests to check antigens and antibodies in the patient.

  • Lyme titer

Lyme titer is done to check the presence of Lyme’s disease, which is a condition cause by deer ticks.

  • Fasting blood glucose determination

Fasting blood glucose is especially performed on patients with drug intoxication because drug intoxication may lead to lowered blood glucose levels.

  • Blood pressure measurement

Blood pressure measurements are also taken to monitor post-stroke patients in order to prevent another brain attack episode that may result in further affectation of the cranial nerves.

Treatments of Internuclear Ophthalmoplegia

Treatment for INO involves managing the root cause in order to improve the communication of the cranial nerves and the medial rectus fasciculus. Treatments may include:


Surgery may be done to remove brainstem lesions that are causing INO. Surgery is carefully performed as the brain stem is a delicate tissue. Surgery may employ resection of the medial rectus and lateral rectus.

Antibiotic and antiviral therapies

Conditions caused by infections such as viral diseases, Lyme disease, and syphilis are usually treated with medications to kill the causative microorganism.


Corticosteroids may be used to reduce cerebral edema and increased intracranial pressure (ICP) in cases of ICP due to a cerebrovascular accident and subdural hematoma.

These treatments regimes specifically correct the underlying problem, which eventually result in correcting the internuclear opthalmoplegia.


Complications of INO is often a result of complications of various causes. Brain stem lesions may result in respiratory problems. The INO, which involves problems in horizontal gaze, may cause loss of self-esteem due to the visual defects that are apparent to other people. They may also withdraw from socialization due to INO. In terms of learning, there may be no problems in cognitive learning, but vision problems may cause the person to have difficulty in school activities.

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