Internuclear Ophthalmoplegia (INO)

What is internuclear ophthalmoplegia (INO)? 

Internuclear Ophthalmoplegia is a disorder of horizontal ocular movement due to a lesion in the brain stem (usually in the pons, specifically along the medial longitudinal fasciculus between the VI and III nuclei).

Horizontal gaze requires the coordinated activity of the lateral rectus muscle of the abducting eye (innervated by the VI nerve) and the medial rectus muscle of the adducting eye (innervated by the III nerve).

This integrated function is regulated by the paramedian pontine reticular formation (or pontine gaze center), which receives inputs from the contralateral occipital and frontal eyefields and sends fibers to the ipsilateral abducens (VI) nucleus and the contralateral oculomotor (III) nucleus.

Fibers from the pontine gaze center run rostrally together with vestibular and other fibers to make up the medial longitudinal fasciculus (MLF). 

The cause is commonly multiple sclerosis in young adults, especially when the syndrome is bilateral.

In older people, the syndrome is often unilateral and caused by occlusion of the basilar artery or its paramedian branches.

Occasionally Internuclear Ophthalmoplegia can be caused by lupus erythematosus and drug overdose (e.g., barbiturates, phenytoin, amitriptyline).

Pseudo-INO occurs rarely as a feature of myasthenia gravis, Wernicke’s encephalopathy, and Guillain–Barré syndrome. 

Many patients with Internuclear Ophthalmoplegia have no symptoms, but some have diplopia or blurred vision. On lateral gaze, the signs of Internuclear Ophthalmoplegia include:

  • Impaired or paralyzed adduction of the eye ipsilateral to the lesion. The deficit can range from complete medial rectus paralysis to slight slowing of an adducting saccade.
  • Horizontal nystagmus of the abducting eye contralateral to the lesion.
  • Bilateral Internuclear Ophthalmoplegia results in defective adduction to the right and left, and nystagmus of the abducting eye on both directions of gaze.
  • Convergence is usually preserved. Skew deviation and vertical gaze nystagmus are sometimes present.
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