Cranial Nerve III, IV and VI

Internuclear ophthalmoplegia (INO)

Internuclear ophthalmoplegia (INO) is a disorder of horizontal conjugate gaze caused by a lesion of the medial longitudinal fasciculus (MLF).

Normal function

• Horizontal conjugate gaze refers to the normal ability of both eyes to move together in the same horizontal direction (either left or right)

• This requires precise coordination between the eye that abducts and the eye that adducts so that the visual axes remain aligned.

• The MLF is a paired white-matter tract located in the paramedian brainstem (pons and midbrain). It coordinates horizontal eye movements by connecting:

  • the abducens (VI) nucleus on one side
  • with the contralateral oculomotor (III) nucleus

• This linkage allows the abducting eye and the adducting eye to move synchronously during horizontal gaze.

  Another important feature of INO is slow aDducting saccadic velocity in the eye ipsilateral to the lesion. This is actually the MOST sensitive finding in INO and the only finding that truly must be present for diagnosis in a subtle case.

Pathophysiology

• During horizontal gaze, the abducens nucleus activates:
  • the ipsilateral lateral rectus (via CN VI)
  • the contralateral medial rectus (via the MLF to CN III)
• A lesion of the MLF interrupts this signal → the eyes can no longer move together horizontally.

Clinical features

• Impaired ADduction of the eye on the same side as the lesion
• Horizontal nystagmus of the ABducting contralateral eye
• Convergence is preserved, as this does not require the MLF

Causes

• Multiple sclerosis
  • commonly causes bilateral MLF lesions
  • optic neuritis by itself does not cause INO
• Vascular disease → Brainstem stroke
  • typically causes unilateral lesions, especially in older patients