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one and a half syndrome
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This is the inability of one eye to move in the horizontal plane during attempted conjugate gaze. The other eye can adduct normally but shows nystagmus on abduction.

The lesion is in the pons resulting in:

  • damage to the pontine lateral gaze centre:
    • failure of abduction in the ipsilateral eye

  • internuclear ophthalmoplegia:
    • failure of adduction in the ipsilateral eye
    • contralateral nystagmus on abduction in the contralateral eye

The common causes of a one and a half syndrome are:

  • stroke
  • demyelination