What is the One and a half syndrome?
This disorder of horizontal ocular movement is characterized by a lateral gaze palsy on looking toward the side of the lesion and INO on looking in the other direction.
The location of the lesion is the paramedian pontine reticular formation or VI nerve nucleus.
MLF fibers crossing from the contralateral VI nucleus are also involved, causing INO.
The common causes of this syndrome are similar to those of INO (e.g., multiple sclerosis, stroke).
Hemorrhage or tumor in the lower pons is also in the differential diagnosis.
Pseudo-one-and-a-half syndromes may occur with myasthenia gravis, Wernicke’s encephalopathy, or Guillain–Barré syndrome. Clinical signs include:
- Horizontal gaze palsy on looking toward the size of the lesion (“one”).
- INO on looking away from the side of the lesion (“half”). This paralyzes adduction and causes nystagmus on abduction. As a result, the ipsilateral eye has no horizontal movement, and the only lateral ocular movement that remains is abduction and nystagmus of the contralateral eye.
- Associated signs include: skew deviation, gaze-invoked nystagmus on vertical gaze, and exotropia of the eye contralateral to the lesion.
- Vertical ocular movements and convergence are usually intact.