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One and a Half Syndrome. Shirley H. Wray, M.D., Ph.D. Professor of Neurology, Harvard Medical School Director, Unit for Neurovisual Disorders Massachusetts General Hospital. The One-and-a-Half Syndrome. On horizontal gaze there is: An ipsilateral gaze paresis or palsy

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One and a Half Syndrome

Shirley H. Wray, M.D., Ph.D.

Professor of Neurology, Harvard Medical School

Director, Unit for Neurovisual Disorders

Massachusetts General Hospital

the one and a half syndrome
The One-and-a-Half Syndrome

On horizontal gaze there is:

An ipsilateral gaze paresis or palsy

An internuclear ophthalmoplegia (INO) on contralateral gaze

At rest, the eyes are:

Orthophoric, or, in acute stage

Ipsilateral eye esotropic or

Contralateral eye exotropic (Paralytic pontine exotropic)

Three possibilities to account for an ipsilateral horizontal gaze palsy: may be due to unilateral lesion affecting

The ipsilateral PPRF only

The ipsilateral abducens nucleus alone

Both the ipsilateral PPRF and abducens nucleus

abducens nucleus
Abducens Nucleus

All the cells necessary for ipsilateral horizontal gaze:

Motoneurons whose axons form the sixth nerve (VIN) to innervate the ipsilateral lateral rectus muscle

Internuclear neurons which send axons across the midline to opposite MLF and ultimately to the medial rectus motoneurons in the contralateral oculomotor nucleus (III N).

pathogenesis of certain signs
Pathogenesis of Certain Signs

Ocular Motor Possible Pathophysiologic Deficit Substrate

Ipsilateral adduction weakness

Ipsilateral slowed abducting saccades

Contralateral abduction nystagmus

Interruption of axons of abducens internuclear motoneurons

Inadequate inhibition of medial rectus motoneurons

Impaired inhibition of contralateral medial rectus or

Interruption of descending fibers to contralateral abducens nucleus or

Involvement of adjacent PPRF

Reported Boston Total cases series

Brainstem Infarct 12 4 16

Multiple Sclerosis 2 14 16

Pontine Glioma 2 1 3

Arteriovenous Malformation 1 0 1

Pontine Hemorrhage 8 0 8

Basilar Artery Aneurysm 0 1 1

Cerebellar Astrocytoma 2 0 2

Metastatic Melanoma 1 0 1

Ependymoma Fourth Ventricle 1 0 1

29 20 49

Table 1. The one-and-a-half syndrome: Etiology


Diplopia 12

Blurred Vision 8

Oscillopsia 4

Difficulty looking to one side 2

“Quivering” of the eye 1

No visual complaint 3

Table 2. One-and-a-half syndrome (N = 20): Visual Symptoms


(N = 20)

Gaze-evoked upbeat nystagmus 12

Skew deviation 8

Horizontal ipsilateral gaze nystagmus 4

Rotary component to horizontal ipsilateral gaze nystagmus 2

Spontaneous nystagmus to the contralateral side 1

Absent or impaired convergence 5

Saccadic vertical pursuit 9

Gaze-evoked downbeat nystagmus 4

Impaired upward gaze 1

(N = 11)

Exotropia 4

Esotropia 3

Orthotropia 4

Table 3. One-and-a-half syndrome (N = 20;11): Associated ocular motility signs


Cranial Nerve Involvement

I 0

II 1


V 3



IX 3

XI 0


Horner’s Syndrome 1

Weakness or spasticity 6

Sensory deficits 7

Abnormally brisk or asymmetric reflexes 5

Extensor plantar responses 9

Incoordination 10

Table 4. One-and-a-half syndrome (N = 20): Associated neurologic signs


Patient 1. The one-and-a-half syndrome (A) Mild left INO looking right. (B) Esotropia OS (ipsilateral) in the primary position of gaze. (C) Horizontal conjugate gaze palsy attempting to look left. (D) Normal convergence.


Patient 2. Paralytic pontine exotropia. (A) Horizontal conjugate gaze paresis looking right. (B) Exotropia OS (contralateral) in the primary position of gaze. (C) Right INO looking left. (D) Right “peripheral-type” ipsilateral facial palsy. (E) Impaired convergence.

Patient 2. Paralytic Pontine Exotropia
  • Horizontal conjugate palsy looking right.
  • Exotropia OS contralateral in the primary position of gaze.
  • Right INO looking left
  • Right “peripheral-type” ipsilateral facial palsy
  • Impaired convergence
In paralytic pontine exotropia the exotropic eye shows:

Abduction nystagmus during attempts to move it laterally

Extreme slowness of adduction saccades when eye fixing to move it to the midline

Paralytic Pontine Exotropia attributed to:

Tonic contralateral deviation of the eyes

Implies acute ipsilateral PPRF lesion

Failure of ipsilateral eye to deviate medially explained by the INO