One and a Half Syndrome
HARVARD MEDICAL SCHOOLDEPARTMENT OF NEUROLOGY
MASSACHUSETTS GENERAL HOSPITAL
HARVARD MEDICAL SCHOOLDEPARTMENT OF NEUROLOGY
MASSACHUSETTS GENERAL HOSPITAL
HARVARD MEDICAL SCHOOLDEPARTMENT OF NEUROLOGY
MASSACHUSETTS GENERAL HOSPITAL
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 palsyAn internuclear ophthalmoplegia (INO) on contralateral gazeAt rest, the eyes are:
Orthophoric, or, in acute stageIpsilateral eye esotropic orContralateral 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
All the cells necessary for ipsilateral horizontal gaze:Motoneurons whose axons form the sixth nerve (VIN) to innervate the ipsilateral lateral rectus muscleInternuclear 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
Ocular Motor Possible Pathophysiologic Deficit SubstrateIpsilateral 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 BostonTotal 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
III 0
V 3
VII 4
VIII 2
IX 3
XI 0
XII 2
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
A. Horizontal conjugate palsy 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
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
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