GENERAL NEUROLOGY

One and a half syndrome

David Goldemund M.D.
Updated on 21/08/2024, published on 21/08/2024

One and a half syndrome is a rare neurological disorder characterized by a unique combination of horizontal gaze palsy and internuclear ophthalmoplegia. This complex oculomotor  disorder was first described by Fisher in 1967

Etiopathogenesis

Anatomic substrate of horizontal gaze 

  • a trio of brainstem structures working in perfect harmony are involved in horizontal gaze
    • paramedian pontine reticular formation (PPRF), also known as horizontal gaze center, is located in the pons and initiates and controls horizontal saccades (under control of cortical centers)
    • the abducens nucleus (CN VI) controls the lateral rectus muscle, which abducts the ipsilateral eye (moves it outward)
    • medial longitudinal fasciculus (MLF) is a white matter tract that coordinates conjugate eye movements between both eyes (it connects the abducens nucleus with the contralateral oculomotor nucleus)
  • the brainstem center is directed by the cortex
    • frontal eye fields (FEF) is located in the frontal lobe and is involved in voluntary saccades and visual attention
    • parietal eye fields (PEF) help with spatial orientation and reflexive saccades
    • supplementary eye fields (SEF) contribute to the planning and execution of complex eye movement sequences
  • the magic happens when all these structures communicate. Here’s a simplified breakdown of the main pathways:
    • cortex → brainstem: signals from the FEF, PEF, and SEF travel to the PPRF, initiating horizontal eye movements
    • PPRF → abducens nucleus: the PPRF activates the abducens nucleus, which stimulates the lateral rectus muscle of the ipsilateral eye
    • PPRF → contralateral CN III: simultaneously, signals cross to the opposite side to activate the medial rectus muscle of the contralateral eye via the MLF

Pathophysiology

  • isolated  lesion in the MLF leads to internuclear ophthalmoplegia (INO)
  • isolated damage to the PPRF results in the inability to generate horizontal eye movements (horizontal gaze palsy)
  • one and a half syndrome results from a lesion in the PPRF and/or the abducens nucleus, combined with interruption of the internuclear fibers of the MLF
  • most common causes:
    • stroke (ischemic or hemorrhagic)
    • multiple sclerosis
    • brainstem tumors
    • infections (e.g., tuberculosis, neurocysticercosis)

Clinical Presentation

  • horizontal gaze palsy to the side of the lesion (one)
  • internuclear ophthalmoplegia on attempted gaze to the opposite side
    • the adducting eye (same side as the lesion) fails to move past the midline (half)
    • the abducting eye (contralateral to the lesion) shows nystagmus
  • vertical eye movements are preserved
  • diplopia, oscillopsia, and balance problems may occur
One and a half syndrome (lesion is in the left pons)

Diagnostic evaluation

  • detailed neurological examination focused on eye movements and other brainstem functions
    • evaluate nystagmus and skew deviation, convergence, and vertical eye movements
  • MRI (Magnetic Resonance Imaging) incl. DWI is the gold standard for the detection of lesions
  • DTI (diffusion tensor imaging) evaluates white matter tract integrity, particularly of the MLF

Management

  • treatment of the underlying cause (thrombolysis, surgery, radiotherapy, etc.)
  • symptomatic therapy
    • prism glasses to alleviate diplopia
    • vestibular rehabilitation exercises
    • occupational therapy to improve daily functioning

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One and a half syndrome
link: https://www.stroke-manual.com/one-and-a-half-syndrome/