👁️ One and a Half Syndrome

• 🧠 Lesion: PPRF / Abducens nucleus + ipsilateral MLF
• 👁️ Ipsilateral eye: no horizontal movement
• 👁️ Contralateral eye: abducts only (half movement) + nystagmus
• 🔼🔽 Vertical movement: preserved
• ⚡ Symptoms: diplopia, oscillopsia, balance issues

Optional: neuron/microstructure emphasis

• 🧠 Central pathway affected: MLF (internuclear fibers), PPRF (horizontal gaze center)
• ⚗️ Functional effect: horizontal gaze palsy + INO
• 👁️ Clinical hallmark: “one eye frozen, other eye half-moves”

Definition:

• One eye cannot move horizontally at all.
• The other eye can only move halfway horizontally.

🔹 Key Features

• Horizontal gaze palsy to the side of the lesion → one eye frozen
• Internuclear ophthalmoplegia (INO) on attempted gaze to opposite side:
  • Adducting eye (same side as lesion) fails to cross midline
  • Abducting eye (contralateral) shows nystagmus
• Vertical eye movements are preserved
• May cause diplopia, oscillopsia, balance problems

🔹 Pathophysiology

• MLF lesion: → internuclear ophthalmoplegia (INO)
• PPRF or abducens nucleus lesion: → horizontal gaze palsy
• Combination lesion: PPRF/abducens nucleus + MLF → One and a Half Syndrome

🔹 Summary Table

✅ Mnemonic for Recall:
“One eye frozen, other eye half-moves → think PPRF + MLF lesion”