A 45-year-old man develops acute right sixth nerve palsy of suspected microvascular origin. Botulinum toxin is injected into the right medial rectus. What is the purpose of this injection?

• A To directly strengthen the paralysed lateral rectus
• B To prevent contracture of the antagonist medial rectus while the lateral rectus recovers ✓
• C To correct the underlying microvascular infarct
• D To paralyse the yoke left medial rectus and reduce diplopia

Explanation

In acute lateral rectus palsy, the unopposed medial rectus progressively contracts, producing a secondary medial rectus overaction that complicates later surgery. Injecting botulinum toxin into the ipsilateral medial rectus chemodenervates it, prevents contracture, and preserves the option of a simpler lateral rectus recession or transposition later. The toxin has no effect on the paretic lateral rectus itself, does not treat the vascular cause, and the yoke muscle is not involved in the rationale.

Reference: Kanski's Clinical Ophthalmology, 9th ed.

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Written and medically reviewed by the StethoPrep medical team.