Sudden onset diplopia

• Patient has changed from having binocular single vision, seeing one image only, to a sudden onset manifest squint and diplopia (double vision)
• The double vision will be horizontal, vertical, or both
• Due to new weakness of extra-ocular muscles

Investigations

Cover test

• Used to detect a manifest squint (tropia) - squint present when the eyes are open and being used
• Horizontal double vision with esotropia/exotropia on cover test suggests LR or MR problem
• Vertical double vision with hypertropia/hypotropia on cover test suggests SO, SR, IR, IO problem

Ocular motility

• Assessed using a pen torch
• If the ocular muscles are normal there should be no visible sclera left in either eye in each position of gaze (visible sclera → weak muscle)

CN III (oculomotor nerve) palsy

• CN III supplies all extraocular muscles except superior oblique and lateral rectus

Aetiology

• Microvascular
• Tumour
• Aneurysm
• MS