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What causes it?
Monocular diplopia persists when one eye is covered. It is caused by an optical aberration (cataract, uncorrected refractive error, presbyopia, keratopathy).

Binocular diplopia disappears when either eye is covered. It results from misalignment of the eyes, and may be caused by:

  • a central nervous system lesion
  • an ocular motor nerve lesion
  • a neuromuscular junction lesion
  • extraocular muscle lesion
Central nervous system lesion: a good example is internuclear ophthalmoplegia.

Ocular motor nerve lesion: examples are lesions of cranial nerve III, cranial nerve IV, and cranial nerve VI.

First examine a patient with third cranial nerve palsy. There are many causes of third nerve palsy, but the chief concern is aneurysm, which could rupture and kill the patient imminently.

Now examine the patient with fourth cranial nerve palsy.

Now examine the patient with sixth cranial nerve palsy. There are many causes of a sixth cranial nerve palsy including increased intracranial pressure, head injury, meningeal inflammation or neoplasm and nerve stroke.

Now examine a patient with a combined third, fourth and sixth cranial nerve palsy also called unilateral ophthalmoplegia.

Neuromuscular junction lesion: the best example is myasthenia gravis.

Extraocular muscle lesion: a good example is orbital myositis, a type of restrictive ophthalmopathy.

What to do?
New onset diplopia that disappears with covering either eye (binocular diplopia) is a call to action.

The most worrisome pattern is third cranial nerve palsy, which could be caused by aneurysm.

Chronic binocular diplopia and monocular diplopia should be referred non-urgently.

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