Double vision: monocular or binocular
- Clinical approach
Cover one eye. If the double image goes, the eyes are misaligned (nerve, muscle or brain); if it stays, the problem is inside that eye.
See it in 3D: Oculomotor nerve (III) · Trochlear nerve (IV) · Abducens nerve (VI) · Lateral rectus · Superior oblique · Medial rectus
What it is
Diplopia is seeing two images of one object. Monocular diplopia persists when the other eye is covered and comes from the optics of one eye: refractive error, cataract, dry eye or corneal irregularity. Binocular diplopia disappears when either eye is covered and means the eyes are misaligned: a cranial nerve palsy (III, IV, VI), myasthenia gravis, thyroid eye disease, an orbital fracture, or a brainstem lesion such as internuclear ophthalmoplegia.
Red flags
- A dilated pupil with ptosis and an eye that is down and out: compressive third nerve palsy (posterior communicating artery aneurysm).
- Headache, papilloedema or vomiting: raised intracranial pressure (sixth nerve palsy can be a false localising sign).
- More than one cranial nerve, or other neurological signs.
- Fluctuating diplopia and ptosis that worsen through the day: myasthenia gravis, which can affect breathing.
- Over 50 with headache or jaw claudication: giant cell arteritis.
- A child, or after facial trauma with restricted upgaze, nausea or a slow pulse.
Step-by-step approach
- Cover each eye in turn: monocular or binocular diplopia.
- For monocular diplopia, check whether a pinhole removes it: refractive or optical cause.
- Ask whether images are side by side or one above the other, and in which direction of gaze they separate most.
- Look for ptosis, pupil size and proptosis.
- Test eye movements in the nine positions of gaze and do a cover test.
- Ask about fatigability, thyroid disease, trauma, vascular risk factors and other neurological symptoms.
Causes of binocular diplopia
| Cause | Pattern | Key sign | Next step |
|---|---|---|---|
| Third nerve palsy, pupil involved | Mixed, with ptosis | Eye down and out, dilated pupil | Emergency CT or MR angiography |
| Third, fourth or sixth nerve palsy, microvascular | By the nerve; over 50 with diabetes or hypertension | Pupil spared | Risk-factor control; image if atypical or not recovering |
| Sixth nerve palsy | Horizontal, worse looking to the side of the lesion | Eye does not abduct | Look for papilloedema; image if present |
| Fourth nerve palsy | Vertical, worse looking down (stairs, reading) | Head tilted away from the affected side | Image if no trauma or vascular cause |
| Myasthenia gravis | Variable, worse late in the day | Fatigable ptosis, pupils normal | Antibodies, chest imaging for thymoma |
| Thyroid eye disease | Vertical or horizontal, worse in the morning | Lid retraction, proptosis, restricted upgaze | Thyroid tests, ophthalmology |
| Internuclear ophthalmoplegia | Horizontal | Adduction fails on side gaze, nystagmus of the other eye | MRI (multiple sclerosis or stroke) |
Keys to the differential
- The pupillomotor fibres run on the outside of the third nerve: compression dilates the pupil, while microvascular ischaemia usually spares it.
- Fourth nerve palsy is the commonest cause of vertical diplopia after head trauma.
- Myasthenia gravis never affects the pupil and can mimic any nerve palsy.
- In a child with a facial blow, restricted upgaze with nausea and a slow pulse suggests a trapped inferior rectus muscle (trapdoor fracture).
Next step / Initial management
- Monocular diplopia: routine optometry or ophthalmology (refraction, cataract, dry eye).
- Third nerve palsy with pupil involvement, pain or other signs: emergency CT or MR angiography for an aneurysm.
- Isolated pupil-sparing palsy in a person over 50 with vascular risk factors: control glucose and blood pressure and expect recovery within about three months; image if young, atypical or not recovering.
- Suspected myasthenia: acetylcholine receptor antibodies, chest CT for thymoma, neurology; urgent if swallowing or breathing are affected.
- Diplopia with papilloedema, headache or other neurological signs: urgent brain imaging.
Test yourself
Think of the answer, then tap the question to see it.
Diplopia persists when the other eye is covered. Where is the problem?
In the optics of the uncovered eye: refractive error, cataract, dry eye or an irregular cornea.
Why is a third nerve palsy with a dilated pupil an emergency?
Because it suggests compression of the nerve, typically by a posterior communicating artery aneurysm that can rupture.
Which clinical feature suggests myasthenia gravis as the cause of diplopia?
Variability and fatigue: diplopia and ptosis that worsen through the day, with normal pupils.
Elsewhere in DurgiPedia
- The visual pathway, pupillary reflexes and eye movements
- Eye exam: visual acuity, visual fields and eye movements
Connections
See it in 3D6
The structures of Durgical's 3D anatomy where it is found.
In Pharmacology1
Educational content; not a clinical guideline and no substitute for assessment by a health professional. For sudden visual loss, severe eye pain, a chemical burn or an eye injury, go to an emergency department at once.
Sources: Diplopia (StatPearls Publishing (NCBI Bookshelf), 2024); Neuroanatomy, Cranial Nerve 3 (Oculomotor) (StatPearls Publishing (NCBI Bookshelf), 2023); Abducens Nerve Palsy (StatPearls Publishing (NCBI Bookshelf), 2023); Myasthenia Gravis (StatPearls Publishing (NCBI Bookshelf), 2023).
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Representative model, for educational purposes only.