Approach to Diplopia: Monocular vs Binocular and the Three-Step Test

Last updated: November 2026

Double vision can come from a dry eye or an aneurysm. A structured history and four quick questions sort most patients in minutes and tell you who needs same-day imaging.

Parks-Bielschowsky three-step test flow chart for vertical diplopia — The three-step test for a vertical deviation (isolated superior oblique palsy example).

Four questions

Question	Why it matters
1. Does it go when one eye is covered?	No (persists with one eye open) = monocular: dry eye, cataract, irregular astigmatism, keratoconus, IOL edge, macular disease. Not neurological.
2. Are the images side by side or one above the other?	Horizontal: VI nerve palsy, INO, convergence problems, decompensated phoria. Vertical: IV nerve palsy, skew deviation, TED, orbital floor fracture, myasthenia.
3. Is it worse in one direction of gaze or at near or distance?	Points to the weak muscle; worse at distance suggests VI nerve palsy or divergence insufficiency.
4. Does it vary through the day?	Fatigable, variable diplopia with ptosis: myasthenia gravis.

The three-step test (vertical diplopia)

Step 1: which eye is higher in primary gaze?
Step 2: is the hypertropia worse in right or left gaze?
Step 3: is it worse with head tilt to the right or left (Bielschowsky)?
Example: right hypertropia, worse on left gaze and right head tilt = right superior oblique (IV nerve) palsy. Check for a skew deviation (upright-supine test) if the pattern does not fit.

Red flags: image the same day

A pupil-involving or painful III nerve palsy (aneurysm until proven otherwise: CT or MR angiography).
Multiple cranial nerve palsies, or palsy with other neurological signs.
Diplopia with headache and papilloedema.
Age over 50 with jaw claudication or scalp tenderness: giant cell arteritis.
A palsy in a young patient without vascular risk factors, or one that progresses or does not recover in 3 months.

Common causes and tests

Cause	Clues	Tests
Microvascular (ischaemic) palsy	Older, diabetic or hypertensive, pupil-sparing III, isolated IV or VI; recovers in 3 months	BP, sugar, ESR and CRP
Myasthenia gravis	Variable, fatigable, ptosis, Cogan lid twitch	Ice-pack test, acetylcholine receptor and MuSK antibodies, single-fibre EMG, CT chest
Thyroid eye disease	Restriction, lid retraction, proptosis; inferior rectus most often	Thyroid tests, orbital imaging
Decompensated phoria	Long-standing tendency, comitant, no other signs	Prism cover test, old photographs

Where residents slip

Imaging every monocular diplopia.
Missing myasthenia because the eye movements look normal at that moment.
Calling a pupil-involving III nerve palsy microvascular.
Not checking ESR and CRP in older patients.

Test yourself: flip cards

Key sources

Kanski JJ, Bowling B. Clinical Ophthalmology: A Systematic Approach (strabismus and neuro-ophthalmology chapters)
Parks MM. Isolated cyclovertical muscle palsy. Arch Ophthalmol. 1958;60:1027-1035
Tamhankar MA, et al. Isolated third, fourth and sixth cranial nerve palsies from presumed microvascular versus other causes. Ophthalmology. 2013;120:2264-2269

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Written by Dr. Dhaval Patel, MD (Ophthalmology, AIIMS New Delhi)

Consultant, Cataract & Refractive Surgery — among a small number of AIIMS-trained ophthalmologists practicing in Gujarat. Read full credentials & experience or view his 28 publications on ResearchGate.

📚 Also the author of OKAT Decoded and other high-yield ophthalmology exam-prep books used by residents and PG aspirants.

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