Reasoning GP · Algorithm · 1-page summary
New double vision (diplopia) in adults
For: adults with new double vision, face to face or remote. Not for: children with new squint or diplopia → Urgent paediatric ophthalmology; with headache, vomiting, ataxia or neurological signs → immediate acute paediatrics (NG127).
Core rule: cover each eye in turn. Binocular diplopia is a nerve, muscle, orbit or brainstem problem: all new binocular diplopia needs prompt specialist assessment. Never assume a "microvascular" palsy in primary care.
0Stop — call 999 now
- Thunderclap headache (peaks within minutes), with or without diplopia → 999 / immediate ED (subarachnoid haemorrhage). No outpatient MRI; a normal examination does not exclude it.
- Sudden binocular diplopia with vertigo, ataxia, dysarthria, dysphagia, facial or limb weakness, sensory loss, severe imbalance or reduced consciousness → 999 stroke pathway. No FAST signs does not exclude posterior circulation stroke.
- Diplopia or ptosis with bulbar or respiratory symptoms (dysphagia, dysarthria, weak cough, secretions, breathlessness) → 999 (myasthenic crisis).
- Orbital infection with sepsis, reduced consciousness, airway compromise or rapid swelling; severe orbital trauma → 999. Do not delay for bloods.
1Triage — cover each eye, then the time course
| Finding | Meaning → next |
|---|---|
| Binocular | Goes when either eye is covered: misalignment → blocks 2–4 |
| Monocular | Persists with the affected eye alone, goes when that eye is covered; pinhole improves it → optical cause |
| Time course | Sudden: vascular or aneurysmal · progressive: compressive, thyroid eye · variable: myasthenia, decompensating squint |
2Screen — one row is enough
- 3rd nerve palsy with dilated or poorly reactive pupil, pain, severe headache, reduced consciousness or other deficit → Emergency hospital now (aneurysm). Any acute 3rd nerve palsy, even pupil-sparing → Same-day specialist.
- New Horner's with neck, face or orbital pain or neck trauma (dissection); papilloedema, severe or progressive headache, persistent vomiting → Emergency hospital now
- Painful ophthalmoplegia with fever or proptosis, chemosis, reduced vision, RAPD → Emergency hospital now
- Aged 50+, new diplopia or visual symptoms (GCA) → high-dose oral glucocorticoid now + Same-day ophthalmology/rheumatology or acute.
- Several cranial nerves over days–weeks, or areflexia and ataxia → Same-day neurology. After trauma → Same-day eye casualty.
3Assess — document the pupils
☐ Acuity each eye ± pinhole, fields ☐ Pupils: size in light and dark, reaction, RAPD ☐ Eye movements ("H"); fatigable ptosis on upgaze ☐ Lids, orbits ☐ Fundi ☐ Full neurological exam ☐ BP, pulse, temperature; temporal arteries if 50+
Targeted tests: GCA → ESR, CRP, FBC if no delay to steroids; microvascular? → glucose/HbA1c; thyroid eye → TSH, free T4; myasthenia → AChR antibodies. Not here: CT/MR angiography; GP MRI for isolated diplopia.
4Classify — only if the screen is negative
- Monocular (refractive, cataract, dry eye, cornea, lens, macula) → optometry; Urgent if sudden visual loss, painful red eye, trauma, proptosis, photophobia or neurological signs.
- 6th nerve — horizontal, worse distance and towards that side. 4th — vertical, worse looking down; head tilt.
- Myasthenia — variable, fatigable ptosis and diplopia, worse later in the day; pupils normal.
- Thyroid eye disease — gritty or bulging eyes, lid retraction, restricted upgaze; refer even if thyroid function is normal.
- Decompensating squint — full movements; diagnosis of exclusion.
- Uncertain → Same-day discussion; never label it microvascular.
5Treat today — only these
- Suspected GCA: high-dose oral glucocorticoid now per the local GCA pathway; IV only via specialists. Doses: NICE/SPC, BNF, local formulary.
- Adults, symptom relief: temporary patch or translucent tape while awaiting assessment; prisms via orthoptics. Children: no patching without specialist advice.
- Driving (statutory): must not drive and must tell DVLA; Group 1 resumes only once DVLA accepts it is controlled. No heights or machinery if vision is unsafe.
6Refer — urgent and routine routes
- Urgent eye / neuro-ophthalmology / neurology (local pathway): isolated binocular diplopia, no emergency features; 4th or 6th palsy; ocular myasthenia; thyroid eye disease.
- Progressive, sub-acute loss of central neurological function, no emergency features → consider urgent direct-access MRI brain (NG12); not for isolated diplopia.
- Optometrist: monocular, no red flags; stable old squint. Orthoptist: prisms.
7Never reassure by
- A normal pupil in a 3rd nerve palsy.
- Diabetes or hypertension ("it's microvascular").
- No FAST signs.
- A normal fundus (raised pressure not excluded).
- A normal rested exam or negative AChR antibodies (myasthenia).
- Watching a new palsy for 12 weeks without specialist contact.
8Safety-net — say it, then document it
999 “Call 999 for a sudden severe headache, your eyelid drops or your pupil looks bigger, weakness, slurred speech, face drooping, trouble swallowing or breathing, or collapse.” Same day “Any loss of vision, the double vision getting worse, new eye pain or bulging, or new numbness of the face — contact us today. Don't drive until DVLA says you can.”
Follow-up: check the specialist saw them within the pathway timeframe. Worse, painful, new ptosis, pupil change or another nerve → Same-day. Not improving or atypical → re-discuss; do not wait 3 months.
✓Safe to leave primary care only if — all five, documented
No block 0–2 row applies
Monocular or binocular, and pupils, recorded
Eye movements and full neuro exam recorded
Referral urgency set; review owner and date
DVLA and safety-net advice given
Summary of the full Reasoning GP diplopia algorithm, which holds the detail, examination, investigations and evidence. Sources: NICE NG228, NG128, NG127, NG12 (updated April 2026); RCOphth emergency eye care 2024; ABN myasthenia 2025; BSR GCA 2020; local GCA pathway; DVLA. Clinical reference — follow local pathways and check doses: NICE/SPC, BNF.