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Ocular motor cranial nerve palsies, INO and Horner syndrome
Sudden double vision usually comes from a 3rd, 4th or 6th nerve palsy, myasthenia or a brainstem lesion — and the critical question is whether the pupil is involved, because a 3rd nerve palsy with a dilated pupil is an aneurysm until proven otherwise.
Why it matters in the exam
Questions present a patient with double vision and a drooping lid, asking you to name the nerve and the cause: 3rd nerve with a dilated pupil — compression from a posterior communicating artery aneurysm, needing urgent CTA; 3rd nerve with a normal pupil in an older diabetic — a microvascular cause that recovers within about 3 months; 4th nerve after head injury with vertical double vision going down stairs; 6th nerve as a sign of raised intracranial pressure. INO (MLF lesion) in a young patient with multiple sclerosis and painful Horner syndrome suggesting carotid dissection are also tested. A classic distractor is reassuring a patient with a 3rd nerve palsy and a dilated pupil, or missing myasthenia, which can mimic any palsy but spares the pupil.
Key points
- A 3rd nerve palsy with a dilated pupil is an aneurysm until proven otherwise — urgent CTA.
- A 3rd nerve palsy with a spared pupil in an older diabetic is usually microvascular and recovers.
- 4th nerve: vertical double vision looking down; 6th nerve: horizontal, and sometimes a sign of raised pressure.
- INO: weak adduction with nystagmus in the opposite eye; in the young — multiple sclerosis.
- Painful Horner requires a search for carotid dissection.
Summary table
| Condition | Findings | Causes and what to do |
|---|---|---|
| 3rd nerve | Ptosis, eye "down and out"; dilated pupil in compressive lesions | Pupil involved — urgent CTA for aneurysm; pupil spared with complete palsy in an older diabetic — microvascular, follow up |
| 4th nerve | Vertical double vision, worse looking down and in (reading, going downstairs); head tilt to the opposite side | Head injury, microvascular, congenital |
| 6th nerve | Horizontal double vision, worse at distance and towards the affected side; weak abduction | Raised intracranial pressure (false localising sign), microvascular, Wernicke |
| INO (MLF lesion) | Weak adduction on the same side with nystagmus of the opposite abducting eye; convergence preserved | Young, bilateral — multiple sclerosis; older — stroke |
| Horner syndrome | Mild ptosis, small pupil, reduced sweating | Painful Horner — carotid dissection (urgent CTA); Pancoast tumour; lateral medullary stroke |
| Cavernous sinus | Palsies of nerves 3, 4 and 6 with reduced sensation in V1 and V2 | Thrombosis, tumour, fistula; urgent MRI |
A free exam question
בת 20, מתלוננת על צניחת עפעפיים דו- .צדדית בשנים האחרונות שמפריעה לה מבחינה קוסמטית אין שינוי בצניחת העפעפיים במהלך היום. שוללת ראיה כפולה, הפרעות בליעה, הפרעות לעיסה. בבדיקה: פטוזיס דו-צדדי ניכר; תגובות אישונים תקינות; חולשה סימטרית בעצימת העיניים; הגבלה סימטרית קשה בתנועות העיניים לכל הכיוונים. מבין הבאים, היכן ממוקמת הפגיעה? (the question as examined, in Hebrew)
- א. acetylcholine receptor
- ב. muscle protein
- ג. brainstem
- ד. mitochondria
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Sources
- AAN Continuum: Neuro-Ophthalmology, 2019 — Approach to diplopia and ocular motor palsies
- AAN Continuum: Neuro-Ophthalmology, 2019 — Pupillary disorders and Horner syndrome
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.