Horner syndrome vs Third nerve palsy
How to tell Horner syndrome and Third nerve palsy apart when a board question makes them look alike.
Horner is small pupil plus mild ptosis; third nerve palsy is big pupil or down-and-out ophthalmoplegia plus ptosis.
Side by side
Horner syndrome causes ipsilateral ptosis, miosis, and anhidrosis from sympathetic pathway disruption, often with dilation lag and a small pupil more obvious in the dark.
Third nerve palsy causes ptosis with ophthalmoplegia, eye down and out, diplopia, and possible mydriasis from parasympathetic fiber involvement.
Interruption of oculosympathetic fibers removes sympathetic drive to the dilator pupillae, Muller muscle, and facial sweating pathways.
Oculomotor nerve dysfunction weakens most extraocular muscles, levator palpebrae, and sometimes parasympathetic pupil constriction.
Miosis, mild ptosis, anhidrosis, and anisocoria worse in the dark support Horner syndrome.
Dilated pupil, severe ptosis, impaired adduction/elevation/depression, and down-and-out eye support third nerve palsy.
The trap
Both can cause ptosis and anisocoria, so pupil size and eye movement decide localization.
Watch it explained
Test it nowThe repair move
For ptosis with anisocoria, decide whether the abnormal pupil is too small or too large.
Try a free question
CoreA patient has mild right ptosis, right miosis, facial anhidrosis, and anisocoria that is more obvious in the dark. Extraocular movements are intact. Which diagnosis is most likely?
Show answer and explanation
Answer: A. Horner syndrome. Ptosis, miosis, anhidrosis, and anisocoria worse in the dark support Horner syndrome.
Why the other choices are tempting
- B. Third nerve palsy causes ophthalmoplegia and often a large pupil when compressive.
- C. Adie pupil is a tonic dilated pupil, not ptosis/miosis/anhidrosis.
- D. Optic neuritis causes painful vision loss and afferent pupillary defect.
- E. Bell palsy causes peripheral facial weakness.
Flashcards
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What triad points to Horner syndrome?
Ptosis, miosis, and anhidrosis point to Horner syndrome.
When is Horner anisocoria most obvious?
Horner anisocoria is more obvious in the dark because the miotic pupil dilates poorly.
Quick answers
What is the fastest way to tell Horner syndrome from Third nerve palsy?
Horner is small pupil plus mild ptosis; third nerve palsy is big pupil or down-and-out ophthalmoplegia plus ptosis.
What is the key clue for Horner syndrome?
Horner syndrome causes ipsilateral ptosis, miosis, and anhidrosis from sympathetic pathway disruption, often with dilation lag and a small pupil more obvious in the dark.
What is the key clue for Third nerve palsy?
Third nerve palsy causes ptosis with ophthalmoplegia, eye down and out, diplopia, and possible mydriasis from parasympathetic fiber involvement.
What is the common trap with Horner syndrome vs Third nerve palsy?
Both can cause ptosis and anisocoria, so pupil size and eye movement decide localization.