---
title: What causes Horner syndrome symptoms?
description: "Horner syndrome is a combination of small pupil, drooping eyelid, and reduced sweating on one side of the face. ## Underlying problem It results from disruption of the sympathetic nerve pathway that runs from the brain to the eye and…"
url: "https://bluefinvision.com/knowledge-base/topics/symptoms-signs/what-causes-horner-syndrome-symptoms/"
updated: "2025-12-31"
category: "Symptoms & Signs"
---

# What causes Horner syndrome symptoms?

Horner syndrome is a combination of small pupil, drooping eyelid, and reduced sweating on one side of the face.

## Underlying problem

It results from disruption of the sympathetic nerve pathway that runs from the brain to the eye and face. Many different conditions can affect this long route.

- Neck or chest tumours compressing the nerve
- Carotid artery dissection tear in the vessel wall
- Brainstem stroke or demyelinating disease
- Trauma or surgery affecting the neck or upper chest

## Symptoms patients may notice

Changes are often subtle but important. Unequal pupils may be more obvious in dim light.

- Mild drooping of the upper lid with a slightly raised lower lid
- Smaller pupil on the affected side
- Different sweating pattern or facial flushing
- Associated neck pain or headache in some serious causes