How is Horner’s syndrome diagnosed?

The diagnosis of Horner’s syndrome may be complex, since other disorders may cause similar symptoms. Once a definite diagnosis is made, further tests may be required to determine the underlying cause of the symptoms.

  • Physical examination: Your doctor will conduct an examination and note any visible signs. He or she will ask if you are experiencing other symptoms, such as pain.
  • Medical history: Your doctor will ask you about your past illnesses, injuries, and surgeries.
  • Eye examination: An ophthalmologist (eye specialist) but, more often, a neuro-ophthalmologist – an ophthalmologist who specializes in neurological conditions – can perform tests to measure the response of the pupils of the eye to certain medications. The doctor will insert eye drops containing a small amount of the drug to dilate the pupils. Failure of one pupil to dilate (open) may indicate that Horner’s syndrome is present. Further tests with other types of drugs can confirm the diagnosis.
  • Imaging tests: If symptoms indicate the presence of a tumor, lesion or structural abnormality, tests may be ordered to locate the growth and determine if it is cancerous or benign. Tests may include chest X-rays, magnetic resonance imaging (MRI), computed tomography (CT scan) or ultrasound.

Last reviewed by a Cleveland Clinic medical professional on 05/15/2018.


  • National Organization for Rare Disorders. Horner’s Syndrome. Accessed 5/17/2017.
  • American Association for Pediatric Ophthalmology and Strabismus. Anisocoria and Horner’s Syndrome. Accessed 5/17/2017.
  • I. Davagnanam, C.L. Fraser, K. Miszkiel, C.S. Daniel and G.T. Plant. Adult Horner’s syndrome: a combined clinical, pharmacological, and imaging algorithm. Eye (2013) 27, 291-298.
  • Sivashakthi Kanagalingam and Neil R. Miller. Horner Syndrome: clinical perspectives. Eye and Brain. April 10, 2015: 7 35-46.

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