What are the causes of Horner's syndrome?

Causes of Horner's syndrome

A. T1 lesion by:

• Pancoast's tumour
• Trauma to brachial plexus
• Cervical rib

B. Neck (sympathetic) lesion: 

• Trauma
• Neck surgery
• Cervical sympathectomy
• Lymphoma
• Thyroid carcinoma

C. Brain stem lesion:

• Vascular (Lateral medullary syndrome)
• Multiple sclerosis

D. Cervical cord lesion (bilateral Horner's syndrome may occur)

• Syringomyelia
• Spinal cord tumour (Glioma and ependymoma)

E. Migraine (temporary Horner's syndrome may occur)

Additional:

The exact location of the site of the lesions can be found considering the presence or absence of anhydrosis in the affected areas of our body.  The picture below shows the levels of the lesion. 

[Source: reproduced from Suda Medica; page 113]