Table 2: Etiologies and features of different types of Horner syndrome.

Subtypes	Etiologies and features of the three types of Horner syndrome
First-order syndrome	Lesions of the sympathetic tracts in the brainstem or cervicothoracic spinal cord can produce a first-order Horner syndrome. The most common cause is a lateral medullary infarction, which produces a Horner syndrome as part of the Wallenberg syndrome. Typically the patient presents with vertigo and ataxia, which overshadow the Horner syndrome. Strokes, tumors, and demyelinating lesions affecting the sympathetic tracts in the hypothalamus, midbrain, pons, medulla, or cervicothoracic spinal cord are other potential causes of a central Horner syndrome.
Second-order syndrome	Trauma or surgery involving the spinal cord, thoracic outlet, or lung apex can cause a second-order Horner syndrome. Other cases are related to malignancy, which can be occult at the time of presentation with the Horner syndrome. Ipsilateral axillary or arm pain often accompanies the Horner syndrome in these cases. Lumbar epidural anesthesia can also produce a Horner syndrome due to pharmacologic disruption of the preganglionic neuron as it exits the spinal cord (as in the present case).
Third-order syndrome	Third-order Horner syndromes often indicate lesions of the internal carotid artery such as an arterial dissection, thrombosis, or cavernous sinus aneurysm. Carotid endarterectomy and carotid artery stenting can also produce a Horner syndrome. Other causes of postganglionic Horner syndrome include neck masses, otitis media, and pathology involving the cavernous sinus.