can include partial closure of the third eyelid, or nictitating membrane. Causes Horner's syndrome is usually acquired as a result of disease, but may also be congenital (inborn, associated with heterochromatic iris) or iatrogenic (caused by medical treatment). In rare cases, Horner's syndrome may be the result of repeated, minor head trauma, such as being hit with a soccer ball. Although most causes are relatively benign, Horner's syndrome may reflect serious disease in the neck or chest (such as a Pancoast tumor (tumor in the apex of the lung) or thyrocervical venous dilatation). Causes can be divided according to the presence and location of anhidrosis: Central (anhidrosis of face, arm and trunk) Syringomyelia Multiple sclerosis Encephalitis Brain tumors Lateral medullary syndrome Preganglionic (anhidrosis of face) Cervical rib traction on stellate ganglion Thyroid carcinoma Thyroidectomy Goiter Bronchogenic carcinoma of the superior fissure (Pancoast tumor) on apex of lung Klumpke paralysis Trauma - base of neck, usually blunt trauma, sometimes surgery. As a complication of tube thoracostomy Thoracic aortic aneurysm Postganglionic (no anhidrosis) Cluster headache - combination termed Horton's headache An episode of Horner's syndrome may occur during a migraine attack and be relieved afterwards Carotid artery dissection/carotid artery aneurysm/trauma Cavernous sinus thrombosis Middle ear infection Sympathectomy Nerve blocks, such as cervical plexus block, stellate ganglion or interscalene block Pathophysiology Horner syndrome is due to a deficiency of sympathetic activity. The site of lesion to the sympathetic outflow is on the ipsilateral side of the symptoms. The following are examples of conditions that cause the clinical appearance of Horner's syndrome: First-order neuron disorder: Central lesions that involve the hypothalamospinal tract (e.g. transection of the cervical spinal cord). Second-order neuron disorder: Preganglionic lesions (e.g. compression of the sympathetic chain by a lung tumor) that releases acetylcholine. Third-order neuron disorder: Postganglionic lesions at the level of the internal carotid artery (e.g. a tumor in the cavernous sinus or a carotid artery dissection) that releases norepinephrine. Partial Horner's syndrome: In case of a third-neuron disorder, anhidrosis is limited to the middle part of the forehead or can be absent, resulting in a partial Horner's syndrome. If patients have impaired sweating above the waist affecting only one side of the body, and they do not have clinically apparent Horner's syndrome, then their lesions are just below the stellate ganglion in the sympathetic chain. Diagnosis Three tests are useful in confirming the presence and severity of Horner