In children Horner syndrome sometimes leads to a difference in eye color between the two eyes (heterochromia).^[2] This happens because a lack of sympathetic stimulation in childhood interferes with melanin pigmentation of the melanocytes in the superficial stroma of the iris.

Causes

Horner syndrome is acquired as a result of pathology but may also be congenital (inborn) or iatrogenic (caused by medical treatment). Although most causes are relatively benign, Horner syndrome may reflect serious pathology in the neck or chest (such as a Pancoast tumor (tumor in the apex of the lung) or thyrocervical venous dilatation).

• Due to lesion or compression of one side of the cervical or thoracic sympathetic chain which generates symptoms on the ipsilateral (same side as lesion) side of the body.
• Lateral medullary syndrome
• Cluster headache - combination termed Horton's headache^[5]
• Trauma - base of neck, usually blunt trauma, sometimes surgery.
• Middle ear infection
• Tumors - often bronchogenic carcinoma of the superior fissure (Pancoast tumor) on apex of lung
• Aortic aneurysm, thoracic
• Neurofibromatosis type 1
• Goitre
• Dissecting aortic aneurysm
• Thyroid carcinoma
• Multiple sclerosis
• Cervical rib traction on stellate ganglion
• Carotid artery dissection
• Klumpke paralysis
• Cavernous sinus thrombosis
• Sympathectomy
• Syringomyelia
• Nerve blocks, such as cervical plexus block, stellate ganglion or interscalene block

Diagnosis

Three tests are useful in confirming the presence and severity of Horner syndrome:

1. Cocaine drop test - Cocaine eyedrops block the reuptake of norepinephrine resulting in the dilation of a normal pupil. Due to the lack of norepinephrine in the synaptic cleft, the pupil will fail to dilate in Horner's syndrome. A more recently introduced approach that is more dependable and obviates the difficulties in obtaining cocaine is to apply the alpha-agonist apraclonidine to both eyes and observe the reversal of miosis on the affected side of Horner syndrome (the opposite effect to cocaine).

1. Paredrine test:- This test helps to localize the cause of the miosis. If the 3rd order neuron (the last of 3 neurons in the pathway which ultimately discharges norepinephrine into the synaptic cleft) is intact, then the amphetamine causes neurotransmitter vesicle release, thus releasing norepinephrine into the synaptic cleft and resulting in robust mydriasis of the affected pupil. If the lesion itself is of the aforementioned 3rd order neuron, then the amphetamine will have no effect and the pupil remains constricted. There is no pharmacological test to differentiate between a 1st and 2nd order neuron lesion.
2. Dilation lag test

It is important to distinguish the ptosis caused by Horner's syndrome from the ptosis caused by a lesion to the oculomotor nerve. In the former, the ptosis occurs with a constricted pupil (due to a loss of sympathetics to the eye), whereas in the latter, the ptosis occurs with a dilated pupil (due to a loss of innervation to the sphincter pupillae). In an actual clinical setting, however, these two different ptoses are fairly easy to distinguish. In addition to the blown pupil in a CNIII (oculomotor nerve) lesion, this ptosis is much more severe, occasionally occluding the whole eye. The ptosis of Horner syndrome can be quite mild or barely noticeable.

When anisocoria occurs and the examiner is unsure whether the abnormal pupil is the constricted or dilated one, if a one-sided ptosis is present then the abnormally sized pupil can be presumed to be the one on the side of the ptosis.