However, in Horner's syndrome the lack of norepinephrine in the synaptic cleft causes mydriatic failure.ДЄ 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 increased mydriatic effect (due to hypersensitivity) on the affected side of Horner syndrome (the opposite effect to what the cocaine test would produce in the presence of Horner's) Three tests are useful in confirming the presence and severity of Horner syndrome:Д¬ocaine eye drops block the reuptake of norepinephrine resulting in the dilation of a normal pupil. This happens because a lack of sympathetic stimulation in childhood interferes with melanin pigmentation of the melanocytes in the superficial stroma of the iris In children, Horner's syndrome sometimes leads to heterochromia, a difference in eye color between the two eyes. The pupil's light reflex is maintained as this is controlled via the parasympathetic nervous system. Sometimes there is flushing on the affected side of the face due to dilation of blood vessels under the skin. Upside-down ptosis (slight elevation of the lower lid)ДЄnhidrosis (decreased sweating on the affected side of the face)Д®nophthalmos (the impression that the eye is sunk in) -because of loss of sympathetic innervation to the orbitalis muscleД«loodshot conjunctiva, depending on the site of lesion. Partial ptosis (drooping of the upper eyelid from loss of sympathetic innervation to the superior tarsal muscle, also known as MГјller's muscle) Signs that are found in patients on the affected side of the face include: Postganglionic involvement has primarily benign causes (ie, usually a vascular headache).Д¬auses of Horner syndrome may also be classified as involving first-order, second-order, or third-order neuron lesions. Preganglionic Horner syndrome indicates a serious underlying pathology and is associated with a high incidence of malignancy. Virtually all lesions producing postganglionic sympathetic dysfunction are located intracranially or intraorbitally because the superior cervical ganglion is near the skull. The common lesions that cause Horner syndrome interfere with preganglionic fibres as they course through the upper thorax. The interruption of the sympathetic fibres may occur centrally (i.e., between the hypothalamus and the fibres' point of exit from the spinal cord ) or peripherally (i.e., in cervical sympathetic chain, at the superior cervical ganglion, or along the carotid artery). Types of causes: Middle ear infection, multiple sclerosis, trauma, goiter Horner syndrome is acquired as a result of disease but may also be congenital (inborn) or iatrogenic (caused by medical treatment).ДЄlthough most causes are relatively benign, Horner syndrome may reflect serious disease in the neck or chest (such as a Pancoast tumour (tumour in the apex of the lung) or thyrocervical venous dilatation). Iris heterochromia also may occur in long-standing Horner syndrome.Д¬an be congenital, acquired, or purely hereditary (autosomal dominant). Iris heterochromia (with the affected eye being hypopigmented) is seen in congenital Horner's syndrome or Horner syndrome that occurs in children younger than 2 years. The pupils react normally to light and accommodation. With postganglionic lesions occurring after vasomotor and sudomotor fibres have branched off the sympathetic chain, anhidrosis is either absent or limited to an area above the ipsilateral brow. Lesions affecting second-order neurons may cause anhidrosis of the ipsilateral face. With central first-order neuron lesions, anhidrosis affects the ipsilateral side of the body. Miosis and dilation lag, where pupillary dilation after psychosensory stimuli is slower in the affected pupil than in the unaffected pupilДepending on the level of the lesion, impaired flushing and sweating may be found ipsilaterally. Slight elevation of the lower lid (upside-down ptosis), owing to denervation of the lower lid muscle (analogous to the denervation of the orbital muscle in the upper lid) Mild-to-moderate ptosis, owing to denervation of the sympathetically controlled orbital muscle May develop from lesions at any point along the sympathetic pathway.ДЄbnormalities found in all patients, regardless of the level of interruption, include the following:

Unequal pupil size with dysfunction