What are the genetic changes related to Horner syndrome ?
Although congenital Horner syndrome can be passed down in families, no associated genes have been identified. Horner syndrome that appears after the newborn period (acquired Horner syndrome) and most cases of congenital Horner syndrome result from damage to nerves called the cervical sympathetics. These nerves belong to the part of the nervous system that controls involuntary functions (the autonomic nervous system). Within the autonomic nervous system, the nerves are part of a subdivision called the sympathetic nervous system. The cervical sympathetic nerves control several functions in the eye and face such as dilation of the pupil and sweating. Problems with the function of these nerves cause the signs and symptoms of Horner syndrome. Horner syndrome that occurs very early in life can lead to iris heterochromia because the development of the pigmentation (coloring) of the iris is under the control of the cervical sympathetic nerves.  Damage to the cervical sympathetic nerves can be caused by a direct injury to the nerves themselves, which can result from trauma that might occur during a difficult birth, surgery, or accidental injury. The nerves related to Horner syndrome can also be damaged by a benign or cancerous tumor, for example a childhood cancer of the nerve tissues called a neuroblastoma.  Horner syndrome can also be caused by problems with the artery that supplies blood to the head and neck (the carotid artery) on the affected side, resulting in loss of blood flow to the nerves. Some individuals with congenital Horner syndrome have a lack of development (agenesis) of the carotid artery. Tearing of the layers of the carotid artery wall (carotid artery dissection) can also lead to Horner syndrome.  The signs and symptoms of Horner syndrome can also occur during a migraine headache. When the headache is gone, the signs and symptoms of Horner syndrome usually also go away.  Some people with Horner syndrome have neither a known problem that would lead to nerve damage nor any history of the disorder in their family. These cases are referred to as idiopathic Horner syndrome.