Final answer:
Horner syndrome is due to a disruption of sympathetic innervation of the head, leading to symptoms like ptosis, miosis, and anhidrosis. A thoracic cavity tumor can interfere with these functions by disrupting the sympathetic chain path that passes through this region before ascending to the head.
Step-by-step explanation:
Horner syndrome results from an interruption of the sympathetic innervation of the head, which can lead to a distinct set of symptoms known as ptosis (drooping eyelid), miosis (constricted pupil), and anhidrosis (decreased sweating on the affected side of the face). A tumor in the thoracic cavity, like a Pancoast tumor, may impact these autonomic functions because the sympathetic pathway that innervates the face and eyes passes through the apex of the lung in the thoracic cavity before ascending to the head. Therefore, a growth in this area can disrupt the normal function of the sympathetic nervous system, leading to Horner syndrome.
The accessory nerve, which controls the sternocleidomastoid and trapezius muscles, is not involved in Horner syndrome. Instead, Horner syndrome is related to the disruption of the sympathetic chain that follows a different pathway, which includes a route that goes over the apex of the lung and ascends to the carotid plexus, then eventually innervates the eye and facial structures. While the accessory nerve is responsible for movements such as head flexion and extension, it does not impact the sympathetic functions affected in Horner syndrome.