Where will a patient experience anhidrosis in postganglionic Horner's syndrome?

In postganglionic Horner's syndrome, anhidrosis occurs due to the disruption of sympathetic innervation, which affects sweat production in the area supplied by the affected sympathetic nerves. The specific location of anhidrosis is related to the fibers that are involved.

In this syndrome, the sweat gland innervation to the face is affected in a way that it leads to decreased sweating. The sympathetic fibers that supply the forehead and scalp come from the cervical ganglia. When the postganglionic fibers are disrupted, the forehead region, especially the medial forehead, loses its sympathetic supply. Therefore, anhidrosis will occur in this specific area.

The ipsilateral eyelid may experience ptosis due to the paralysis of the Müller muscle, but this area does not display anhidrosis directly, as the eyelid has more complex innervation. The contralateral forehead and the entire face are not affected by the disruption in the same manner as the ipsilateral area, given the unilateral nature of the syndrome.

Consequently, the correct identification of the anhidrosis in postganglionic Horner's syndrome is localized to the ipsilateral medial forehead only, highlighting the selective nature of the sympathetic innervation loss in this condition.