Horner’s syndrome is a monocular loss of sympathetic innervation to the eye. This causes a loss of function in all of the ocular structures that are sympathetically controlled. The pupil is smaller, but the light reaction remains normal. In 90% of cases, there is a ptosis of the upper lid, caused by paresis of Muller’s smooth muscle within the palpebral levator muscle complex. At the same time there is a small elevation of the lower lid as well, since the lower lid retractors are also sympathetically innervated. The narrowing of the palpebral fissure causes the appearance described as “apparent enoph-thalmos.” If the site of damage to the sympathetic path lies proximal to the branching of the fibers that mediate sweating and temperature regulation in the face, these functions will also be impaired. The face will appear flushed on one side, with a sharp dividing line that runs precisely along the sagittal midline.
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