Horner’s Syndrome results from damage to the sympathetic fibres at any level along the sympathetic pathway to the eye. The lesion may affect the central neurones in the brain stem or spinal cord, eg mid-brain tumour, cervical disc disease, pre-ganglionic neurones (which synapse in the superior cervical ganglion), eg apical lung tumour, thoracic aortic aneurysm, or the post-ganglionic neurones to the eye, eg carotid artery surgery, cavernous sinus tumour. Clinical features include anisocoria with the affected pupil smaller (myosis) due to paralysis of the pupillary dilator muscle. The pupil asymmetry is greater in dim illumination as the abnormal pupil does not dilate fully, but both pupils have intact light and near reflexes. In addition there is typically a mild ptosis on the same side caused by a paresis of Müller’s muscle in the upper lid.
