Among the clinical manifestations of bronchopulmonary cancer, one of them is “Horner’s syndrome”, which is often used as a multiple-choice question in exams, or appears in terminology explanation questions. Some students may not be able to read the Chinese name of the word when they see it, let alone get it right. I. Definition Lung cancer occurring in the apical part of the lung, in the process of intra-thoracic expansion, easily compresses the cervical sympathetic nerve, causing ptosis of the eyelid, pupil narrowing, inversion of the eyeball, and little or no sweating on the ipsilateral forehead and chest wall. This is Horner’s syndrome. So, does Horner’s syndrome only occur in lung cancer? No, we can infer from the above definition that Horner’s syndrome occurs in all diseases that tend to compress the cervical sympathetic nerve. Then, we can conclude its real definition: Horner syndrome, also known as cervical sympathetic nerve palsy, is a syndrome in which any section of the pathway from the cervical sympathetic nerve center to the eye receives compression and damage, causing pupillary constriction, normal response to light, sunken eyeballs on the diseased side, ptosis, and little or no sweating on the affected side. Etiology 1, cervical to mediastinal lesions: including cervical tumors, tuberculosis, lymphosarcoma, lymphadenitis, cervical crestal tumors, esophageal, thyroid and mediastinal tumors, cervical hematoma, etc.; pulmonary apical pleurisy, pneumothorax, lung cancer, etc. 2, crestal medullary lesions: crestal medullary hemorrhage, crestal cavitation, crestal tumor, cervical spondylosis, crestal myelitis, etc. 3.Cranial base lesions: post-trigeminal ganglion dissection, cranial trauma, etc. 4.Intracranial lesions: encephalitis, vascular lesions of the medulla oblongata and crista medullaris, etc. In summary, common causes are inflammation, trauma, tumor compression, surgery, hemorrhage and other factors. Clinical manifestations 1. Pupil narrowing: but the light reflex exists. 2. Eyelid ptosis: The eyelid can still open and close on its own. 3. Intraocular sagging: associated with paralysis of the posterior ophthalmic sphincter. 4.Little or no sweating on the ipsilateral face: disorder of glandular secretion, facial fever, flushing, and no sweating. 5.Decreased intraocular pressure: related to the influence of vascular nerve regulation.