Horner syndrome: Condition due to a disruption in the sympathetic nerve supply comprising of:

1. Partial ptosis: Drooping or falling of upper eyelid
2. Miosis: Constricted pupil
3. Facial anhidrosis (loss of sweating)

It can be of 3 types:

1. Central (1st-order) lesion: Lesions of the hypothalamus, brainstem and spinal cord such as stroke, demyelination or neoplasms
2. Preganglionic (2nd-order) lesion: Cervical rib, Pancoast's tumor, thyroid malignancies
3. Postganglionic (3rd-order) lesion: Lesions of the superior cervical ganglion, ICA (dissection, thrombosis, invasion by tumors), lesions of the sellar and parasellar regions