A syndrome is a term given to a group of signs that have significance when they all occur together. It is important to realise that having a syndrome is not the same as having a diagnosis. A syndrome, however, often has a limited number of causes such that recognising a specific syndrome allows us to look more clearly for a small number of possible disease causes (ie the diagnosis).
Horner syndrome is a collection of signs resulting in a constricted, small pupil (miosis); drooping of the upper eyelid; a slightly sunken eyeball in the eye socket (enophthalmos) and protrusion of the third eyelid. The condition is caused by damage to sympathetic nervous system as it supplies the eye on the affected side of the head.