Horner's syndrome is caused by:

(a) Nasopharyngeal carcinoma metastasis(Ref. Scott Brown, 8th ed., Vol 3; 98)Nasopharyngeal carcinoma metastasis or direct extension laterally to the parapharyngeal space post styloid compartment leads to the involvement of cervical sympathetic chain which is known as Horner's syndrome. Signs of Horner's syndrome:(Mnemonic: AMPLE)1. Anhidrosis (decreased sweating on the affected side of the face)2. Miosis (small pupils)3. Ptosis (drooping of the upper eyelid from loss of sympathetic innervation to the superior tarsal muscle, also known as Muller's muscle)4. Loss of ciliospinal reflex (i.e. no dilation of the ipsilateral pupil in response to pain stimuli applied to the neck).5. Enophthalmos (the impression that the eye is sunk in)