Miosis (or myosis, from Ancient Greek μύειν, mūein, "to close the eyes") is a term with various definitions, which generally include constriction of the pupil.
Aside from cancer general symptoms such as malaise, fever, weight loss and fatigue, Pancoast tumour can include a complete Horner's syndrome in severe cases: miosis (constriction of the pupils), anhidrosis (lack of sweating), ptosis (drooping of the eyelid) and enophthalmos (sunken eyeball).
Damage to the T1-T3 regions of the spinal cord is responsible for drooping of the eyelids (ptosis), constriction of the pupil (miosis), and sinking of the eyeball (Enophthalmos).