Complex sentence: A common sign is a constricted pupil (Horner's syndrome) on the ipsilateral side due to loss of sympathetic innervation to the eye, caused by damage to the sympathetic trunk in the neck.
Split into simpler sentences: A common sign is a constricted pupil (Horner's syndrome) on the ipsilateral side (side where damage has occurred). This is due to loss of sympathetic innervation to the eye, caused by damage to the sympathetic trunk in the neck.