Yes, and here's something for you to go find out...
Sympathetic effects dilate the pupils whereas parasympathetic
effects constrict the pupils. The brain stem controls a majority of
nerves that juts out from it to serve most of the physiological
functions of the head and neck (tears, salivary glands, tongue
movements, head and neck movements, facial expression, heart and
lung control...)
Brain dead patients means that the brainstem is no longer
functioning...
One such nerve called the oculomotor nerve has parasympathetic
effects and it subserves the pupils and as it is, it constricts the
pupil especially during relaxation , resting and response to light.
But as death is imminent, the parasympathetic effects lose control
over the pupillary sphincters, hence leading to the opposite effect
of constriction, which is dilatation... 
