Step I What causes the downward gaze when there is increased ICP?

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3rd ventricle enlarges and exerts pressure on the superior colliculus and the rostral interstitial nucleus deep to it.

The 3 causes of parinaud syndrome they've given are tumor (pinealoma), stroke, and hydrocephalus.

Would a colloid cyst of the 3rd ventricle cause parinuad? I would guess not, since the hydrocephalus is generally limited to the lateral ventricles (usually obstructs the foramen of monro).
 
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You get failure of upward gaze and hence tonically downward deviated eyes in both pinealoma and increased ICP in babies. The superior colliculus is the conjugate vertical gaze center. The rostral interstitial nucleus is the rostral most part of the MLF. The MLF connects CNs III, IV, VI, and has a role in vertical and horizontal conjugate gazes as well as the oculocephalic and vestibuloocular reflexes.
 
hydrocephalus in children may causes dilation of suprapineal recess of 3rd ventricle which pushes downward onto tectum of midbrain (the dorsal part of the midbrain) so that will result in impairment of vertical gaze especially upward gaze (so eye will deviate downward). also hydrocephalus cause increase ICP so there will be 6th nerve palsy.
so hydrocephalus in children produces sitting sun sign in which the eye are deviated inward(6th palsy) and downward(parinaud's syndrome).