Vertical strabismus is termed hypotropia (downward turn of the eye) or hypertropia (upward turn of the eye). Therefore, hypotropia is a form of vertical strabismus where one eye deviates downwards in comparison to the other eye. The most common cause is weakness (called palsy) in the nerves in the brain that control eye movement. Some people are born with this eye weakness, but it can also happen later in life. Other common causes in adults include: Stroke.