Why can person with down's syndrome not take kwell (lindane) travel sickness tablets?

Kwells (lindane) active ingred: Int is scopolamine, commonly prescribed in the united kingdom for motion sickness. For adults, a 300 µg tablet is swallowed. For children ages 4 to 10, a 150 mg tablet is sucked, chewed, or swallowed. Drugs.com states that people affected with Down syndrome may have an abnormal increase in pupillary dilation and acceleration of heart rate. Tthere a long list of other medical conditions in which this medication should be avoided.