High rise syndrome relates to a collection of injuries sustained by an animal who falls from a substantial height (usually greater than 2 stories) such as a balcony or window. Every year, the summer months see a rise in deaths and serious injuries as cats fall from unscreened balconies & windows which have been left accessible to cats by owners who open them to enjoy the weather.
Many owners don't believe their cat would jump from an open window and typically they won't deliberately leap from a great height. However, it is possible for a cat to become fixated on a bird or other object & leap without realising or accidentally slip and fall.
Common injuries incurred in high-rise syndrome:
- Thoracic trauma (chest, lungs, heart and diaphragm)
- Forelimb fractures
- Facial trauma (broken jaws, teeth, hard palate fractures)
- Rib fractures
- Pelvic fractures
- Vertebral fractures
- Ruptured bladder
- Abdominal wall hernia
Interestingly, it has been observed that the greatest injury results from falls from two to seven stories. After this, injuries either level off or decrease. It is believed that in cats falling greater than seven stories have reached their terminal velocity of fall, at which point the vestibular apparatus is no longer stimulated. Prior to reaching terminal velocity, continued vestibular stimulation is believed to result in limb rigidity & failure to maximally prepare for a horizontal landing. After reaching maximum velocity, it is believed that cats assume a less rigid, more horizontal posture. These cats are more prepared for landing & have the force of impact evenly distributed throughout the body, preventing an increase, and in some cases causing a decrease in the number of injuries. 
About 90% of cats affected by high-rise syndrome will survive , although this certainly isn't a reason to become lax when it comes to giving cats access to high ledges etc. Injuries can be serious, extremely painful & traumatic for the cat, not to mention expensive for you. And there is still a chance that the fall will kill your cat.
How can I avoid high-rise syndrome?
- Place screen on all windows & balconies which cats have access to.
- Regularly check that the screens are still fitted tightly & have no holes in which your cat can get through.
- If you have friends or tradesmen open, be extra diligent to ensure they don't accidentally let your cat out.
References:  The Feline Patient - Gary D. Norsworthy, Mitchell A. Crystal, Sharon K. Fooshee, Larry P. Tilley.