Misophonia

Misophonia (meaning "hatred of sound") is a rarely diagnosed mental disorder. It is a condition where certain sounds can cause someone to be angry or enraged. The sounds could be as simple as someone chewing food with their mouth open. Other examples include a ballpoint pen clicking (repeatedly), tapping, typing and other common sounds. The disorder has only been identified and named in the last 20 years. According to one sufferer, "It’s like a fight-or-flight response: Your muscles get tense, you’re on edge, your heart races, and you feel the urge to flee". Misophonia is often misdiagnosed as one of several psychological disorders. Sufferers often keep away from trigger sounds. There is no cure but it can be treated.
People with misophonia have different amounts of tolerance for triggering sounds. They all, however, result in anger and irritability. Many also have trouble focusing on normal activities while exposed to triggering sounds.
The cause of misophonia is not know. Some studies say that it affects more than 5% of the population.