Meltdown: Meltdowns typically look like severe tantrums but they are actually panic anxiety attacks if you compare them to the definition of these attacks as found in the DSM IV (The Diagnostical and Statistical Manual of the American Psychiatric Association). They may include aggression to others and to themselves in a variety of ways. In this state they are out of control of what their body is doing. They will not be able to take in information or learn while in this state so it is a waste of our time to try and teach them anything at this point.
Solution: The best treatment is to allow them to withdraw to a quiet place until they can calm themselves down. Each person will have their own calming techniques. It is important that we respect them and allow them to use them. Any input from anyone else at this point may only cause more distress.
Examples of various meltdowns (panic anxiety attacks) as found on the interent: