The interesting thing about this form of amnesia is that new information is still able to go through the storage, recollection etc process while the older memories before the trauma cannot be recalled. This type of issue can be either temporary or permanent. Here is a very fascinating website that talks about how to diagnose retrograde amnesia: http://www.ehow.com/how_2121497_diagnose-retrograde-amnesia.html
Example: The patient was knocked unconscious for several minutes. She can’t recall the accident or anything from the preceding three days.