When you do a remote code review with one person it is easiest to do it in real time. Just pick up the phone and talk about the code while both parties are looking at it. However, time constraints can make it advantageous to do a code review asynchronously. Performing code review via email is very common in the open source community. There is no reason a code review must be done in real time.
There is also a growing number of online code review tools. These tools will show you differences, let you make comments, and generally help you organize the information for your code review. Some of these systems run on servers which your company must install and other run on third party servers. If you are using a tool hosted on a third-party server, remember that your code will be out of your control and possibly available to the public.