Systematic reviews need to be evaluated on a periodic basis to determine if they need to be updated. If it's likely that new studies have come out and the topic is still of interest to clinicians and decision makers then it should be updated.
It can be challenging to search the databases and retrieve only the new studies that have been added since the last time you searched. Each database uses different date fields and some databases are not able to restrict a search to a specific month and year. Another challenge is that sometimes older articles are added to the databases so if you only search the new time period you might miss articles that were retrospectively added.
There are two solutions.
1. Run new searches on the entire time period, even the years you've already looked at. De-duplicate your new search results against the previous search results which you have hopefully saved. This is relatively easy to do in EndNote but can be cumbersome and slow if you have thousands of results.
2 Run new searches not by publication date but by the date they were entered into the databases. This will retrieve articles that weren't in the databases the last time you searched no matter what their publication date is. Unfortunately, this isn't possible in all databases but it is in most. Here are the methods to do it:
"Living" systematic reviews are updated on a periodic basis, often monthly. Any new evidence is immediately incorporated into the published review. Needless to say, this is very time consuming so this type of updating is usually only used for topics that are of high importance to decision makers, on which new evidence is frequently published. More information and many examples are available at the Cochrane community.