Amend text on summary email

We use summary notification emails to our service desk program to automatically raise incidents. To avoid one outage raising lots of incidents we use summary emails but I cant find any way of amending the summary email template to give a more meaningful (to our helpdesk system) first line instead of "[PRTG Network Monitor (%sitename)] 9 Summarized Notifications". I can amend regular emails on the notifications settings, but not summary ones.