Go directly to the spec which describes the case.
TS 3GPP 23.401, subclause 126.96.36.199 EUTRAN Initial Attach.
Or here is a copy of it http://www.lteandbeyond.com/2012/01/lte-attach-procedure.html
For an Emergency Attach the UE shall set both the Attach Type and the Request Type to "Emergency" and the IMSI shall be included if the UE does not have a valid GUTI or a valid P-TMSI available. The IMEI shall be included when the UE has no IMSI, no valid GUTI and no valid P-TMSI.