THE HELICOPTER WAS DISPATCHED AT NIGHT ON AN EMERG MED SVC (EMS) FLT FM A HOSPITAL AT CAPE GIRARDEAU, MO. PURPOSE OF THEFLT WAS TO TRANSPORT A PATIENT FM A HOSPITAL AT MARION, IL, TO CAPE GIRARDEAU. WX AT CAPE GIRARDEAU DETERIORATED AFTER THE ACFT DEPARTED. DRG VFR FLT BACK TO CAPE GIRARDEAU, THE PLT WAS UNABLE TO LND AT THE DESTN HOSPITAL. THE FLT NURSE COMMENTED THEY WOULD HAVE TO 'GET DOWN' AS THE PATIENT WAS GETTING WORSE. THE PLT ELECTED TO TRACK TO THE CAPE GIRARDEAUARPT BY USING THE ILS COURSE TO SEE IF THE ARPT LIGHTING WOULD HELP HIM TO 'LET DOWN.' HE STATED THAT WHILE TRACKING INBND & HOLDING 'ABOUT 300 FEET,' HE NOTICED THE HELICOPTER WAS LEFT OF COURSE. BEFORE THE ACDNT OCCURRED, HE HAD STARTED TO CORRECT BACK TO COURSE, BUT COULDN'T REMEMBER ANYTHING BYD THAT POINT. ALSO, HE RPRTD HE HAD LEFT THE STROBE LIGHT ON & EXPERIENCED FLICKER VERTIGO. SUBSEQUENTLY, THE HELICOPTER STRUCK A POWER LINE & CRASHED ON LVL TERRAIN IN AN OPEN FIELD. THE FLT NURSE, THERAPIST & PATIENT WERE FATALLY INJURED IN THE ACDNT.