Largely I assume they can function at the same level as someone without an ABI and work "down" (for want of a better word) from there if necessary. In the sense that I may then simplify the words I use, the sentence length and structure, hand signals, body language, etc.
Its way less embarrassing to ask an ABI pt what they think of the carbon emissions tax and have them gurgle in response than it is to wave a telly tubby doll in their face and have them throw their PhD at you.
The carers will often be able to provide a reasonable picture of what level they communicate at.
I certainly don't speak to them in the same way I would a child, unless of course they are a child. Although occasionally I use similar techniques of distraction/discussion if appropriate. Listening to their own heart through a stethoscope, discussing football (I've yet to meet an intellectually disabled/ABI pt who doesn't passionately follow the football), having them assist in performing tasks (putting on ECG dots, taking temps). Teaching them to take their own BP if you've got the time.
If they're non-verbal/barely conscious then I just treat them as if they were fully conscious/competent. Explain procedures (BP, temp etc), drug side affects, apologise for bumping them/dropping something on them accidentally (I do it to everyone :unsure: ) just like you would to any other pt etc.
+1
IF the person is nervous and is non-verbal/deaf a rub on the back doesn't hurt.