It sounds that you had some bruising of the subcutaneus tissue of the forehead with your accident >> the edema would be associated with this....
It sounds normal for the type of impact that you must have had to deploy an airbag.....
I would expect that it should resolve within a few days.
I hope this information helps! Let me know if you have further questions!! I will be happy to get back with you!
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Anthony Bray MD