Q:  Can Head Injury Result in Bipolar Disorder?

My son had two concussions at age 17 within 3 weeks resulting from 2 car accidents (neither was his fault). We have had many diagnosis for him and the most recent is bipolar 1. Can this be the result of this type of injury? I thought it was hereditary. Before now neurologist have said he had PTSD, anxiety attacks, depression, post-concussion syndrome, etc. Now we have added this new diagnosis. Does this sound reasonable to you?

Dear Ms. M' -- 
Unfortunately, yes, this is reasonable: cases of bipolar disorder associated with head injury have been reported.  Here is an example (you could dig up the reference if you wish; note that the abstract points out this is rare):

Dr. Phelps

Published May, 2001