Adult-onset Hirschsprungs ????

So I don't have IBS (this seemed like the most relevant forum though), but I do have severe chronic constipation.  I am a healthy 27 yr old female with the exception that I can't poo.  I am sick and tired of "helpful" people telling me that I need more fiber or to eliminate cheese or something.  I clearly have problems, we just don't have a complete diagnosis yet.

Anyway, I've tried Metamucil, Miralax and Amitiza and haven't had any success.  I had a colonoscopy in early Nov and that was normal except that there was still a lot of stool in the rectum.  Last week I did the Sitz Marker study.  It was supposed to be a 5-day study, but I am on day 8 now and still haven't gone.  It appears that I have normal transit because all the markers flowed on through to the rectum and then they just got stuck there.

Now my gastroenterologist told me today that he doesn't know what is wrong with me.  All he knows is that there is a problem with my rectum.  He mentioned trying botox in the rectum to relax the muscles, but he has never done the procedure before.  He is also thinking that maybe I have adult-onset Hirschsprungs disease.  I thought this only happened to children.  Has anyone heard of this?  I know I need to get a second opinion, but any advice is welcomed (well....except advice that recommends more fiber )

Thanks.

Yes, I've tried prune juice, Fleet saline enemas and soap suds enemas.  Prune juice does nothing.  While enemas are effective (most of the time), I don't want to have to depend on them to have a bowel movement.  I do use them about once a week though so that I don't get impacted.  I drink plenty of water and avoid caffeine and alcohol except on occasion.  I could probably use more cardio exercise in my life, but I am certainly not sedentary.  I love yoga and my instructor has me do poses and flows that are specfic to helping with constipation.

I'm in the process of making an appointment with a colon rectal surgeon for a consultation and then an Anorectal Manometry test.  I read about the procedure online.  Has anyone out there had this done?  Does it hurt?