Emily Titterington had not been to the toilet for three months when she suffered a cardiac arrest at home.
An inquest heard the condition was so bad that some body parts had been displaced, including her diaphragm.
The Coroner for Cornwall concluded the cause of death was "natural causes contributed by psycho-social factors".
Summing up, Dr Emma Carlyon said it could have been prevented.
"Something went wrong with the patient-parent-professional triangle which meant she did not get the care she needed," she said.
Paramedics described their "shock" at Emily's "grossly extended abdomen" when she collapsed at St Austell in February 2013 and said she was "vomiting faeces".
The inquest was told that Emily, who was home-educated, had a history of constipation that could be linked to her anxiety and mild autism.
Her sister Hannah Herbert raised concerns about the impact "parental anxieties" had on her, and said she had warned social services "something terrible might happen" the year before.
Her brother-in-law Brian Herbert told the inquest Emily's decision not to be examined or go to hospital would have been influenced by the fact her parents "could not cope" with that.
However Emily's GP, Dr Alistair James, said her mother Geraldine Titterington had tried to persuade her to be examined and he found no evidence of safeguarding issues.
Consultant paediatrician Dr Paul Davis said "self-neglect" played a part in her death but she should have been persuaded to go to hospital.
A serious case review by Cornwall Council's Safeguarding Children Board has made several recommendations including that agencies share concerns raised by a family member.
Dr Carlyon said she was satisfied "most of the recommendations have been actioned and embedded".

The Welsh have never been involved in a penalty shoot out, which could happen in Saturday's last 16 match in Paris.
Coleman admits it is impossible to replicate the anxiety of sudden death penalty shoot out.
''If it comes our way we will have to try and deal with it,'' said Coleman, who has a fully-fit squad after beating Russia 3-0 to top Group B.
Individual players have penalty experience, including top-scorer Gareth Bale who scored from the spot in Real Madrid's shoot-out win over Atletico Madrid in the Champions League final in May.
The current squad have been sharpening their spot kick skills in training, but Coleman recognises its limitations.
''I would say on three days out of five, the players naturally just gravitate after training to take penalties," he said.
''If it comes to that (on Saturday), again it is new territory for us.
''How do you recreate 50,000 people breathing down your neck;  or the eyes of the world upon you as you are stepping up to take the penalty and you need to put it in the back of the net. You cannot recreate that.''
Coleman reported no injury worries after clinching Group B top spot with Monday night's commanding win over Russia in Toulouse.
Eight Wales players started all three games in the opening stages of the competition and Coleman says he will not hesitate to alter the line up on Saturday, if required.
"'I would make changes if I think physically, they cannot do it," he added.
"But I would be surprised if any of them admit to me that they are a bit tired and may be they don't want to start.
"The guys who have not been on the grass or have not been starting are chomping at the bit. They are not very happy they have not been on the grass. That is a positive.
"But I can only pick 11 . . . I look at the opposition and I think if we can create a few problems with a different player in there, or in a different place I will do it.
"That does not mean the player who is being replaced in the last game has not done very well. It's just horses for courses."
A 16-year-old died from chronic constipation because either she, or she and her parents, resisted the care she needed, a coroner has said.
[NEXT_CONCEPT]
Wales have been practising penalties since arriving at Euro 2016, manager Chris Coleman says.