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Meconium ileus LOCATION question

Discussion in 'Step I' started by Phloston, Jun 19, 2012.

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  1. Phloston

    Phloston SDN Lifetime Donor Lifetime Donor Gold Donor

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    From GT:

    "A 52-hour-old neonate has developed abdominal distension and bilious vomiting. She has also failed to pass meconium per rectum. She is diagnosed with meconium ileus. Where is the obstruction most likely to be found?

    A. Distal jejunum
    B. Mid-ileum
    C. Ascending colon
    D. Sigmoid colon
    E. Proximal duodenum

    Answer Explanation

    The correct answer is B.

    Meconium ileus usually is the result of inspissated meconium plugs in the mid-ileum. The three cardinal signs of intestinal obstruction are abdominal distention, bilious vomiting, and failure to pass meconium within 48 hours."

    ---


    When I read this question, I initially thought CF or Hirschsprung's, but then decided Hirschsprung's based on the bilious vomiting and abdominal distension. I would have assumed that the obstruction would be as distal as possible. Notice that the explanation doesn't give a diagnosis.

    Could someone please share thoughts here?

    Cheers,
  2. MrBeauregard

    MrBeauregard Soon-to-be PGY-1

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    The way I understand it is that meconium ileus is due to thick, inspissated meconium (just as the GT explanation states) plugging up the mid- and distal-ileum. The meconium lodging in this portion of the small bowel results in ileus, or a disruption in the normal peristaltic/propulsive activity of the GI tract. The infant presents just as the question stated.

    Hirschsprung's, or congenital aganglionosis, is due to failure of the neural crest cells to migrate into the rectum. As a result, the rectum loses the ability to relax and is thus chronically contracted, resulting in the characteristic distal narrowing with proximal megacolon. As I understand it, this isn't meconium ileus, because the meconium isn't causing the bowel to lose propulsive activity, but rather the inability of the colon to adequately propel bowel contents is resulting in the back-up of bowel contents. In the infant, this content happens to be meconium.

    So, meconium ileus is due to meconium plugging the distal small bowel and causing obstruction. Hirscsprung's is due to the absence of both the submucosal and myenteric plexus, leading to intestinal obstruction and failure of the passage of stool, which is meconium in the infant.

    If anyone sees this as being completely incorrect, feel free to make corrections. I hope this offered some explanation.
  3. loveoforganic

    loveoforganic -Account Deactivated-

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    I don't think anything you said is incorrect, but I don't think that's addressing what Phloston had the main issue with - making the diagnosis of meconium ileus (despite the title to the contrary, so correct me if I'm wrong).

    Is bilious vomiting a feature of Hirschsprung's? I would think it theoretically could be, but in actually it will never present that way. The constipation would present before that ever presented due to the distal nature of the impaction
    Last edited: Jun 19, 2012
  4. Phloston

    Phloston SDN Lifetime Donor Lifetime Donor Gold Donor

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    I understand the Hirschsprung's mechanism, but both CF and Hirschsprung's would present with failure to pass meconium. I've seen a practice Q explanation talk about meconium ileus associated with Hirschsprung's disease.

    My guess is that GT is once again just vague. In this particular question's case, I believe that the implication is that meconium ileus secondary to cystic fibrosis is associated with inspissated contents of the mid-ileum. The reason I'm saying that is because GT had followed up with a question asking about the most appropriate test to follow (which was CFTR mutation analysis), suggesting that this location is specific to CF, not Hirschsprung's. In the latter, the inspissated contents would most certainly be proximal-peri-rectal, not mid-ileal.

    Loveoforganic, I've seen bilious vomiting in a practice question with Hirschsprung's on USMLE Rx. At first I had thought the same as you, that this wouldn't particularly occur with Hirschsprung's, but it does. If the neonate has bilious vomiting, constipation and distended abdomen --> Hirschsprung's, not duodenal atresia or annular pancreas, since these latter two don't present with constipation or abdominal distension to the same extent as the former, despite also presenting with bilious vomiting.
  5. loveoforganic

    loveoforganic -Account Deactivated-

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    Right you are

    http://www.sciencedirect.com/science/article/pii/S0022346884802559

    Seems to actually be fairly common (35%, although this is a small sample size).

    I would agree then that you can't really make an answer here

    If you come across stuff you're pretty sure is too vague to answer correctly or is actually incorrect, GT is very responsive to feedback on their questions if you have the motivation to correct them (there's a feedback button on the top of the screen you can click when you're quizzing)
  6. MrBeauregard

    MrBeauregard Soon-to-be PGY-1

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    I guess what I was trying to get at is that because the question asks for the most likely site of the obstruction after giving the diagnosis of meconium ileus, it isn't going to be Hirschsprung's, because the meconium is causing the ileus, not the aganglionic segment of the rectum. Does that make more sense?

    I also saw a practice question that spoke about meconium ileus occuring in the distal ileum at the ileocecal junction, so I don't necessarily know that the mid-ileum is the specific location, but instead the small bowel is the site of obstruction for meconium ileus as opposed to rectum for Hirschsprung's.
  7. Phloston

    Phloston SDN Lifetime Donor Lifetime Donor Gold Donor

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    Maybe we have different definitions of meconium ileus (even though this is medical, so we shouldn't).

    Meconium ileus, as far as I'm aware, is merely the lack of movement of meconium. Meconium never causes ileus. There's always a primary aetiology. In CF, it's the abnormal pancreatic secretions; in Hirschsprung's, it's the aganglionosis. In the case of CF, MI should be ileal (and as you've said, maybe not just mid-, but also distal-); in Hirschsprung's, it should be pre-peri-rectal.

    Thanks for the input btw.
  8. MrBeauregard

    MrBeauregard Soon-to-be PGY-1

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    eMedicine makes the point that meconium ileus and Hirschsprung's are completely separate diagnoses, and therefore distinguishing between the two is paramount.

    Also, from Nelson, 18th ed.:
    "Meconium ileus occurs primarily in newborn infants with cystic fibrosis. Approximately 10% of infants with cystic fibrosis develop meconium ileus; 80–90% of infants presenting with meconium ileus have cystic fibrosis. In simple meconium ileus, the last 20–30 cm of ileum is collapsed and filled with pellets of pale-colored stool, above which a dilated loop of varying length appears obstructed by meconium of the consistency of thick syrup or glue. Peristalsis fails to propel this very viscid material forward, and it becomes impacted in the ileum." That's probably a little bit better definition than GT and it's from a respected pediatrics text.

    "Contrast studies of the upper and lower bowel can delineate the level of obstruction and differentiate atresia from meconium ileus, meconium plug, and Hirschsprung disease."

    "In neonates, Hirschsprung disease must be differentiated from meconium plug syndrome, meconium ileus, and intestinal atresia."

    "More than 90% of full-term newborn infants pass meconium within the 1st 24 hr. The possibility of intestinal obstruction should be considered in any infant who does not pass meconium by 24–36 hr. Intestinal atresia, stricture, or stenosis; Hirschsprung disease; milk bolus obstruction; meconium ileus; or meconium plugs may manifest as constipation or, more often, obstipation."

    All are saying the same thing: the two must be differentiated. The GT question gave you the diagnosis of meconium ileus, and therefore, by definition, the obstruction is in the mid- to distal ileum.
    Last edited: Jun 19, 2012
  9. Phloston

    Phloston SDN Lifetime Donor Lifetime Donor Gold Donor

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    If you have a source saying that meconium ileus doesn't occur in Hirschsprung's, that would be phenomenal.
  10. MrBeauregard

    MrBeauregard Soon-to-be PGY-1

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    Well, I just searched pretty exhaustively in my copies of Cecil, Nelson, and Robbin's, and none of them actually stated meconium ileus occurs with Hirschsprung's; it was always mentioned in the context of CF. The texts will instead say that an infant with Hirschsprung's "fails to pass meconium within the first 24 hours", but it never says, "patient's with Hirschsprung's have meconium ileus". I've never actually noticed this distinction before which is why I'm searching through my texts.

    This is about the most direct quote I can give you, and it's from the chapter on Hirschsprung's, not CF:
    "Hirschsprung disease in older patients must be distinguished from other causes of abdominal distention and chronic constipation ( Table 329-1 and Fig. 329-1 ). The history often reveals increasing difficulty with the passage of stools, starting in the 1st few weeks of life. A large fecal mass is palpable in the left lower abdomen; the rectum is usually empty of feces. The stools, when passed, may consist of small pellets, be ribbon-like, or have a fluid consistency; the large stools and fecal soiling of patients with functional constipation are absent. In neonates, Hirschsprung disease must be differentiated from meconium plug syndrome, meconium ileus, and intestinal atresia. In older patients, the Currarino triad must be considered (anorectal malformations—ectopic, anus, rectal stenosis; sacral bone anomalies—hypoplasia, poor segmentation; or presacral masses—anterior meningoceles, teratoma, cysts)."
  11. Phloston

    Phloston SDN Lifetime Donor Lifetime Donor Gold Donor

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    Thanks for posting that.
  12. Go Ducks

    Go Ducks

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    I think that I can clear things up here. It seems that you are thinking "meconium ileus" = failure to pass meconium + having ileus. This is NOT the case. "Meconium ileus" = thick meconium stuck in the ileum. Stupid name, huh? It is more specific than you'd expect given just the words in the name.

    Your practice question was simply to see if you know the definition of meconium ileus, because if you did, then you'd know that it MUST be in the ileum. You don't need to know the diagnosis in order to know the answer to the question.

    You can get meconium ileus-type presentation from Hirshprung's if the area of deinervation happens to be in the ileum, but that's rare. Hirshprung's is usually in the rectum. Also, usually the meconium is not thick enough to qualify as meconium ileus anyway. That's why when people say "meconium ileus," they are almost always talking about CF.
    Last edited: Jun 20, 2012
  13. AngryBird69

    AngryBird69

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    Phloston "inspissated" is the key here to cut your Dx down to CF. Even tho the term was not in the question stem, you should just make this word synonymous with Mec il and CF in your mind. It means dehydrated and as you know thick dehyrdated mucous is the reason why CF does a number on your resp and GI tract . The ileum is the location of the most mucous secretion in the gut and thus where the blockage will occur due to massive dehyrdated mucous build up.

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