You have been asked to see Travis, a baby who was born yesterday at 38 weeks gestation. Mum reports that he has been feeding well, but his abdomen has become more swollen.
When you check his nappy, he doesn’t have any inguinal hernias, but his bottom just doesn’t look right.
Bottoms are very important! Life becomes much harder if your bottom doesn’t work properly. But what makes an anus?
For an excellent anus, you have to have an adequate hole within the muscular ring and for the entire complex to be located in a good position.
How do I tell if Travis has a normal bottom?
Getting a good view, with adequate light and a good surface to examine the baby on are important for success. Clean all the poo away and lift the baby’s legs right up to see where the meconium comes from.
Beware of fistulae (check out the photos below). Not all poo comes from an anus. Sometimes meconium can come out in the urine (if there is a fistula to the urinary tract), the posterior fourchette (rectovestibular fistula), the perineum or scrotum (perineal fistula), or, less commonly, the vagina (rectovaginal fistula).
If there is a lot of meconium and the baby has chubby buttocks, it can be easy to miss the fact that the hole the meconium is coming from is not the anus, particularly in poor light or if you are busy and in a rush.
Make sure you ‘see the sun’. This pearl of wisdom came from a neonatal consultant. When babies’ anorectal malformations are missed, it is often, in retrospect, an obvious abnormality. You must visualise both the creasing of the anal sphincter and the anal opening.
Main types of anorectal malformations
Anorectal malformations are classified by where the bowel connects to – there is usually a connection between the bowel and the skin or urogenital tract but not always.
Higher anorectal malformations are associated with more long-term problems with bowel control, and the surgical repair is more challenging.
Sometimes, the bowel lacks an external connection (an imperforate anus without a fistula). This is more common in people with trisomy 21.
Basic classification
Milder forms of anorectal malformation
Mucosal bridge/Perineal groove: Happily, this is mostly a cosmetic issue that does not require surgical intervention. As long as the baby is decompressing well (no abdominal distension, feeding well), this may well just need outpatient surgical review.
Anterior ectopic anus: The whole anal complex (hole surrounded completely by muscle) is just more anterior than it should be (closer to the vagina or scrotum). Children with an anterior ectopic anus are more likely to get constipated as their continence mechanisms ‘aren’t right’.
Again, as long as they are decompressing well (no abdominal distension, feeding well), then they may just need outpatient surgical review. This should, ideally, occur within a month or so, as when the baby starts solids, the poo can become thicker, which can cause issues if there is a true anorectal malformation.
Stenotic anus: The anal canal is tight and narrow. This doesn’t let the baby or child decompress well enough (they become distended or constipated and may have bilious vomiting).
It can be associated with sacral and pre-sacral abnormalities so imaging of the spine / pelvis is important.
This condition can also present later when the baby starts solids. The change in diet can result in thicker poo which can block the narrow opening, so it is always worth looking at the anus of children who present with constipation.
What do I do about Travis’ bottom?
This depends on the type of problem Travis has. Generally, it falls into one of 3 areas: true anorectal malformation and obstruction, a well baby with an odd-looking bottom who is pooing well, or something that is just a bit uncertain.
If the baby has an anorectal malformation and is obstructed – they need urgent management:
Examine them for associated anomalies e.g. VACTERL.
Manage their obstruction by placing them nil by mouth, inserting an NG tube, and setting it to free drainage with 4-hourly aspirates and ml:ml replacement of NG losses.
Start maintenance fluids +/- resuscitation.
Perform an abdominal X-ray.
If the baby is male and you can’t see an anal opening, start trimethoprim prophylaxis to cover a fistula to the urinary tract.
Refer to your friendly surgeons who will probably want to see the X-ray and review the baby.
To treat the obstruction, the baby may have a colostomy formed, followed by another operation later in life to create a new bottom after other tests have been carried out to identify a potential fistula, but this varies depending on their anatomy.
If the baby has an unusual bottom but is pooing well, feeding well and not obstructed – this often doesn’t need urgent management and is less likely to be associated with other abnormalities:
Perform a head-to-toe examination to double-check for associated anomalies. It is less likely for babies with very subtle anorectal malformations to have an associated syndrome such as VACTERL, but they should be thoroughly examined.
Discuss with / refer to the paediatric surgeons. This may be done as an outpatient.
If I’m just not sure. I’ve examined the baby, cleaned their bottom and had a good look, but I’m still not sure if there is a problem with their bottom or if they’re obstructed:
Take a full history and examine them thoroughly, double-checking that they don’t have another problem that could cause similar symptoms, e.g., an inguinal hernia or sepsis.
An abdominal x-ray can help spot dilated bowel loops and increase suspicion that there is a problem (which may or may not be due to their bottom).
Putting something in the bottom is a good way to prove that the anus is patent; nurses often take a rectal temperature, and if there is meconium on the probe, that is usually reassuring.
A lubricated 8Fr urinary catheter will fit easily in a neonate’s anus and only needs to go in a cm or 2 to prove it is patent.
Ask for help: getting a second opinion from a colleague (e.g., a registrar/ consultant) or a period of observation may help put matters to rest. Discuss with the surgical team if there are any ongoing concerns about obstruction or anorectal malformation.
What would the surgical team do?
The surgical team will assess the baby, looking for associated abnormalities, signs of obstruction and checking the anatomy of the baby’s anus. They can use Hegar dilators to check the size of the anus/fistula and may do a rectal examination.
If the baby has a high anorectal malformation, they will have a colostomy and a surgical repair at a later date.
If they have a perineal fistula, sometimes the baby can have an anoplasty in the neonatal period, or some go home with regular dilatation done by their parents until they are big enough for reconstructive surgery.
Babies with more mild abnormalities might be followed up as an outpatient (sometimes by their paediatric team rather than the surgical team) and may well not need any surgery.
So, how big should an anus actually be and where should it be located?
There is often someone somewhere, probably with some neuro-atypical superpowers, who has put a lot of time and thought into these questions. Fundamentally, if the anus looks ‘OK’ and works well, it is OK. If you want to get into the fine detail, there are a couple of formulae you can use:
The anal position index [2,3]
The ratio of the anus: genitalia distance and the anus: coccyx distance.
Normal: <0.46 in males and <0.34 in females
The size of the anus (based on calibration with Hegar dilators)
. These aren’t hard-and-fast numbers, but they provide a rough estimate – if you are going to pass a dilator, start small and increase stepwise. If you meet resistance or get blood, then stop.
The numbers from the literature:
Birth: Anal diameter (mm) = 7 + (1.3 × weight in kg).
E.g. a 3kg baby should have a diameter of 10.9 mm
Older children are harder, but taking the post op dilation targets following anorectal reconstruction, these seem to be standard accepted values from the Children’s Hospital Colorado (Hegar sizes)
–1-4 months: 12
– 4-8 months: 13
– 8-12 months: 14
– 1-3 years: 15
– 3-12 years: 16
>12 years: 17
What happened to Travis?
To get a better look at Travis’ bottom, you bring him to a better examining surface with good lighting, clean his bottom and notice that the meconium is actually coming from the end of his penis and he has an anorectal malformation with rectourethral fistula (formally known as an imperforate anus).
You clean him up and change his nappy (because you have been brought up well by your NICU nursing team), perform a general examination (focusing on features of VACTERL, which are happily absent) and explain your findings to Travis’ parents.
You discuss Travis with your NICU team and arrange to bring him round to NICU, place him nil by mouth, arrange the appropriate fluids, take some routine blood tests including a group and save and coagulation screen. You start IV fluids and request an abdominal X-ray.
The X-ray shows some dilated bowel loops, and the surgical team are happy to take your referral and review Travis on NICU.
Travis’ reconstruction goes well – he needs a stoma initially, then has his reconstruction and then stoma closure later. Travis’ parents are grateful to the team for his new, designer bottom.
Several years later, you happen to see Travis and his parents in clinic for a different, unrelated problem. He has thrived!
Travis still takes some laxatives, and toilet training took a little while longer than most, but he sees his surgical team regularly, and his new bottom is generally working well.
Interesting articles/websites
More details about anorectal malformations:
De Blaauw I, Stenström P, Yamataka A, Miyake Y, Reutter H, Midrio P, Wood R, Grano C, Pakarinen M. Anorectal malformations. Nature Reviews Disease Primers. 2024 Nov 21;10(1):88.
Details about missed anorectal malformations in the UK:
Murthi GV, McEwan T, Walker GM, Bustani P. Missed or delayed diagnosis of anorectal malformations: a review of the literature, current training and practice in the UK in relation to detection via the NIPE programme. Archives of Disease in Childhood-Fetal and Neonatal Edition. 2024 Jan 1;109(1):4-9.
NASA’s space place: https://spaceplace.nasa.gov/all-about-the-sun/en/
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