Anorectal Malformations (ARM)
Failure of normal development of the rectum and anus during embryologic
development
By the 6th wk of gestation, the urorectal septum moves caudally and the
cloaca divides into the anterior urogenital sinus and posterior
anorectal canal. If these steps fail, a fistula results between the
bowel and urinary tract (in boys) or vagina (in girls). Complete or
partial failure of the anal membrane to open results in imperforate
anus or stenosis. If the cloacal membrane has a discontinuity anywhere
along its course, the result is an external anal opening anterior to
the sphincter complex (anterior displacement).
Anorectal malformations have been described as high or low lesions
based on the relationship of the rectum to the sphincter complex,
specifically the levator ani. (The muscle fibers of the sphincter
complex surround the anorectum and include: the puborectalis, levator
ani, external and internal sphincters, and the superficial external
sphincter muscles)
If an anocutaneous fistula is observed anywhere on the perineal skin of
a boy or external to the hymen of a girl, a low lesion can be assumed
* The incidence is
1/3,000-1/5,000 live births
* Significant
long-term concerns focus on bowel control and urinary and sexual
functions
Evaluation
Normal position of anus:
Anal position index (API), measured in cm
- Males:
- [Anus to Scrotum]/[Coccyx to scrotum] = 0.53+/-0.12 (abnormal
if <0.41)
- Females:
- [Anus to Fourchette]/[coxxyx to Fourchette] = 0.4±0.1
(abnormal if <0.3)
If an anocutaneous fistula is observed anywhere on the perineal skin of
a boy or external to the hymen of a girl, a low lesion can be assumed
A persistent cloaca is a defect where the rectum, vagina, and urethra
all remained together as a single common channel. In girls, a single
orifice in the perineum indicates a cloaca.
A flat bottom (no midline groove and absence of an anal dimple)
suggests a high defect
An ARM patient with no high or low fistula, think T21
Rule out additional anomalies
If a patient on newborn exam is noted to have an anorectal
malformation, within 24HOL
answer the following 2 questions:
1. Is there an associated defect (see list below)
e.g. urologic or cardiac, that requires immediate Tx?
2. Will this require a colostomy, or can the malformation
be repaired in a primary way without a colostomy?
Obtain:
- Xray of spine and sacrum
- Ultrasound of Spine (tethered cord) Use US in 1st 3mo of
life, MRI preferred if age >3mo
- Ultrasound of Kidneys
- Ultrasound of the Abd/Pelvis
- UA – look for traces of mec signifying rectourinary fistula (high
lesion, needs colostomy)
- Echo
- Pass NG r/o TEF/EA, confirm with imaging
- Consider obtaining a Voiding cystourethrogram
- Males with high lesions, the voiding cystourethrogram often
identifies the rectourinary fistula
- Females with high lesions, vaginogram and endoscopy to
elucidate the cloacal anatomy
- At 24HOL get a prone cross table lateral plan film (place a
radio-opaque marker at the anal impression in order to determine the
distance of the gas filled rectum to the perenium or the location of
the anal impression)
Anomalies associated with anorectal
malformations
- Anomalies associated with anorectal malformations
- Most common associated anomalies are of the kidney and urinary
tract along with the sacrum
- Called “caudal regression syndrome”
- Cardiac w or w/o TEF can appear in any combination
- Think VACTERL ( v ertebral, a nal, c ardiac, t racheal, e
sophageal, r enal, l imb)
- Others
- Ventricular septal defect
- Transposition of the great vessels
- Hypoplastic left-heart syndrome
- Tracheoesophageal fistula
Imperforate Anus notes
- Most patients will have a fistula
- Low lesions – rectum has descended through the sphincter complex
- Males – usually manifest with mec staining on perenium along
median raphe
- Females – spectrum from slightly anterior anus to fourchette
fistula that opens on the mucosa of the introitus distal to the hymen
- High lesions – rectum has NOT descended through the sphincter
complex
- Males – no visible cutaneous fistula, usually to urinary tract
(urethra or bladder)
- Females – cloacal anomalies where rectum, vagina, and urethra
all empty into common channel or stem or varying length (or may have
rectovaginal fistula)
- Males with imperforate anus and no fistula usually occurs in the
setting of T21
- Most common lesion
- Males – rectourethral bulbar fistula
- Females – rectovestibular fistula
- Second most common in both sexes is perianal fistula
At Birth, the abdomen is not distended. Over the next 18 to 24 hours,
the abdomen becomes distended and the intraluminal pressure in the
bowel increases significantly, forcing meconium through the lowest part
of the rectum, surrounded by the sphincter mechanism. Expect that
meconium will pass through a fistula, usually after 18 to 24 hours. The golden rule is to wait at least 18 to
24 hours before making a decision.
Prognosis
- Correlation exists between the degree of sacral development and
future function
- Patients w/ absent sacrum usually have permanent fecal and
urinary incontinence.
- Tethered cord occurs in approx 25% of patients with ARM
- Untethering of the cord might improve urinary and rectal
continence, however; it seldom reverses established neuro defects
- Of Note, The diagnosis of spinal defects can be screened for in
the first 3 mo of life by spinal ultrasound, although MRI is the
imaging method of choice if a lesion is suspected. In older patients,
MRI is needed.
Elements required for bowel control
- Anal canal and Rectal sensation
- Sensation resides in the anal canal (from a few mm above the
pectinate line to the anal verge)
- Here pts can discriminate between gas/liquid/solid and
temperature changes
- Proprioception is the sensation of distension of the rectum
(sensors in muscle surrounding the rectal mucosa)
- Sphincters
- Requires the individual to detect the presence of stool,
contract voluntary sphincters to prevent defecation, and relax at the
appropriate time to defecate
- Colonic motility
- Once daily or every other day depending on personal “rhythm,”
the contents of the rectosigmoid colon are pushed down toward the anal
canal. Contact with the anal canal requires the individual to decide to
relax and defecate or to contract and withhold the bowel movement.
Voluntary relaxation allows the next peristaltic wave to empty the
rectosigmoid colon
Difficulties with ARM patients
- 75% of patients with ARM will be able to achieve rectal continence
- 25% will not
- 50% will have some degree of constipation and / or soiling
- Patients with ARM will have deficiencies in all three areas of
bowel control
- Rectal atresia is an unusual case where typically there is a
normal anus (and usually normal sensation)
- Patients will have varying degrees of proprioception
- Usually can be toilet trained
- Liquid stool would be more difficult to sense and manage
- The sphincter mechanism has varying degrees of
hypodevelopment based on the lesion
- Typically patients will not be able to efficienctly empty the
rectum at once but will instead keep passing small amounts of stool
throughout the day
- Removal of the rectosigmoid removes the reservoir and
patients tend to have worse control and liquid incontinence constantly
Goals of Management
- Anatomically reconstruct all malformations
- Long term monitoring of patient
- Correction of functional disturbances
- Provide optimal circumstances to maintain bowel control
Bowel Regimen
- Must be provided so patient can remain clean and socially accepted
- Constipation dominant
- Constipation is typically worse if the original rectum was
preserved
- Large reservoir due to incomplete emptying
- Incomplete emptying accumulates stool and the rectum becomes
larger (megarectum) and the cycle worsens
- Can correlate directly with degree of initial rectal dilation
- The lower the original defect , usually the worse the
constipation
- Requires cleaning the colon once daily with enemas
- The amount and specific make-up of the enema is typically done
on trial /error basis until the correct volume and ratios are achieved
(usually over the course of one week)
- Saline enema may need to be combined with glycerin, soap, or
phosphate to make it stronger
- Follow current U.S. Food and Drug Administration (FDA) or
comparable guidelines re: phosphate enemas in pediatric patients
- Incontinence with diarrhea predominance
- A “constipating” diet use of medication such as loperamide and
pectin to decrease peristalsis
- This type of bowel management program has been successful in
95% of cases
- Once patient is able to maintain cleanliness ofr 24hrs on a
regular basis, Offer Malone antegrade continence enema (MACE) Malone,
or appendicostomy, or chait cecostomy (neoappendicostomy)
- Soiling
- Usually the result of chronic fecal impactions
- Overflow pseudo-incontinence
- Use of laxatives and bulking agents may help to eliminate
this type of soiling
- If significant and interfering with the social life of the
patient, recommend implementing the bowel management program with a
daily enema as above
References:
-Sabistons Text
-Wylie
-Nelsons
-Google