ANORECTAL MALFORMATIONS

Anorectal malformations (ARMs) include a group of heterogeneous developmental defects involving rectum and anal canal with absence, stenosis or ectopic opening of anus and/or fistulous connections with urogenital tract.

Embryogenesis: Rectum and anal canal develop from common cloaca along with lower urogenital tract, which separate with each other by formation of a urogenital septum by 7-8^th week. ARMs represent defects in the formation of urogenital septum or failure of its fusion with cloacal membrane.

Continence in these cases depends on involvement of: (a) neurogenic components, i.e. sacral outflow from S₂₄, (b) myogenic components, i.e. levator ani and external sphincters, and (c) sensory perception of pain, touch, temperature and propioception.

Classification: ARMs are best classified according to their relationship with the main muscle of continence, i.e. levator ani, (which is present as a sling around the rectum and controls the flow of feces along with external sphincter) as high, intermediate or low defects (Table 14.12). Recto-vestibular and recto-urethral fistulas are commonest ARMs in females and males respectively.

Clinical spectrum: While most ARMs, e.g. imperforate anus are diagnosed at birth during routine orificial

TABLE 14.12: Classification of anorectal malformations

• Supralevator (HIGH) anomalies

- Anorectal agenesis

- Persistent Cloaca (in females)*

- Rectal atresia (normal anal canal/opening)

• Infralevator (LOW) anomalies

- Ano-rectal membrane (imperforate anus)

- Ectopic anus (perineal fistulas)

- Anal stenosis

- Ano-vulval fistula

- Recto-vaginal fistula

• Translevator (intermediate) anomalies

- Anal agenesis

- Recto-vestibular fistula (commonest ARM in females)

- Recto-urethral fistula (commonest ARM in males)

- Recto-vesical fistula

*Rectum, vagina and urinary tract fuse to form a common channel, opening as a single orifice behind the clitoris.

examination, many are missed in first 24-48 hours and present with-(a) failure to pass meconium, (b) passage of meconium from abnormal sites, e.g. urine or vagina, (c) gradual abdominal distension with signs of intestinal obstruction. Rarely, some malformations, e.g. anal stenosis, presents in older children with chronic constipation and fecaloma.

Perineal examination may help to identify the type of defect and localize ectopic anal opening. While perineum is usually flat without median raphe and anal dimple in high types, presence of median raphe and anal dimple indicate low or intermediate defects with relatively spared sphincteric mechanisms and better post-operative chances for continence.

High ARMs are frequently associated with sacrospinal deformities (gt;25%), urogenital malformations (gt;50%) and other gut malformations, which should be excluded before surgery.

Diagnosis aims to-(a) identify the precise anatomical defect before surgery and (b) search for associated urological, spinal and neurological problems. Important investigations include:

• Ultrasonography is the preferred choice to differentiate between high and low ARMs as well as to identify associated urological abnormalities. A distance from rectal pouch to perineum lt;10 mm indicates low imperforate anus, while a distance of gt;15 mm suggests high defects. Spinal canal may also be scanned by USG for a latent tethered cord during first 3 months of life, though MRI is more reliable.

• Invertograms have been conventionally used for diagnosis of ARMs, though gradually being replaced with USG. Invertogram should be performed at least 12 hours after birth to allow intestinal gases to reach till most distal part of rectal pouch. Baby is held upside down (inverted) or knee-chest position for 3 minutes, before X-ray exposure. Location of gas shadow in the

Fig. 14.14: Anorectal malformations (Invertogram).

rectal pouch is then compared with pubococcygeal and puboischial lines, (i.e. lines representing levator ani muscle) to differentiate between low or high ARMs. When the rectal gas pouch in invertogram is gt;1 cm away from skin, patient probably has high/ intermediate defect and needs initial colostomy before corrective surgery (Fig. 14.14).

Treatment: The choice of surgical procedure depends on the type of defect, general condition of patient and associated urological or neurospinal defects.

Low malformations like simple imperforate anus may be managed with single-step Anoplasty with transpositioning for fistulous tracts in neonatal period, followed by repeated anal dilatations, till the normal anal size is achieved.

High/intermediate defects need a two-step surgery, involving preliminary colostomy followed by definite corrective repair after 6-12 months. Corrective surgery involves a pull-through procedure to bring the colon below the level of levator ani sling (for continence), to open it at expected anal site and to close the fistulous tracts.

Prognosis depends on the type of defect and co-existing defects. Success rate for fecal continence is excellent in low-defects (gt;80-90%) and moderate (50-60%) in intermediate defects. However, children with persistent cloaca of gt;3 cm length or recto-vesical fistula rarely achieve adequate continence. Surgical complications include anorectal stenosis, prolapse and recurrence of fistula.

14.9.4