Incidence

The incidence of anorectal malformations (ARM) is reported as 2.0-2.5 per 10,000 live births. However, there are significant variations in the prevalence between regions throughout the world. Confusion regarding the incidence is further increased with the addition and/or exclusion of anal stenosis in the studies in the literature. While no significant association between ethnicity and ARM has been identified, there are several studies that identify variations between ethnic groups [1,2].

In North America the Texas Birth Defects Registry reported 542 cases during the period of 1999-2001, thus a prevalence of 5.03 per10,000 live births for all cases of "stenosis or atresia of large intestine, rectum, or anal canal" [3]. In California, similar to Texas, a lower rate of incidence has been identified within the African-American community than within the Hispanic [3,4]. Between 1981 and 1986, the Centre for Disease Control monitored 4,617,613 births. Information on specific race and ethnicity was available for 92.6% of these births. These data revealed a higher incidence of ARM amongst Native American Indians and a lower incidence amongst African Americans than the rest of the population [2].

ARM are a common cause of bowel obstruction amongst neonates in Nigeria, where in one centre ARM accounted for 13.4% of all neonatal malformations and 39% of emergency neonatal surgery [5-7]. Louw reported an incidence of 5.5 per 10,000 live births in South Africa, with the highest rates noted among the Caucasians (5.7 per 10,000) and lower rates amongst the Bantu (4.4 per 10,000) [8]. However, further detailed prevalence rates are not available for sub-Saharan Africa.

The incidence rate in British Columbia was 4.0 per 10,000 live births (273 cases out of 689,118 consecutive liveborn infants) between 1964 and 1982 [9]. According to the Latin American Collaborative Study for Congenital Malformations (ECLAMC, Estudio Colaborativo Latino Americano de Malformaciones Congenitas), its frequency is 4.1 per 10,000 live births. The highest recorded incidence is 7.7 per 10,000 in Chile [10].

In Europe, several countries have reported prevalence rates: for example in Denmark an incidence of 3.8 per 10,000 live births was reported in Denmark over a 13-year period from 1980 to 1993; in Budapest, Hungary, 1.8 per 10,000 live births between 1970 and 1977 [11,12]. A study of different ethnic groups from Birmingham, UK, revealed that rectal atresia occurs more frequently among Europeans and South Asians than amongst those from the Caribbean [13]. A more detailed analysis was performed by EUROCAT (European network of population-based registries for the epidemiologic surveillance of congenital anomalies). Data from 1980 to 1994 from 33 registries revealed a prevalence of 4.05 per 10,000 births from a total of 4,618,840 births throughout Europe. It also revealed significant differences between registries; the lowest being in Luxembourg, with a prevalence of 1.14 per 10,000, to the highest in Finland with 6.13 per 10,000. There were also significant differences between registries within the same country, such as between Gal-way (1.2 per 10,000) and Dublin (4.1 per10,000) in Ireland [14]. This may reflect different ethnic groups within nations or Europe, or different embryological factors.

In the Indian subcontinent, numerous studies have revealed a high prevalence, with ARM accounting for 15% of admissions to one unit; however, overall incidence figures are not presently available [15,16]. It is important to note the concept of the pouch colon with regard to the Indian subcontinent. Pouch colon

Table 8.1 Summary of incidences of anorectal malformations (ARM) from registries

ARM Registry

Region

Incidence (per) 10,000 live births

Texas Registry [3]

USA

5.0

Kansas [24]

USA

2.5

Louw [8]

South Africa

5.7

ECLAMC [10]

South America

4.1

EUROCAT [14]

Europe

4.1

Haeusler [25]

Europe

1.4

Finley [26]

Sweden

2.5

Stoll [27]

France

4.8

syndrome, or congenital short colon, is defined as a sac-like dilation of the shortened colon, which can lead to a massively dilated distal "pouch", and which fistulates into the genitourinary tract. This unusual condition, which is associated with ARM, is much more common in North India than in the rest of the world. Indian patients account for 92% of all reported cases and pouch colon syndrome accounts for 6-13% of all ARM in Northern India [17-19].

In the Far East, the prevalence is reported as 3.5, 5.04 and 3.38 per 10,000 for South Korea, Japan and China [20,21]. The incidence in the Singapore population is reported as 0.86 per 10,00 live births [22]. In Victoria, Australia they have reported an increase in the overall prevalence between 1983 and 1995 to 5.6 per 10,000, followed by a decline to 2.9 per 10,000 between 1999 and 2000. Similar to the increase in the incidence of gastroschisis born to teenage mothers within the state, they noted a significantly increased prevalence of births to mothers less than 20 years old (11.2 per 10,000) as opposed to those 25-29 years old (4.5 per 10,000) [23].

Some argue that the differences in the prevalence results are due to problems with case inclusion; however, ARM patients who are captured by a registry or a study are generally assessed by paediatric surgeons who are quite specific about the type of abnormality [14]. Thus, despite the variable prevalence rates ranging from 0.86 to 7.7 per 10,000, the overall incidence is 2.0-2.5 per 10,000 live births (Table 8.1).

8.1.1 Associated Anomalies

ARM occur in 2.2% of children with trisomy 21, with a range of 2-8% [28-31]. This incidence is 15 times more common than the rate within the general popu lation [32]. In racial groups such as African Americans, the incidence of ARM is much higher in children with Down's syndrome than its incidence in the general population [33]. Black and Sherman were the first to describe the particular association between Down's syndrome and ARM without fistula [34].

A small number of studies have demonstrated an increased incidence among first-degree relatives of affected patients. Data from the 1950s placed the risk of a second child in a family being born with ARM at 1%; however, this has not been confirmed in later studies [35]. There are case reports of siblings and parents and children both having the anomalies, especially anal stenosis [36]. In 1957, Kaijser et al. reported the case of a mother and two daughters in whom each had an imperforate anus with a rectovagi-nal fistula [37]. In 1961, VanGelder commented on a family with three known and one possible ARM extending over four generations. The pattern suggested a dominant inheritance with reduced penetrance [38]. Seitz et al. reported anterior ectopic anus in a mother and her two daughters [39].

In India, Mittal et al. reported that a detailed family history revealed 21 out of 140 patients (15%) had a sibling with an ARM [40]. In a report from Manchester, 15 out of 186 patients (8%) had a strong family history of ARM [41]. Schwoebel et al. describe two families with nine members affected with ARM over two to three generations, with nine members affected with ARM, with a recurrence risk of 10-20% for first degree relatives [42]. Some studies reveal evidence of autosomal recessive inheritance, while others demonstrate an autosomal dominant syndrome [38]. Christensen et al. found an increased frequency of chromosome anomalies among children with ARM [43].

Genetic factors are believed to be more important in anorectal stenosis than other anal anomalies. However, there is little evidence to imply that genetic predisposition plays a significant role in the aetiology of this condition [44].

In 1981, Currarino et al. described the triad of anorectal stenosis (low ARM), an anterior sacral defect and a presacral mass [45]. The complete Curra-rino triad is rare and is familial in 50% of cases [46]. A screening program with lumbosacral ultrasound and plain radiography for siblings of those with the Cur-rarino triad should be obligatory [47].

Cuschieri and the EUROCAT group demonstrated that of the 1,846 patients in the registry with ARM, 1,174 children had other defects [31]. Chromosomal abnormalities occurred in 11%, most frequently those with anal stenosis. VACTERL association was present in 181 patients (10%) and multiple congeni tal anomalies in unrecognised patterns were present in 711 (38%) [31]. VACTERL stands for Vertebral, Anorectal, Cardiac, Tracheo-oEsophageal, Renal and Limbs particularly radial, anomalies. When three or more anomalies are present together then VACTERL is considered to exist.

Associated anomalies are more common in boys (52-63%) and the higher the ARM, the higher the risk of associated anomalies, commonly ranging from 44% to 67% [48]. Associated anomalies can be twice as prevalent in patients with higher anomalies than in those with lower lesions [40]. However, Javid et al. reported a 61% incidence of associated anomalies in girls with low ARM [49].

Vertebral and spinal anomalies are commonly associated, especially with supralevator lesions, with an incidence of 4.6-40%. Abnormalities such as vertebral defects, tethered spinal cord or diastematomy-elia can occur, but by far the most frequent is a sacral anomaly consisting of the absence of one or more sacral vertebrae [14].

Ratan et al. from Haryana in India presented data from 416 patients demonstrating that males had a significantly higher incidence of genital anomalies in association with low ARM, and gastrointestinal tract anomalies in association with high ARM. Unlike other reports, girls with high ARM had more uro-logical anomalies compared with the boys with high ARM [50].

Genitourinary anomalies occur in 21-61% of patients [14]. Up to 26% of boys had genitourinary problems, as opposed to 5% of girls, with upper urinary tract anomalies present in 50% of boys and 30% of girls, respectively. The risk for both sets of problems increased with the level of the anorectal lesion [51]. Low lesions such as perineal cutaneous fistula have less than a 10% chance of having a urinary anomaly, while rectovesical fistulas have a 90% risk. The incidence of genital malformations increased in the presence of renal or spinal lesions. Conversely, the incidence of urinary and spinal anomalies increased in patients with genital malformations [51]. Unde-scended testes are commonly associated with ARM and are reported to occur in up to 19% [52].

Gastrointestinal anomalies are less common, with reported incidence of 10-25%, the commonest deformity being tracheoesophageal fistula (13%) followed by duodenal atresia [31,44]. Cardiac malformations occur in 9-20% of defects equally in patients with high and low lesions, with tetralogy of Fallot being the commonest diagnosis [44]. Several hypotheses exist for this association of abnormalities. Abnormal notochord development may be pivotal in producing neural tube defects and ARM, possibly by altering sonic hedgehog signalling [53].

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