Embryology and Pathogenesis

Indirect inguinal hernias in infants and children are congenital and result from an arrest of embryologic development, failure of obliteration of the processus vaginalis, rather than an acquired muscular weakness. The pertinent developmental anatomy of congenital indirect inguinal hernia relates to development of the gonads and descent of the testis through the internal ring and into the scrotum late in gestation. The gonads develop near the kidney as a result of migration of primitive germ cells from the yolk sac to the genital ridge, which is completed by 6 wk of gestation. Differentiation into testis or ovary occurs by 7 or 8 wk of gestation under hormonal influences. The testes descend from the urogenital ridge in the retroperitoneum to the area of the internal ring by about 28 wk of gestation. The final descent of the testes into the scrotum occurs late in gestation between weeks 28 and 36. The testis is preceded in descent to the scrotum by the gubernaculum and the processus vaginalis. The processus vaginalis is present in the developing fetus at 12 wk of gestation as a peritoneal outpouching that extends through the internal inguinal ring and accompanies the testis as it exits the abdomen and descends into the scrotum.

The gubernaculum testis forms from the mesonephros (developing kidney), attaches to the lower pole of the testis, and directs the testis through the internal ring and inguinal canal and into the scrotum. The testis passes through the inguinal canal in a few days but takes about 4 wk to migrate from the external ring to the scrotum. The cordlike structures of the gubernaculum occasionally pass to ectopic locations (perineum or femoral region), resulting in ectopic testes.

In the last few weeks of gestation or shortly after birth, the layers of the processus vaginalis normally fuse together and obliterate the patency from the peritoneal cavity through the inguinal canal to the testis. The processus vaginalis also obliterates just above the testes, and the portion of the processus vaginalis that envelops the testis becomes the tunica vaginalis. In girls, the processus vaginalis obliterates earlier, at about 7 mo of gestation. Failure of the processus vaginalis to close permits fluid or abdominal viscera to escape the peritoneal cavity and accounts for a variety of inguinal-scrotal abnormalities seen in infancy and childhood. The ovaries descend into the pelvis from the urogenital ridge but do not exit from the abdominal cavity. The cranial portion of the gubernaculum in girls differentiates into the ovarian ligament, and the inferior aspect of the gubernaculum becomes the round ligament, which passes through the internal ring and attaches to the labia majora. The processus vaginalis in girls extends into the labia majora through the inguinal canal and is also known as the canal of Nuck.

Androgenic hormones, adequate end-organ receptors, and mechanical factors such as increased intra-abdominal pressure influence complete descent of the testis through the inguinal canal. The testes and spermatic cord structures (spermatic vessels and vas deferens) are located in the retroperitoneum but are affected by increases in intra-abdominal pressure as a consequence of their intimate attachment to the processus vaginalis. The genitofemoral nerve also has an important role: It innervates the cremaster muscle, which develops within the gubernaculum, and experimental division or injury to both nerves in the fetus prevents testicular descent. Failure of regression of smooth muscle (present to provide the force for testicular descent) might have a role in the development of indirect inguinal hernias. Several studies have investigated genes involved in the control of testicular descent for their role in closure of the patent processus vaginalis, for example, hepatocyte growth factor and calcitonin gene-related peptide. Unlike in adult hernias, there does not appear to be any change in collagen synthesis associated with inguinal hernias in children (Fig. 338-1).

Figure 338-1 Hernia and hydroceles.

(Modified from Scherer LR III, Grosfeld JL: Inguinal and umbilical anomalies, Pediatr Clin North Am 40: 1121–1131, 1993.)

Genetics

The genetics of inguinal hernias in children are not well elucidated. There is some genetic risk incurred for siblings of patients with inguinal hernias; the sisters of affected girls are at the highest risk, with a relative risk of 17.8. In general, the risk of brothers of a sibling is approximately 4-5, as is the risk of a sister of an affected brother. Both a multifactorial threshold model and autosomal dominance with incomplete penetrance and sex influence have been suggested as an explanation for this pattern of inheritance.

Pathology

Closure of the processus vaginalis is often incomplete at birth; closure continues postnatally, and the rate of patency is inversely proportional to the age of the child. It has been estimated that ~40% close during the first months of life and that ~20% of boys have a persistent patency of the processus vaginalis at 2 yr of age. Patency of the processus vaginalis after birth is a potential hernia, but not all patients with a patent processus vaginalis develop a clinical hernia. An inguinal hernia occurs when intra-abdominal contents escape the abdominal cavity and enter the inguinal region through a patent processus vaginalis. Based on their location in the inguinal canal (lateral to the inferior epigastric vessels), they are indirect inguinal hernias but are rarely associated with a muscular weakness or defect, as is typical of an adult hernia. Depending on the extent of patency of the distal processus, the hernia may be confined to the inguinal region or pass down into the scrotum. Complete failure of obliteration of the processus predisposes to a complete inguinal hernia characterized by a protrusion of abdominal contents into the inguinal canal and possibly extending into the scrotum. Obliteration of the processus vaginalis distally (around the testis) with patency proximally results in the classic indirect inguinal hernia with the protrusion in the inguinal canal.

A hydrocele is when only fluid enters the patent processus vaginalis and the swelling may exist only in the scrotum (scrotal hydrocele), only along the spermatic cord in the inguinal region (hydrocele of the spermatic cord), or extend from the scrotum through the inguinal canal and even into the abdomen (abdominal-scrotal hydrocele). A hydrocele is termed a communicating hydrocele if it demonstrates fluctuation in size, often increasing in size after activity and, at other times, being smaller when the fluid decompresses into the peritoneal cavity. Occasionally, hydroceles in older children follow trauma, inflammation, or tumors affecting the testis. Although reasons for failure of closure of the processus vaginalis are unknown, it is more common in cases of testicular nondescent and prematurity. In addition, persistent patency of the processus vaginalis is twice as common on the right side, presumably related to later descent of the right testis and interference from the developing inferior vena cava and external iliac vein. Risk factors identified as contributing to the development of congenital inguinal hernia relate to conditions that predispose to failure of obliteration of the processus vaginalis and are listed in Table 338-1. Incidence of inguinal hernia in patients with cystic fibrosis is ~15%, believed to be related to an altered embryogenesis of the wolffian duct structures, which leads to an absent vas deferens in males with this condition. There is also an increased incidence of inguinal hernia in patients with testicular feminization syndrome and other forms of ambiguous genitalia. The rate of recurrence after repair of an inguinal hernia in patients with a connective tissue disorder approaches 50%, and often the diagnosis of connective tissue disorders in children results from investigation after development of a recurrent inguinal hernia.

Incidence

The incidence of congenital indirect inguinal hernia in full-term newborn infants is estimated at 3.5-5.0%. The incidence of hernia in preterm and low birthweight infants is considerably higher, ranging from 9% to 11%, and approaches 30% in very low birthweight infants (<1,000 g) and preterm infants <28 wk of gestation. Inguinal hernia is much more common in boys than girls, with a male : female ratio of ~6 : 1. About 60% of inguinal hernias occur on the right side, 30% are on the left side, and 10% are bilateral. The incidence of bilateral hernias is higher in girls and appears to be 20-40%. The increased frequency on the right side is presumably related to the later descent of the right testis and interference with obliteration of the processus vaginalis from the developing inferior vena cava. An increased incidence of congenital inguinal hernia has been documented in twins and in family members of patients with inguinal hernia. There is a history of another inguinal hernia in the family in 11.5% of patients.

Clinical Presentation

An inguinal hernia typically appears as a bulge in the inguinal region or extending through the inguinal region into the scrotum. In girls, the mass typically occurs in the upper portion of the labia majora. The bulge or mass is most visible at times of irritability or increased intra-abdominal pressure (crying, straining, coughing). It may be present at birth or might not appear until weeks, months, or years later. The bulge is most often 1st noted by the parents or on routine examination by the primary care physician. The classic history from the parents is of intermittent groin, labial, or scrotal swelling that spontaneously reduces but that is gradually enlarging or is more persistent and is becoming more difficult to reduce. The hallmark signs of an inguinal hernia on physical examination are a smooth, firm mass that emerges through the external inguinal ring lateral to the pubic tubercle and enlarges with increased intra-abdominal pressure. When the child relaxes, the hernia typically reduces spontaneously or can be reduced by gentle pressure, 1st posteriorly to free it from the external ring and then upward toward the peritoneal cavity.

Methods used to demonstrate the hernia on examination vary depending on the age of the child. A quiet infant can be made to strain the abdominal muscles by stretching him or her out supine on the bed with legs extended and arms held straight above the head. Most infants struggle to get free, thus increasing the intra-abdominal pressure and pushing out the hernia. Older patients can be asked to perform the Valsalva maneuver by blowing up a balloon or coughing. The older child should be examined while standing, and examination after voiding can also be helpful. With increased intra-abdominal pressure, the protruding mass is obvious on inspection of the inguinal region or can be palpated by an examining finger invaginating the scrotum to palpate at the external ring. In the female infant, the ovary and fallopian tube can be contained within the hernia sac, presenting as a firm, discrete, nontender mass in the labia. Another test is the “silk glove sign,” which describes the feeling of the layers of the hernia sac (processus vaginalis) as they slide over the spermatic cord structures, with rolling of the spermatic cord beneath the index finger at the pubic tubercle; this physical finding has a sensitivity of 91% and a specificity of 97.3%. If the bulge is located below the inguinal canal on the medial aspect of the thigh, a femoral hernia should be suspected. In the absence of a bulge, the finding of increased thickness of the inguinal canal structures on palpation also suggests the diagnosis of an inguinal hernia. It is important on examination to note the position of the testes because retractile testes are common in infants and young children and can mimic an inguinal hernia with a bulge in the region of the external ring. Because in the female patient ~20-25% of inguinal hernias are sliding hernias (the contents of the hernia sac are adherent within the sac and therefore not reducible), a fallopian tube or ovary can be palpated in the inguinal canal.

Evaluation of Acute Inguinal-Scrotal Swelling

Occasionally, an inguinal-scrotal mass appears suddenly in an infant or child and is associated with discomfort. The differential diagnosis