The round ligament is attached to the uterus and a small evagination of the parietal peritoneum accompanies the round ligament through the inguinal ring into the inguinal canal in the female¹⁾. This small evagination of the parietal peritoneum is the canal of Nuck in the female, which is homologous to the processus vaginalis in males. The canal of Nuck is normally obliterated in the first year of life. Failure to achieve complete obliteration results in an indirect inguinal hernia or hydrocele of the Canal of Nuck1,²⁾. Here, we report two cases with ultrasound and magnetic resonance (MR) findings and we review the literature. Several cases with this rare anomaly have been reported, but MR findings have not previously been reported in the Korean literature. Kim et al.³⁾ reported three cases which were diagnosed with sonography and CT. Park et al.⁴⁾ reported one case which were diagnosed with sonography and MR imaging. In both cases, the occurrence of contralateral side inguinal hernia was noted although there have been no reports about the recurrence of this anomaly.

The processus vaginalis, which is a homologue to the canal of Nuck, may be obliterated at any point between the internal inguinal ring and the scrotum, or it may happen incompletely. These variations account for the diverse classification of hernias (the extraabdominal passage of viscus) and hydroceles (the extra-abdominal passage of peritoneal fluid), including complete or scrotal hernias, communicating or non-communicating hydroceles, hydroceles of the spermatic cord in males, and the canal of Nuck in girls⁵⁾.

Hydroceles are common in children, especially in boys. But hydrocele of the canal of Nuck is an uncommon clinical presentation⁶⁾.

Although the exact process is unclear, it is generally agreed that obliteration of the processus vaginalis occurs only after the seventh month of gestation⁷⁾, thus accounting for the higher incidence of inguinal hernias in the premature infant⁸⁾. Although not clearly defined, there is a known heredity factor associated with the occurrence of hernias that is more frequent in twin gestations and in infants who have a family history of hernia⁹⁾.

Swelling of the inguinal region in a female may result from a number of conditions, including inguinal hernia, tumor (lipoma, leiomyoma, sarcoma), cyst, abscess, lymphadenopathy, or hydrocele of the canal of Nuck¹⁾.

In most cases, the canal of Nuck should be distinguished from hernia. Bowel sounds over the swelling are strongly suggestive of a hernia.

Hydrocele of the canal of Nuck typically presents as a painless, translucent swelling in the inguinolabial region⁶⁾. There is no nausea or vomiting. If possible, the child should be examined in the supine and standing positions⁶⁾. In older children, Valsalva's maneuver should be performed to rule out hernia. If the bulge is apparent in the standing position and disappears while supine, a hernia is more plausible¹⁰⁾.

Infection is an uncommon complication of hydroceles in children, and until now only 5 cases have been reported in the English language literature, all in boys. Infected hydrocele of the canal of Nuck is rare¹¹⁾.

In the past, a palpable inguinal mass was removed via surgery without an imaging study. Currently, sonography is the imaging modality of choice for evaluation of this type of mass¹²⁾.

Because sonography is a relatively easy, accessible and accurate procedure⁴⁾, we recommend that sonography be performed routinely in those females with an inguinal hernia containing an asymptomatic movable mass. But, we cannot identify whether there is a communication between cystic mass and peritoneal cavity with sonography. MR imaging can give more precise images including septation and a communication between cystic lesion and peritoneal cavity, and information on the anatomical relation with adjacent structures4,¹³⁾. Therefore, MR imaging can help to diagnose in patients with inguinal cystic mass. The differential diagnosis of a cystic mass in the female groin region includes round ligament cysts, varicosities of the round ligament, inguinal herniation of the ovary, cystic lymphangiomas, epidermal inclusion cysts, abscesses, and pseudoaneurysms¹⁴⁾.

The final diagnosis is made during surgery and confirmed by pathological examination⁶⁾. Surgical resection of the hydrocele and ligation of the neck of the processus vaginalis should be considered as standard therapy.

Han et al.¹⁵⁾ reported that in 2.6% of the patients who had been operated of unilateral hydrocele, contralateral hernia occurred. They also reported that the occurrence of the contralateral hydrocele after the operation of unilateral hydrocele was 2.0% in males. Flum et al.¹⁶⁾ reported that the recurrence rate in inguinal hernia is about 10% but has not been reported for a hydrocele of the canal of Nuck yet. In our cases, the occurrence of inguinal hernia was on the contralateral side from the hydrocele and operation was needed in one case. Therefore, one should be aware that the ocurrence rate of the contralateral hernia is not low in patients with hydrocele of the canal of Nuck. So, we should consider the hydrocele of the canal of Nuck in patients who have a inguinal cystic mass and have a history of inguinal hernia on the other side.

In these cases, we suggest that MR imaging can help to identify an inguinal cystic mass when the diagnosis is vague by its sonographic findings in female. Because the inguinal hernia on the other side was detected either before or after the operation of hydrocele in both cases, we should consider the hydrocele of the canal of Nuck in patients who have an inguinal cystic mass and have a history of an inguinal hernia on the other side. Also, we should consider the risk of occurrence of contralateral inguinal hernia in patients with hydrocele of canal of Nuck.