Teaching Case

A 65-year-old female patient presented with a two-month history of discomfort and swelling in her left groin. On physical examination, a soft, well-circumscribed groin swelling was noted. This was non-tender, irreducible and non-pulsatile. No cough impulse was present.

Ultrasound examination using a linear transducer was undertaken in the left inguinal region which revealed a cystic structure measuring 3.6 x 4 cm. This was thin-walled with no septations (Fig. 1). Colour Doppler examination excluded a vascular abnormality. Ultrasound-guided aspiration of the cystic structure revealed seromucinous clear fluid and scattered inflammatory cells.

Contrast-enhanced CT of the abdomen and pelvis demonstrated the presence of bilateral cystic structures within both the left and right inguinal canal along the course of the round ligaments. These were large with thin walls. There was no evidence of concomitant herniation of bowel (Fig. 2). Interestingly, the patient did not report any symptoms from her right inguinal region.

Follow-up ultrasound after 1 year demonstrated bilateral significant increase in cyst volume (Fig. 3). The patient underwent abdominal and pelvic CT for further assessment. This demonstrated a bilateral interval increase in cyst volume with formation of multiple septations (Fig. 4).

The canal of Nuck was first described by the Dutch anatomist Anton Nuck van Leiden in the 17^th century. It originates as an abnormal evagination of parietal peritoneum which extends anteriorly along the round ligament of the uterus descending into the inguinal canal and may extend into the labia majora (similar to processus vaginalis in men). [1, 2]

The canal of Nuck is normally obliterated in the first year of life. Persistence of the canal is pathological and can lead to formation of congenital inguinal hernias and cystic fluid collections in adults. The fluid collection is attributed to imbalance between the secretory and absorptive balance of the parietal peritoneal membrane. This can lead to fluid collecting in the canal and this is often referred to as the female hydrocele. [1, 2]

Cysts in the canal of Nuck are rare with about 400 cases reported worldwide. The majority of these are unilateral at presentation and most commonly occur in young women. [3] This case is a unique presentation of bilateral canal of Nuck cysts.

Ultrasonography is the primary diagnostic imaging modality. Canal of Nuck cysts are diagnosed as an anechoic or hypoechoic thin-walled cystic structure. They are described as tubular, oval or comma-shaped with extension into the inguinal canal. Colour Doppler signal demonstrates no blood flow. [4] Ultrasonography is helpful in differentiating a canal of Nuck cyst from an inguinal hernia. No omental fat or bowel loops will be evident upon preforming the Valsalva manoeuvre in a patient with a canal of Nuck cyst. [4, 5]

Contrast-enhanced CT better defines a cyst in the canal of Nuck. A large fluid attenuating, well-circumscribed structure is described in the inguinal canal. It can be seen to follow the course of the round ligament of the uterus. Thin septations can also be seen. [2, 3] Calcification within the cyst may also be noted (Fig 4). MRI investigation is also a useful diagnostic technique. Cysts appear hyperintense on T2-weighted images and hypointense on T1-weighted images. [5] MRI investigation was not available for this patient.

Surgical intervention is the treatment of choice, with excision and high ligation of the canal shown to reduce recurrence. [3]

This rarely encountered entity can be confidently diagnosed with ultrasonography and should be considered in the differential diagnosis of a cystic groin lump in female patients.