A canal of Nuck cyst is a fluid collection trapped in the inguinal canal of a female, lying along the course of the round ligament. It forms when the embryonic peritoneal evagination that accompanies the round ligament fails to obliterate. It is the female counterpart of a hydrocele of the processus vaginalis, and ultrasound is usually diagnostic on its own.
The diagnosis is the easy part. The type is what changes the operation, because most of these cysts do not come alone.
| At a glance | |
|---|---|
| What it is | Fluid in a persistent peritoneal diverticulum accompanying the round ligament into the inguinal canal |
| Typical age | Most often girls under 5 years; uncommon in adults |
| Commonest adult form | Hydrocele |
| Presentation | Painless inguinal or labial swelling; pain suggests infection or incarceration |
| First-line imaging | Ultrasound, scanned at rest and during straining, which is usually diagnostic |
| The thing not to miss | An ovary inside a hernia sac, which is a risk of ischaemia and needs urgent attention |
| The thing usually missed | A concurrent indirect inguinal hernia, present in most operated adult cases |
Why the canal of Nuck exists
In the female fetus the round ligament descends from the uterine cornu through the inguinal canal towards the labium majus, and it drags a finger of parietal peritoneum with it. That diverticulum is the canal of Nuck, named after the Dutch anatomist Anton Nuck. It normally closes before birth. If the opening persists, the peritoneal cavity has a patent route into the groin, and either peritoneal contents follow it (a hernia) or fluid accumulates within a segment of it (a hydrocele). This is the same embryology as the processus vaginalis in the male, which is why the entity is described as its exact counterpart.
Cyst, hydrocele or hernia: the three types
“Cyst of the canal of Nuck” and “hydrocele of the canal of Nuck” are used interchangeably in the literature and in practice. What is not interchangeable is the three-way split of the hydroceles, because it records whether the lesion still communicates with the abdomen and how far it extends.
| Type | Anatomy | What you see |
|---|---|---|
| Communicating | Canal remains patent to the peritoneal cavity | Inguinal fluid collection that changes in size, and may vary with position or Valsalva. Continuity with the peritoneum can sometimes be traced |
| Encysted | Closed at both the deep and superficial rings, isolating a segment | A self-contained, non-communicating cystic lesion confined to the inguinal canal. This is the classic “canal of Nuck cyst” |
| Bilocular | Spans the deep ring, with components inside and above the inguinal canal | Two connected compartments, one in the inguinal canal and one above the deep ring, so the lesion is not fully shown by scanning the groin alone |
A canal of Nuck hernia is the other category: the patent canal transmits actual contents rather than just fluid. Reported contents include bowel, omentum, fallopian tube, uterus and ovary. The ovary is the one that changes urgency, because an ovary in the sac carries a risk of ischaemia and should be treated as needing prompt surgical attention rather than elective follow-up.
Imaging features
| Modality | Findings |
|---|---|
| Ultrasound (first line) | Anechoic or hypoechoic, well-defined lesion in the inguinal canal, elongated along the round ligament, with a thin or imperceptible wall and increased posterior through-transmission as expected of a simple fluid collection. Internal septa, debris or wall thickening suggest prior infection or haemorrhage. Scan both at rest and during straining or Valsalva, which is what separates a hydrocele from a hernia and shows whether the lesion communicates |
| CT | Well-circumscribed fluid-attenuation lesion in the inguinal canal, with or without internal septations. CT is most useful for mapping a bilocular lesion above the deep ring and for excluding bowel in the sac |
| MRI | T1 hypointense and T2 markedly hyperintense, following fluid, thin walled, sometimes with fine T2 hypointense septa. Best for showing the relationship to the round ligament and the deep inguinal ring |
Differential diagnosis
| Mimic | How to separate it |
|---|---|
| Fluid-containing inguinal hernia, or ascites tracking into the canal | Look for a neck at the deep ring, peritoneal continuity and fluid elsewhere in the abdomen. A patient with ascites and inguinal fluid usually has the former |
| Inguinal endometriosis of the round ligament | Cyclical pain, heterogeneous or haemorrhagic contents, T1 hyperintense foci, solid enhancing tissue rather than simple fluid |
| Lymphadenopathy | Rounded rather than tubular, hilum, vascular flow, solid echotexture |
| Abscess | Thick enhancing wall, surrounding inflammatory change, clinical signs of infection |
| Soft tissue tumour of the groin, including round ligament leiomyoma and sarcoma | Solid components and internal vascularity. A truly anechoic avascular lesion essentially excludes it |
What to put in the report
- Say it is in the inguinal canal and related to the round ligament. That one sentence is what converts “groin cyst” into a diagnosis.
- State the type: communicating, encysted or bilocular, and whether any part extends above the deep inguinal ring.
- Report whether there is a concurrent indirect inguinal hernia, and whether the sac contains anything other than fluid. Name the ovary explicitly if it is there.
- Note complexity: septations, debris, wall thickening or hyperaemia, which point to infection or haemorrhage.
- Give the size and the craniocaudal extent, since these drive the surgical approach.
Management
Surgical excision is the definitive treatment: the sac is dissected off the round ligament and the canal ligated near the deep inguinal ring, by either an open or a laparoscopic approach. Recurrence is rare.
The practical point for the radiologist is the associated hernia. In a 2026 series of 31 adult patients, 27 were diagnosed on pre-operative imaging and 4 only at operation during an inguinal hernia repair. Every patient operated through an extraperitoneal approach had a concurrent indirect inguinal hernia that was repaired with mesh in addition to excision of the cyst, and the authors conclude that most patients have an associated indirect or occult hernia. Two patients had multiple cysts along the round ligament. So a report that describes the cyst and says nothing about the deep ring or a coexisting hernia is the report most likely to be contradicted in theatre.
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Frequently asked questions
References
- Thomas AK, Thomas RL, Thomas B. Canal of Nuck abnormalities. J Ultrasound Med. 2020;39(2):385-395. PMID: 31378959.
- Nasser H, King M, Rosenberg HK, et al. Anatomy and pathology of the canal of Nuck. Clin Imaging. 2018;51:83-92. PMID: 29448124.
- Saguintaah M, Eulliot J, Bertrand M, et al. Canal of Nuck abnormalities in pediatric female patients. Radiographics. 2022;42(2):541-558. PMID: 35061516.
- Co C, Wijerathne S, Dacaynos L, et al. Hydrocele of the canal of Nuck: a case series and literature review. Hernia. 2026;30(1):244. PMID: 42268444.
- Bazuaye-Ekwuyasi E, Odogwu-Hall O, Bushey H, et al. Multimodality imaging features of Canal-of-Nuck hydrocele in adults. Clin Imaging. 2022;92:101-108. PMID: 36270153.
- Keeratibharat N, Chansangrat J. Hydrocele of the canal of Nuck: a review. Cureus. 2022;14(4):e23757. PMID: 35402114.
- Safak AA, Erdogmus B, Yazici B, et al. Hydrocele of the canal of Nuck: sonographic and MRI appearances. J Clin Ultrasound. 2007;35(9):531-532. PMID: 17551936.
- Park SJ, Lee HK, Hong HS, et al. Hydrocele of the canal of Nuck in a girl: ultrasound and MR appearance. Br J Radiol. 2004;77(915):243-244. PMID: 15020367.
- Ozel A, Kirdar O, Halefoglu AM, et al. Cysts of the canal of Nuck: ultrasound and magnetic resonance imaging findings. J Ultrasound. 2009;12(3):125-127. PMID: 23397010.
- Adhikari S, Bhatta OP. Hydrocele of the canal of Nuck: a report of five cases. Int J Surg Case Rep. 2023;112:108993. PMID: 39491851.
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