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St. Michael's Hospital
Toronto, ON, Canada
We present a rare case of endometriosis in the canal of Nuck. The canal of Nuck is an embryologic remnant of the processus vaginalis peritonei that can remain patent and has been described as a site for endometriosis seeding [1, 2]. It is a dilatation of peritoneum that accompanies the round ligament and extends from the inguinal canal into the vulva.
A healthy 31-year-old premenopausal woman presented with a mass overlying her right pubis. The mass had been present for 6 months and had been slowly increasing in size over that time period. The size changes were noted to concur with menses. Her medical history was unremarkable. She denied catamenial pain at the time of presentation. Physical examination revealed a 2-cm subcutaneous thickening to the right of the midline overlying the patient's right pubis that was painful on palpation.
Sonography revealed a subcutaneous, well-defined hypoechoic nodule measuring 1.6 x 1.0 cm (Fig. 4A). Aspiration biopsy yielded insufficient information for a diagnosis. Contrast-enhanced CT of the abdomen and pelvis revealed a small homogeneous soft-tissue nodule comparable to that seen on sonography. Subsequent T1- and T2-weighted MRI of the pelvis showed an intermediate-signal-intensity nodule with homogeneous enhancement that measured 1.0 x 1.8 x 1.2 cm (Figs. 4B, 4C, 4D). The lesion was felt to be superficial to the abdominal wall musculature. No associated pelvic disease was identified. Imaging features suggested that the mass could represent inflammatory disease. No specific features of endometriosis or a neoplastic process, such as a soft-tissue sarcoma, were present. Excisional biopsy of the lesion was performed, and pathologic examination provided a diagnosis of endometriosis.
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Sonography and MRI are the primary diagnostic imaging tools in classical cases of endometriosis. Sonography typically shows homogeneous hypoechoic focal lesions with diffuse low-level internal echoes [3]. MRI has high sensitivity and specificity compared with other imaging techniques in the diagnosis of pelvic endometriosis. Sensitivity and specificity in diagnosing endometrial cysts have been reported as high as 90% and 98%, respectively [4]. The high sensitivity and specificity relate to high signal intensity on T1-weighted images and the "shading" phenomenon on T2-weighted images. MRI in this case was nonspecific because of the intermediate signal intensity on both T1- and T2-weighted images. Although inflammatory disease and endometriosis were considered, a soft-tissue sarcoma could not be excluded from the differential diagnosis. Turpin et al. [1] described a similar case of endometriosis in which MRI was performed and the results facilitated a diagnosis. In their patient, however, T1- and T2-weighted images showed high signal intensity, as is seen in classic cases of pelvic endometriosis.
This case illustrates the challenges in diagnosing atypical presentations of well-recognized diseases. A palpable mass in the subcutaneous tissues of the pelvis is a nonspecific finding. However, in women at risk for endometriosis, we believe that the traditional dogma of uncommon manifestations of a common lesion must be considered in the differential diagnosis as illustrated by the unusual imaging findings in our patient with extrapelvic endometriosis in the canal of Nuck.
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  D. M. Yang, H. C. Kim, W. Jin, C. W. Ryu, J. K. Ryu, D. H. Nam, S. I. Choi, and S. J. Lim Inguinal Endometriosis Presenting as a Multicystic Mass on Sonography J. Ultrasound Med., October 1, 2007; 26(10): 1449 - 1451. [Full Text] [PDF]
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