Discussion: This 15 year old girl presented with a 3 day history of lower abdominal pain and vomiting. Bilateral adnexal cysts were demonstrated sonographically. At surgery, bilateral parovarian cysts were found. The larger right sided cyst appeared torsed and hemorrhagic.

Parovarian cysts constitute 10% of adnexal masses [1]. They are found in the broad ligament, specifically, in the mesosalpinx between the ovary and fallopian tube. Three main types of cysts have been described, based on the histopathology of the cyst lining. Most (68%) are of mesothelial origin, 30% arise from paramesonephric elements, and 2% are of mesonephric origin [2].

Mesothelial type cysts are thought to be inclusion cysts arising from the mesothelium adjacent to the pelvic peritoneum and are lined with flattened epithelium. At 9 weeks gestation, the caudal extent of the paramesonephric (Mullerian) ducts fuse to form the uterus and superior aspect of the vagina with the cranial ends of the ducts developing into the fallopian tubes. Compression of the tubal ostia results in multiple adjacent invaginations which form the fimbriae of the tubes. If accessory lumina or blind sacs form in this process, they may enlarge to form a paramesonephric cyst [1,2,3]. The cysts are lined with secretory, ciliated columnar or cuboidal epithelium characteristic of paramesonephric tissues [2,4]. The mesonephric (Wolffian) duct runs parallel to the fallopian tube and joins the embryonic remnants of the mesonephric tubules in the hilum of the ovaries. Mesonephric type cysts develop from the mesonephric ducts or tubular remnants and are lined with cuboidal or flattened epithelium [4].

Parovarian cysts are found over a wide age range, and are most common in the third and fourth decades [2]. Paramesonephric cysts rarely occur before puberty. Parovarian cysts also vary greatly in size, ranging from tiny to massive. Larger cysts tend to be seen in younger patients and are usually of mesothelial origin [2]. Cysts are rarely bilateral or multiple. Hemorrhage, torsion, rupture, or secondary infection are infrequent complications [1]. Neoplasms occur in approximately 2% of cysts [5]. Pathologically, these include serous cystadenomas, serous adenofibromas, papillary serous malignancy, and cystadenocarcinoma [1,2]. Neoplasms have been seen in cysts as small as 4 cm. in diameter [5].

At surgery, parovarian cysts have a characteristic appearance. They are typically located in the mesosalpinx, separate from the ovary, with the fallopian tube often stretched over the cyst [1]. Cysts are intimately blended with the peritoneal lining of the broad ligament and contain serous type fluid [6].

Most parovarian cysts are symptomatic, presenting with lower abdominal pain, menstrual irregularities or increased abdominal girth. Symptoms are thought to be due to displacement of pelvic organs by the cyst and are similar to those of ovarian cysts. Preoperative diagnosis of parovarian cyst, however, is rarely made [3]. Sonographically, parovarian cysts are usually thin walled, unilocular with anechoic contents. Although, it was previously thought that parovarian cysts were difficult to differentiate from ovarian cysts by sonography [1,6,7], a recent report by Kim et al. detected a normal ipsilateral ovary in 76% of patients [8]. Large parovarian cysts may be located superior to the uterus mimicking the urinary bladder. Unlike physiologic ovarian cysts, parovarian cysts do not change over the course of the menstrual cycle or with hormonal administration [1,6].

Eccentric ovarian cysts occurring at the ovarian edge, can be mistaken for parovarian cysts. They can be differentiated by identifying crescentic shaped ovarian tissue surrounding a portion of the ovarian cyst margin. Hydrosalpinx may also appear as an anechoic, thin walled cyst. They typically have a tubular appearance, however. Additionally, clinical findings of pelvic inflammatory disease help distinguish this from a parovarian cyst [8]. Cystic ovarian tumor, endometrioma or tuboovarian abscess may simulate the diagnosis but more typically will demonstrate a thick or irregular wall, septations and internal echoes [1,3].

References:
1. Athey PA, Cooper NB. Sonographic features of parovarian cysts.
AJR 1985; 144:83- 86.

2. Genadry R, Parmley T, Woodruff J. The origin and clinical behavior of
the parovarian tumor. Am J Obstet Gynecol 1977; 129:873-879

3. Ravo B, Metwally N, Pal P, Ger R. Developmental retroperitoneal cysts
of the pelvis, a review. Dis Colon Rectum 1987; 30:559-564.

4. Stenbäck F, Kauppila A. Development and classifcation of parovarian
cysts. an ultrastructural study. Gynecol Obstet Invest 1981; 12:1-10.

5. Stein AL, Koonings PP, Schlaerth JB, Grimes DA, d'Ablaing III G. Relative
frequency of malignant parovarian tumors: should parovarian tumors be aspirated?
Obstet Gynecol 1990; 75:1029-1031.

6. Alpern MB, Sandler MA, Madrazo BL. Sonographic features of parovarian cysts
and their complications. AJR 1984; 143:157-160.

7. Ralls PW, Rotter AJ, Halls JM. Non-ovarian adnexal pathology. Seminars in
Ultrasound 1983; 4:193-205.

8. Kim JS, Woo SK, Suh SJ, Moretin LB. Sonographic diagnosis of parovarian cysts:
value of detecting a separate ipsilateral ovary. AJR 1995; 164:1441-1444.

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