Accessory and cavitated uterine mass (ACUM)

Accessory and cavitated uterine mass (ACUM) also known as non-communicating accessory uterine cavity or cystic adenomyoma, is a rare uterine anomaly comprised of an accessory uterus-like mass in the uterus along the wall but with no communication to the main uterus.

Clinical presentation

This pathology generally presents at a young age (before 30 years) with pelvic pain and dysmenorrhea.

Pathology

Many authors consider ACUM a congenital anomaly, however other theories of development including heterotopias and metaplasia exist. The condition is different from cystic adenomyosis where the cysts are usually small and do not have a typical endometrial lining or uterus-like muscle which is seen in ACUM. The main uterus does not show features of adenomyosis but the myometrium of ACUM may show features of adenomyosis.

Radiographic features

Imaging criteria

• accessory cavitated mass along the uterine wall under the round ligament
• normal uterus, fallopian tubes, and ovaries
• blood degradation products in the cavity/cystic part
• no adenomyosis in the main uterus but features of adenomyosis can be seen in ACUM itself

Ultrasound

Ultrasound is usually the first investigation. Ultrasound can demonstrate a solid mass along the uterine wall with a variable cystic component that can resemble an endometrioma.

It can be difficult to differentiate ACUM from a unicornuate uterus with obstructed rudimentary horn. Hysterosalpingography can help in such cases.

MRI

MRI will typically show an accessory cavitated mass along the uterine wall under the round ligament with T2 hyperintense endometrial lining and blood degradation products in the cavity.

It will further demonstrate a normal uterus, cornua, and ovaries which helps rule out other lesions in the region.

Differential diagnosis

Main differential diagnoses include:

• cystic degeneration in leiomyoma
• unicornuate uterus with obstructed rudimentary horn