Complete hydatidiform mole

Complete hydatidiform moles (CHM) are a type of molar pregnancy and fall at the benign end of the spectrum of gestational trophoblastic disease. 

Epidemiology

A complete hydatidiform mole is the most common type of gestational trophoblastic disease.

Pathology

Complete moles are characterized by the absence of a fetus or fetal parts (i.e. no embryonic tissues). There is a non-invasive, diffuse swelling of chorionic villi.

Significant difference is seen among the pathologists in the diagnosis of molar pregnancies just based on histopathological examination of the products of conception (POC) ⁸. The p57KIP2 gene is paternally imprinted and expressed from the maternal allele ^8,9. Polymer-based immunohistochemistry (IHC) with p57, shows absent staining in the complete mole (CM) and positive staining in the hydropic abortus (HA) and partial mole (PM) ^8,9. This IHC staining is a useful and inexpensive tool that can help distinguish a complete mole from its mimics and avoid DNA analysis 8,9.

Genetics

Approximately 90% of complete hydatidiform moles have a 46XX diploid chromosomal pattern, with ~10% having a 46XY composition. All the chromosomes are derived from the sperm, suggesting fertilisation of a single egg that has lost its chromosomes.

Markers

Serum beta HCG levels are markedly elevated, out of proportion to the pregnancy. 

Radiographic features

Ultrasound

• enlarged uterus

• may be seen as an intrauterine mass with cystic spaces without any associated fetal parts

  • the multiple cystic structures classically give a "snowstorm" or "bunch of grapes" type appearance.

• may be difficult to diagnose in the first trimester ⁶

  • may appear similar to a normal pregnancy or as an empty gestational sac

  • <50% are diagnosed in the first trimester

• bilateral theca lutein cysts may also be seen on ultrasound

• color Doppler interrogation may show high velocity with a low impedance flow

CT

CT evaluation is not usually performed due to its low resolution for the uterine assessment. CT may show an enlarged uterus with areas of low attenuation, or hypoattenuating foci surrounded by highly enhanced areas in the myometrium.

MRI

MRI may demonstrate a heterogeneous mass with cystic spaces distending the uterine cavity. Fetal parts are notably absent. Uterine zonal anatomy is often distorted although a hypointense irregular myometrial boundary may be seen ³. MRI may also demonstrate bilateral theca lutein cysts ^ref.

Signal characteristics

• T1: may be areas of high signal corresponding to foci of hemorrhage

• T2: heterogeneous high signal from the cystic spaces

• T1 C+ (Gd): often intense enhancement due to hypervascularity

Treatment and prognosis

Suction and curettage are used for evacuation and are usually the routine treatment. Serial beta hCG levels are performed until it is no longer detectable on the assay, as it is the main indicator of residual disease.

A complete mole is itself benign but is considered a premalignant lesion. Degeneration into more invasive and malignant types of gestational trophoblastic disease can occur in ~15% (range 10-20%) of cases ^ref.

See also

• partial hydatidiform mole

• choriocarcinoma