Ovarian torsion

Ovarian torsion (also sometimes termed adnexal torsion) refers to rotation of the ovary and portion of the fallopian tube on the supplying vascular pedicle (also termed tubo-ovarian torsion).
This can be intermittent or maintained and results in venous, arterial and lymphatic stasis. It is a gynaecologcial emergency and requires urgent surgical intervention to prevent ovarian necrosis. 

Epidemiology

Ovarian torsion has a bimodal age distribution occurring mainly in young women (15-30 years) and in post-menopausal women. Approximately 20% of the cases occur during pregnancy. 1

Torsion occurs for two main reasons: 2

hypermobility of the ovary ~ 50%

adnexal lesion ~ 50-80%

most lesions are dermoid cysts or para-ovarian cysts

Clinical presentation

Most patients present with severe lower abdominal and pelvic pain, nausea, and vomiting. A raised white cell count is common.

Pathology

The end-result of ovarian torsion is haemorrhagic infarction and necrosis. This can occur within hours of torsion onset.
Torsion of a normal ovary more commonly occurs in young children when developmental abnormalities may be implicated in the pathogenesis, such as excessively long fallopian tubes or an absent mesosalpinx. In adulthood causes include both benign and malignant ovarian tumours, polycystic ovaries and adhesions. In early pregnancy a torsion can occur secondary to a corpus luteum cyst or laxity of the adjacent tissues.

Radiographic features

The main feature of torsion is ovarian enlargement due to venous / lymphatic engorgement, oedema and haemorrhage. Secondary signs include free pelvic fluid, an underlying ovarian lesion, reduced or absent vascularity and a twisted dilated tubular structure corresponding to the vascular pedicle. Adnexal torsion is generally unilateral, with a slight (3:2) right-sided predilection (presumably due effects of the sigmoid colon being on the left) 6,8.

Ultrasound

Ultrasound is the imaging modality of choice. Sonographic features include:

enlarged hypo or hyperechoic ovary

peripherally displaced follicles 

Doppler 3

little or no intra-ovarian venous flow (common)
absent arterial flow (less common)
absent or reversed diastolic flow
normal vascularity does not exclude intermittent torsion
normal Doppler flow can also occasionally be found due to dual supply from both the ovarian and uterine arteries
free pelvic fluid: may be seen in > 80% of cases 10.
whirlpool sign of twisted vascular pedicle 3
underlying ovarian lesion can often be found
the uterus may be slightly deviated towards the torted ovary

CT

good at ruling out ovarian torsion if a normal ovary/adnexa is see
twisted ovarian pedicle is pathognomonic for ovarian torsion if demonstrated 11
torsion appears as a complex adnexal lesion representing:
enlarged ovary (>4.0cm 11)
distended pedicle
possible underlying ovarian lesion
HU > 50 on non-contrast CT suggests haemorrhagic necrosis 2
lack of enhancement may be seen
surrounding fat stranding oedema and free fluid

Pelvic MRI

Usually not the imaging modality of choice as urgent imaging is required.
If haemorrhagic infarction is present signal changes include 4
T1
thin rim of high signal (methaemaglobin) without contrast enhancement
endometriomas and haemorrhagic corpus luteal cysts are less likely to have high T1 rim and do not usually involve the entire ovary
T2 - can have low signal due to interstitial haemorrhage
Treatment and prognosis
Urgent surgery is required to prevent ovarian necrosis. Most ovaries are not salvageable in which case a salpinogo-oophorectomy is required. If not removed the necrotic ovary can become infected and cause abscess or peritonitis. In the case of a non-infarcted adnexa, surgical untwisting can be performed. Mortality resulting from ovarian torsion is rare. Spontaneous de-torsion has also been reported.

Differential diagnosis

For an enlarged oedematous ovary +/- Fallopian tube, consider:
pelvic inflammatory disease (PID) - has a very different clinical presentation
massive ovarian oedema (MOO)
References