Ovarian Cysts

Introduction

Ovarian cysts are fluid-filled sacs arising within or on the surface of the ovary. They are extremely common - detected incidentally on a significant proportion of pelvic ultrasounds in premenopausal women - and the great majority are benign, resolving spontaneously or requiring no more than surveillance.¹

The clinical priority when a cyst is found is not simply to characterise it, but to stratify the risk of malignancy, since this determines whether a woman is managed conservatively in primary or general gynaecology care, or referred to a specialist gynaecological oncology multidisciplinary team (MDT).

Classification

Functional cysts

Arise from the normal ovulatory cycle and are the most common type overall, particularly in younger women. A follicular cyst forms when a dominant follicle fails to rupture at ovulation and continues to grow. A corpus luteum cyst forms when the corpus luteum fails to regress after ovulation, sometimes filling with fluid or blood; these are more likely to cause pain or, occasionally, spontaneous haemorrhage. Both types typically resolve spontaneously within one to three menstrual cycles.¹

Pathological (non-functional) cysts

Malignant ovarian tumours (primary epithelial cancers, germ cell tumours, sex cord-stromal tumours) are a separate and important differential, particularly in postmenopausal women, and are covered in the ovarian cancer article.

Risk factors

Risk factors differ depending on whether the concern is a functional cyst or malignancy.¹

For functional cysts

• Reproductive age (any cycle with ovulation)
• Early menarche
• Subfertility treatment with ovulation induction
• Tamoxifen use

For malignancy

• Postmenopausal age
• Family history of ovarian or breast cancer, or known BRCA1/BRCA2 mutation
• Nulliparity
• Early menarche and late menopause (more ovulatory cycles over a lifetime)
• Hormone replacement therapy (weak association)

Clinical features

Many ovarian cysts are entirely asymptomatic and found incidentally. When symptomatic, features include chronic pelvic or abdominal pain or a dragging sensation, bloating, a palpable abdominal or pelvic mass in large cysts, dyspareunia, and menstrual irregularity (more likely with functional or hormonally active cysts). Pressure effects from a large cyst can cause urinary frequency or, rarely, venous compression causing leg swelling.²

Acute presentations occur when a cyst undergoes torsion, ruptures, or bleeds internally - these present as an acute abdomen and are discussed below under complications.

Investigations and risk stratification

Transvaginal ultrasound

First-line and most informative investigation, assessing size, laterality, and morphological features (simple/unilocular versus complex/multilocular, presence of solid components, septations, papillary projections, and blood flow on Doppler) that predict benign versus malignant behaviour.

IOTA simple rules

The International Ovarian Tumor Analysis (IOTA) simple rules classify a cyst as benign, malignant, or inconclusive based on five benign (B-) and five malignant (M-) ultrasound features - for example, a unilocular cyst with no blood flow is a benign feature, while an irregular solid tumour with strong blood flow is a malignant feature. They are widely used in specialist ultrasound assessment to guide onward management.³

Tumour markers

CA-125 is requested in all postmenopausal women with an ovarian cyst, and in premenopausal women if the ultrasound is suspicious. It is non-specific and can be raised by endometriosis, fibroids, pregnancy and pelvic infection, so it is interpreted alongside imaging rather than in isolation. In women under 40 with a complex mass, additional markers (AFP, hCG, LDH) are checked to screen for a germ cell tumour.

Risk of Malignancy Index (RMI)

RMI = ultrasound score × menopausal status score × CA-125 level, and is the standard tool used to decide whether a woman needs referral to a gynaecological oncology MDT.⁴ A high RMI (locally defined threshold, commonly ≥200) warrants urgent referral; a low RMI supports conservative or general gynaecology management.

Management

Management depends on age, cyst characteristics, symptoms and risk stratification.²

Conservative management

Simple, unilocular cysts in a premenopausal woman with a normal or reassuring RMI are managed conservatively, as the great majority resolve spontaneously - typically re-scanned after a period of observation (often around 6-12 weeks) to confirm resolution or stability. RCOG guidance stratifies this by size:²

Surgical management

Indicated for large cysts (typically over 5-7 cm), persistent or enlarging cysts, symptomatic cysts, complex morphology, or a raised RMI. Laparoscopic ovarian cystectomy (removing the cyst while preserving the ovary) is preferred where possible, particularly in women wishing to preserve fertility. Oophorectomy is used for large cysts, when malignancy cannot be excluded, or in postmenopausal women, where the ovary's reproductive function is no longer relevant. Any cyst with suspicious features on imaging or a high RMI should be managed via a gynaecological oncology MDT, with surgery planned to allow full staging if malignancy is confirmed intraoperatively.

Acute complications

Ovarian cysts are an important cause of the acute abdomen in women and should always be considered alongside appendicitis and ectopic pregnancy in a woman presenting with acute lower abdominal pain.²

Ovarian torsion

The ovary (with or without the fallopian tube) twists on its pedicle, compromising its blood supply - a surgical emergency, since delay risks irreversible ovarian necrosis. Larger cysts (particularly dermoid cysts, given their weight) increase the risk. Presents with sudden-onset, severe, unilateral pelvic pain, often with nausea and vomiting, and pain that may wax and wane if the ovary intermittently detorts. Diagnosis is supported by ultrasound with Doppler showing reduced or absent ovarian blood flow, but a normal Doppler does not exclude torsion, and diagnostic laparoscopy is the definitive investigation and treatment (detorsion, with cystectomy or oophorectomy depending on ovarian viability).

Cyst rupture

Presents with sudden, severe pain, often triggered by intercourse or exercise, sometimes with signs of peritonism if there is significant intraperitoneal bleeding or cyst content spillage. Most cases are managed conservatively with analgesia and observation; surgery is reserved for haemodynamic instability or ongoing bleeding.

Haemorrhage into a cyst

Bleeding within a cyst (most often a corpus luteum cyst) causes acute pain from capsule distension. Managed conservatively unless there is evidence of significant intraperitoneal haemorrhage causing haemodynamic compromise.

Ovarian cysts in pregnancy and postmenopause

Corpus luteum cysts are common in early pregnancy, supporting progesterone production until the placenta takes over, and typically resolve by the second trimester without intervention. Any persistent or enlarging cyst in pregnancy is monitored with ultrasound because torsion risk rises as the enlarging uterus displaces the ovary.

In postmenopausal women, ovaries should no longer be forming functional cysts, so any new ovarian cyst is assessed with a lower threshold for concern - CA-125 and RMI calculation are performed in all cases, and simple cysts under 5 cm with a normal CA-125 may still be managed conservatively with surveillance, but the threshold for surgical assessment is lower than in premenopausal women.

Prognosis

The overwhelming majority of ovarian cysts are benign and either resolve spontaneously or are cured by conservative surgery, with excellent long-term outcomes and no impact on fertility when cystectomy preserves normal ovarian tissue. The key task in every case is correctly identifying the minority of cysts that harbour malignant risk through careful ultrasound assessment and RMI calculation, since outcomes for ovarian cancer depend heavily on the stage at which it is found.