Ovarian hyperstimulation syndrome (OHSS) occurs in a third of women undergoing IVF cycles. Ovarian stimulation with follicle stimulating hormone (FSH) followed by exposure to either lutenising hormone (LH) or beta human chorionic gonadotrophin (β-hCG) causes release of pro-inflammatory mediators; chiefly vascular endothelial growth factor (VEGF). This causes a sudden increase in vascular permeability, massive fluid extravasation to the extravascular space and intravascular volume depletion.
Loss of fluid into the extravascular space causes a profound fall in intravascular volume, haemoconcentration and suppression of urine formation. Loss of protein into the extravascular space causes a fall in plasma oncotic pressure, which results in further loss of intravascular fluid.
Alongside this there is a re-setting of osmotic thresholds leading to hyponatraemia and a lower serum osmolality. OHSS is a self-limiting condition that requires supportive treatment until recovery occurs.
Mild OHSS occurs in up to 33% of women and 3.1–8% of IVF cycles are associated with moderate–severe OHSS ( Balen 2005 ), but only 3% experience severe OHSS requiring hospital admission.
Most cases occur after gonadotrophin ovarian stimulation in IVF cycles but it may occasionally occur with non-IVF clomiphene ovulation inductionor very rarely with pregnancy.
|Younger age||Higher doses of exogenous gonadotrophin|
|Low bodyweight||Higher estradiol levels|
|Previous OHSS||Higher number of follicles|
|Use of GnRH agonists||Renders the ovary more amenable to stimulation of multifollicular development by high-dose gonadotrophin treatment.|
|Development of multiple follicles during treatment|
Indicates an exuberant response to gonadotrophic stimulation
|Exposure to LH/hCG||Stimulates the processes that mediate neovascularisation and vascular permeability|
|Successful IVF cycles||High levels of endogenous β-hCG augment exogenous β-hCG|
Diagnosis is clinical, though important differential diagnoses should be ruled out:
- ovarian cyst haemorrhage
- ovarian cyst rupture
- ovarian torsion
- pelvic infection
- ectopic pregnancy.
It is important to note that severe pain is uncommon in OHSS.