Introduction

Ovarian hyperstimulation syndrome (OHSS) is a serious and life-threatening complication of induction of ovulation or superovulation. The syndrome is characterized by ovarian enlargement and a shift of fluid from the intravascular to the extravascular space. This leads to accumulation of fluid in the peritoneal, pleural and, rarely, the pericardial cavities, resulting in intravascular fluid depletion and haemoconcentration.

Any woman undergoing ovarian stimulation is at risk, although OHSS occurs more frequently in women with polycystic ovaries. Women with high oestradiol levels and/or a large number of follicles during ovulation induction have an increased risk of OHSS. The incidence of OHSS is between 0.25% and 4%, and severe hyperstimulation is seen in about one in 200 patients.

During the last 10 years the pathophysiology of the syndrome has been extensively investigated. Vascular endothelial growth factor (VEGF) plays a central role in the cascade of events leading to OHSS. The renin–angiotensin–aldosterone system, luteinizing hormone (LH), histamine, prostaglandins or ovarian prorenin may also have a role in the development of OHSS.

Complications are more frequent when conception does occur and a protracted course may evolve.

Classification of ovarian hyperstimulation syndrome

Mild OHSS:

• Abdominal distension.

• Mild pain.

• Ovarian size usually <8cm.

Moderate OHSS:

• Features of mild OHSS and ultrasonic evidence of ascites.

• Nausea, vomiting and diarrhoea.

• Ovarian size usually 8–12 cm.

Severe OHSS:

• Clinical ascites and/or hydrothorax.

• Haemoconcentration (haematocrit >45%, wbc > 15 000/ml).

• Coagulation and/or electrolyte disturbances.

• Hypovolaemia.

• Oliguria with elevated serum creatinine.

• Renal failure.

• Thromboembolic phenomena.

• Ovarian enlargement >12 cm.