Severity Classification Mild OHSS (grades 1 and 2) is associated with high serum estradiol levels and ovarian enlargement to less than 5 cm and has minimal clinical significance. Grade 3 moderate OHSS is accompanied by mild abdominal distention and has minimal clinical significance (397). Grade 3 OHSS occurs in one-third of all COH cycles (398). Grade 4 moderate OHSS is accompanied by gastrointestinal upset and ovarian enlargement to 5 to 12 cm (397). The presence of ﬂuid shifts from intra-to extravascular spaces are the hallmark of severe disease, and grade 5 OHSS includes tense ascites or hydrothorax. Grade 6 OHSS is accompanied by hemoconcentration, coagulation abnormalities, respiratory failure, and renal dysfunction (397). Other markers of severe OHSS include hyponatremia, hyperkalemia, elevated liver function tests, and an elevated white blood cell count (397,398). Concerning symptoms for severe disease include rapid weight gain (≥2 lb per day), increased measurable abdominal girth, hypotension, tachypnea, tachycardia, oliguria, and severe abdominal pain. The latter symptom is suggestive of ovarian cyst rupture or hemorrhage (398).
Risk Factors Mild OHSS occurs in 13.5% of clomiphene cycles, but more severe disease is rare. With gonadotropin injections, the incidence of moderate disease is 3% to 6% and severe OHSS occurs in 0.1% to 2% of patients. Polycystic ovary syndrome, polycystic ovarian morphology with attendant elevated antim¨ullerian hormone levels (>3.36 ng/mL), and previous episodes of OHSS are major risk factors for the disease; the impact of young age and lean body mass is controversial (351,398,400). Estradiol levels greater than 800 pg/mL on day 9 of stimulation have been associated with a 55.8% risk for the development of OHSS (severity not specified). Estradiol concentrations of greater than 3,500 pg/mL and greater than 6,000 pg/mL at the time of hCG trigger were associated with severe OHSS in 1.5% and 38% of patients, respectively. During antagonist cycles, the presence of 13 or more follicles at 11 mm or greater average diameter was predictive of the development of OHSS. More than 20 preovulatory follicles were associated with a 15% incidence of severe OHSS. When 20 to 29 oocytes were collected at retrieval, 1.4% of patients developed severe OHSS; this raised to 22.7% with 30 or more oocytes (400).