Oligohydramnios

Oligohydramnios

Description

Oligohydramnios is defined as a generalised decrease in amniotic fluid so that the vertical diameter of the largest pocket is less than 1 cm, or the largest pocket measured in two perpendicular planes is 1 cm or less, or the AFI is below the 5th percentile for gestational age. Oligohydramnios may also be defined as an AFI less than 5.1 cm. Normally amniotic fluid volume increases steadily from about 250 ml at 16 weeks of gestation to 800 ml at 28 weeks, 1000 ml at 34 weeks and declines to 800 ml at 40 weeks. The volume is determined by a steady state between input: fetal urination 400-1000 ml/da, alveolar exudate 600/900 ml/day, and outflow: fetal swallowing 250-450 ml/day and reabsorption through the chorioamniotic membranes 80 ml/day. Input is affected by maternal blood flow to the placenta and the capacity for fetal maternal exchange at the intervillous space, modulated by maternal serum osmolality and maternal intervascular volume. Oligohydramnios is associated with increased perinatal morbidity and mortality and develops in 0.5-4.0 percent of all pregnancies. It has been associated with congenital anomalies such as renal dysgenesis or obstruction, intrauterine growth retardation (IUGR), post maturity and placental problems related to maternal fetal interaction. When oligohydramnios develops early in pregnancy, the fetal outcome is generally poor due to development of pulmonary hypoplasia, fetal compression syndrome, and possible amniotic band syndrome. Barss et al. found second trimester oligohydramnios to be associated with no survivors in their series of twelve pregnancies. Without adequate lung development, extrauterine life is impossible. Premature rupture of membranes is a cause of oligohydramnios and occurs in approximately 10 percent of all pregnancies. It has been correlated with an increased incidence of perinatal death, fetal distress in labour, and infection. Oligohydramnios associated with IUGR may result from intrauterine hypoxemia which causes shunting of fetal blood flow away from the kidneys and lungs to more vital organs, the brain and heart. Decreased urine production and alveolar transudate result in a decrease of amniotic fluid production. Maternal hypertension may result in IUGR from placental insufficiency. When hypertension is a manifestation of pre-eclampsia, reduction of the maternal intravascular volume may also be reflected by fetal volume contraction with resultant fetal oliguria and oligohydramnios. Certain maternal drug therapies such as maternal indomethacin treatment, often used as an adjunct in treatment of premature labour may induce oligohydramnios. The mechanism is thought to relate to decreased renal perfusion.

Diagnosis

Oligohydramnios is diagnosed by subjective assessment by an experienced examiner of a markedly decreased or absent amniotic fluid volume. A largest pocket with a vertical diameter of less than 1 cm or a diameter of 1 cm or less in two perpendicular planes is consistent with severe oligohydramnios. An AFI below the 5th percentile for gestational age also indicates oligohydramnios, and an AFI less than 5.1 cm is also considered consistent with oligohydramnios. When oligohydramnios is diagnosed, careful assessment for fetal malformations is indicated including observation of fetal urinary bladder filling. Unfortunately, the presence of oligohydramnios makes such a search more difficult. Colour flow doppler studies, amnioinfusion, and the transvaginal rather than the transabdominal approach may be helpful in selected cases.

Differential Diagnosis

No differential diagnosis found with oligohydramnios but aetiology varies as discussed under definition

Sonographic Features

Markedly decreased or absent amniotic fluid volume

The biggest pocket of amniotic fluid with a vertical diameter of less than 1 cm or a diameter of 1 cm or less in two perpendicular planes is consistent with severe oligohydramnios

For Amniotic Fluid Index (AFI), percentile values are available from 16-42 weeks.

An AFI below the 5th percentile at those gestational ages indicates oligohydramnios

An AFI less than 5 cm. is also consistent with oligohydramnios

Associated malformations frequent

Early oligohydramnios is associated with pulmonary hypoplasia (and poor prognosis)

Associated Syndromes

  • Amniotic band
  • BRA
  • Deletion 4p
  • Ellis Van Creveld
  • Goltz
  • Infantile Polycystic Kidney Disease (IPKD)
  • Maternal indomethacin exposure
  • Meckel Gruber
  • Megacystis
  • Megacystis-microcolon-intestinal hypo-peristalsis (MMIH)
  • Mosaic Trisomy 9
  • Multicystic dysplastic kidneys (MKD)
  • Posterior urethral valve (PUV)
  • Sirenomelia
  • Triploidy
  • Trisomy 13
  • Trisomy 18
  • Ureterovesico junction (UVJ)
  • VATER

References

Chervenak FA, Isaacson GC, Campbell S In: Ultrasound in Obstetrics and Gynecology Vol I Little, Brown, & Co, p555-563, p565-568
Chervenak FA, Isaacson GC, Campbell S In: Ultrasound in Obstetrics and Gynecology Vol I Little, Brown, & Co, p1063-1081
Nyberg D, Mahony B, Pretorius D In: Diagnostic Ultrasound of Fetal Anomalies Vol II Mosby Year Book: St. Louis, p38-50
Fleischer A, Romero R, Manning F, Jeanty P, James Jr. A In: The Principles and Practice of Ultrasonography and Obstetrics and Gynecology, 4th Edition Appleton & Lange: Norwalk, CT, p196
Scott RJ, Goodvurn SF Potter’s syndrome in the 2nd trimester – prenatal screening and pathological findings in 60 cases of oligohydramnios sequence Prenatal Diag 15:519-29
Newboald MJ, Barson AJ Oligohydramnios sequence: spectrum of renal malformations Br J Obstet Gynecol Vol 101:598-604
Barss VA, Benacerraf BR, Frigoletto FD Second trimester oligohydramnios. A predictor of poor fetal outcome Obstet Gynecol 64:608