Anhydramnios

Did You Know?

• Amniotic fluid helps maintain a constant temperature around the fetus and cushions it from injury.
• Untreated anhydramnios can lead to pulmonary hypoplasia or skeletal deformities.
• The volume of amniotic fluid typically increases until 36 weeks and starts decreasing after 40 weeks in post-term gestations.

Etiology

Anhydramnios may arise from maternal, fetal, placental, or idiopathic causes. Fetal renal abnormalities are a prominent cause: bilateral renal agenesis (Potter syndrome) is the most prevalent cause of anhydramnios, and cystic renal disease also impairs urine production. Obstruction of the lower urinary tract, such as posterior urethral valves or urethral atresia, can prevent urine from entering the amniotic sac. Medications that decrease fetal renal blood flow—including angiotensin converting enzyme inhibitors (e.g., lisinopril), prostaglandin synthetase inhibitors (NSAIDs, anti-inflammatory steroids), and trastuzumab—may lead to reduced urine output. Nimesulide and certain chemotherapeutic agents have been linked specifically to anhydramnios. Maternal conditions causing uteroplacental insufficiency, such as preeclampsia, chronic hypertension, collagen vascular disease, nephropathy, and thrombophilia, can diminish fetal urine production. Other contributors include maternal dehydration from severe diarrhea, vomiting, or excessive fluid loss; TORCH infections (Toxoplasma gondii, rubella, cytomegalovirus, herpes simplex virus) and parvovirus B19; chromosomal abnormalities like Down syndrome associated with gastrointestinal anomalies; intrauterine demise; post-term pregnancy; rupture of membranes; and intrauterine growth restriction due to placental insufficiency.

Diagnosis

Diagnosis of anhydramnios is made by transabdominal ultrasound. Two measurement methods are used: the amniotic fluid index (AFI) and the single deepest pocket (SDP). An AFI of 0 cm or an absent SDP indicates anhydramnios. For the AFI, the sonographer measures fluid in each of the four abdominal quadrants (right upper, left upper, right lower, left lower) and sums the values; a normal AFI is 5–25 cm. For the SDP, the sonographer scans all four quadrants for the deepest pocket free of fetal parts or umbilical cord, measuring from the 12 o'clock to the 6 o'clock position; a normal SDP is 2–8 cm. Randomized controlled trials have shown that use of AFI can lead to more false-positive diagnoses of oligohydramnios, and the SDP method is recommended to reduce unnecessary interventions without increasing adverse perinatal outcomes. In multiple gestation pregnancies, measuring a single deepest pocket is the most accurate method for determining adequate amniotic fluid levels. Clinical suspicion may arise from discrepancies between fundal height measurements and gestational age, decreased fetal movement, a decrease in the size of the baby's abdomen, or uterine contractions not associated with pain.

Management

After an initial diagnosis of oligohydramnios, a detailed history and physical exam are conducted, with further diagnostic tests as needed. To rule out premature rupture of membranes, a nitrazine test, ferning pattern, or fluid pooling in the cervix is checked. An ultrasound examines the fetus for structural anomalies, chromosomal issues, growth restriction, or placental problems; the National Institute of Health advises careful inspection of the kidneys, bladder, and umbilical cord. If fluid is too low for clear imaging, an MRI may be used. Genetic testing is helpful when anomalies are found. A high maternal serum alpha-fetoprotein level suggests amniotic fluid leakage from membrane or placental damage, which carries a poor outlook. If a TORCH infection is suspected, maternal blood or amniotic fluid is tested. There is no permanent way to increase amniotic fluid, but temporary increases can allow a full anatomy scan. Treatment depends on the cause and may include amnioinfusion to support fetal lung development and lower risks, close fetal monitoring, and adjusting delivery timing based on the severity and fetal condition.

Prognosis

The prognosis of anhydramnios depends on the underlying cause, severity, gestational age at diagnosis, and presence of other complications. Babies with anhydramnios caused by fetal renal abnormalities have a much poorer prognosis, with a high mortality rate. Those with anhydramnios due to other factors, such as premature rupture of membranes (PPROM), have a better outlook. Severe anhydramnios, diagnosis early in pregnancy, and the presence of additional complications like fetal growth restriction or skeletal deformities worsen the prognosis. With early diagnosis and appropriate treatment, many babies with anhydramnios can be born healthy, but those with fetal renal abnormalities may require long-term medical care and may have developmental disabilities.