Anhydramnios is the complete absence of amniotic fluid, the protective liquid surrounding a baby that is generated primarily from fetal urine. The condition is distinct from oligohydramnios, which is a low level of amniotic fluid; anhydramnios signifies no measurable fluid at all. The absence of this fluid has significant implications for fetal development and is identified during routine prenatal check-ups.
Causes and Diagnosis of Anhydramnios
The origins of anhydramnios are most often traced to the fetus’s inability to produce urine. A primary cause is bilateral renal agenesis, a condition where both kidneys fail to develop. Severe blockages in the fetal urinary tract, such as posterior urethral valves, or non-functional kidneys like multicystic dysplastic kidneys, can also prevent urine from reaching the amniotic sac.
Problems with the placenta can also lead to a lack of fluid. Severe placental insufficiency can diminish blood flow to the fetus, impairing kidney function and urine output. Another cause is Preterm Prelabor Rupture of Membranes (PPROM), where the amniotic sac breaks and a large, continuous leak depletes the fluid.
Diagnosing anhydramnios is accomplished through a prenatal ultrasound. A sonographer scans the uterus to find any measurable pockets of amniotic fluid. To formalize this finding, clinicians use a measurement called the Amniotic Fluid Index (AFI). An AFI of zero confirms the diagnosis of anhydramnios.
Fetal Development Complications
Amniotic fluid serves several functions for a developing fetus. It creates a cushioned, temperature-stable environment that allows for free movement for proper musculoskeletal growth. The fluid also keeps the umbilical cord from being compressed, which would cut off oxygen and nutrient supply. The fetus also swallows and “breathes” the fluid, a process needed for the lungs to grow and mature.
The most significant consequence of anhydramnios is pulmonary hypoplasia, or underdeveloped lungs. Without amniotic fluid to inhale and exhale, the fetal lungs cannot expand and develop the structures needed for breathing air after birth. This lack of development means the lungs are too small and immature to support life. The severity is directly related to how early in gestation the anhydramnios occurs and how long it persists.
The sustained lack of amniotic fluid leads to a pattern of physical characteristics known as Potter sequence. Without the buoyancy of fluid, the fetus is compressed by the uterine walls. This compression restricts movement and leads to limb deformities, such as contractures and clubfoot, and distinct facial features called Potter facies, which include a flattened nose and low-set ears.
Medical Management and Prognosis
Management of anhydramnios is dictated by its underlying cause and the gestational age at diagnosis. The first step involves a detailed ultrasound to identify the reason for the lack of fluid, such as evaluating the presence and functionality of the fetal kidneys and bladder. This clarification is important because the cause heavily influences potential outcomes.
One intervention that may be considered is amnioinfusion, which involves instilling a sterile saline solution into the amniotic cavity. Amnioinfusion can be used diagnostically to create a fluid-filled space for better ultrasound visualization of fetal anatomy. Some centers perform serial therapeutic amnioinfusions to provide fluid for lung development, but this approach is considered experimental and is not standard practice.
The prognosis for a fetus with anhydramnios diagnosed in the second trimester is poor, primarily due to lethal pulmonary hypoplasia. If the cause is bilateral renal agenesis or another untreatable kidney abnormality, the outlook is poor as the fetus cannot produce its own fluid. The prognosis can be different if anhydramnios is caused by PPROM later in pregnancy, as the lungs may have already reached a more mature stage.
Post-Birth Considerations
Should the pregnancy continue and delivery be attempted, the newborn will require immediate and intensive medical care. The primary challenge is severe respiratory distress due to underdeveloped lungs. This necessitates intervention from a neonatal team, often involving mechanical ventilation and other respiratory support in a neonatal intensive care unit (NICU).
If the infant survives the initial postnatal period and the cause was a treatable urinary tract obstruction, long-term medical challenges remain. The focus shifts to managing kidney function. This could involve complex surgeries to correct the blockage, and the child may face chronic kidney disease requiring lifelong care, including dialysis or an eventual kidney transplant.
For pregnancies where the prognosis is understood to be lethal due to conditions like bilateral renal agenesis, families may be offered perinatal hospice or palliative care. This approach focuses on providing comfort, dignity, and quality of life for the baby. It also provides comprehensive emotional, spiritual, and psychological support for the parents and family, helping them navigate a difficult time.