Recognizing the Signs: Symptoms and Diagnosis of CSD

When a baby is born with Congenital Sodium Diarrhea (CSD), the symptoms often begin before they even enter the world. Because the condition is so rare, it is frequently confused with more common newborn issues like a blocked bowel or a kidney disorder. Understanding the specific signals your baby is sending can help the medical team reach the correct diagnosis faster. Diagnosis generally integrates the clinical pattern, stool electrolytes and osmotic gap, acid-base findings, urine studies, exclusion of mimics, and molecular results.

Early Warning Signs

The signs of CSD usually appear in two stages: during pregnancy and immediately after birth.

• Prenatal Signs: Many parents are told during the third trimester that there is polyhydramnios (excess amniotic fluid) ^[1][2]. This happens because the baby is passing large amounts of watery diarrhea into the amniotic sac. On an ultrasound, the baby’s intestines may look dilated (swollen) or show a “honeycomb” pattern, which can be mistaken for a physical blockage ^[2].
• The “Urine” Illusion: After birth, the primary symptom is secretory diarrhea—a constant flow of watery stool that persists even if the baby is not eating ^[3]. This stool is often so clear and yellow that it is mistaken for urine in the diaper, leading some to believe the baby is simply urinating a lot rather than having diarrhea ^[4][5]. (Care must be taken to avoid contamination of the diaper sample with actual urine).
• Body Chemistry Changes: Because the stool contains massive amounts of salt and water, the baby quickly becomes dehydrated. This leads to hyponatremia (low blood sodium) and metabolic acidosis (a buildup of acid in the blood due to the loss of bicarbonate in the stool) ^[1][6].

Distinguishing CSD from “Look-alikes”

Because several conditions cause watery diarrhea or swollen bowels in newborns, doctors must use specific tests to tell them apart.

CSD vs. Congenital Chloride Diarrhea (CCD)

While both cause massive watery stool, the “flavor” of the salt lost is different. In CSD, the stool is high in sodium and the blood becomes acidic ^[5]. In Congenital Chloride Diarrhea, the stool is high in chloride, and the blood becomes alkaline (the opposite of acidic), a state called metabolic alkalosis ^[5][7]. Doctors look at the stool sodium-to-chloride ratio and the stool osmotic gap in conjunction with blood chemistry.

CSD vs. Mechanical Obstruction (Blockage)

This is one of the most common and dangerous misdiagnoses. Because the bowels look so swollen on X-rays and ultrasounds, doctors may suspect a physical blockage (like intestinal atresia) ^[1]. In some cases, babies have even undergone surgery only for the surgeon to find no blockage at all ^[8][1]. However, ongoing diarrhea does not exclude an obstruction. A partial obstruction, early obstruction, or a second process can still allow liquid stool to pass. Bilious (green) vomiting, progressive distension, severe pain, or a sudden change in output always requires urgent assessment and imaging.

CSD vs. Bartter Syndrome

Bartter Syndrome is a rare kidney disorder that also causes low salt levels and dehydration in newborns ^[9]. However, children with Bartter syndrome have high levels of salt in their urine, while children with CSD usually have very low levels of salt in their urine because their kidneys are desperately trying to hold onto whatever sodium is left in the body ^[10][11].

CSD vs. Microvillus Inclusion Disease (MVID)

MVID also causes severe, life-threatening diarrhea from birth. The difference is found under a microscope. In MVID, the surface of the gut (the microvilli) is physically damaged or missing, requiring appropriate histology, electron microscopy, or immunostaining (like MYO5B testing) to identify intracellular inclusions ^[12]. In CSD, the gut surface may look mostly normal, or it may show small “tufts” of cells (called epithelial tufting), particularly in certain genetic types ^[13][3].

The Path to a Diagnosis

To confirm CSD, the medical team will typically perform several specific checks:

1. Stool Electrolyte Test: They will measure the exact amount of sodium and chloride in the stool. High sodium (often over 100 mmol/L) points toward CSD ^[5][1].
2. Blood Gas Analysis: This checks if the blood is too acidic or too alkaline ^[6].
3. Endoscopy and Biopsy (Selective): A specialist will decide if a biopsy is needed to evaluate alternatives like MVID. It is an invasive procedure and is not necessarily needed in every child if the clinical and genetic findings are convincing ^[12].
4. Genetic Testing: By looking at genes like SLC9A3, GUCY2C, or SPINT2, doctors can confirm the diagnosis and determine which specific subtype of CSD your child has when a causative variant is found ^[13][14]. However, genetic panels can be negative or return variants of uncertain significance, meaning diagnosis may still require clinical interpretation.