Kernicterus is a type of brain damage most often seen in babies. It’s caused by an extreme buildup of bilirubin in the brain. Bilirubin is a waste product that’s produced when your liver breaks down old red blood cells so your body can remove them.

It’s normal for newborns to have high bilirubin levels. This is known as newborn jaundice. About 60 percent of babies have jaundice, because their bodies can’t remove bilirubin as well as they should yet. Kernicterus is much rarer. It involves dangerously high bilirubin levels.

Kernicterus is a medical emergency. Babies with this condition need to be treated right away to bring down their bilirubin levels and prevent further brain damage.

Signs of jaundice can appear within the first few days of a newborn’s life. Jaundice causes the baby’s skin and whites of the eyes to turn a yellowish color. Kernicterus symptoms are more severe.

Babies with kernicterus are also lethargic. This means they are unusually sleepy. All babies sleep a lot, but lethargic babies sleep even more than is typical and are very hard to wake up. When they do wake up, they often fall right back to sleep.

Other symptoms of kernicterus include:

  • high-pitched crying
  • decreased appetite and less feeding than usual
  • inconsolable crying
  • floppy or limp body
  • missing reflexes
  • arching of the head and heels back, like a bow
  • uncontrollable movements
  • vomiting
  • unusual eye movements
  • lack of wet or dirty diapers
  • fever
  • seizures

If you notice these symptoms, see a doctor or take your baby to the hospital right away.

Kernicterus is very rare in adults. The conditions that cause it most often affect infants. It’s possible for adults to develop high bilirubin levels, but almost never kernicterus.

Conditions that can cause very high bilirubin levels in adults include:

  • Crigler-Najjar syndrome: An inherited condition that makes it harder for the body to break down bilirubin.
  • Dubin-Johnson syndrome: A rare, inherited disorder that prevents the body from effectively removing bilirubin. This condition does not cause kernicterus.
  • Gilbert’s syndrome: A condition in which the liver can’t properly process bilirubin.
  • Rotor syndrome: An inherited disorder that causes bilirubin levels to build up in the blood. This condition does not cause kernicterus.

Kernicterus is caused by severe jaundice that isn’t treated. Jaundice is a common problem in newborns. It happens because a newborn’s liver can’t process bilirubin quickly enough. Bilirubin builds up in the baby’s bloodstream as a result.

There are two types of bilirubin in the body:

  • Unconjugated bilirubin: This type of bilirubin travels from your bloodstream to your liver. It’s not water-soluble, meaning it doesn’t dissolve in water, so it can build up in your body’s tissues.
  • Conjugated bilirubin: This is converted from unconjugated bilirubin in your liver. Conjugated bilirubin is water-soluble, so it can be removed from your body through your intestines.

If unconjugated bilirubin isn’t converted in the liver, it can build up in the baby’s body. When the level of unconjugated bilirubin gets very high, it can move out of the blood and into the brain tissue. Unconjugated bilirubin can lead to kernicterus if something causes it to build up. Conjugated bilirubin does not cross from the blood into the brain and can usually be removed from your body. Therefore, conjugated bilirubin doesn’t lead to kernicterus.

There are several potential causes that can lead to unconjugated bilirubin building up:

Rh disease or ABO incompatibility

Sometimes the baby’s and mother’s blood types are not compatible. If a mother is Rh-negative, it means her red blood cells don’t have a certain type of protein attached to them. It’s possible for her baby to have a different Rh factor than her. If her baby is Rh-positive, it means they do have that protein attached to their red blood cells. This is known as Rh incompatibility.

In Rh incompatibility, some of the fetus’ red blood cells can cross the placenta and get into the mother’s bloodstream. The mother’s immune system recognizes these cells as foreign. It produces proteins called antibodies that attack the baby’s red blood cells. The mother’s antibodies can then get into the baby’s body through the placenta and destroy the baby’s red blood cells.

As these blood cells are destroyed, the baby’s bilirubin levels rise. After the baby is born, bilirubin builds up in the bloodstream and brain. Rh disease is rare today, because mothers can be treated for it during pregnancy.

A similar, but less severe condition can sometimes occur when a mother has type O blood and her baby has a different type (ABO incompatibility). This is still fairly common. Though these babies are also at higher risk for kernicterus, it can almost always be prevented with proper monitoring and early treatment if necessary.

Crigler-Najjar syndrome

Babies with this inherited condition lack an enzyme needed to convert unconjugated bilirubin to conjugated bilirubin for removal. As a result, high levels of bilirubin build up in their blood.

Kernicterus and sulfonamides

Certain drugs — particularly antibiotics — have also been linked to kernicterus. Sulfonamides (also called sulfa drugs) are a group of antibiotics that kill bacteria. One common antibiotic combines the sulfonamide sulfamethoxazole with trimethoprim (SMX-TMP) to treat bacterial infections. Studies have raised concerns that sulfonamides might increase the risk for kernicterus.

Unconjugated bilirubin normally travels through the bloodstream to the liver bound to the protein albumin. In the liver, it’s converted to conjugated bilirubin so it can be removed from the body. Sulfonamides may unbind bilirubin from albumin, which increases blood levels of bilirubin. The unbound bilirubin can cross into the brain and cause kernicterus.

Learn the difference between sulfonamides and sulfites »

Babies are more likely to get severe jaundice and kernicterus if they:

  • Were born prematurely. When babies are born before 37 weeks, their livers are even less well developed and will take longer to effectively remove bilirubin.
  • Don’t feed well. Bilirubin is removed in stool. Poor feeding prevents babies from making enough dirty diapers.
  • Have a parent or sibling who had jaundice as an infant. This condition can run in families. It may be linked to certain inherited disorders, such as G6PD deficiency, which causes red blood cells to break down too early.
  • Were born to a mother with type O or Rh-negative blood type. Moms with these blood types sometimes give birth to babies who have high bilirubin levels.

Kernicterus is most often diagnosed in babies. One test that may be used to check bilirubin levels is a light meter. A doctor or nurse will check your baby’s bilirubin levels by placing the light meter on your baby’s head. The light meter tells how much bilirubin is in your baby’s skin, or their transcutaneous bilirubin (TcB) level.

If your baby’s TcB level is high, it could be an indication that bilirubin is building up in their body. Your doctor will order a bilirubin blood test.

The goal of treatment is to reduce the amount of unconjugated bilirubin in a baby’s body before it gets to levels that cause brain damage by kernicterus.

Babies with high bilirubin levels are often treated with phototherapy, or light therapy. During this treatment, the baby’s bare body is exposed to a special light. The light increases the rate at which the baby’s body breaks down unconjugated bilirubin.

Learn more about light therapy and how it’s used to help depression »

Babies with a very high bilirubin level may need a blood exchange transfusion. During this procedure, the baby’s blood is removed a little bit at a time. Then it’s replaced with matched blood from a donor.

Increasing the number of feedings you give your baby can also help treat high bilirubin levels. Bilirubin is removed through the intestines in stool. The more babies eat, the more waste they produce, and the more bilirubin is removed.

Keep reading: A doctor-recommended feeding schedule for your 6-month-old »

Babies with kernicterus can develop these complications:

  • athetoid cerebral palsy, a type of movement disorder caused by brain damage
  • lack of muscle tone
  • muscle spasms
  • trouble coordinating movement
  • hearing loss and deafness
  • problems with eye movement, including trouble looking up
  • difficulty speaking
  • intellectual disability
  • stained baby teeth

Once the symptoms of kernicterus appear, brain damage has already begun. Treatment can stop but not reverse this damage. That’s why it’s important to monitor newborns for high bilirubin levels — especially if they’re at risk — and treat them quickly.