Carnitine Uptake Defect (CUD)

Carnitine uptake defect (CUD), also known as primary carnitine deficiency, is a rare genetic disorder that prevents your body from transporting carnitine into cells. Carnitine is a natural substance that helps move certain fats into cells, where they’re turned into energy.

If you have CUD, your body can’t bring enough carnitine into cells. As a result, fats can’t be used properly for energy. This can lead to serious health issues, including liver problems, heart disease and brain damage. You may also hear the term “systemic primary carnitine deficiency.”

CUD is one type of carnitine deficiency, which is a general term for having low carnitine levels in your blood. There are two main types:

• Primary carnitine deficiency: This type is inherited and caused by a genetic change that affects a carnitine transporter in your body.
• Secondary carnitine deficiency: This type happens when there’s not enough carnitine in your blood due to another health issue.

This article focuses on primary carnitine deficiency.

Symptoms of carnitine deficiency

Symptoms of primary carnitine deficiency can vary widely. Some people have mild symptoms or none at all. But other people can become seriously ill. Symptoms often appear during infancy or early childhood, but they can also begin later in life. Illness, infection or long periods without eating (fasting) can trigger symptoms.

Common symptoms may include:

• Low blood sugar
• Tiredness or low energy
• Poor feeding or loss of appetite
• Vomiting or diarrhea
• Irritability or behavior changes
• Muscle weakness
• Confusion or trouble thinking clearly

Systemic primary carnitine deficiency causes

A genetic change in the SLC22A5 gene causes primary carnitine deficiency. This gene gives your body instructions for how to make a protein called OCTN2. This protein acts like a transporter. Its job is to move carnitine into cells.

When there’s a change in the SLC22A5 gene, the OCTN2 transporter doesn’t work properly. As a result, your cells don’t get enough carnitine. Without enough carnitine inside cells, your body can’t break down certain fats to make energy. This causes the symptoms and health issues that happen with CUD.

Risk factors

The only risk factor is genetics because CUD is inherited. It happens when both biological parents pass down a nonworking copy of the SLC22A5 gene.

With each pregnancy, there’s a 1 in 4 chance your child will inherit both nonworking copies and develop the condition. Genetic counseling can help you understand your risk and options.

Complications of primary carnitine deficiency

In primary carnitine deficiency (carnitine uptake defect), low carnitine levels can cause fat to build up in certain organs. Without treatment, it may lead to serious complications, including:

• A weak or enlarged heart
• Enlarged liver
• Breathing problems
• Seizures or loss of consciousness
• Permanent brain damage

Some people with primary carnitine deficiency don’t have symptoms. But you still have an increased risk of heart issues if you have untreated CUD.

How is primary carnitine deficiency diagnosed?

Your healthcare provider will usually find primary carnitine deficiency (carnitine uptake defect) during routine newborn screening. A provider will take a small blood sample from your baby’s heel shortly after birth. This sample measures carnitine levels in their blood. Babies with low levels may have CUD.

If the screening result is out of the normal range, your baby will need follow-up testing to confirm the diagnosis. These tests may include:

• Blood and urine tests to check carnitine levels
• Genetic testing to look for changes in the SLC22A5 gene
• A small skin sample to check how well cells take up carnitine

Carnitine uptake defect false positives

Not every out-of-range newborn screening means primary carnitine deficiency. False positives can happen for several reasons:

• Premature babies may have temporarily low carnitine levels.
• If a provider collects the blood sample too early (before 24 hours of age), it can affect the results.
• If you have low carnitine levels, your baby’s levels may appear low on the test even if they’re healthy.

Because of these risks, follow-up testing for both you and your baby is very important. Confirming the diagnosis early ensures treatment starts quickly and prevents serious complications.

How do you treat carnitine uptake defect?

The main treatment for carnitine uptake defect is a supplement called L-carnitine. L-carnitine supplements replace the missing carnitine in your body. These supplements also prevent harmful fat buildup in your heart, liver and muscles.

If your child has CUD, their healthcare provider may recommend following a specific eating plan. This plan will include frequent meals and snacks to avoid long periods without food. You’ll want to focus on high-carb, low-fat foods to provide energy without overloading their body with fats it can’t process.

During illnesses that cause poor appetite, vomiting or diarrhea, your child might need emergency care. A healthcare provider may give your child intravenous (IV) fluids with sugar to prevent dangerously low blood sugar.

When should I see my healthcare provider?

Contact your provider anytime your baby has certain symptoms, including:

• Weakness or low energy
• Poor feeding or vomiting
• Confusion or unusual behavior
• Low blood sugar
• Breathing trouble
• Fast or irregular heartbeat

What is the outlook (prognosis) for carnitine uptake defect?

The outlook for children with primary carnitine deficiency is very good when treatment starts early. Daily L-carnitine supplements and a healthy eating plan allow most children to grow and develop normally. Early treatment can also prevent serious problems involving the heart, liver and brain. Even children who have already shown symptoms of the condition often improve once treatment begins.

Without treatment, primary carnitine deficiency can be life-threatening. It can lead to severe heart problems, dangerously low blood sugar, liver damage or brain injury. These can all shorten life expectancy.