UCD 101

Get an overview about urea cycle disorders, or UCDs, to understand what is happening inside the body.  

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Being diagnosed with a urea cycle disorder can feel overwhelming. Understanding more about it can help. A urea cycle disorder, commonly called a UCD, is a genetic disorder a child is born with. The child inherits a defective gene or genes from his or her parents, or has a spontaneous genetic mutation. People can be diagnosed with a UCD at any point in their life. No matter the age, when food made up of carbs, fats, and protein enters the body it is broken down in the digestive tract. Protein is broken down and leftover ammonia, or urea, is eliminated from the body through a healthy urea cycle in the liver. For a person living with a UCD, the urea cycle isn’t working correctly and ammonia can’t be removed properly.

There are different types of UCDs. The most common one is ornithine transcarbamylase deficiency or OTC. The name of the UCD is based on which enzyme or transporter isn’t working properly in the urea cycle. A healthy urea cycle turns toxic ammonia into urea so the body can get rid of it naturally. For a patients living with a UCD, the ammonia stays in the body and builds up to toxic levels in the blood. 

Common signs and symptoms of high ammonia levels to watch out for include vomiting, headaches, aggression, and feeling very tired. When this happens it could signal a hyperammonemic crisis, which is very serious and requires immediate medical attention, because it could cause a coma, brain damage, or even death. 

There are options to help control high ammonia levels due to a UCD. They include a low-protein diet, amino acid supplements, and other options your doctor can discuss with you. A UCD affects everyone differently, so make sure to always talk to your doctor to learn more about your UCD and possible ways to manage your ammonia levels. 

A urea cycle disorder (UCD) is an inherited disease. When we eat protein our bodies break it down into amino acids, which we need to grow and stay healthy. Extra amino acids, that our body doesn't need, are turned into a waste product called ammonia. Enzymes in the liver turn the ammonia into urea so the body can get rid of it through the urine. 

In someone with a UCD, the enzyme is missing or is not working correctly. Because of this, ammonia builds up in the blood, and it can be harmful. It can travel to the brain and cause memory loss, behavior changes, coma, seizures, or brain damage. UCDs affect about 1 in 35,000 births in the United States. Just because it’s rare doesn’t mean there isn’t information to help you or someone you love who is living with a UCD. 

Consultar mas informacion sobre los UCD en espanol.

UCD symptoms can be subtle and similar to many other conditions, which can lead to misdiagnosis. These symptoms can include headaches, fatigue (feeling tired), confusion, and trouble concentrating. Any level of elevated ammonia, even if it’s not high enough to cause severe symptoms or a hyperammonemic crisis, should be avoided in order to prevent brain damage.

Learn about the long-term effects of a UCD on the body and brain

Long Term Effects

Make the connection of urea cycle (UCD) symptoms and the potential long term effects on the body.  

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Most people think a urea cycle disorder, or UCD, is a condition that only affects the liver. But it might surprise you to learn that the most serious damage caused by a UCD actually happens in the brain. Symptoms like forgetfulness, a short attention span, moodiness, and trouble with problem solving, all result from a UCD. It all starts with ammonia. Ammonia can travel through your blood to your brain, and it is harmful to your brain cells. High levels of ammonia may cause your brain cells to swell and malfunction which may damage them to the point that they no longer work at all. It’s important to remember that even though you can’t see this happening, higher than normal ammonia levels are dangerous to your brain. One instance of elevated ammonia may not cause a permanent or obvious problem. But the more often your ammonia levels are elevated over time, the more likely it is that the damage to your brain will be serious and permanent. Even if you’re not having a hyperammonemic crisis, high ammonia levels that are high enough to cause symptoms can still lead to permanent damage. So what can happen when brain cells are damaged? You may not notice the effects right away or even realize that they’re related to your UCD, but the problems that result from the damaged brain cells can affect your everyday life at home, at work, or at school. You may have issues like remembering things, difficulty making decisions, mood swings, and problems with thinking. Damaged brain cells may also cause you to function at a lower level than a typical person your age. Keeping ammonia levels low can help prevent damage to your brain over time. The best way to do this is by sticking to the management plan recommended by your doctor, which may include a low-protein diet, dietary supplements, and an ammonia-removing medicine (also called a nitrogen-scavenger medicine). Not following your low-protein diet or not taking your dietary supplements and medicine, even when you’re feeling healthy, can cause ammonia levels to rise. It’s very important to stick to the management plan your doctor has created for you and to take all the supplements and medicines as prescribed. So, remember, the major effects of high ammonia are actually on your brain which can cause serious symptoms. Tell your doctor about any and all symptoms you experience, even if you don’t think they are related to your UCD. And make sure to talk to him or her about your management plan and whether you should make any changes to help keep your ammonia levels under control.

Healthy urea cycle diagram with normal serum ammonia levels and urea production
Urea Cycle Disorder diagram showing how break in cycle causes high ammonia levels

Healthy Urea Cycle

Protein, in the food you eat, is needed for the growth and development of your body.

Our bodies break down the protein we eat into amino acids, which the body needs to grow and stay healthy. The extra protein that our bodies do not need contains nitrogen, which is turned into ammonia.

The ammonia is then removed from the blood, turned into urea, and passed from the body through urine.


Urea Cycle Disorder

When a person has a urea cycle disorder, the urea cycle can’t change the ammonia into urea as well as it should. The ammonia builds up because the body can’t get rid of it. 

The ammonia can reach toxic levels in the blood and can trigger vomiting, confusion, and swelling in the brain.

While UCDs start in your liver, it’s important to remember that elevated ammonia levels have the biggest effect on your brain.

UCD genes icon

A UCD is a genetic disorder. This means it is caused by a defective gene, which can be inherited from one or both parents. UCDs can also be caused by a random genetic mutation. There are different types of urea cycle disorders. The most common type of UCD is ornithine transcarbamylase deficiency (OTC). It is typically passed from a mother to her child. A mother, who is an “OTC carrier," may not know she carries the OTC gene until her child is diagnosed. However, mothers who are “OTC carriers” can have mild UCD symptoms that should not be ignored.

Map of Ornithine Transcarbamylase Deficiency, Citrullinemia, Arginase Deficiency and other UCD subtypes

Fortunately, there are options for people with urea cycle disorders to help control high levels of ammonia in the body. Options can include a low-protein diet, amino acid supplements, and ammonia-controlling medicines. Because every person’s needs are different, it’s important to talk to your doctor to develop a plan for how you can best manage your ammonia levels. 

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