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In Health, What Is the Difference between Bilateral and Unilateral?

By Clara Kedrek
Updated Mar 03, 2024
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The terms bilateral and unilateral are used to refer to which sides of the body are affected by certain pathologic conditions. Bilateral conditions affect both sides of the body, and unilateral conditions only affect one side. Only parts of the body that are located symmetrically on both sides of the body are typically described with these terms. Whether a disease affects one side or both sides of the body can usually offer clues regarding why that disease developed.

Bilateral conditions affect both sides of the body. Due to the symmetry of the human body, many body features and organs are replicated and located on both sides of the body, including the kidneys, eyes, ears, arms, and legs. A number of different pathologic processes could affect both of the organs, causing bilateral involvement.

Other medical conditions may only affect one side of the body, and would be considered unilateral. For example, there are two kidneys present in the body. If a disease only affects one of the kidneys, the affected individual is said to have unilateral kidney involvement.

For some organs and features of the body, using the terms bilateral and unilateral as descriptors does not make sense. For example, there is only one liver located in the the human body. Disease in the liver would not be referred to as unilateral liver disease, because saying this would imply that there was a normal liver located on the other side of the body. This is not the case because there is only one liver. As such, bilateral and unilateral are typically only used to describe organs or features present on both sides of the body.

The difference between bilateral and unilateral involvement of the body can often suggest that a certain type of pathological process is occurring. The presence of arthritis in the hands is an illustrative example of the difference in causes of bilateral and unilateral conditions. Having pain and stiffness in both hands at the same time could suggest an underlying systemic process, such as the inflammatory disease rheumatoid arthritis. In contrast, if a person had arthritis in only the left wrist and in no other joints, these symptoms could have been caused by a past injury to that joint. Bilateral disease typically results from conditions that affect the entire body, whereas unilateral disease might result from a more localized problem.

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Discussion Comments

By Sporkasia — On Feb 13, 2014

When a little pain of bruise pops up on one side of my body I simply attribute it to some little accident that I can't remember and I don't give it a second thought. When the symptoms are bilateral, I get a bit more concerned.

By Animandel — On Feb 13, 2014

Drentel - You couldn't be more right. Where disease shows up in the body as compared to where it is not present can go a long way in determining what caused the disease. Whether a disease or condition is unilateral or bilateral also makes a difference in the way treatment is administered.

The body is great at giving us little indicators as to what is going on inside. We just have to make sure we are listening and watching closely so we can stay healthy and treat disease aggressively.

By Drentel — On Feb 12, 2014

This is a helpful article. It does a great job of explaining how doctors can use unilateral vs bilateral symptoms to help diagnose the cause of a condition.

Recently, I went to the doctor because I have been having pain and weakness in my right hand. I thought the pain might be coming from arthritis based on what I have heard from people who have the disease. This seemed even more likely since I am getting older and the condition often pops up in older people.

The doctor concluded that arthritis was most likely the culprit. He asked whether I had any symptoms in my other hand, and he asked about my family medical history. Neither of my parents ever had painful arthritis.

After the physical exam and a series of questions, the doctor concluded that the condition was most likely related to an injury I suffered while playing basketball in college. Until he began asking the questions, I had forgot all about jamming my hand against a backboard when I was 20 years old.

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