In health, what is the difference between bilateral and unilateral?

The terms bilateral and unilateral are used to refer to which sides of the body are affected by certain pathological conditions. Bilateral conditions affect both sides of the body, and unilateral conditions only affect one side. Only body parts that lie symmetrically on either side of the body are typically described by the terms bilateral and unilateral. Whether a disease affects one or both sides of the body can often offer clues as to why the 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. Several different pathological processes could affect both 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 renal involvement.

For some organs and features of the body, it makes no sense to use the terms bilateral and unilateral as descriptors. For example, there is only one liver located in the human body. Disease in the liver would not be called unilateral liver disease, because to say 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 processes are generally 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 disease process is occurring. The presence of arthritis in the hands is an illustrative example of the difference in the 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. Conversely, if a person had arthritis only in the left wrist and no other joints, these symptoms could have been caused by a previous injury to that joint. Bilateral disease is usually the result of conditions that affect the entire body, while unilateral disease may be the result of a more localized problem.

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