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.