A lesion is a broad medical term used to describe any area of tissue that exhibits a structural or functional abnormality compared to the surrounding healthy tissue. A lesion is a descriptive observation, not a formal diagnosis in itself, and it can occur in any organ or tissue, ranging from a minor skin change to a significant internal mass. A lesion can result from many causes, including simple injury, infection, or a chronic disease process, and its severity can vary greatly from harmless to life-threatening.
Defining the Medical Scope of a Lesion
The term “lesion” serves as a foundational observation point in medicine, indicating a physical or structural change within the body. Physicians use the word to communicate the presence of an identifiable area of damage, disease, or abnormal growth before the exact cause is known. This structural alteration is distinct from a symptom, which is a subjective experience like pain or nausea; a lesion is an objective, measurable change in tissue.
For example, a doctor might describe a “lesion on the lung” seen on an X-ray, which then requires follow-up tests like a biopsy or computed tomography (CT) scan to determine if it is a scar, an infection, or a tumor. Lesions can be classified by whether they are anatomic and visible to the unaided eye, histologic and seen only under a microscope, or biochemical and detectable only through chemical analysis.
Classification Based on Appearance and Characteristics
In fields like dermatology, lesions are systematically categorized based on their appearance and evolution to help standardize descriptions and aid in diagnosis. This classification primarily separates skin lesions into two groups: primary and secondary. Primary lesions are the initial, direct manifestation of a disease process and develop on previously unaltered skin.
Primary Lesions
Primary lesions are the initial, direct manifestation of a disease process and develop on previously unaltered skin. Examples include:
- A macule, which is a flat area of discoloration like a freckle.
- A papule, which is a small, raised, solid bump like a mole or a wart.
- A vesicle, which is a small blister filled with clear fluid.
- A plaque, which is a flat, elevated area larger than a papule, commonly seen in conditions like psoriasis.
Secondary lesions, by contrast, result from the modification of a primary lesion, either through natural progression, trauma, or healing. A common example is a crust, which is a dried collection of blood, pus, or serum that forms over a wound, like a scab. If a primary fluid-filled lesion ruptures and the top layer of skin is lost, the resulting shallow depression is classified as an erosion, which typically heals without scarring. A scar is another secondary lesion, representing the permanent fibrous tissue that replaces normal skin following an injury that extends into the dermis.
Common Causes and Mechanisms of Lesion Formation
Lesions arise from events that disrupt the normal structure and function of cells and tissues. One common mechanism is physical trauma, where an external force damages the tissue, including simple cuts, abrasions, burns, or surgical incisions.
Another major category involves infection and inflammation, which trigger the body’s immune response. Pathogens like bacteria, viruses, or fungi can directly damage cells, or the resulting inflammation can lead to tissue destruction or swelling. For instance, an inflammatory lesion may appear as a swollen lymph node during a throat infection.
Lesions can also be caused by various internal disease processes that lead to cellular dysfunction. This includes the uncontrolled cell growth seen in cancer, which forms a tumor, or autoimmune disorders where the body’s own immune system attacks healthy tissue, such as the demyelination seen in multiple sclerosis. Circulatory issues, such as a blocked blood vessel, can cause tissue death, known as necrosis, which is another form of lesion.
Internal vs. External Lesions: Context Matters
The word lesion is often associated with the skin, but its medical scope extends throughout the entire body, with the context of the location dictating its significance and detection method. External lesions, or cutaneous lesions, are those visible on the skin, and they are typically identified through visual inspection.
In contrast, internal lesions occur within organs such as the brain, liver, lungs, or bones, and their presence is often not apparent until they cause symptoms or are discovered incidentally. A brain lesion, for example, can cause neurological deficits depending on the affected region, while a coronary lesion describes an abnormality in the heart’s arteries.
Because they are hidden from direct view, internal lesions are primarily detected and monitored using medical imaging technologies like Magnetic Resonance Imaging (MRI), CT scans, or ultrasound. The location of a lesion heavily influences its effect on the body, making a small lesion in the central nervous system potentially more serious than a much larger one on a less functionally sensitive area of the skin.