A lesion is a medical term describing any area of tissue that has undergone an abnormal structural or functional change. This broad classification applies to a wide variety of abnormalities, ranging from a common blister to complex damage within an internal organ. The presence of a lesion is not a diagnosis itself, but rather an initial observation indicating that a pathological process is occurring. Clinicians use the term to communicate that a part of the tissue looks different from the surrounding healthy tissue.
Defining a Lesion
A lesion is technically defined as a localized change in tissue caused by disease or injury. This change represents a deviation from the normal state of the tissue’s architecture or biological function. Lesions vary dramatically in size, capturing everything from a minor scratch to a significant growth. Those large enough to be seen with the unaided eye are described as gross lesions. Others are microscopic, requiring examination to detect the cellular-level alteration, which is referred to as a histologic lesion. A lesion’s severity is determined by the underlying cause and the extent of the tissue disruption.
How Lesions Are Described
Clinicians use precise language to describe a lesion’s physical characteristics, or morphology, which is the first step in determining a diagnosis. This description involves noting the size, shape, color, and texture of the abnormality. Lesions are first categorized as either primary or secondary, which communicates their stage of development.
Primary lesions are the initial, spontaneous manifestation of the disease process. Examples include a macule (a flat, discolored spot) or a papule (a small, solid elevation less than 10 millimeters in diameter). Other primary forms include vesicles (small blisters filled with clear fluid) and plaques (broad, elevated areas often seen in conditions like psoriasis). Identifying the initial lesion type provides clues about the underlying condition.
Secondary lesions develop from a primary lesion altered by trauma, healing, or disease progression. Examples include an ulcer, which is an open sore resulting from the loss of the entire top layer of tissue. A crust, commonly known as a scab, is dried serum, blood, or pus. A scar is also a common secondary lesion, representing fibrous tissue that has replaced normal skin after an injury. Describing both the primary form and any secondary changes allows medical professionals to accurately communicate complex findings.
Primary Causes of Lesion Development
Lesions form due to three major categories of biological insult: trauma, infection and inflammation, or abnormal cellular growth. Physical trauma involves any external force that directly damages tissue structure. This ranges from a simple abrasion to a severe burn causing widespread tissue necrosis. Trauma also includes internal forces, such as mechanical stress leading to a pressure sore or damage to blood vessels resulting in a bruise.
Infection and the resulting inflammatory response are a frequent cause of lesion formation. Pathogens like bacteria, viruses, or fungi invade tissue, prompting the immune system to attack. This leads to localized swelling, redness, and fluid accumulation. For example, a bacterial infection can lead to a pustule (a small, pus-filled lesion), while a viral infection, such as herpes simplex, can cause a cluster of vesicles. Autoimmune diseases also fall into this category, involving the immune system mistakenly attacking healthy tissue and causing chronic inflammatory lesions.
The third major cause is neoplasia, which refers to the uncontrolled, abnormal proliferation of cells. These lesions are commonly called tumors, and they can be either benign or malignant. Benign tumors are non-cancerous and typically slow-growing, while malignant tumors are cancerous with the potential to spread. Neoplastic lesions develop from genetic mutations that disrupt normal cell cycle regulation. The resulting abnormal growth physically displaces or destroys healthy surrounding tissue.
Lesions Beyond the Skin Surface
While the most visible lesions occur on the skin, the term applies to abnormalities in any organ or tissue. Internal lesions are often detected through medical imaging techniques like magnetic resonance imaging (MRI) or computed tomography (CT) scans. For instance, a lung lesion may represent scarring, a nodule from a fungal infection, or a malignant tumor.
Lesions within the brain are particularly significant because their functional impact depends entirely on the specific location of the damage. A small lesion controlling motor function can cause severe disability, while a much larger lesion in a non-functional area might produce few noticeable symptoms. A bone lesion may indicate a fracture, chronic infection, or the presence of a metastatic cancer deposit. The function of the affected organ dictates the consequences, making location and tissue type the primary determinants of a lesion’s medical importance.