The skin, our body’s largest organ, often displays various changes known as skin lesions. Medical professionals categorize these to aid in understanding and diagnosing dermatological issues. This article focuses on primary skin lesions.
Understanding Primary Skin Lesions
Primary skin lesions are the initial, direct expressions of a disease or condition on the skin. They are original alterations in appearance or texture, unmodified by external factors like scratching, infection, or healing processes. These lesions arise de novo, directly reflecting the underlying pathology.
Identifying primary lesions is a significant aspect of a dermatologic physical examination, offering valuable diagnostic insight. These lesions can be present at birth or acquired over time due to various disorders, including infections, allergies, or autoimmune conditions. While primary lesions may remain stable, they can also evolve or be altered by trauma or other factors, potentially leading to secondary lesions.
Common Types of Primary Skin Lesions
Macule
A macule is a flat, non-palpable lesion characterized by a change in skin color. It is typically small, measuring less than 1 centimeter (cm) in diameter, and does not involve any alteration in skin thickness or texture. Examples include freckles, flat moles, and the discolored spots seen in conditions like measles.
Patch
A patch is similar to a macule but is larger, typically measuring 1 cm or more in diameter. It is also a flat, non-palpable area of altered skin color. Vitiligo, a condition causing areas of skin depigmentation, is a common example of a patch.
Papule
A papule is a small, elevated, and solid lesion that can be felt or palpated. These raised bumps are generally less than 1 cm in diameter and can have distinct or indistinct borders. Common examples include moles, warts, and the small bumps associated with acne. Papules can vary in shape, color, and size, sometimes appearing dome-shaped or flat-topped.
Plaque
A plaque is a palpable lesion that is elevated, flat-topped, and typically larger than 1 cm in diameter. These solid, superficial lesions often appear rough and can be red, scaly, and irritating. Psoriasis, characterized by raised, silvery, scaly patches, is a common example of a plaque. Plaques can result from the merging of multiple papules.
Nodule
A nodule is an elevated, solid lesion that extends deeper into the skin layers, often into the dermis or subcutaneous tissue. These firm, palpable bumps are generally larger than 1 cm in diameter, with some sources indicating up to 2 cm. Lipomas, which are benign fatty tumors, and cysts are examples of nodules.
Tumor
In dermatology, a tumor refers to a solid mass that develops on or under the skin. While often larger than nodules, typically measuring more than 2 cm, tumors are essentially larger nodules. They can be firm and may represent a range of growths, including benign and malignant neoplasms.
Wheal
A wheal is a suddenly formed, elevated area of skin caused by localized swelling or edema. These lesions are often itchy, red, or pale pink, and can change shape rapidly. Hives, also known as urticaria, are a classic example of wheals, often occurring as a reaction to allergens like insect bites or certain foods.
Vesicle
A vesicle is a small, elevated, fluid-filled blister, typically less than 1 cm in diameter. The fluid within vesicles is usually clear, and they can appear on various parts of the body. Conditions like chickenpox and herpes simplex infections are characterized by the presence of vesicles.
Bulla
A bulla is a larger fluid-filled blister, measuring greater than 1 cm in diameter. These can form when fluid becomes trapped beneath the thin outer layer of the skin. Bullae can be caused by various factors, including burns, allergic reactions, and certain autoimmune blistering disorders.
Pustule
A pustule is an elevated lesion similar to a vesicle but filled with purulent fluid, or pus. These blister-like sores can be red with white or yellow centers and may be tender to the touch. Acne pimples and the lesions seen in folliculitis are common examples of pustules.
Cyst
A cyst is a benign, round, dome-shaped lesion that is encapsulated and contains fluid or semi-fluid material. These lumps appear just beneath the skin and can vary in size, sometimes growing up to several centimeters wide. Epidermoid cysts, often mistakenly called sebaceous cysts, are a common type.
Differentiating Primary and Secondary Lesions
Understanding the distinction between primary and secondary skin lesions is important for accurate diagnosis. Primary lesions appear on previously unaffected skin, representing the initial manifestation of a disease process.
Secondary lesions, conversely, develop from primary lesions or are caused by external factors. These changes occur due to progression or regression of a disease, or as a result of trauma such as scratching, rubbing, or infection. Examples of secondary lesions include crusts, which form from dried fluids over an injured primary lesion, and scales, which are flaky skin cells that build up from conditions like psoriasis. Other secondary lesions include ulcers, which are open sores, and scars, which result from the healing process of damaged skin.