discoid
Summary
Discoid refers to a disc- or coin-shaped lesion or structure, most commonly used in dermatology (e.g., discoid lupus erythematosus) to describe round, well-demarcated plaques, and in hematology to describe the normal biconcave shape of red blood cells.
Detail
The term 'discoid' describes a flat, round, disc-like morphology. Its most high-yield clinical use is in Discoid Lupus Erythematosus (DLE), a chronic cutaneous form of lupus characterized by well-demarcated, erythematous, scaly plaques with follicular plugging, atrophy, and dyspigmentation, typically on sun-exposed areas (face, scalp, ears). DLE can occur in isolation or as part of systemic lupus erythematosus (SLE), though most patients with isolated DLE do not progress to SLE (~5-10%). Histology shows interface dermatitis, basement membrane thickening, and follicular plugging. Scarring alopecia can result if the scalp is involved. Discoid lesions are typically ANA-negative or low-titer positive compared to SLE. The term is also used to describe normal biconcave disc-shaped red blood cells, important for gas exchange efficiency and flexibility through capillaries; loss of this shape (e.g., spherocytes, sickle cells) indicates pathology. Additionally, 'discoid meniscus' refers to a congenital anomaly where the knee meniscus is abnormally disc-shaped rather than crescent-shaped, predisposing to tears and clicking/locking symptoms, often diagnosed in children or adolescents via MRI.
Sources
- First Aid for the USMLE Step 1
- Robbins and Cotran Pathologic Basis of Disease
- UpToDate: Discoid lupus erythematosus
- Fitzpatrick's Dermatology in General Medicine
Reviewed by Medusa editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.