- A hypersensitivity reaction pattern that occurs secondary to multiple causes, including gastrointestinal, fungal and other infections; inflammatory bowel disease; sarcoidosis; oral contraceptive pills; malignancy and certain drugs.
- Female predominance with a peak age between 20 and 30 years
- Acute onset of tender bright red nodules, most commonly on the anterior shins, resolving in a few weeks
- Septal panniculitis comprised mainly of neutrophils, often with some admixed lymphocytes and eosinophils
- The septa also display some degree of edema and/or hemorrhage.
- The inflammation extends into the adjacent periphery of the fat lobules, but the lobules are otherwise largely unaffected.
- Older lesions show thickening of the septa with a predominantly mononuclear infiltrate progressing to eventual granulomatous inflammation and greater involvement of the lobules.