Panniculitis is a group of diseases whose hallmark is inflammation of subcutaneous adipose tissue. Symptoms include tender skin nodules, and systemic signs such as weight loss and fatigue.

"Erythema nodosum" is a form of panniculitis characterised by tender red nodules, 1–10 cm, associated with systemic symptoms including fever, malaise, and joint pain. Nodules may subside over a period of 2–6 weeks without ulcerating or scarring. Erythema nodosum is associated with infections, including Hepatitis C, EBV and tuberculosis, pregnancy, Non-Hodgkin lymphoma, and pancreatic cancer.

  • Tuberculosis is one of the significant causes.
  • The legs are a commonly affected area.
References Erythema Nodosum: A Practical Approach and Diagnostic Algorithm 33683567 
Erythema nodosum is the most common form of panniculitis and is characterized by tender erythematous nodules mainly in the lower limbs on the pretibial area. The exact cause of erythema nodosum is unknown, although it appears to be a hypersensitivity response to a variety of antigenic stimuli. Although the etiology is mostly idiopathic, ruling out an underlying disease is imperative before diagnosing primary erythema nodosum. Erythema nodosum can be the first sign of a systemic disease that is triggered by a large group of processes, such as infections, inflammatory diseases, neoplasia, and/or drugs. The most common identifiable causes are streptococcal infections, primary tuberculosis, sarcoidosis, Behçet disease, inflammatory bowel disease, drugs, and pregnancy.
 Panniculitis in Children 34449587 
Panniculitides form a heterogenous group of inflammatory diseases that involve the subcutaneous adipose tissue. These disorders are rare in children and have many aetiologies. As in adults, the panniculitis can be the primary process in a systemic disorder or a secondary process that results from infection, trauma or exposure to medication. Some types of panniculitis are seen more commonly or exclusively in children, and several new entities have been described in recent years. Most types of panniculitis have the same clinical presentation (regardless of the aetiology), with tender, erythematous subcutaneous nodules.
 Erythema nodosum - a review of an uncommon panniculitis 24746312
Panniculitis, inflammation of the subcutaneous fat, usually presents with inflammatory nodules. Erythema nodosum (EN) is clinically the most frequent form of panniculitis. Whilst up to 55% of EN is considered idiopathic, the most common causes include infections, drugs, systemic illnesses such as sarcoidosis and inflammatory bowel disease, pregnancy, and malignancy. EN typically presents in the teens and 20s, and is seen more commonly in females. It is often preceded by a non-specific prodrome of one to three weeks, which may include fever, malaise, and symptoms of an upper respiratory tract infection. Cutaneous lesions then follow, typically localized on the extensor aspect of the limbs. The lesions are painful rounded or oval, slightly raised, non-ulcerative red nodules. The exact pathogenesis of EN is not understood, although is thought to result from deposition of immune complexes in the venules of the septae in subcutaneous fat, causing a neutrophilic panniculitis. Even without specific therapy for a causative condition, EN resolves without treatment in most cases.