Panniculitis is a general term describing inflammation of the layer of fat situated directly beneath the skin, also known as the hypodermis or subcutis. The condition typically presents as firm, tender lumps or plaques that form deep within the tissue. While this inflammation can occur anywhere, it is most frequently observed on the lower extremities, particularly the shins. Because many different underlying processes can lead to this inflammation, panniculitis is considered a diagnostic challenge.
Understanding the Subcutaneous Fat and General Symptoms
The layer of subcutaneous fat acts as an essential insulator, helping to regulate the body’s temperature and conserve heat. This fatty tissue also serves as a shock absorber, protecting deeper structures like muscle and bone from external trauma. Furthermore, the fat cells within this layer function as a significant energy reserve. This deepest layer of the skin is where the inflammatory process of panniculitis is centered.
The primary physical manifestation of panniculitis is the presence of subcutaneous nodules that can measure several centimeters in diameter. These lumps are often painful or tender to the touch, and the overlying skin frequently appears discolored, ranging from red to purple or brown. The discoloration can sometimes resemble a bruise. These localized skin changes are often accompanied by generalized systemic symptoms that indicate body-wide inflammation.
Patients often experience a general feeling of illness, referred to as malaise, alongside fever and joint pain. These accompanying symptoms vary in severity depending on the specific cause and type of panniculitis. In some cases, the inflammation can resolve, but it may leave behind a localized indentation or depression in the skin due to the destruction of underlying fat cells.
Classifying the Major Types of Panniculitis
Pathologists categorize panniculitis based on the precise location where the inflammation occurs within the subcutaneous fat layer. This classification system distinguishes between two main types: septal panniculitis and lobular panniculitis. This distinction helps narrow down potential causes and informs the treatment approach.
Septal panniculitis is characterized by inflammation that primarily targets the fibrous connective tissue structures, known as septa. Erythema Nodosum (EN) is the most recognized and common example. Inflammation in this type is largely confined to these dividing walls, often without significant destruction of the fat lobules.
Conversely, lobular panniculitis involves inflammation focused directly on the fat lobules. Conditions such as Pancreatic panniculitis and Lupus panniculitis are examples that fall into this category. The inflammation tends to be more destructive to the fat cells compared to the septal types.
Some forms of the condition can exhibit a mixed pattern, showing inflammation in both the septa and the lobules. Furthermore, the presence or absence of vasculitis (inflammation of the blood vessels within the fat layer) is used as a secondary criterion to sub-classify both septal and lobular types. Microscopic analysis of the affected tissue is necessary to properly place the inflammation within this structural framework.
Identifying Underlying Causes and Triggers
Panniculitis is not a single disease but a reaction pattern triggered by a wide range of factors, making the identification of the underlying cause paramount. Infectious agents are a common trigger, including bacterial pathogens like Mycobacterium tuberculosis or Streptococcus, as well as various viral and fungal infections. The body’s immune response to these infections can initiate the inflammatory cascade in the subcutaneous fat.
Systemic conditions can also manifest as panniculitis. This includes autoimmune disorders such as Systemic Lupus Erythematosus (which can cause Lupus panniculitis), and inflammatory diseases like Crohn’s disease or ulcerative colitis. Certain metabolic or enzymatic deficiencies, such as alpha-1 antitrypsin deficiency or conditions involving the pancreas like pancreatitis, are also associated with this type of inflammation.
Physical factors can directly induce panniculitis in the absence of internal disease. Exposure to extreme cold can cause cold panniculitis, sometimes seen in children. Direct trauma to the skin, or the injection of medications or other substances into the fat layer, can similarly trigger a localized inflammatory response.
A reaction to certain medications is another cause of this condition. Drugs such as oral contraceptives, some antibiotics, and nonsteroidal anti-inflammatory drugs (NSAIDs) have been implicated in triggering episodes. Identifying and removing the specific trigger is a fundamental step in managing the disease.
Diagnostic Procedures and Management Strategies
The initial diagnosis of panniculitis begins with a thorough physical examination and a review of the patient’s medical history, focusing on the appearance and tenderness of the nodules. Because the clinical presentation of the many types of panniculitis can look similar, a definitive diagnosis often requires a tissue sample. A deep excisional biopsy is the preferred procedure, as it provides a large enough sample of subcutaneous fat to allow a pathologist to classify the inflammation as septal or lobular.
The microscopic analysis of the biopsy sample is crucial for determining the specific subtype, which guides the search for the underlying cause. Blood tests may also be ordered to look for evidence of systemic disease, such as elevated inflammatory markers, or to screen for specific infectious or autoimmune conditions. If infection is suspected, the tissue sample may also be sent for microbiological culture.
Management of panniculitis is primarily focused on treating the underlying cause, as the inflammation is often a symptom of another condition. If the trigger is a medication, discontinuing the drug is usually necessary. For infectious causes, appropriate antimicrobial therapy is prescribed to clear the pathogen.
For symptomatic relief, nonsteroidal anti-inflammatory drugs (NSAIDs) are commonly used. Supportive measures like resting and elevating the affected limbs, along with using compression garments, can help reduce swelling and discomfort. In severe, persistent, or recurring cases, systemic therapies such as oral corticosteroids or other immunosuppressive medications may be introduced to suppress the immune response.