Panniculitis is inflammation of the layer of fat that sits just beneath your skin. It typically shows up as tender, red or purplish bumps (nodules) that can appear on the shins, thighs, arms, or other areas where subcutaneous fat is present. The term covers a broad group of conditions rather than a single disease, and the underlying cause ranges from infections and autoimmune disorders to medication reactions and, in some cases, no identifiable trigger at all.
How Subcutaneous Fat Becomes Inflamed
The fat beneath your skin is organized into small compartments called lobules, separated by thin walls (septa) made of connective tissue. Those walls carry blood vessels, lymphatic channels, and nerves. Each lobule is fed by its own small artery, which is essentially a dead-end blood vessel. When that vessel is damaged or when an immune response targets the fat tissue, the fat cells in that lobule can become inflamed and even die off. This is what produces the painful, palpable nodules characteristic of panniculitis.
Because the inflammation can center on different structures within the fat layer, doctors classify panniculitis into two broad patterns. Septal panniculitis mainly affects the connective tissue walls between fat lobules. Lobular panniculitis targets the fat cells inside the lobules themselves. This distinction matters because it helps narrow down the cause and guides treatment decisions.
What It Looks and Feels Like
The hallmark of panniculitis is one or more firm, tender bumps under the skin. They often start out red and can turn purplish over several days, eventually fading to look like deep bruises. The nodules are typically felt more than seen in the early stages, since the inflammation sits below the skin surface. Some forms produce nodules that ulcerate or drain an oily substance as fat cells break down.
Beyond the skin, many people experience systemic symptoms: fatigue, fever, joint pain or swelling, and general malaise. The combination of skin nodules with these whole-body symptoms often points toward an underlying condition driving the inflammation.
Erythema Nodosum: The Most Common Type
Erythema nodosum is the form of panniculitis people encounter most often. It causes painful red bumps primarily on the shins, though nodules can also appear on the ankles, thighs, arms, or face. The bumps typically become purplish within a few days, then fade through stages resembling a bruise. Fever, fatigue, joint pain, and enlarged lymph nodes in the chest can accompany the skin lesions.
Strep throat is one of the most common triggers, and doctors often order a throat swab as part of the workup. Other known triggers include sarcoidosis, tuberculosis, fungal lung infections, inflammatory bowel disease (both Crohn’s disease and ulcerative colitis), and certain cancers. Medications can also set it off, including antibiotics containing sulfa or penicillin, birth control pills, and some anti-inflammatory drugs. In many cases, no cause is ever identified.
Autoimmune and Systemic Causes
Panniculitis frequently shows up alongside autoimmune and connective tissue diseases. Lupus panniculitis (sometimes called lupus profundus) is one of the best-described forms, producing deep nodules that often heal with scarring and visible depressions in the skin where fat has been lost. Dermatomyositis, morphea, and scleroderma can all produce their own patterns of fat-layer inflammation as well.
Less commonly, panniculitis appears in patients with overlapping autoimmune conditions. There are documented cases in people with Hashimoto’s thyroid disease, insulin-dependent diabetes, and juvenile rheumatoid arthritis occurring together. The shared thread is an immune system that has turned against the body’s own tissues, with the fat layer becoming one of the targets.
Alpha-1 Antitrypsin Deficiency
One genetic cause worth knowing about is alpha-1 antitrypsin deficiency, a condition where the body doesn’t produce enough of a protein that protects tissues from damage by immune cells. In people with the most severe form of this deficiency, panniculitis occurs in roughly 1 in 1,000 cases. The nodules tend to relapse over time and heal with atrophic scarring, leaving thinned, sunken areas of skin.
How Panniculitis Is Diagnosed
A skin biopsy is the cornerstone of diagnosis. Standard superficial biopsies don’t reach deep enough because the inflammation sits in the fat layer well below the skin surface. Doctors need a deep punch biopsy or an incisional biopsy that captures a wedge of tissue extending down into the subcutaneous fat. The pathologist examines whether the inflammation is concentrated in the septa or the lobules, whether blood vessels are involved, and what types of immune cells are present. These details narrow the diagnosis from the broad category of “panniculitis” to a specific subtype, which then determines treatment.
Blood work, imaging, and cultures may follow depending on what the biopsy suggests. If an autoimmune disease is suspected, antibody testing helps confirm it. If infection is a possibility, tissue cultures can identify the organism.
Treatment Approaches
Treatment depends entirely on the underlying cause. When panniculitis is triggered by an infection, treating that infection resolves the skin lesions. When a medication is responsible, stopping the drug is usually sufficient.
For autoimmune and idiopathic forms, corticosteroids are the first-line treatment. They reduce inflammation broadly and can bring significant improvement. If symptoms return after steroids are tapered, or if the condition doesn’t respond adequately, doctors may add immunosuppressive medications to achieve longer-term control. Some forms of panniculitis, particularly those linked to connective tissue diseases, require ongoing management because they tend to relapse.
Anti-inflammatory medications are used for milder cases, particularly erythema nodosum, where bed rest, leg elevation, and compression stockings can also help the nodules resolve faster.
Healing Timeline and Long-Term Outcomes
How quickly panniculitis resolves and whether it leaves lasting marks depends heavily on the type. Many forms have an excellent outlook. Cold panniculitis, which can develop after skin exposure to freezing temperatures, resolves on its own within weeks to months. Subcutaneous fat necrosis in newborns also has an excellent prognosis, with spontaneous resolution within several weeks being common.
Other types leave more of a mark. Lupus panniculitis often heals with scarring and lipoatrophy, a visible loss of fat beneath the skin that creates depressions or contour changes. Recurrent forms in children can produce striking ring-shaped areas of fat loss. Panniculitis linked to alpha-1 antitrypsin deficiency tends to relapse and leaves atrophic scars.
A rare and historically significant form, sometimes called Weber-Christian disease, involves recurring episodes of tender nodules on the lower extremities accompanied by fevers, joint pain, and muscle aches. It predominantly affects women and can follow a relapsing course over years. The nodules in this form involve lobular inflammation, and episodes may include fat-laden immune cells and tissue death visible under the microscope.
For most people, the key to a good outcome is identifying and addressing the underlying trigger. When that’s possible, the panniculitis itself resolves. When the cause is chronic or autoimmune, long-term treatment keeps flares manageable and limits permanent skin changes.