Rheumatoid arthritis (RA) is an autoimmune condition that primarily affects the joints, but it can also manifest outside of the joint capsule. Rheumatoid nodules are the most common non-joint manifestation, appearing as benign lumps that form under the skin. Their presence is often an indicator of advanced or active RA. These lumps are masses of inflammatory tissue, and understanding their appearance is important for people managing the condition.
Understanding the Physical Characteristics
Rheumatoid nodules feel firm, often described as rubbery or hard masses beneath the skin’s surface. The texture depends on whether the nodule is anchored to an underlying structure like bone or tendon, making it feel fixed, or if it is floating freely in the soft tissue, making it feel movable. These lumps are usually non-tender, but they can become painful if they are located in an area of constant friction or if the overlying skin breaks down.
The size of these nodules can vary widely, from a small pea measuring about two millimeters in diameter, up to the size of a walnut or plum, which can be five centimeters or larger. While most nodules are solitary, they can sometimes appear in clusters. Nodules typically present with a rounded or oval shape, though some may develop a more linear appearance when they form alongside a tendon.
The skin covering a rheumatoid nodule generally maintains a normal appearance and color. They are not typically red or inflamed unless the skin has become irritated, infected, or ulcerated due to trauma or pressure. The nodule’s dense, firm consistency comes from a central area of dead tissue surrounded by immune cells.
Typical and Atypical Locations on the Body
Rheumatoid nodules form most frequently in areas subject to repeated pressure or mechanical stress. The most common site is the forearm, especially near the elbow joint, where the bone is close to the skin. They are also frequently observed over the knuckles of the hands and fingers, and occasionally over the Achilles tendon at the back of the heel.
Patients who are bedridden or less mobile may develop nodules on other pressure points, including the hips, the sacrum at the base of the spine, or the back of the scalp. These locations are particularly prone to nodule formation because the constant weight or friction stimulates the underlying tissue. This pattern of formation over pressure points is a defining characteristic of subcutaneous rheumatoid nodules.
While most rheumatoid nodules are visible or palpable under the skin, less common or atypical locations can include internal organs. These rare internal nodules are usually only detected through medical imaging, as they do not produce noticeable symptoms. They have been documented in several areas:
- The lungs
- The vocal cords
- The heart
- The white part of the eye, known as the sclera
Clinical Significance and Distinguishing Features
The presence of rheumatoid nodules is often associated with seropositive rheumatoid arthritis. Patients with nodules are highly likely to have detectable levels of certain autoantibodies in their blood. This seropositivity suggests a more aggressive form of the disease.
Because a variety of other conditions can cause lumps under the skin, distinguishing a rheumatoid nodule from other masses is important. Gout tophi, for example, are also firm lumps but are deposits of uric acid crystals and can be tender during a gout flare, sometimes discharging a chalky white material. Lipomas are soft, benign tumors composed of fatty tissue, which are easily movable and feel distinctly less firm than a rheumatoid nodule.
A physician may need to confirm the nature of a new lump. A tissue biopsy is the definitive way to differentiate a rheumatoid nodule from other masses, such as a calcified cyst or a malignancy. Any new or rapidly changing lump should be promptly evaluated by a rheumatologist to ensure accurate diagnosis and appropriate management of the underlying condition.