Induration

Induration refers to the thickening and hardening of soft tissues of the body, specifically the skin, and is the result of an inflammatory process caused by various triggering factors. Indurated areas commonly appear on the hands and face but can also be found on the chest, back, abdomen, breasts, or buttocks. 

Induration can be caused by various medical conditions that trigger an inflammatory process, including infections, infiltration by malignant cells, and other polysystemic conditions. 

Infections
Several types of infections can lead to induration of the soft tissue. For instance, cysts usually infected by microorganisms, like Staphylococcus aureus, can cause hardening of the cystic walls that can be identified as indurated. If the inflammation proceeds to form an abscess, (i.e., a collection of pus that builds up within tissues) further induration of the skin can be identified. Induration of the skin can also be caused by smaller inflammatory processes, such as infected insect bites.

Notably, induration of the skin is also used as a diagnostic method for individuals who are concerned if they have ever been in contact with the bacteria that causes tuberculosis (TB). More specifically, the Purified Protein Derivative (PPD) test is a test performed by injecting a small amount of fluid containing tuberculin (i.e., purified protein from culture of TB) under the top layer of the skin (i.e., intradermal), usually on the lower part of the arm. After 48 to 72 hours, a trained healthcare worker must evaluate the reaction by measuring the induration created around the site of injection. Depending on the individual’s risk factors, there are different cutoffs to indicate a positive reaction. A positive reaction represents prior exposure to the antigen; therefore, the induration that results is an activated T-cell mediated response. Individuals with a positive PPD test may have an active TB infection, a past TB exposure (i.e., latent TB infection), may have had the BCG vaccine, or may even be infected by a variety of non-tuberculosis Mycobacteria.

Malignant Cell Infiltration
Cancerous cells that migrate from a primary lesion may infiltrate various tissues and cause induration. For example, cutaneous metastasis may occur by cancerous cells that spread to the skin from different parts of the body, like the breast tissue. Additionally, skin cancer may similarly cause induration.

Systemic Conditions
Scleroderma
Scleroderma, also known as systemic sclerosis, is a rare autoimmune condition involving inflammation and fibrosis of the skin and internal organs that can cause induration in soft tissue areas. This autoimmune polysystemic condition has three phases with the second phase causing skin induration. Induration is usually widespread on both sides of the body; however, some individuals may present with localized scleroderma and indurated spots localized to the trunk, face, or extremities.

Diabetes Mellitus
Diabetes mellitus can also be associated with induration as it frequently causes recurrent ulcers, especially on the bottom of the feet due to poor circulation and decreased sensation of the lower extremities. Therefore, the areas of skin that have open sores remain chronically inflamed, causing induration of the surrounding skin.

Panniculitis
Panniculitis, an inflammation of the subcutaneous (i.e., under the skin) fat tissue can also cause induration in the affected areas. Similarly, an inflammatory process triggered by inflammatory disorders (e.g., Crohn disease), trauma, exposure to cold, or connective tissue disorders (e.g., lipoatrophy) can cause thickening, formation of firm nodules, and plaques on the skin.

Induration has a very characteristic presentation. On inspection, induration can be identified as raised areas that look smoother and shinier than the rest of the skin. On palpation, the tissues may feel thicker and firmer with a more resistant feeling than the surrounding soft tissues.

The time it takes for induration to resolve varies depending on the underlying cause. With supportive care, such as warm compresses of the indurated areas, analgesic drugs (e.g., acetaminophen), and elevation of the affected limb, induration may take days (e.g., as seen in infections, exposure to cold) or weeks (e.g., as seen in scleroderma, diabetes) to resolve. However, sometimes, further interventions are necessary in order for induration to resolve more quickly. For example, an abscess can resolve promptly if it is drained with a surgical incision and proper antibiotics are administered. Accordingly, in inflammatory disorders like scleroderma, induration can only resolve if the underlying cause is under control with immunosuppressants and topical steroid creams. Lastly, proper treatment of diabetes mellitus can minimize the time needed for the skin induration to resolve. 

Induration is when the soft tissue of different parts of the body, especially the skin, becomes thicker and harder due to an inflammatory process caused by various triggering factors. These include infections, infiltration by malignant cells, and other polysystemic conditions. Induration can be recognized on inspection and palpation due to its characteristic presentation. Treatment of induration usually depends on the underlying cause.