Definition

Prurigo nodularis is a chronic inflammatory skin disease characterized by symmetrical, intensely itchy skin rashes. The itching can be so severe that it frequently disrupts sleep and daily activities.

Causes

The exact cause of prurigo nodularis is unknown, but it is believed to be related to immune system and nerve function disorders in the skin. Microscopic examination of skin biopsies from individuals with prurigo nodularis reveals an increase in nerve fibers in the skin, leading to heightened itching sensations that prompt scratching. Persistent scratching can also cause skin changes such as thickening.

Compared to healthy individuals, the skin of prurigo nodularis sufferers contains more immune cells that produce chemicals involved in the inflammatory response, contributing to increased itching.

Prurigo nodularis may occur independently or in association with other conditions such as:

• Skin diseases: Prurigo nodularis is most often found in people with other itchy skin conditions, such as eczema (atopic or allergic dermatitis), lichen planus, xerosis cutis (dry skin), keratoacanthoma (skin tumor), and bullous pemphigoid.
• Infections: Associated infections include bacterial infections like tuberculosis and H. pylori, viral infections such as herpes zoster, hepatitis C, HIV, and parasitic infections.
• Neurological conditions: Neurological diseases causing prurigo nodularis are linked to nerve damage, as seen in herpes or shingles, polyneuropathy, and post-burn sensitive and itchy skin.
• Psychiatric conditions: Psychiatric causes of prurigo nodularis include psychological itching, related to depression, anxiety, and dissociative disorders that lead to excessive scratching and subsequent skin changes.
• Other medical conditions: Conditions such as diabetes, chronic kidney disease, liver disease, thyroid disorders, hormonal imbalances, gout, iron-deficiency anemia, polycythemia vera (blood cancer), amyloidosis (amyloid protein buildup), HIV/AIDS, and various cancers (particularly myelodysplasia, leukemia, Hodgkin's and non-Hodgkin's lymphoma, multiple myeloma, bladder cancer, lung cancer, gastrointestinal cancers, and female reproductive cancers).
• Medications: Certain medications, including chemotherapy agents like pembrolizumab, paclitaxel, and carboplatin, may cause prurigo nodularis, potentially due to immune system activation following therapy.

Risk factor

Prurigo nodularis can occur at any age but is more common in older individuals, particularly between the ages of 40 and 69. The condition affects both genders but is more prevalent in women. People with HIV are more frequently affected by prurigo nodularis compared to the general population. Ethnicity and genetics also play roles, with African-Americans being 3.4 times more likely to develop prurigo nodularis than Caucasians.

When prurigo nodularis occurs in younger individuals, it is often associated with other inflammatory skin diseases, usually eczema (atopic or allergic dermatitis).

Symptoms

The appearance of prurigo nodularis rashes varies among individuals and is thought to result from prolonged and excessive scratching due to intense itching, burning, and stinging sensations. The itching in prurigo nodularis is usually intense, episodic but can also be continuous, and long-lasting, often persisting for more than six weeks. Itching typically worsens with sweating, hot weather, and stress.

The number of rashes can range from a few to hundreds. The size of the rashes also varies from 1/2 cm to 2 cm. Rashes are usually symmetrically distributed and can appear as solid dome-shaped spots, larger bumps, or plaques.

Affected skin areas thicken due to an increase in keratin, similar to callus formation and skin thickening seen in conditions like eczema, where repetitive scratching leads to thickened skin.

Prurigo nodularis rashes can be reddish-brown, pink, red, brown, or black. They are most commonly found on the back of the scalp, abdomen, back, arms, and legs. The mid-back area is typically spared, likely because it is difficult to scratch.

Diagnosis

Diagnosis of prurigo nodularis is typically made by a dermatologist based on symptoms and response to treatment. However, because its symptoms resemble other conditions causing intense itching, it is often misdiagnosed as another skin disease. Doctors usually use a magnifying glass with light to examine the skin's structure. For accurate diagnosis and differentiation from similar conditions, a skin biopsy may be performed to examine a rash sample under a microscope.

Once prurigo nodularis is diagnosed, doctors will conduct a complete blood count, metabolic tests including liver and kidney function tests, thyroid function tests, and other evaluations to identify any underlying conditions contributing to prurigo nodularis.

Management

Symptoms of prurigo nodularis require medical therapy, as they typically do not resolve spontaneously without treatment. Currently, there is no FDA-approved therapy for prurigo nodularis. Treatment generally involves medications used for other skin conditions.

Topical medications and injections into the rash area can be used for prurigo nodularis. Topical treatments include corticosteroids, calcineurin inhibitors (immune system suppressants), and vitamin D. Injectable therapy often involves corticosteroids.

Behavioral therapy can also help prevent scratching and dry skin. Strategies include trimming nails short, wearing long sleeves and gloves, wrapping itchy skin, cleansing the skin gently, maintaining skin moisture with non-irritating moisturizers, and avoiding warm environments to reduce sweating. Doctors may prescribe anti-itch creams containing calamine, menthol, and camphor.

If initial therapies are unsuccessful, additional treatments such as cryotherapy (using cold to remove lesions), phototherapy (using light therapy), and medications to control symptoms can be considered. Other therapies include antidepressants, gabapentin (a nerve disorder medication), anticonvulsants, and sedatives, especially if nighttime itching causes sleep disturbances. These medications, combined with psychotherapy and relaxation techniques, can help manage the psychological effects of prurigo nodularis.

Complications

Complications can occur if the rashes become infected with bacteria. Healed lesions may leave scars and darkened pigmentation, leading to a loss of self-confidence.

Prevention

Until the exact cause is known, preventing prurigo nodularis is challenging. Avoiding scratching may be the only way to prevent it. If you have risk factors such as genetic predisposition or certain medical conditions, monitor your skin regularly. Consult a doctor if you experience persistent itching. Try to break the itch-scratch cycle before it becomes established and difficult to stop.

When to see a doctor?

Consult a doctor if you experience persistent itching. Early treatment can minimize complications and improve daily quality of life.

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