Prurigo nodularis (PN) is a skin condition that causes extremely itchy bumps.

Skin itchiness often occurs before PN bumps appear. Some people experience stinging or burning instead of an itch. Others report unusual sensations like crawling bugs under their skin, although this is rare.

About 6 weeks after skin sensations begin, itchy bumps appear. Experts believe they occur as the result of scratching or rubbing the affected skin areas.

It’s not always clear what prompts PN itching, but in some instances, there may be a connection to certain underlying conditions.

Experts haven’t found a single specific cause of PN. Instead, there are factors that contribute to the condition.

Neural changes

PN nodules have a neural structure that’s different from unaffected skin. PN nodules have more:

  • Merkel cells: These are cells located just below the skin’s surface, very close to nerve endings that receive touch sensation.
  • Papillary dermal nerves: These nerves are in the papillary dermis (inner skin layer), which regulates temperature and provides nutrients to the epidermis (outer skin layer).

This increase in sensory structures is specific to PN. It isn’t found in neurodermatitis or lichen simplex chronicus, two other skin conditions that feature chronic itch.

Immunological differences

PN features an increased density of immune system cells called masts and neutrophils. Another type of immune cell called eosinophils remains at typical levels but shows evidence of increased activity.

Also present in higher numbers are immune cell chemicals called cytokines, which are involved in inflammatory responses that may lead to itch.

Risk factors

There are factors that increase the chance a person may develop PN. They include:

  • Age: PN usually occurs in people ages 51 to 65, although it can develop at any age.
  • Ethnicity: A 2018 study found that Black people were 3.4 times more likely to develop PN than white people.
  • Allergic conditions: The British Association of Dermatologists estimates that about 80% of people living with PN are atopic, which means that they’re sensitive to allergens.

Itch-scratch cycle

The itch-scratch cycle refers to the way that scratching to relieve an itch can cause more itching. This cycle is a key part of PN.

Researchers have identified four categories of itch:

  • Pruriceptive: This refers to a dermatological itch that originates in the skin.
  • Neurogenic: This type is also known as systemic itch and originates from areas of the body other than the skin, like blood, liver, and kidneys. Neurogenic itch is not connected to nerve damage or psychiatric issues.
  • Neuropathic: This itch results from damage to central or peripheral nerves that receive sensory information.
  • Psychogenic: This type of itch may stem from psychiatric issues if no other cause for the itch is found and certain mental health symptoms are present.

Researchers believe that many cases of PN itch are connected to neurogenic skin inflammation.

Underlying conditions

Underlying conditions may be the cause of itching in some cases of PN. These conditions are usually one of four types:

  • infectious
  • dermatological
  • systemic
  • psychiatric

This isn’t always the case, though. In many instances, doctors are unable to identify any underlying condition accompanying PN.

Dermatologists often conduct thorough exams and order additional tests when assessing a person with PN. This is because PN may be the first sign that an underlying condition is present.

Thyroid disease

Thyroid disease and hyperparathyroidism are examples of systemic conditions associated with PN.

  • Thyroid disease: The thyroid gland produces too much or too little thyroid hormone.
  • Hyperparathyroidism: The parathyroid gland produces too much parathyroid hormone.

Both thyroid disease and hyperparathyroidism can cause itchy skin.

Hepatitis C

Hepatitis C is a viral and blood-borne liver infection, usually the result of unsafe needle use or nonsterile medical procedures. Hepatitis C sometimes leads to liver cirrhosis or cancer.

As many as 20% of people with chronic hepatitis C report itchy skin as a symptom.


A weakened immune system can lead to PN. As a result, it’s one of the skin conditions commonly associated with HIV.

PN occurs more frequently in people with HIV when their CD4 T-cell count is below 50 microliters. The CD4 T-cell count is a measure of immune system function, and the usual range is between 500 and 1500 microliters.

Since HIV is transmitted through bodily fluids, anyone can contract this virus. Skin changes like the onset of PN are sometimes the first sign of an HIV infection.

Kidney failure

PN can occur with chronic kidney disease-associated pruritus (CKD-aP). CKD-aP is itching found in chronic kidney disease (CKD) and end-stage kidney disease (ESKD).

CKD-aP has multiple possible causes relating to changes in the body from CKD and ESKD. One example is uremic pruritus, which is itching that results as damaged kidneys accumulate toxins and waste.

CKD-aP can occur alone as itch or with PN or another skin disease.


A 2018 case study demonstrated how PN could be the first indicator of lymphoma. The subject, a 35-year-old woman, experienced PN itching and lesions that didn’t subside until her Hodgkin’s lymphoma was treated.

Doctors aren’t fully certain why cancer can cause itching, but there are theories. Substances released by tumors may trigger itch, or the cause might be the way the body reacts to cancer.

Liver disease

Liver disease is another condition dermatologists may test for when assessing a person with PN.

A 2019 study found that people living with chronic liver disease experienced itching more frequently than those without liver disease. The participants with chronic liver disease had no allergies or dermatological sources of itch.

Cholestasis, or impaired bile secretion from the liver, can occur as part of inflammatory liver disease. Cholestasis is a condition that can cause itching that leads to PN.

Mental health conditions

Depression and anxiety are two examples of mental health conditions that may be linked to PN.

A 2022 study found that people with PN had a higher chance of also having a psychiatric disorder than age-matched controls.

In addition to depression and anxiety, other issues identified included:

  • increased risk of self-harm
  • eating disorders
  • attention deficit hyperactivity disorder
  • schizophrenia

In the same study, 17% of participants with PN had a psychiatric diagnosis other than anxiety or mood disorders.

If you live with PN, you can identify and avoid triggers to lessen the severity of your symptoms. Some of triggers may include:

  • heat
  • stress
  • sweat
  • humidity
  • dryness
  • wool, polyester, or rough-fabric clothing
  • personal care products
  • allergens

Your dermatologist may be able to offer solutions for the times when it’s difficult to avoid triggers.

PN is a skin condition featuring persistent itch that leads to bumps. PN can occur on its own or with another condition.

Since PN is sometimes the first sign of an underlying condition, dermatologists often conduct thorough exams when assessing a person with PN symptoms. They may order additional tests or consult with a specialist in a different area of medicine.