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PubMed This is a summary of 20 peer-reviewed journal articles Updated
Pediatrics · Chronic Nonbacterial Osteomyelitis

Getting Your Bearings: What is CNO/CRMO?

At a Glance

CNO and CRMO are related forms of bone inflammation without infection. They mainly affect children and may cause pain in one or several bones, including areas seen only on MRI, with possible links to skin, joint, or bowel inflammation.

If you have spent months or even years searching for an answer to your child’s bone pain, finally hearing the words Chronic Nonbacterial Osteomyelitis (CNO) or Chronic Recurrent Multifocal Osteomyelitis (CRMO) can bring a complicated mix of relief and anxiety. It is common to feel frustrated by a long diagnostic journey; because this condition is rare, many primary care doctors and even specialists may not have encountered it before [1][2].

This page provides an orientation to what is happening inside your child’s body and why this diagnosis is different from the infections or injuries you might have first suspected.

Understanding “Sterile” Inflammation

The most important thing to know is that CNO is not caused by bacteria, viruses, or fungi [3]. In a typical bone infection (osteomyelitis), germs invade the bone and the immune system attacks them. In CNO, the immune system attacks the bone even though no infection is present [3][4].

Doctors call this sterile inflammation [4]. Because there are no germs to kill, traditional antibiotics do not fix the underlying problem. Instead, the “internal thermostat” of the immune system is stuck in the “on” position, causing the body to act as if it is fighting a phantom infection [5][6]. It is important to know that CNO is not contagious, and it is not caused by anything you or your child did.

The Role of the Innate Immune System

The part of the immune system responsible for CNO is the innate immune system—your body’s first line of defense. Specifically, research points to a pathway involving a protein called IL-1β (interleukin-1 beta) [6][7].

In a healthy body, IL-1β acts like a chemical “fire alarm” that tells the body to start an inflammatory response. In children with CNO, this alarm system (often involving a structure called the inflammasome) is overactive [5][8]. This is an area of active research, and while it doesn’t explain every case perfectly, this biological process leads to:

  • Osteoclast Activation: These are cells that naturally break down old bone. Overactive inflammation signals these cells to work too hard, leading to the bone lesions seen on scans [7][9].
  • Imbalance of Signals: The body also produces fewer “calm down” signals, such as the anti-inflammatory protein IL-10, which would normally turn the inflammation off [10][11].

Who Does This Affect?

CNO is a rare condition, which makes exact numbers difficult to confirm. Current data from the UK suggests it affects approximately 0.65 out of every 100,000 children [1]. Some older global estimates place the prevalence at roughly one in a million people, highlighting why it is often not the first diagnosis doctors consider and why these statistics can vary widely depending on the country or year studied [2].

  • Age of Onset: The average age when symptoms first appear is roughly 9 to 10 years old [12][13]. While it is primarily a pediatric disease, it can occasionally be diagnosed in adults [12].
  • Sex: The condition is more common in girls than boys, with studies showing a ratio of roughly 2:1 [1][12].

A Spectrum of Disease

Doctors view CNO and CRMO as parts of the same spectrum [3].

  • CNO is the broad umbrella term for this sterile bone inflammation.
  • CRMO is often used when the inflammation occurs in multiple bones (multifocal) or disappears and returns over time (recurrent) [3][14].

It is common for a child to have pain in only one bone (unifocal) while an MRI reveals other “silent” areas of inflammation in bones that do not yet hurt [15].

Associated Conditions and Overlaps

Because CNO is an autoinflammatory disease—meaning the innate immune system is misfiring—it can sometimes overlap with other inflammatory conditions. Your child’s doctor may review their symptoms for signs of:

  • Skin Conditions: Some children develop psoriasis (scaly skin patches), palmoplantar pustulosis (small blisters on the hands or feet), or severe acne [16][12].
  • Joint Issues: Arthritis (joint swelling and pain) or sacroiliitis (inflammation where the lower spine meets the pelvis) occurs in a significant number of patients [16][17].
  • Digestive Issues: There is a known link between CNO and Inflammatory Bowel Disease (IBD), such as Crohn’s disease or ulcerative colitis [17][18]. Routine screening of every child is not usually necessary unless specific gut symptoms are present.

While these overlaps do not happen to every child, knowing they are connected helps your medical team provide more comprehensive care. In very rare cases, if a child is very young or has a strong family history of similar issues, doctors may test for specific rare genetic syndromes like Majeed syndrome or DIRA [19][20].

Common questions in this guide

What is the difference between CNO and CRMO?
CNO is the broad term for sterile inflammation of the bone without an infection. CRMO is often used when inflammation affects multiple bones or returns over time, so the two terms describe a related disease spectrum rather than two completely separate diseases.
Is CNO or CRMO a bone infection, and do antibiotics treat it?
No. CNO and CRMO are not caused by bacteria, viruses, or fungi, so they are called sterile inflammation; antibiotics do not treat the underlying inflammatory process. Doctors still need to rule out infection before confirming the diagnosis.
Who usually develops CNO or CRMO?
CNO and CRMO most often begin in childhood, with symptoms appearing around ages 9 to 10 on average, although adults can occasionally be diagnosed. The condition is reported more often in girls than boys, at roughly a two-to-one ratio.
Can a child have CNO in bones that do not hurt?
Yes. A child may feel pain in one bone while an MRI shows additional areas of inflammation that are not causing symptoms. Doctors may describe this as unifocal when one bone is involved or multifocal when several bones are affected.
What other health problems can occur with CNO or CRMO?
CNO and CRMO can overlap with psoriasis, palmoplantar pustulosis, severe acne, arthritis, or sacroiliitis. They are also associated with inflammatory bowel disease such as Crohn's disease or ulcerative colitis, but these problems do not occur in every child.
Should every child with CNO be tested for inflammatory bowel disease?
Not usually. Doctors generally review digestive symptoms and consider evaluation when symptoms suggest inflammatory bowel disease, such as Crohn's disease or ulcerative colitis. This decision should be individualized by the child's medical team.
How do doctors rule out infection or other serious conditions before diagnosing CNO?
Doctors use the child's symptoms, physical examination, and imaging such as MRI to assess the pattern of bone inflammation. They also work to exclude bacterial infection and other conditions, including cancer, before confirming CNO.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Does my child's case fit the standard age and symptoms for CNO, or are there features that make it unusual?
  2. 2.Based on the MRI results, does my child have 'unifocal' or 'multifocal' involvement, and how does that affect the treatment plan?
  3. 3.How did you rule out bacterial infection or other conditions like malignancy before arriving at this diagnosis?
  4. 4.Given that CNO can overlap with other conditions, should we review symptoms of inflammatory bowel disease, psoriasis, or arthritis?
  5. 5.Are you following a specific set of guidelines, such as the CARRA or EULAR/ACR criteria, for my child's care?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

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    Chronic Non-bacterial Osteomyelitis: A Review.

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This page explains CNO/CRMO and sterile bone inflammation for informational purposes only; it does not constitute medical advice or diagnose your child. Discuss your child's symptoms and test results with their pediatrician or specialist.

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