Diagnosis of Inclusion Body Myositis

Inclusion body myositis is diagnosed by combining information from your medical history, a physical examination, and results from several tests. Doctors look for a characteristic pattern of gradually worsening muscle weakness—often affecting the thighs and hands—and then use additional tests to help confirm the diagnosis and rule out similar conditions. These tests may include blood work, electromyography (EMG) and nerve conduction studies, MRI, and, when needed, muscle biopsy. This page summarizes the key findings used to diagnose inclusion body myositis and explains how each test contributes to an accurate diagnosis.

Although IBM can be difficult to diagnose, an experienced neuromuscular physician can often make a tentative diagnosis based on the patient’s story and physical examination, with tests used to confirm and rule out similar conditions.

Symptoms

Inclusion body myositis symptoms include a long, gradual history of slowly worsening muscle weakness, often demonstrated by frequent falls, difficulty standing up from a chair, and decreasing hand grip strength. Trouble swallowing, known as dysphagia, is common, and can occasionally be the first symptom.

More information: Symptoms of Inclusion Body Myositis

Physical Examination

During the physical examination, doctors test muscle strength and look for muscle wasting (atrophy). In IBM, weakness is often most noticeable in the thighs and in the muscles used to flex the fingers and grip objects.

More information: Physical Examination for Inclusion Body Myositis

Blood Tests

Blood tests can be helpful, but they cannot diagnose inclusion body myositis by themselves. Creatine kinase (CK) levels can provide an early clue that a muscle disease may be present, and a positive anti-cN1A (NT5C1A) antibody test can support the specific diagnosis of IBM.

More information: Blood Tests for Inclusion Body Myositis

Electromyography (EMG) and Nerve Conduction Studies

EMG and nerve conduction studies are electrodiagnostic tests that are often performed together. They help doctors determine whether muscle weakness is primarily due to a muscle problem or a nerve problem. These tests involve a small needle exam of muscles (EMG) and mild electrical stimulation of nerves (nerve conduction study).

More information: Electromyography (EMG) and Nerve Conduction Studies for Inclusion Body Myositis

Magnetic Resonance Imaging (MRI)

MRI can show edema (swelling), atrophy (muscle wasting), and replacement of muscle tissue by fat. These findings in a specific pattern of muscles can help support the diagnosis of IBM. In some cases, MRI can also help choose the best muscle to biopsy by identifying a muscle that is affected but not too severely.

More information: Magnetic Resonance Imaging (MRI) for Inclusion Body Myositis

Muscle Biopsy

Muscle biopsy has long been considered the gold standard test for diagnosing inclusion body myositis, although doctors now place increasing weight on the overall clinical picture and other tests. When needed, biopsy can provide direct evidence to support the diagnosis, and it is often required for participation in clinical drug trials. Typical biopsy findings include inflammatory immune cells around muscle fibers, “rimmed vacuoles,” and abnormal protein clumps inside the fibers.

More information: Muscle Biopsy for Inclusion Body Myositis

Misdiagnosis and Differential Diagnosis

Misdiagnosis is common in inclusion body myositis because the disease is rare and early symptoms can overlap with other conditions. Doctors use the term “differential diagnosis” to mean the list of conditions that can look similar and must be considered before confirming IBM. Common mistaken explanations include “normal aging,” while common alternative diagnoses include sarcopenia, polymyositis, and ALS. Some inherited muscle diseases can mimic IBM and genetic testing is sometimes used to rule out mimics.

More information: Differential Diagnosis and Misdiagnosis of Inclusion Body Myositis

Why Diagnosis is Often Delayed

The typical patient with inclusion body myositis does not receive an accurate diagnosis for at least several years after symptoms begin.

Early symptoms are subtle and worsen gradually. Mild muscle weakness is not very noticeable, and patients can lead a normal life and perform normal day-to-day activities despite some loss of strength. A person can lose half of their leg or grip strength and not even know it.
Patients often do not go to their doctor to discuss their early symptoms, believing they are just a sign of aging. A man who plays basketball with his grandchildren and finds that he can no longer shoot the ball far enough to make free throws is unlikely to see his doctor to complain.
Most of the symptoms are not specific to inclusion body myositis. If a woman has an unexplained fall or two, it is doubtful that her doctor would think of IBM. Falling is common and has many causes.
IBM is rare, and many primary care physicians have never seen a case.
There is no simple, reliable test for IBM. The most helpful test, a muscle biopsy, requires minor surgery, and even a biopsy does not always give a definite answer.

Finding the Right Specialist

The doctors who are most expert at diagnosing inclusion body myositis are usually neurologists who specialize in neuromuscular medicine. These specialists evaluate disorders across the entire pathway that controls movement—including the brain, spinal cord (motor neurons), peripheral nerves, the neuromuscular junction, and the muscles themselves. They commonly diagnose muscle diseases, peripheral neuropathies, myasthenia gravis, and motor neuron diseases such as ALS. Rheumatologists, who specialize in inflammatory and autoimmune diseases that affect muscles and joints, may also have experience diagnosing IBM.

ENMC Diagnostic Criteria

The European Neuromuscular Centre (ENMC) publishes diagnostic criteria for inclusion body myositis that are widely used in research studies and clinical trials, and may also help guide clinical diagnosis.

Many patients first encounter the term “ENMC criteria” when reading eligibility requirements for clinical trials, especially on ClinicalTrials.gov. Trials often require that participants be “diagnosed with inclusion body myositis (IBM) based on the ENMC IBM Research Diagnostic Criteria,” or similar wording. These criteria help researchers enroll participants who meet consistent diagnostic standards across study sites.

Detailed explanation: New ENMC Criteria for the Diagnosis of Inclusion Body Myositis

ICD-10 Code

Doctors and hospitals document diagnoses using International Classification of Diseases (ICD) codes. Using the correct ICD code for inclusion body myositis helps with accurate medical records and can improve the quality of research data by making it easier to identify IBM patients in large health datasets. The ICD-10 code for inclusion body myositis is G72.41. Cure IBM encourages patients to politely ask whether this code is being used in their medical record.