IMNM / Immune-Mediated Necrotizing Myopathy 4.33/5 (3)

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Definition

Immune-mediated necrotizing myopathy (IMNM) is distinguished from other types of myositis by a tissue sample (biopsy) from muscle, most often taken from the thigh. Typical is the absence or near absence of inflammatory changes (inflammatory cells) in the tissue, even if the cells are damaged and die (necrotic). One can divide IMNM into three subgroups based on which (myositis-specific) antibody which are: 1) HMGCR-positive, 2) SRP-positive and 3) antibody-negative type. IMNM is in some cases associated with statin treatment (against high cholesterol), statin myopathy. Anti-HMGCR is typically antibody. The course of the disease is often also typical of relatively severe muscular weakness (reference: Pinal-Fernandez I, 2018).

Occurrence

About 10% of all myositis are IMNM with SRP or HMCR antibody. SRP myositis often begins at the age of 40, while HMGR myositis at the age of 55, but there are large variations, so that children can also be attacked (reference: Pinal-Fernandez I, 2018).

Symptoms

The disease picture is characterized by pronounced muscle weakness, especially in younger people. The disease is thus often more aggressive towards the muscles than other types of myositis. Pulmonary manifestations, on the other hand, are relatively rare (5% in HMGCR myositis and 10-20% in HMGCR myositis). Nor is the skin attacked particularly frequently (in fewer than 10%). Cardiac manifestations are rarely seen in HMGCR myopathy and are very uncommon in SRP myositis. 

Examinations

IMNM myositis
Immune-Mediated Necrotizing Myopathy. IMNM O'Grady J, Harty L, Mayer N, Critcher V, Ryan J - J Clin Med Res (2015)  CC BY-2.0

Medical history inquire about cases in the immediate family, although heredity is uncommon. Any weakness, pain or stiffness is mapped together with symptom onset and course. Medication consumption (statins, others)? Symptoms of other systemic connective tissue diseases? Symptoms from the joints, oesophagus, lungs or heart are requested.

Clinical examination includes the lungs, heart, nerves and other organs. The musculature is assessed in particular for localization of weakness and any visible changes - Tests of muscle strength can be done by seeing if it is difficult to get up from the chair and from squatting without support, as well as lifting the arms against weight. Physiotherapists test both direct muscle strength and endurance according to standardized methods.

Laboratory tests. In general, the tests may include CRP, SR, Hgb, white blood cells (leukocytes with differential counts), electrolytes, liver, kidney and thyroid function tests, creatine kinase (CK), LD and glucose. if a cardiac manifestation is suspected, troponin may be relevant. ANA and myositis-specific and associated antibodies in the form of anti-SRP or anti-HMGCR detected, but "antibody-negative cases also exist. Urine sting. -Muscle enzymes; CK, LD, ASAT. These are expected to be clearly elevated.

EMG (electromyography)) can help distinguish between myositis and other muscle diseases

Imaging diagnostics: MRI (magnetic resonance imaging) of thigh muscles often shows swelling (edema) in the muscles and adjacent muscle fascia. Most often done CT (computer tomography) of lungs to rule out pulmonary manifestation. X-ray examination of the esophagus (dynamic with contrast agent that is swallowed) maps whether the swallowing function is affected.

Tissue sample (biopsy). Typical changes: has typical histology with pronounced necrosis and cell regeneration, little lymphocyte infiltration. Biopsy results are similar then rhabdomyolysis and toxic myositis. To detect antibody-negative IMNM, biopsy is a necessity.

Cancer Association is so rare that there are no general recommendations on systematic cancer investigation at IMNM (reference: Pinal-Fernandez I, 2018). 

Treatment

Adapted physiotherapy with training and strengthening of the muscles has proven useful, as in the treatment of other types Myositis. At IMNM, the choice of supplementary drug is partly based on which antibody is present. People with HMGCR myositis usually have a good effect of intravenous gamma globulins (IVIG), while in SRP myositis early treatment with rituximab beneficial. IVIG is often dosed at 2g/kg body weight every 4 weeks. The dose is spread over 3 days (for example, a body weight of 60 kg would require IVIG 40g/day = 120g in total over three days). If there is a risk of blood clots, Fragmin or similar is given preventively. Kidney function is checked. In general, the treatment response is worse than with other types of myositis (apart from inclusion body myositis), so that one sometimes tries combinations of drugs (reference: Weeding E, 2021).

Literature

Myositis, BINDEVEVSSYKDOMMER.no


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