Diagnostics

The diagnosis of myositis is usually complicated because it is difficult to differentiate between different clinical pictures. The clinical symptoms should be the guide, as these can give an indication of the type and location of the inflammation. However, the majority of myositis is a creeping disease that is noticed only late.

This increases the risk of developing permanent secondary damage. Basically, the examining physician has three diagnostic instruments that can be used: a laboratory examination, electromyography (EMG; measuring tension in the muscles) and a muscle biopsy (invasive procedure in which muscle tissue is removed. Laboratory examination: When examining the laboratory parameters in the blood of the patient, the main focus is on enzymes that are abundant in muscle cells and are released when the cells are damaged.

The most important enzyme is creatine kinase (CK). Other parameters such as the activity of lactate dehydrogenase, aldolase and aspartate aminotransferase in the blood are also measured. General signs of inflammation such as increased C-reactive protein, increased leukocyte count or prolonged BSG are also recorded, but only prove the presence of inflammation.The amount of myoglobin, a specific protein of the skeletal muscles, can also be determined and included in the diagnosis.

However, the value does not say anything about the location of the damage, only that muscle cells have been lost. If an infestation with pathogens is suspected, it is possible to detect antibodies formed by the body against the pathogen and thus indicate an existing infection, or to duplicate the DNA of the pathogen by means of a PCR (polymerase chain reaction) and display it in such a way that computer-controlled exact identification is possible. Myositis-specific antibodies, which are produced in the course of the disease in some patients, are in most cases not conclusive, since they are also produced in other diseases, such as inflammation of the alveoli (alveolitis) or inflammation of the joints (arthritis).

Electromyography (EMG): In an EMG, two tiny needles are inserted into the muscle to be examined. The needles conduct electric current and measure changes in tension in muscle tissue. The changes are recorded and evaluated at rest and under tension.

In most patients with myositis, conspicuous patterns appear, but these are not automatically indicative of the disease. Nevertheless, the EMG represents an uncomplicated examination, which can provide the indication for further diagnostics. In addition, an electroneurography can be performed, in which the nerve conduction velocity and the muscle reaction time are measured.

Here a nerve is excited with the help of applied electrodes and attention is paid to the resulting muscle twitch. Accompanying nerve damage or other diseases can be proven or excluded, which plays an important role in differential diagnosis (other diseases with corresponding symptoms). Muscle biopsy: Since muscle biopsy is an invasive examination, the location of the intervention should be planned.

This is usually done by an MRI (magnetic resonance imaging). The biopsy should not be performed at a site where an EMG has previously taken place. The punctures through the needles lead to local cell death, which in retrospect cannot be distinguished from myositis.

Once the correct site for the biopsy has been found, general characteristics of myositis can be detected in the biopsy specimen (biopsied tissue) during light microscopic examination, but also tissue changes specific to various forms can be observed. Characteristically, both muscle fiber destruction (dead/necotic muscle fibers) and regenerated sections of muscle fibers and typical signs of inflammation – tissue infiltration (immigration) by inflammation mediating cells – can be seen. If the disease activity is low, the diagnosis may be complicated by missing or difficult to detect cell signs.

Since myositis is one of the inflammatory skeletal muscle diseases that can be caused by an autoimmune reaction, i.e. an erroneous reaction of the body’s own defense system against endogenous structures, it is therefore possible to detect certain antibodies in the blood of the affected patients. These antibodies are components of the immune system, are produced by the so-called B-lymphocytes and are directed against – here in the case of myositis – structures of the skeletal muscle culture, so-called antigens, in the context of an autoimmune disease. In myositis, a distinction is made between myositis-specific and myositis-associated antibodies.

The former are found in blood serum in about 15-50% of patients and can be measured by blood sampling. Myositis-specific antibodies include mainly antibodies against tRNA synthetases, such as Jo-1 antibodies, PL-7 antibodies, EJ antibodies or KS antibodies. Myositis-associated antibodies include anti-Mi-2, anti-SRP and anti-Pm-Scl.