Diagnosis
The diagnosis of polymyositis is difficult to make due to its manifold appearance. Usually one thinks first of a flu-like infection, a rheumatological illness or a medicine reaction (e.g. Simvastatin), before the suspicion of a Polymyositis is steered. When making a diagnosis, it is first important to rule out the other possible causes.
One of the most important diagnostic tools is probably the blood test, in which specific inflammation values (white blood cells, CRP value, rheumatoid factors if necessary) are determined. Only then can the diagnosis of polymyositis often be made. Due to the fact that polymyositis is a form of idiopathic myositis and thus belongs to the autoimmune diseases, so-called autoantibodies in the blood serum can be detected in 90% of patients.
In about 60% of cases, however, these are mostly antibodies directed against unspecific molecular biological cell components of the body’s own cells, only in about 30% of cases are there actually myositis-specific or myositis-associated antigens, which the body incorrectly recognizes as foreign and fights them. Myosite-specific antigens include so-called tRNA synthetases, such as Mi-2, SRP and Jo-1. The corresponding myosite-specific autoantibodies (MSA), which may exist and be detectable in the blood of the patient for these antigens, are therefore anti-Jo-1 antibodies, Mi-2 antibodies and SRP antibodies.
Myositis-associated antigens include PM-Scl, SS-A-Ro.These antigens are produced by the cells in a wide variety of muscle damage of various types and are presented on the cell surface. In patients who are genetically predisposed, these antigens can then be recognized as foreign by the body’s own immune system, so that an antibody-antigen immune reaction takes place, which leads to a chronic (muscle) inflammation process. In the course of the diagnosis of polymyositis, blood values are determined, among other things, which is another examination method in addition to physical examination, tissue removal (muscle biopsy) and imaging procedures (ultrasound, electromyography, MRI).
Due to the fact that this is a (muscle) inflammatory process, which is triggered by the autoimmune reaction, increased inflammation values (CRP=C-reactive protein, BSG=Blood sedimentation rate) are usually found in the blood of the affected patient. Increased muscle enzymes are also noticeable, which are released from the damaged muscle cells into the blood (CK-MM, GOT, LDH, aldolase, RF). The level of these enzyme values can also be used to draw conclusions about the extent of the disease or muscle damage and thus provide an impression of the current state of the disease. In addition, certain autoantibodies in the blood can be found which are involved in the characteristic autoimmune reaction. A distinction is made between myositis-specific autoantibodies (MSA) and myositis-associated autoantibodies (MAA).
