The most common clinical pictures

Polymyositis is the rarest form of the common inflammatory muscle diseases. It occurs more frequently in two stages of patients’ lives: in childhood and adolescence from 5 to 14 years and in advanced adulthood from 45 to 65 years. On average, twice as many women as men are affected by polymyositis.

Clinically, the disease is characterized by mostly symmetrical muscle weakness in the area of the shoulder-neck-belt and the hip – i.e. the muscles close to the trunk. Compared to inclusion body myositis, the weaknesses develop relatively quickly, over weeks to months.The lack of muscular strength can lead to painful malpositions, and scarring of inflamed muscle sections can lead to malpositions of joints. The tissue sample taken by biopsy shows inflammatory cells that have migrated between the muscle fibers.

The disease process of polymyositis is not yet fully understood. However, it is assumed that it is an autoimmune disease. In contrast to dermatomyositis, however, it is mediated by a direct cell response of the body and not by corresponding proteins.

Dermatomyositis, if age is disregarded, is generally more common than polymyositis. An age-specific accumulation is observable as with the Polymyositis. Women are more likely to be affected than men.

To the symptoms, which concern the skeletal musculature, changes of the skin show up with the Dermatomyositis. Lilac-colored rashes (erythema) form in light-exposed areas of the body, which is why the name lilac disease has been given. The skin becomes flaky, especially in places above joints, such as the fingers, elbows and knees.

As part of the rash, swelling of the upper eyelids may occur, giving the patient a tearful expression. This can be intensified by scarring of the scaly skin. The described changes can appear to a greater or lesser extent.

If a muscle biopsy is performed, the preparation can identify the vascular (perivascular) inflammatory cells surrounding the vessels. Corresponding cells also collect between the individual muscle fibre bundles (interfascicular). Peripheral muscle fibers become narrower in relation to the rest of the bundle.

This is known as perifascicular atrophy. The pathomechanism (disease process) is based on an autoimmune reaction directed against the capillaries (smallest vessels) located in the muscles. These are attacked and damaged by the body’s own inflammatory proteins (e.g. immunoglobulins).

As a result, the muscle fibers can no longer be supplied and die. This leads to local necrosis (cell death) and vascular thrombosis (occlusion of a vessel by a blood clot/thrombus) – the muscle fiber bundles lose strength and eventually atrophy. In more than a quarter of all dermatomyositis diseases, a malignant tumor is the cause of its development.

Here too, the body produces substances that are directed both against the tumor and against healthy body tissue. Inclusion body myositis is a chronic progressive, inflammatory, degenerative disease. The causative process in the body is not yet fully understood, but an interaction of inflammatory and degenerative factors is suspected.

It affects men in 75 percent of cases of illness and occurs mainly after the age of 50. The course of the disease is rather insidious – it sometimes takes months to years before the first clinical symptoms appear. Patients first notice problems when climbing stairs or getting up from a sitting position.

Difficulties in holding a firm grip or performing a firm grip in the first place are also characteristic. The symptoms are caused by the progressive weakness of the forearm and thigh muscles. 60% of patients report difficulty swallowing, as this also requires muscles that can be affected by the inflammation.

The preparation prepared from a biopsy is in itself similar to that of polymyositis. Immigrated inflammatory cells and lost fibre strands can be detected. In addition, there are inclusions in the tissue, so-called ̈rimmed vacuoles ̈ (in German: umrandete Vakuolen; vacuole = cell vesicle).

The inclusion bodies contain various protein structures, ? amyloid and tau proteins. These compounds are also found in other degenerative diseases, such as Alzheimer’s disease.