Myositis

This article is still missing information.

Myositis is a group of rare autoimmune diseases that cause inflammation of the skeletal muscles. Myositis can be difficult to diagnose due to its varied and sometimes nonspecific symptoms

Classification

The original Bohan and Peter classification which includes 5 groups from 1975 has been the most widely used classification criteria. The criteria are sensitive but not specific for diagnosing myositis.Over the years there has been increasing recognition of involvement of other organ systems such as interstitial lung disease. In 2017 EULAR-ACR developed a new classification criteria. These new criteria are more specific.

There are four main subtypes of myositis: dermatomyositis, polymyositis, immune-mediated necrotising myopathy, and inclusion body myositis.

The clinical subclassification schema is limited by overlapping clinical and histopathological features. Subgrouping by myositis-specific autoantibodies is a newer classification method to overcome this. These autoantibodies are very specific and can help diagnose myositis. The autoantibodies are associated with distinct clinical features, including arthritis, lung disease, and skin manifestations.

Dermatomyositis

Figure 1. Gottron's papules

This is the easiest subtype to diagnose due to the skin manifestations. Gottron's papules are found over the dorsal aspects of the MCPJs and PIPJs and even over the DIPJs. These can be itchy. The patient may have a very photosensitive V-neck distribution rash, very similar to lupus.

The muscle weakness pattern is proximal. including the neck, arm abductors, and hip flexors. On muscle biopsy there is peri-fascicular atrophy.

Immune Mediated Necrotising Myopathy

This was first described in 2010. These patients have the same pattern of weakness as dermatomyositis. However on biopsy there is necrosis of the muscle fibres. The myofibres are different in size which indicates a true intrinsic myopathic process. With treatment the fibres regenerate.

The important question is whether they have had statin exposure which has a strong link to autoimmune necrotising myositis. It is an extremely rare adverse event.

Polymyositis

This is a diagnosis of exclusion. On biopsy you should see primary inflammation , i.e. inflammatory cells invading the muscle fibres.

The polymyositis category is controversial as to whether it actually exists as many of these patients evolve to develop more typical features of another subtype or actually have a genetic neuromuscular disease such as facioscapulohumeral dystrophy or limb girdle muscular dystrophy.

Inclusion Body Myositis

Inclusion body myositis (IBM) is a chronic, inflammatory myopathy that primarily affects older adults, particularly men. The condition is characterized by proximal muscle weakness and the presence of inflammatory cells surrounding and invading myofibres and rimmed vacuoles on biopsy. IBM typically presents with asymmetric weakness of the proximal and distal limbs. Initial symptoms are often difficulty climbing stairs and rising from a seated position. In advanced cases, dysphagia and respiratory muscle weakness may also be observed.

The pathogenesis of IBM is not fully understood, but it is thought to be autoimmune. Overall, the prognosis for patients with IBM is poor, by 15 years most patients require assistance with basic daily activities.

Unfortunately the term IBM also stands for an unrelated category of diseases called the inclusion body myopathies. The myopathies are inherited muscle disorders (h-IBM), while the other type are autoimmune. The two different "IBMs" only have superficial resemblence.

Evaluation

The most important aspects for evaluation are:

• Pattern of muscle weakness, i.e. proximal or distal. IBM also has distal weakness
• Muscle enzyme elevation (CK)
• Muscle biopsy
• MRI of the muscles
• Neurophysiology studies
• Autoantibody myositis

Neurophysiology

Figure 2. In myopathies the disease affects the muscle fibers within the motor unit. As the motor neurons and axons are unaffected each motor unit actional potential (MUAP) leads to activation of less muscles fibres. This is seen by an increase in polyphasic activity, a decrease in duration, and a decrease in amplitude. From Paganonii et al.

In myopathies, there is a drop-out of individual muscle fibres. Motor neurons and axons are not affected. In myopathies each motor unit action potential (MUAP) is generated by fewer motor fibres.

Therefore with EMG you see polyphasic, shorter duration, and low amplitude results. This is how you determine that it is an intrinsic muscle process. ALS on the other hand will have normal EMG. In comparison, nerve conduction studies will be abnormal in ALS but normal in myopathies.

Electrodiagnostic Evaluation of Myopathies - Paganoni 2013 - 247 KB (f)

MRI

On T2 weighted images and STIR you see increased oedema signal. This doesn't diagnose myositis as such but it can tell you which muscle is involved and it can be helpful in planning muscle biopsy to get the best biopsy specimen This is important because in myositis the inflammation can be patchy.

In chronic cases, such as in untreated myositis or in genetic muscular dystrophies where there is no effective treatment, there is increased signal on T1 indicating fatty atrophy and irreversible damage.

Resources

Myositis - Members Subpage

https://www.youtube.com/watch?v=sz5AVWo-Suo