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Myositis

Contents

Introduction

Myositis is inflammation and degeneration of skeletal muscle tissue. In this article we will discuss the various types of myositis, their symptoms, investigation and management.

Causes

There can be multiple causes of myositis including:

  • bacteria (staphylococcus, streptococcus etc)
  • viruses (influenza A and B, hepatitis B etc)
  • parasites
  • some drugs and toxins like statins, Rifampicin, Sulphonamide, cyclosporine, alcohol etc.

Many at times the cause is unknown, commonly known as idiopathic inflammatory myositis. Among these, bacterial infections cause acute intramuscular infection with abscess formation. Some rare forms of myositis are focal nodular myositis, giant cell myositis and eosinophilic myositis, which can be diagnosed on muscle biopsy.

Two most common types of idiopathic inflammatory myositis are polymyositis and dermatomyositis, which affects approximately 60-80/million of population.  

Polymyositis

Definition

It can be defined as an inflammatory myopathy of gradual onset (weeks to months) and steady progression.

Aetiology

The exact cause of polymyositis is as yet unknown. The leading theories behind what could be underlying the disease process are either an autoimmune mechanism or an as yet unidentified infectious cause.

Pathophysiology

In this condition, the systemic inflammation impacts the endomysium of the muscles. This differentiates it from the next condition mentioned below (dermatomyositis) where perimysial inflammation is the focus of inflammation. There is a T cell mediated response in this condition against the surface of muscle fibers, leading to necrosis. There is a debate about the triggering factors, but the exact cause is still unclear.

Endomysium - histological slide

Endomysium - histological slide

Epidemiology

The condition affects people with African or Caribbean ancestry at almost double the rate that it affects Caucasians. The disease affects people of all ages, but does not often present in middle-aged adults (50 to 70 years old), as well as in children (5 to 15 years old). The disease is more likely to affect women than men.

Signs and Symptoms

Common signs include weakness in the proximal muscles i.e. weakness of the shoulders and hips. There is a difficulty in getting up from a chair, climbing stairs, lifting objects or even combing hair.

At later stage of the disease some fine-motor movements that depend on the strength of distal muscles, such as buttoning a shirt, sewing, knitting, or writing, are also affected. Atrophy of muscles may also appear at this stage. Polymyositis never affects occular muscles, even at the advanced stage of the disease. Facial and respiratory muscles are rarely affected by this condition. Pain and tenderness in the muscles can occur at the earlier stage of the disease.  

This condition usually occurs in association with some connective tissue disorders, systemic autoimmune diseases, or viral infections. Cardiac diseases and interstitial lung disease might occur in some patients affected with this condition.

Differential Diagnosis

Some muscular dystrophies, conditions and diseases might mimic the polymyositis and need to be excluded. They include:

  • fascio-scapulohumeral dystrophy
  • Becker’s muscular dystrophy
  • metabolic myopathies
  • neurogenic muscular atrophies
  • biochemical muscle diseases

Diagnosis

Lab investigations reveal elevated serum muscle enzymes in this condition, such as:

  • creatine kinase
  • serum glutamic-oxaloacetic transaminase
  • serum glutamic-pyruvic transaminase
  • lactate dehydrogenase

The definitive diagnosis is based on histopathological examination of muscle biopsy. Inflammation with foci of endomysial infiltrates are seen surrounding the muscle fibers. Presence of “CD8/MHC-1 complex” is considered specific for polymyositis diagnosis.  

Management

Management of acute flares is intravenous corticosteroids and intravenous immunoglobulin (IVIG). Treatment consists of corticosteroid medications which will gradually be reduced over a month to a month and a half. The maintenance of the treatment is usually a long term low dose of steroids. As long-term use of steroids can result in fractures, these patients are also likely to be on bone protecting medications such as bisphosphonates, and vitamin D supplements to ensure they are calcium replete.

Response to treatment is variable, but around 20% of patients with the condition die within 5 years of diagnosis. Immunosuppressant therapy is also an option for unmanageable and severe disease e.g. mycophenolate.

Some patients may also benefit from regular physiotherapy and strengthening exercises. Cancer and lung pathology are the commonest causes of deaths in these patients.

Dermatomyositis

Definition

This is an inflammatory myopathy with cutaneous manifestations.

Epidemiology

Like the other inflammatory myopathies, the disease is more likely to affect women than men. It most commonly affects adults between 40 and 60, and is up to three times more common in those with African of Caribbean ancestry. There is also a juvenile form the disease which affects girls more than boys and usually presents between 5-14 years of age.

Aetiology

The exact cause of dermatomyositis is as yet unknown, and there a number of theories as to what may be the mechanism underlying the disease. These include:

  • genetic dysfunctions
  • immune dysfunction
  • an environmental trigger such as ultraviolet radiation,
  • a viral or bacterial infection 
  • drugs, such as statins and penicillamine (a chelating agent)

Pathophysiology

As mentioned above, this condition does not yet have a clear or known mechanism or trigger. Polymorphism of tumor necrosis factor promoter and an allele of the HLA-DQA gene are recognised as risk factors in genetic studies. There is autoantibody proliferation and this overactivation of immune system causes tissue damage. More specifically, the perimysium of the muscle is affected. In 50-70% of dermatomyositis myositis-specific autoantibodies have been found.

In the early stage of the disease the membrane attack complex cells are also there, which can cause cell death and attack endothelial cells to mediate vascular damage. Agents such as interferon alpha and beta also have a role in the disease process.

Perimysium - histological slide

Perimysium - histological slide

Signs and Symptoms

Distinctive cutaneous features in this disease make the early recognition possible. Classical skin symptoms include 6 characteristics features including:

  • dis-pigmentation of poikiloderma
  • heliotrope rash (this is a purplish dusky periorbital rash)
  • scaling alopecia
  • Gottron’s papules (violaceous flat red papules that form on the backs of the elbow, knees, knuckles and ankles) on the backs of the hands
  • Itchy rash mainly in sun-exposed areas
  • Nail fold telangiectasia and dystrophic cuticles.

Other non-classical signs and symptoms can include:

  • proximal muscle weakness
  • photosensitivity
  • itching
  • weight loss
  • shortness of breath (due to respiratory muscle involvement)
  • muscle and joint pain
  • palpitations (due to cardiac involvement)
  • Raynaud’s phenomenon, but it is not specific to dermatomyositis only

The condition is also associated with increased risk of malignancy, interstitial lung disease, and heart problems. The disease may have a degree of overlap with rheumatoid arthritis and systemic lupus erythematosus among other connective tissue diseases.

Differential Diagnosis

Polymyositis is often confused with dermatomyositis, but cutaneous manifestation is the differentiating factor. Other cutaneous diseases like systemic lupus erythematous, dermatitis, tinea corporis, psoriasis, or rosacea must be ruled out.  

Investigations

The diagnosis of dermatomyositis is again, usually a clinical one that is based on examination, and the discovery of a symmetrical proximal myopathy. There are also tests that can confirm and support the diagnosis. These include:

  • raised muscle enzymes (creatine kinase and aldolase)
  • anti-Mi2
  • myopathic changes on electromyography
  • positive anti-nuclear antibody
  • reduced left ventricular ejection fraction
  • raised troponin (due to cardiac involvement)
  • inflammatory infiltrate, revealed by muscle biopsy

Management

Management of acute flares is intravenous corticosteroids and intravenous immunoglobulin (IVIG). Protection from the sun with sunscreen is also highly recommended, as are antipruritic drugs e.g. topical soft paraffin. Immunosuppressant therapy is also an option for unmanageable and severe disease e.g. topical tacrolimus or oral methotrexate.

Hereditary Inclusion Body Myositis

Definition

Inclusion Body Myositis (IBM) is characterised by distinctive inclusions, containing tubulo-filaments in a subset of patients with polymyositis. It is classified into 'sporadic inclusion body myositis' which shows inflammation, and 'hereditary inclusion body myositis'. It is a group of disorders that comprised of both autosomal dominant and autosomal recessive conditions. They result in myopathy, which causes in muscle weakness that gradually progresses over 10-20 years.

Epidemiology

This group of disorders is rare in the general population. It presents earlier than some of the other myopathies i.e. polymyositis or dermatomyositis.  The autosomal form of the disease results in sparing of the quadriceps, but they are affected in advanced disease. The autosomal recessive form of the disease affects those of middle eastern and Jewish lineage more often than the general population.

Aetiology

The cause of the disease is as yet unknown. The underlying genetic inheritance has been traced, but the malfunctioning element in the muscle tissue pathway is not yet known.

Pathophysiology

On muscle biopsy, the muscle cells show small rimmed vacuoles, and no evidence of inflammation. This separates it from the inflammatory myopathies such as polymyositis and dermatomyositis.

Signs and Symptoms

Early symptoms include balance issues, making running or walking on the heels difficult or impossible. Weakness of the index finger may also present as an early feature. Unique feature of this disorder is the partial or complete sparing of the quadriceps muscles, even in the advanced stages of the disease.

Investigations

As always, a thorough history and examination assist in the detection of the disease. Muscle biopsy reveals inclusion bodies i.e. rimmed vacuoles formed due to the collection of abnormal proteins. T1 W magnetic resonance imaging show fatty replacement of hamstring muscles. Nerve biopsy and a buccal swab for genetic testing will all support and confirm the diagnosis. The diagnosis of HIBM is based on both clinical symptoms as well as the histopathology of a muscle biopsy.

Management

There is no curative treatment for this chronic and progressive disease. Orthoses for the lower limb may assist if the patient has foot drop. Dietary modifications are proposed, including avoidance of excess selenium, copper and zinc.

Other Forms

Granulomatous Myositis

This form of myositis presents in an individual with a chronic granulomatous disease such as sarcoidosis. It usually impacts the proximal muscles.

Eosinophilic Myositis

This is characterised by eosinophilic disorders, with muscle biopsy forming a cornerstone of diagnosis. The main causes of eosinophilic myositis are believed to be:

  • parasitic infections
  • drugs
  • toxic substances
  • haematological disorders

Focal Myositis

This is a rare disorder that presents with an inflammatory mass or ‘tumour.’ As such it is commonly confused with inflammatory disorders and neoplastic disorders. It is important to exclude trauma as a potential cause for the mass, and other causes of inflammation.

Orbital Myositis

This is inflammation of the extra ocular muscles and is a rare disease. It usually occurs in the young and muddle ages. Like the other disorders in this article, women are affected more than men. It is associated with other disorders such as systemic lupus erythematosus, rheumatoid arthritis, Crohn’s disease, other autoimmune diseases, diabetes mellitus and myaesthenia gravis.

Signs include drooping of the eyelids and bulging eyes (may be mistaken for Grave’s disease). There is often swelling around the eye (may be mistaken for periorbital oedema). Symptoms include eye pain that worsens on movement, and double vision.

Highlights

Causes

Myositis is inflammation and degeneration of skeletal muscle tissue. There can be multiple causes of myositis including:

  • bacteria (staphylococcus, streptococcus etc)
  • viruses (influenza A and B, hepatitis B etc)
  • parasites
  • some drugs and toxins like statins, Rifampicin, Sulphonamide, cyclosporine, alcohol etc.

Many at times the cause is unknown, commonly known as idiopathic inflammatory myositis.

Polymyositis

Polymyositis can be defined as an inflammatory myopathy of gradual onset (weeks to months) and steady progression. In this condition, the systemic inflammation impacts the endomysium of the muscles. Common signs include muscular weakness, pain, tenderness and atrophy.

Polymyositis is diagnosed by the presence of elevated serum muscle enzymes together with histopathological examination of muscle biopsy. Management includes intravenous corticosteroids and immunoglobulin, bone protecting medications and immunossuppressant therapy.

Dermatomyositis

This is an inflammatory myopathy with cutaneous manifestations. In this condition, the inflammation impacts the perimysium of muscles. The damage is caused by an overactivation of the immune system and the presence of autoantibodies. Tissue damage is also mediated by membrane attack complex cells and interferons. Signs and symptoms range from classical ones seen on the skin, to non classical ones such as photosensitivity, weight loss, itching, etc.

Dermatomyositis is diagnosed by the presence of symmetrical proximaly myopathy, but also certain tests. Management includes intravenous corticosteroids and immunoglobulin, antipuritic drugs and immunosuppressant therapy.

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References:

Article, Review and Layout:

  • Shahab Shahid
  • Uruj Zehra
  • Adrian Rad

Illustrators:

  • Endomysium - histological slide - Smart In Media
  • Perimysium - histological slide - Smart In Media
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