Inflammatory myopathies: Clinical practice

Inflammatory myopathies refers to a group of three disorders - polymyositis, dermatomyositis, and inclusion body myositis.

They’re all autoimmune disorders that cause muscle inflammation, leading to progressive muscle weakness and wasting.

Sometimes, the muscles can be tender or painful. In severe cases, it can involve the respiratory muscles, which can be life threatening.

Usually, there’s also systemic symptoms like fever, fatigue, and weight loss; as well as other organ-specific symptoms.

There can be subcutaneous calcification in the skin; joint pain and arthritis; cardiovascular conditions like arrhythmias, myocarditis, and pericarditis; gastrointestinal conditions like gastroesophageal reflux disease and dysphagia; respiratory problems like aspiration pneumonia; and vascular problems like Raynaud’s phenomenon, where arterial spasm causes reduced blood flow to the fingers.

Let’s start off with polymyositis, which typically affects adults, and it’s characterized by bilateral muscle weakness that mostly affects proximal, large muscle groups, like the shoulder or hips and spares the distal muscles like muscles in the hands.

Individuals might have difficulty rising from a chair, lifting their arms, or climbing stairs.

The muscle weakness usually worsens gradually over several months, and over time there can be muscle atrophy.

Dermatomyositis has the same muscular presentation as polymyositis, but it mainly affects children, and in addition to muscle weakness, children can have a skin rash.

One type of rash is the heliotrope or lilac rash, which is a pruritic purplish rash that can appear on or around the eyelids.

A similar rash may appear on sun-exposed areas, like the chest, shoulders, or thighs.

This rash is similar to the malar rash of individuals with lupus, but it typically extends beyond the nasolabial folds, which is a region that’s usually spared in lupus.

Another rash is one that causes Gottron’s papules, which are flat, red, scaly papules, typically located on bony prominences, especially on hands, elbows, and knees.

There’s also Gottron’s sign, which is when these red or violet, sometimes scaly, slightly raised papules erupt on the metacarpophalangeal joints or on the interphalangeal joints of the fingers.

Next, there’s inclusion body myositis which is the most common inflammatory myositis in individuals older than 50 years.

There’s usually a slowly progressive weakness and wasting of both distal and proximal muscles - especially involving the quadriceps, the wrists and fingers, and the muscles that lift the front of the foot.

In inclusion body myositis, the involvement may not be bilateral.

The most frequent initial complaint is weakness of grip strength, like having difficulty opening jars.

Individuals may also have difficulty getting up out of a chair or have frequent falls. Sometimes there’s also mild muscle pain.

In individuals with gradually progressive muscle weakness, an inflammatory myopathy should be suspected.

Typically, there’s an elevated ESR and CRP; as well as positive titers for myositis-specific autoantibodies like anti-Jo12 and anti-SRP, most often associated with polymyositis, and anti-Mi2, most often associated with dermatomyositis.

There can also be elevations in muscle enzymes like creatine kinase, lactate dehydrogenase, aldolase, and also the AST and ALT; and a complete blood count may also show anemia of chronic disease.

Then, electromyography can be done on affected muscles to detect regions of dead muscle cells that cause abnormal electrical signal conduction and evidence of muscle membrane irritability.

An MRI can show large areas of muscle inflammation, edema, fibrosis, and calcification.

Ultimately, a muscle biopsy is usually done to help with the diagnosis.

In polymyositis, CD8+ T lymphocytes infiltrate the endomysium, while in dermatomyositis, CD4+ T lymphocytes infiltrate the perimysium.

Inclusion body myositis is called that because there are inclusion bodies made of amyloid plaques that collect within the muscle fibers.

Five criteria are used to diagnose polymyositis and dermatomyositis.

The first one is bilateral proximal muscle weakness, involving both thighs or both upper arms.

The second one is elevated muscle enzymes.

The third one is abnormal electrical signal conduction on electromyography.

The fourth one is finding lymphocyte infiltration and abnormal muscle degeneration on biopsy.

And finally, the fifth one is the presence of skin rashes.

To diagnose polymyositis, all of the first four criteria must be met, while for dermatomyositis, three out of the first four criteria plus the fifth criteria must be met.

Dermatomyositis and polymyositis can both be associated with an underlying cancer of the cervix, breasts, ovaries, lungs, pancreas, or bladder.

So whenever an individual is diagnosed with dermatomyositis or polymyositis, they should get screening with a thorough history and physical, including a pelvic exam.

In addition, a CBC, liver function tests, urinalysis, fecal occult blood test, colonoscopy, chest X ray, Pap test, and mammography should all be done.

There is no cure for inflammatory myopathies, but treatment includes physical therapy and rehabilitation to help prevent muscle atrophy and regain muscle strength and range of motion.

For both dermatomyositis and polymyositis, initial treatment includes systemic glucocorticoids for four to six weeks.

Severely ill individuals may get intravenous glucocorticoid therapy, followed by systemic glucocorticoids for four to six weeks.

During that time, muscle enzymes begin to normalize and muscle strength begins to recover.

Glucocorticoids should be tapered to the lowest effective dose and continued over a total of 9 to 12 months.

In individuals that don’t respond to glucocorticoids, repeat muscle biopsies may be needed to confirm the diagnosis.

Sometimes there may be an occult underlying cancer.

Alternatively, there may be a glucocorticoid-induced myopathy, in which case lowering the glucocorticoid dose can help.

In individuals who don’t respond to glucocorticoids, and in whom these other diagnosis are ruled out, immunosuppressant medications like azathioprine or methotrexate can be added. In some specific cases, IVIG may be added on as well.

In inclusion body myositis, treatment relies heavily on exercise and physical therapy, because anti-inflammatory or immunosuppressive drugs aren’t particularly effective.

Summary

Alright, as a quick recap… Inflammatory myopathies refers to polymyositis, dermatomyositis, and inclusion body myositis.

Diagnosis of inflammatory myopathies involves physical examination showing bilateral muscle weakness, laboratory tests showing positivity for myositis-specific autoantibodies and elevations in serum muscle enzymes, electromyography showing abnormal electrical signal conduction, and muscle biopsy showing white blood cell infiltration and abnormal muscle degeneration.

To diagnose polymyositis, all four of these must be present, while for dermatomyositis, three out of the four must be present, plus there has to be a skin rash.

On the other hand, inclusion body myositis generally has slowly progressive weakness and wasting of both distal and proximal muscles that may not be bilateral - especially involving the quadriceps, the wrists and fingers.

Physical therapy and rehabilitation should begin early in the course of treatment.

The standard initial treatment for dermatomyositis and polymyositis involves the administration of glucocorticoids for four to six weeks.

Individuals that don’t respond to glucocorticoids may be started on immunosuppressant medications, and in some settings IVIG can be used.

For inclusion body myositis, there’s no optimal standard immunomodulatory therapy, so treatment relies heavily on exercise and physical therapy.