History and exam

Key diagnostic factors

common

difficulty with motor tasks

Most patients would experience difficulties with tasks requiring the use of proximal muscles (e.g., getting up from a chair, climbing steps, lifting objects, and combing hair).

However, fine motor movements that depend on the strength of distal muscles (e.g., sewing, knitting, or writing) could be affected late in the course of dermatomyositis and polymyositis, and earlier in the course of inclusion body myositis.[22]

muscle weakness

Having the patient stand from the sitting position (especially from a low foot stool) with the arms crossed is a good way to examine proximal leg muscle strength. Often a patient with a myopathy is unable to do this. Patients with inclusion body myositis often walk with a 'locked knee' gait and have early falls and difficulties climbing and descending stairs.

Distal muscles could be affected late in the course of dermatomyositis and polymyositis, and earlier in the course of inclusion body myositis where wrist and finger flexor weakness is characteristic.[27]

muscle atrophy

Severe muscle weakness is usually associated with muscle atrophy and is a sign of muscle damage which is often not present in patients early in the disease course.

In inclusion body myositis, quadriceps and distal wrist and finger flexor atrophy is common.​[72][77]

uncommon

heliotrope rash with eyelid oedema

A blue-purple discoloration of the upper eyelids, with oedema.[1][2][72]​​​[77]

Highly suggestive of dermatomyositis.

Rarely observed in other disorders.

Gottron's papules

Erythema over the knuckles, accompanied by raised violaceous scalp eruption, frequently found in patients with dermatomyositis and anti-synthetase syndrome, may also be seen in patients with immune-mediated necrotising myopathy or overlap myositis but this is less common.​[4][5][72]​​[77]

Other diagnostic factors

common

frequent falls

Falling is frequent, particularly in the course of sporadic inclusion body myositis, because of early involvement of the quadriceps muscle.[72]

fatigue and generalised malaise

A non-specific symptom when the disease is active.

weight loss

A non-specific symptom when the disease is active.

May also occur in association with concomitant malignant disease.

signs of heart failure and/or myocardial infarction, arrhythmias, shortness of breath, abnormal breath sounds, palpitations, syncope

Results from cardiopulmonary complications. Cardiac involvement includes conduction defects, tachyarrhythmias, cardiomyopathy, and low ejection fractions.

Myocarditis may occur in patients with dermatomyositis, overlap myositis and anti-synthetase syndrome. This has not been reported in patients with sporadic inclusion body myositis.[7] However, overt cardiac disease has only been reported in around 10% of IIM patients, subclinical involvement is more common, in up to 75% of patients.[4]

IIM-associated interstitial lung disease occurs more commonly among the anti-synthetase syndrome, dermatomyositis and overlap myositis subtypes, and may precede, or be present without muscular symptoms.[1][6]

Shortness of breath is not a feature of inclusion body myositis.

mild fever

A common symptom when the disease is active or associated with a connective tissue disorder.[27]

However, this does not occur in sporadic inclusion body myositis.

uncommon

dysphagia

Results from weakness of the oropharynx and distal oesophagus.

May occur in all subtypes of IIM, but is most prominent in dermatomyositis, inclusion body myositis and overlap myositis.[1][4]

In inclusion body myositis, dysphagia has been reported in 9% of patients at presentation and 40% to 66% of patients with well-established disease.[27][83][84]

myalgia

Prominent muscle pain and tenderness are uncommon, but has been reported in some IIM-subtypes, such as immune-mediated necrotising myopathy and anti-NXP2 dermatomyositis.[1][4]

It is not a feature of sporadic inclusion body myositis.

arthralgia and/or swollen joint

Joint involvement (presenting as arthralgia and/or swollen joint) may occur in patients with dermatomyositis (especially with positive anti-MDA5 antibody), anti-synthetase syndrome, and overlap myositis.[4]

facial rash

Often red or bluish-purple, accompanied by pruritus or burning sensation.​[72][77]

erythematous rash

Over the extensors (such as knees, elbows, and malleoli) and at sun-exposed areas such as the base of the neck and upper chest, forming a V sign, in IIM patients with skin manifestations.[1][2]​​[72][77]

nail fold changes

Dilation of capillary loops (visualised under nail fold capillaroscope) of periungual area seen macroscopically as periungual erythema.​[72][77] Nail fold changes can also occur in patients with systemic sclerosis.

facial muscle weakness

Facial weakness should prompt the investigation of a myositis mimicker such as myasthenia gravis or facio-scapulo-humeral muscular dystrophy.

skin calcinosis

Occurs more commonly in juvenile dermatomyositis, especially in association with the presence of anti-NXP2 antibody.[85]

malignancy

IIM may be associated with malignancy. The highest risk is near the time of diagnosis and up to 3 years after the diagnosis of IIM.[72][86]​ Presence of anti-TIF1 or anti-NXP2 in adult-onset dermatomyositis are associated with the highest risk for developing cancer-associated myositis.[1][4]

systemic signs of autoimmune disease

May be seen in at least 20% of patients.[27]

Raynaud’s phenomenon

Raynaud’s phenomenon may occur in patients with dermatomyositis, anti-synthetase syndrome and overlap myositis.[1][4]

Risk factors

strong

age >40 years

There is evidence to suggest that the incidence of dermatomyositis and polymyositis is bimodal, with peaks in childhood and/or adolescence, and again in later life.[1]​ In adults, the median range of reported age of onset is wide, but most occur between 50 and 60 years.

genetic predisposition

Specific human leukocyte antigen (HLA) subtypes are believed to confer increased risk of development of idiopathic inflammatory myopathies (IIMs).[49] Of note, the 8.1 ancestral haplotype alleles HLA-DRB1*03:01 and HLA-B*08:01 are most strongly associated with IIM.[1][50] 

female sex and/or black ethnicity (polymyositis and dermatomyositis)

Evidence suggests that polymyositis and dermatomyositis have a female to male ratio of 2:1, and are more commonly found in black people compare with white people with a ratio of 2.8:1.[8][9][10][11]

Immune-mediated necrotising myopathy as a subtype occurs more frequently in females, but gender bias seems to be less marked in anti-HMGCR-positive immune-mediated necrotising myopathy.[1]

male sex and/or white ethnicity (inclusion body myositis)

Patients with inclusion body myositis have characteristics distinct from the other idiopathic inflammatory myopathy subtypes, typical onset of symptoms occurs after the age of 50 years, with a male preponderance of 60% to 70%, and has been demonstrated to be occur more frequently in white people, although the exact racial differences are not clear.[1][12]

use of statins

Statins have been associated with a spectrum of muscle toxicity ranging from myaliga to immune-mediated necrotising myopathy and rhabdomyolysis.[28][29]

tobacco smoking

Smoking has been associated with an increased risk of polymyositis in white people, leading to an increased risk of developing interstitial lung disease compared with people who have never smoked.[34]​ Anti-synthetase antibodies were more prevalent in white smokers compared with non-smokers, regardless of the duration of tobacco exposure.[19][34][40]

weak

exposure to high intensity of global UV radiation

There is some evidence to suggest that UV radiation intensity is the strongest contributor to the relative proportion of dermatomyositis and is strongly related to the proportion of anti-Mi2 autoantibodies.[41] Conversely, in subsequent studies this correlation was not found when comparing the capital cities of the Australian states, with latitudes ranging from Hobart (42.9°S) to Darwin (12.5°S), or when examining the north-south gradient of idiopathic inflammatory myopathy incidence within Sweden.[42]​​[43]

infections

Viruses including coxsackievirus, influenza virus, retroviruses, cytomegalovirus, Epstein-Barr virus, and SARS-CoV-2 seem to be associated with idiopathic inflammatory myopathy.[20][21][22][23][24] Other associated infectious agents include protozoa, cestodes, nematodes, and Borrelia species.[27]

vaccination

SARS-CoV-2 (predominantly reported after mRNA vaccine administration), influenza (reported following pandemic H1N1 vaccination and seasonal trivalent vaccinations), hepatitis B (unclear if autoimmune complications are attributable to other components such as yeast proteins, aluminium adjuvant, neomycin).[37][38][39]

use of certain drugs

Immune-checkpoint inhibitors, antiretrovirals, hydroxycarbamide, gemfibrozil, carbimazole (possible increased risk in people of Asian ethnicity), and penicillamine have been linked to idiopathic inflammatory myopathy-onset in case reports.[1][30]​​[31][32][33][34][35][36]​​

other environmental factors

Silicone breast implants, graft-versus-host disease, chimerism (associated with juvenile dermatomyositis), silica (describe in association with overlap myositis with systemic sclerosis), certain mushrooms/red yeast rice/Pu-erh tea/Bacopa (linked to statin-naive anti-HMGCR immune-mediated necrotising myopathy) have been suggested to trigger idiopathic inflammatory myopathy (IIM).[1][41][44][45][46][47]​​[48]​​

Environmental factors such as latitude, living in urban areas, and living close to waterfront have also been suggested to be associated with IIM.[34]

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