Tests
1st tests to order
serum creatine kinase (CK)
Test
The most sensitive muscle enzyme test.[83]
Should be ordered for any patient with suspected disease, even if the patient presents with only cutaneous involvement, as CK may be elevated in clinically amyopathic dermatomyositis (CADM).[2] Although CK level may be elevated up to 50-fold in active disease, it can be normal in some cases of active DM.[3][115][116] Therefore, further investigations to assess for muscle involvement should be performed if the CK is normal where clinical suspicion of DM is high.
Where initial CK is elevated, serial CK measurement can be used as part of monitoring response to treatment.
Result
high levels
serum aldolase
Test
A glycolytic pathway enzyme found in all tissues but mainly skeletal muscle, brain, and liver.
Not as specific or sensitive for muscle disease as creatine kinase (CK), but is occasionally elevated in myositis when CK is normal.
Result
high levels
muscle biopsy
Test
Should be performed in all cases of suspected DM.
In symptomatic patients the biopsy should be taken from a weak but not severely atrophied muscle. Quadriceps and deltoid are common sites.
Open surgical biopsy provides a larger specimen than closed needle biopsy. Biopsy using a conchotome also yields useful specimens.[84] Correct processing and interpretation by a laboratory experienced in muscle histology is essential for correct diagnosis.
Muscle involvement can be patchy and prior electromyogram (followed by biopsy from the contralateral muscle) or MRI to direct muscle biopsy may increase yield.[95][117][118] This is of particular relevance in clinically amyopathic DM.[2]
Perifascicular atrophy is seen due to phagocytosis and necrosis of muscle fibers. This pattern of atrophy is diagnostic of DM.[53][119]
Result
perivascular or interfascicular inflammation; endothelial hyperplasia in the intramuscular blood vessels; perifascicular atrophy
electromyogram (EMG)
Test
Although EMG is recommended, it is not essential for diagnosis if elevated creatine kinase (CK) and typical muscle biopsy findings are present.
The EMG abnormalities are not specific but are seen more frequently in idiopathic inflammatory myopathies.[85][86] Similar patterns may be seen in toxic, infectious, or metabolic myopathies.
Inflammation at the site of needle insertion can cause artifact in subsequent muscle biopsy samples and may cause transient increases in CK.[117] A muscle that has been studied by EMG should not be biopsied; instead, the same muscle on the opposite side should be sampled.
Result
abnormal spontaneous activity (fibrillation and positive sharp waves) and abnormal voluntary activity (low-amplitude, short-duration polyphasic motor potentials)
skin biopsy
Test
In patients who present with classic DM and have the diagnosis confirmed by elevated muscle enzymes and muscle biopsy ± electromyogram, skin biopsy may not be required.
Skin biopsy is required to confirm diagnosis in clinically amyopathic DM, where overlap connective tissue disease may be present, or in cases where the cutaneous diagnosis is in doubt.
The histologic appearances seen with systemic lupus erythematosus (SLE) are very similar to those seen with DM, making distinction between SLE and atypical DM difficult.[108][110][120]
Result
vacuolar alteration of the basal layer of the epidermis; necrotic keratinocytes; vascular dilation; perivascular lymphocytic infiltrate
antinuclear antibody (ANA)
Test
Positive in approximately 80% of patients.[87]
Nonspecific and commonly positive in other connective tissue diseases (CTDs) where myositis may be a feature.
Further testing for autoantibodies against specific nuclear antigens (e.g., anti-double-stranded DNA, anti-Ro, anti-La, anti-Sm, anti-ribonucleoprotein [RNP]) is useful to help differentiate between DM and other CTDs, especially cutaneous lupus, which has a similar biopsy appearance.
Result
positive
myositis-specific antibodies (MSAs) and myositis-associated antibodies (MAAs)
Test
MSAs and MAAs are found in most patients with DM and are associated with clinical subsets that have varying presentations, clinical courses, and therapeutic responses.[24][88][89] They are increasingly used in clinical practice, although are not widely available.[50] Most are directed against cytoplasmic RNA-protein complexes.[46]
Anti-Mi-2 antibodies are directed against a helicase involved in transcriptional activation.[90] They are strongly associated with the classical presentation of DM but are found in only approximately 10% of patients.[91] Anti‐U1 ribonucleoprotein (RNP) and anti‐Ku are found in overlap syndromes that present similarly to anti-Mi-2 positive DM.[50]
Anti‐transcription intermediary factor 1 gamma (TIF1-gamma) is the most commonly isolated antibody.[88] It is strongly associated with malignancy and therefore an intensive search for cancer and careful follow‐up is recommended.[45][92]
Anti-signal recognition particle (SRP) antibodies are found almost exclusively in polymyositis and occasionally in DM. When present they are associated with acute severe, treatment-resistant necrotizing myositis.[46]
Anti-Jo-1 and other anti-aminoacyl-tRNA synthetase antibodies (anti-ARS antibodies; including anti‐PL‐7, anti‐PL‐12, anti‐EJ, anti‐OJ, and anti‐KS) result in a distinct clinical picture known as antisynthetase syndrome. The main clinical features include interstitial lung disease (ILD), fever, myositis, polyarthritis, mechanic's hands, and Raynaud phenomenon. Anti-Jo-1 is associated with a higher frequency of myositis, whereas the other anti-ARS antibodies are associated with higher frequency of ILD.[45]
Anti-melanoma differentiation-associated gene 5 (MDA5; also known as CADM‐140) is specific to clinically amyopathic dermatomyositis (CADM) and is strongly associated with a rapidly progressive ILD with poor prognosis. Tender palm papules and diffuse alopecia may be present.[47]
Anti-NXP2 is strongly associated with juvenile DM with a high proportion suffering from cutaneous calcinosis due to dystrophic calcification.[48]
Anti-PM-Scl antibodies have been found in patients with overlapping myositis and scleroderma.[49]
Result
positive
Tests to consider
muscle MRI
Test
A noninvasive technique that can be used to assess large areas of muscle for active disease.
The MRI appearances are not specific and biopsy is still required for diagnosis.[95]
May improve biopsy yield by identifying affected muscles to target for biopsy.[94]
Serial MRI measurements may be performed to assess response to treatment.
Result
increased edema; fibrosis and calcification depending on disease stage
ECG
Test
Should be performed in all patients. About 30% of patients with idiopathic inflammatory myopathies may have ECG abnormalities.[80]
Patients with normal resting ECG but symptoms of palpitations or syncope merit further investigation with ambulatory ECG.
Result
atrial or ventricular dysrhythmias; conduction defects
echocardiography
Test
Should be performed in all patients. Myocardial involvement severe enough to cause congestive cardiac failure is rare (<5% of patients), but when present indicates poor prognosis.[77]
Result
reduced left ventricular ejection fraction; hypokinesis
cardiac troponin I
Test
Creatine kinase may be elevated in patients with myositis in the absence of cardiac involvement.[121] Troponin I is a sensitive and specific marker of cardiac damage. It is used as a biomarker to screen asymptomatic patients at presentation with idiopathic inflammatory myopathies.[98]
Result
high levels
chest x-ray (CXR)
Test
Required in all patients to evaluate respiratory involvement and to screen for malignancy.
In early ILD, x-ray changes may be minimal or nonspecific and ILD may be missed if further respiratory investigations are not performed.
Result
poor inspiration or atelectasis; diffuse reticulonodular interstitial changes; primary or secondary neoplasm or lymphadenopathy
pulmonary function tests (PFTs)
Test
Required in all patients to evaluate for respiratory muscle weakness and ILD.[99]
Result
restrictive pattern with reduced diffusing capacity; respiratory muscle weakness
high-resolution CT of chest
Test
Should be performed in all patients to evaluate for ILD.[99]
More sensitive than chest x-ray in detecting ILD and provides information on prognosis not provided by chest x-ray. The presence of ground glass opacification is associated with a better prognosis compared with honeycomb fibrotic changes.[100][101]
Result
ground glass opacification; honeycomb fibrotic changes
barium swallow or videofluoroscopic assessment of swallow
Test
Should be performed in patients with dysphagia, nasal regurgitation of fluid, or dysphonia.
Silent aspiration may also occur and further assessment is indicated in patients presenting with lower respiratory tract infection.[102]
Result
pharyngeal or esophageal dysmotility
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