Dermatomyositis

Dermatomyositis (DM) is an autoimmune inflammatory myopathy that combines proximal, symmetric skeletal muscle weakness with a set of highly characteristic — in several cases genuinely pathognomonic — skin findings. It belongs to the broader family of idiopathic inflammatory myopathies (IIM), which also includes polymyositis, antisynthetase syndrome, immune-mediated necrotizing myopathy, inclusion body myositis, and the overlap myositides.^[1] DM is set apart from these other IIM subtypes by its underlying mechanism: a humoral, complement-mediated microangiopathy in which autoantibodies and activated complement — culminating in the C5b-9 membrane attack complex — deposit in the small blood vessels of both skin and endomysium, causing capillary loss, tissue ischemia, and the perifascicular pattern of muscle-fiber atrophy that is the biopsy hallmark of the disease.^[1] This is a fundamentally different mechanism from polymyositis, in which cytotoxic CD8+ T cells directly invade and destroy individual myofibers with no comparable capillary-mediated process and, critically, no skin involvement at all — the presence or absence of the cutaneous findings below is the single most useful bedside feature separating the two conditions.^[1]

DM affects both children (as juvenile dermatomyositis, JDM) and adults, with a roughly 2- to 3-fold female predominance in most adult series.^[1] A genuinely Canadian cohort of 114 adults followed at 2 Ontario tertiary care centres over 13 years found a mean age at diagnosis of 59 years and a female predominance of 80%, with skin and muscle involvement presenting together in 63% of patients and skin preceding muscle disease in a further 11%.^[2] The same cohort found interstitial lung disease (ILD) in 39% of patients and creatine kinase elevation in only 66% of confirmed cases — a normal CK does not exclude the diagnosis.^[2]

Two features of DM carry outsized clinical importance and run through this entire chapter. First, adult-onset DM has a well-established paraneoplastic association: a clinically meaningful proportion of adults — particularly those diagnosed at age 40 or older — have or will develop an underlying malignancy within a few years of DM onset, and age-appropriate cancer screening plus a formal risk-stratified assessment for enhanced screening is a mandatory, non-negotiable part of the initial work-up, not an optional add-on.^[3] Second, DM can present with its characteristic skin findings and little or no detectable muscle disease — clinically amyopathic dermatomyositis (CADM) — a presentation that is anything but reassuring, since it is strongly linked to a specific autoantibody (anti-MDA5) and a rapidly progressive, often fatal interstitial lung disease.

Heliotrope rash — confluent violaceous-to-erythematous macular discoloration confined to the upper eyelids, with periorbital edema — a pathognomonic finding in dermatomyositis. (Dugan EM, Huber AM, Miller FW, Rider LG, Wikimedia Commons, CC BY-SA 3.0)

Gottron's papules — erythematous-to-violaceous, flat-topped papules overlying the metacarpophalangeal and interphalangeal joints — the single most pathognomonic cutaneous finding in dermatomyositis. (Dugan EM, Huber AM, Miller FW, Rider LG, Wikimedia Commons, CC BY-SA 3.0)

Cutaneous findings

The cutaneous findings of DM are frequently the presenting feature and, in several cases, are pathognomonic enough on their own to prompt the diagnosis before any weakness is apparent.^[1]

• Heliotrope rash — violaceous to erythematous discoloration confined to the upper eyelids, often with periorbital edema; the name comes from its resemblance to the purple heliotrope flower. Found in 66% of the Canadian Ontario cohort.^[2]
• Gottron's papules — the most pathognomonic finding in DM: flat-topped, erythematous-to-violaceous papules and plaques over bony prominences, most classically the metacarpophalangeal and interphalangeal joints, but also the elbows, knees, and medial malleoli. Present in 69% of the Canadian cohort — its single most common cutaneous finding.^[2] Gottron's sign refers to the same distribution without discrete papules — flat macular erythema instead.
• Shawl sign and V-sign — symmetric, photodistributed erythema across the posterior neck, upper back, and shoulders (shawl sign) or the anterior neck and upper chest in a V-shaped distribution (V-sign); found in 56% and 61% of the Canadian cohort respectively.^[2]
• Mechanic's hands — hyperkeratotic, fissured, rough, "dirty"-appearing skin along the radial and palmar aspects of the fingers, resembling the hands of a manual labourer; classically associated with antisynthetase syndrome (below) but seen in DM more broadly.
• Holster sign — poikilodermatous erythema over the lateral hip and thigh.
• Periungual changes — periungual telangiectasia, ragged cuticles, and cuticular overgrowth are common and easy to miss without deliberately examining the nail folds.
• Poikiloderma — a combination of hyper- and hypopigmentation, telangiectasia, and skin atrophy in photodistributed areas.
• Calcinosis cutis — subcutaneous or intracutaneous calcium deposits; uncommon in adult-onset DM but a genuinely important complication in juvenile DM (see Special Populations and Complications, within Monitoring and Follow-Up).

The rash is frequently intensely pruritic — more so than the rash of cutaneous lupus erythematosus, which it can otherwise closely resemble — and photosensitive, with sun exposure a genuine trigger for flares.^[1]

Muscle findings

Classic DM produces symmetric, proximal muscle weakness developing over weeks to months — difficulty rising from a low chair, climbing stairs, or lifting the arms overhead (to comb hair or reach a shelf) are the typical presenting complaints. Neck flexors are often disproportionately weak relative to neck extensors. Muscle involvement is usually painless or only mildly tender, which can mislead a clinician expecting muscle disease to hurt; myalgia and mild tenderness do occur but are not the dominant feature. Fatigue, low-grade fever, and modest weight loss are common associated constitutional symptoms.^[1]

Clinically amyopathic dermatomyositis (CADM) — a distinct, high-stakes presentation

A meaningful subset of patients — historically estimated at around 20% of all DM — have the characteristic skin findings above with no objective muscle weakness, and normal or only minimally abnormal creatine kinase, EMG, and muscle biopsy findings sustained for 6 months or longer from rash onset. This is clinically amyopathic dermatomyositis (CADM), sometimes further split into true amyopathic DM (no biopsy, EMG, or enzyme abnormality at all) and hypomyopathic DM (subclinical laboratory or biopsy abnormality without clinically apparent weakness).

CADM is not a reassuring, milder variant of DM. It is strongly associated with the anti-MDA5 autoantibody, and anti-MDA5-positive CADM carries a disproportionate risk of rapidly progressive interstitial lung disease (RP-ILD) — a fulminant, often treatment-refractory pneumonitis that can progress to respiratory failure and death within days to weeks, frequently before muscle weakness ever develops. Do not let a normal strength examination lower your suspicion for serious internal-organ disease in a patient with classic DM skin findings — pursue anti-MDA5 testing and pulmonary work-up urgently regardless of muscle strength (see Red Flags and Investigations, within Assessment and Diagnosis).

Antisynthetase syndrome overlap features

A separate but overlapping presentation is defined by antibodies against aminoacyl-tRNA synthetases, anti-Jo-1 being the most common. Antisynthetase syndrome combines myositis — which may have DM-type skin findings or occur without them — with some combination of ILD, inflammatory polyarthritis, mechanic's hands, Raynaud phenomenon, and fever. Its associated ILD typically follows a more indolent, chronic course than anti-MDA5-associated RP-ILD, though it still carries meaningful morbidity and needs the same proactive pulmonary surveillance described below.

• Female sex — adult-onset DM shows an approximately 2- to 3-fold female predominance in most series, though paraneoplastic DM specifically skews toward older age and male sex.^[1]
• Age — DM has a bimodal age distribution: a childhood peak (juvenile DM, most commonly presenting between about 5 and 15 years) and an adult peak in the 40s to 60s; the Canadian Ontario cohort's mean age at diagnosis was 59 years.^[2]
• Genetic susceptibility — certain HLA class II alleles are overrepresented in DM, though genetic risk alone is insufficient to cause disease and no single gene test is used clinically.
• Ultraviolet light exposure — a recognized environmental trigger and exacerbant of cutaneous disease activity; geographic and seasonal patterns of DM-specific autoantibodies (particularly anti-TIF1-gamma and anti-Mi2) correlate with regional UV intensity.^[1]
• Certain medications and exposures — statins are more classically linked to a related but distinct entity, immune-mediated necrotizing myopathy (anti-HMGCR antibody), but drug-induced dermatomyositis-like eruptions (for example with hydroxyurea) and immune-checkpoint-inhibitor-associated inflammatory myopathy (which can include DM-pattern disease) are both recognized.

Malignancy association — a critical safety point

Adult-onset DM carries a well-established paraneoplastic association: cancer is diagnosed within roughly 3 years before or after DM onset in a clinically meaningful proportion of adult patients, with risk significantly elevated relative to the general population in this window.^[3] The most commonly reported associated malignancies vary by population studied but consistently include breast, lung, ovarian, gastric, colorectal, and pancreatic cancers, non-Hodgkin lymphoma, and — particularly in some Asian cohorts — nasopharyngeal cancer.^[3]

Specific clinical and serologic features meaningfully raise the probability of an associated malignancy and drive the risk-stratified screening approach detailed in Investigations (within Assessment and Diagnosis):^[3]

• Age 40 years or older at DM diagnosis — the single most consistently reported clinical risk factor.
• Anti-TIF1-gamma or anti-NXP2 myositis-specific antibody positivity in an adult — both are strong markers of cancer-associated DM (see the antibody table in Assessment and Diagnosis); critically, this same antibody association does not hold in children (see Special Populations).
• Male sex, rapid disease onset, cutaneous ulceration or vasculitis, dysphagia, and disease that is persistently active or resistant to first-line immunosuppression.^[3]

Treat this as a genuine safety issue rather than a background statistic: every newly diagnosed adult with DM needs both age-appropriate cancer screening and a formal risk-stratified assessment for enhanced screening, pursued in parallel with starting immunosuppressive treatment — not deferred until treatment response is assessed (see Investigations and Red Flags).

Diagnostic approach

No single test confirms DM in isolation. Diagnosis rests on a combination of the characteristic clinical picture (the muscle-weakness pattern plus the cutaneous findings in Recognize the Condition), muscle-enzyme and autoantibody testing, electromyography, imaging, and — where the diagnosis is not clinically clear-cut — muscle and/or skin biopsy.^[1] The 2017 EULAR/ACR classification criteria formalize this combination into a validated probability score (below).^[5]

Laboratory testing

Creatine kinase (CK) is the most commonly used muscle enzyme and is elevated in most, but not all, patients with active myositis — the Canadian Ontario cohort found CK elevated in only 66% of confirmed DM cases, underscoring that a normal CK does not exclude the diagnosis, particularly in clinically amyopathic disease.^[2] Aldolase is a useful adjunct precisely because it is enriched in fascia as well as muscle and can be elevated even when CK is normal. AST, ALT, and LDH are also frequently elevated as nonspecific markers of muscle breakdown, and are easy to misattribute to primary liver disease if the clinician is not thinking about myositis. ANA is positive in the majority of patients but is nonspecific. CRP and ESR are often only mildly elevated and correlate poorly with disease activity — do not rely on a normal inflammatory-marker panel to exclude active DM.

Myositis-specific and myositis-associated antibodies

Testing for myositis-specific antibodies (MSAs) and myositis-associated antibodies (MAAs) is central to modern DM care: these antibodies define clinically distinct subgroups with different organ-involvement patterns, malignancy risk, and prognosis, and directly inform both the intensity of interstitial lung disease surveillance and the intensity of cancer screening.^[1]

Electromyography, imaging, and biopsy

Electromyography (EMG) in active myositis classically shows increased insertional and spontaneous activity (fibrillation potentials, positive sharp waves) together with short-duration, low-amplitude, polyphasic motor unit potentials — a myopathic rather than neurogenic pattern. EMG is abnormal in roughly 70-90% of DM cases and is most useful for confirming a myopathic process and excluding a primary neurogenic cause of weakness.

MRI, using fat-suppressed T2-weighted or short-tau inversion recovery (STIR) sequences, shows muscle edema corresponding to active inflammation, and can distinguish this from the fatty replacement of chronic, burnt-out muscle damage or from corticosteroid-induced myopathy — a genuinely useful distinction when a patient on treatment is not improving and it is unclear whether under-treated disease or a steroid-related complication is to blame. MRI findings can also help select the optimal muscle biopsy site.

Muscle biopsy remains the most direct confirmatory test and is particularly valuable when the diagnosis is not clinically clear-cut. The hallmark finding is perifascicular atrophy — atrophic muscle fibers concentrated at the periphery of the fascicle, reflecting the capillary-mediated ischemia described in Overview — together with a perimysial and perivascular (rather than purely endomysial) inflammatory infiltrate and, on immunohistochemistry, evidence of complement membrane-attack-complex deposition on small vessels. Perifascicular atrophy is not universally present and can also occur in antisynthetase-syndrome-associated myositis, so biopsy findings are interpreted alongside the full clinical and serologic picture rather than in isolation.^[1] Skin biopsy can support the diagnosis when muscle disease is absent or minimal (clinically amyopathic disease), though the histologic findings (an interface dermatitis) substantially overlap with cutaneous lupus erythematosus and are not reliably distinguishing on their own.

2017 EULAR/ACR classification criteria

The 2017 EULAR/ACR classification criteria are the most widely used, data-driven framework for classifying suspected IIM and its major subgroups, developed and validated against 976 IIM patients and 624 patients with mimicking conditions.^[5] The criteria combine weighted items across several domains into a single probability score, calculated with or without muscle-biopsy data:

A weighted score of 5.5 (55% probability) without biopsy data, or 6.7 with biopsy data, classifies a patient as having probable IIM; a score of 7.5 (90% probability) without biopsy, or 8.7 with biopsy, classifies definite IIM — sensitivity and specificity both improve when muscle-biopsy findings are included in the calculation.^[5] Once a patient meets the overall probability threshold, a separate classification tree subclassifies the case into DM, polymyositis, inclusion body myositis, or (in children) juvenile DM, weighted heavily by the presence or absence of the characteristic skin findings above.^[5] Use this framework as a structured classification aid alongside clinical judgement, not as a substitute for it — a patient with a textbook heliotrope rash and Gottron's papules who does not yet meet the full probability threshold should still be treated clinically as having DM.

Differential diagnosis

• Polymyositis — proximal weakness without the DM-specific skin findings; endomysial (rather than perifascicular/perivascular) inflammatory infiltrate on biopsy, with T cells directly surrounding and invading non-necrotic fibers.
• Antisynthetase syndrome — see Recognize the Condition; can overlap substantially with DM.
• Immune-mediated necrotizing myopathy — often statin-associated (anti-HMGCR antibody) or paraneoplastic/idiopathic (anti-SRP antibody); biopsy shows myofiber necrosis with minimal inflammatory infiltrate and no perifascicular atrophy.
• Inclusion body myositis — typically older patients, insidious asymmetric weakness affecting both proximal and distal muscles (classically finger flexors and quadriceps), and a poor response to immunosuppression; biopsy shows rimmed vacuoles.
• Overlap myositis — myositis occurring alongside another connective tissue disease (systemic lupus erythematosus, systemic sclerosis, mixed connective tissue disease, Sjögren disease).
• Drug-induced or toxic myopathy — statins, corticosteroids themselves (a non-inflammatory steroid myopathy that can be mistaken for undertreated DM), colchicine, and others.
• Endocrine myopathy — hypothyroidism and Cushing syndrome (see that chapter) can both produce proximal weakness without the inflammatory or cutaneous features of DM.
• Cutaneous lupus erythematosus — for skin-predominant presentations; malar or discoid lesions typically spare the nasolabial folds and lack the periungual and Gottron-distribution findings of DM.
• Psoriasis and seborrheic dermatitis — psoriatic plaques over the knuckles can superficially resemble Gottron's papules; periorbital seborrheic dermatitis can be mistaken for a mild heliotrope rash.

Mandatory malignancy screening

Every newly diagnosed adult needs age-appropriate cancer screening per standard population guidelines (mammography, cervical screening, colorectal screening, and so on) as a non-negotiable baseline. Layered on top of this, the 2023 International Myositis Assessment and Clinical Studies Group (IMACS) guideline provides an evidence-based, risk-stratified framework for additional, DM-specific cancer screening.^[3] Patients are stratified into standard, moderate, or high cancer-risk categories based on IIM subtype, myositis-specific antibody status (anti-TIF1-gamma and anti-NXP2 confer high risk in adults), age 40 years or older at onset, and the clinical features in Risk Factors. Screening intensity is matched to risk category using a basic panel (history and examination, routine bloodwork, and chest radiography) for standard-risk patients, escalating to an enhanced panel — contrast CT of the chest, abdomen, and pelvis, tumour markers, and consideration of mammography or breast imaging, gynecologic ultrasound, and, in appropriate populations, nasoendoscopy or PET-CT — for moderate- and high-risk patients.^[3] For high-risk patients, the guideline recommends repeating this structured screening at 1, 2, and 3 years after diagnosis, reflecting the well-documented multi-year risk window around DM onset.^[3]

Baseline interstitial lung disease screening

Because ILD is common (39% in the Canadian Ontario cohort) and can be clinically silent until it is advanced or rapidly progressive, obtain baseline high-resolution CT (HRCT) of the chest and pulmonary function testing (including diffusing capacity, DLCO) in every newly diagnosed patient, not only those with respiratory symptoms.^[2] Repeat and intensify surveillance based on antibody status — anti-MDA5- and antisynthetase-antibody-positive patients need closer, more proactive follow-up — and with any new respiratory symptom (see Red Flags and Monitoring and Follow-Up).

• Suppress inflammation and induce remission of both the muscle and skin components of disease — treating the myositis alone and assuming the skin will simply follow is a common and avoidable error (see Common Errors to Avoid).
• Prevent and limit muscle damage, weakness, and long-term functional disability by starting effective treatment promptly.
• Screen for, and where found, coordinate treatment of an underlying malignancy in parallel with immunosuppressive therapy — a genuine treatment goal in its own right, not a one-time box to check at diagnosis.
• Detect and manage interstitial lung disease early, before it can progress to rapidly progressive, treatment-refractory respiratory failure.
• Protect against aspiration and malnutrition in patients with dysphagia.
• Minimize corticosteroid-related morbidity through an early steroid-sparing strategy rather than prolonged high-dose monotherapy.
• Support function, quality of life, and psychosocial wellbeing through a chronic, relapsing-remitting disease course with a disfiguring and often intensely pruritic rash.

Photoprotection

Ultraviolet exposure is a genuine trigger for cutaneous disease activity. Advise broad-spectrum, high-SPF sunscreen used consistently (not only on sunny days), sun-protective clothing, and avoidance of peak-UV hours — simple, low-risk measures that meaningfully reduce flares of the rash.

Exercise and rehabilitation

Once inflammation is reasonably controlled, a graded, supervised exercise and physical/occupational therapy program improves strength and function without provoking a disease flare — this reflects a genuine shift from older teaching that recommended rest; current evidence supports early, carefully supervised mobilization. Energy conservation strategies help with the disabling fatigue many patients report even outside of active flares.

Skin care and pruritus management

The DM rash is frequently more pruritic than that of cutaneous lupus erythematosus. Gentle, fragrance-free skin care, avoidance of known irritants, and standard antipruritic measures (oral antihistamines, topical agents) provide meaningful symptomatic relief alongside definitive immunosuppressive treatment of the underlying disease.

Swallowing and nutrition

Arrange a formal speech-language pathology swallowing assessment for any patient with dysphagia (see Red Flags), with diet-texture modification, safe-swallowing positioning strategies, and close nutritional monitoring; escalate to enteral feeding support if oral intake cannot safely meet nutritional needs.

Bone health and infection prevention around corticosteroid use

Weight-bearing exercise where feasible, adequate calcium and vitamin D intake, and confirmation of age-appropriate vaccination status (with avoidance of live vaccines during significant immunosuppression) before or early in a corticosteroid/immunosuppressant course are standard supportive measures (see Monitoring and Follow-Up for the full corticosteroid-monitoring bundle).

Smoking cessation

Advise smoking cessation in every patient, but particularly those with interstitial lung disease, where ongoing smoking compounds respiratory risk.

Patient and caregiver education

• Set expectations early: DM is typically a chronic, relapsing-remitting disease, and steroid-sparing agents (methotrexate, azathioprine, mycophenolate) take weeks to months to reach full effect — explain this delay explicitly so a patient does not lose confidence in a new medication that has not yet had time to work.
• Explain clearly why extensive cancer screening is being arranged even though the patient may feel well and has no cancer symptoms — this is a routine, evidence-based part of DM care, not a sign that something specific has already been found.
• Review the specific symptoms that should prompt urgent contact rather than waiting for the next scheduled visit (see Red Flags) — new shortness of breath or cough, worsening swallowing, and rapidly progressive weakness chief among them.
• For families of children with juvenile DM, explain the calcinosis risk and the rationale for aggressive early treatment specifically to reduce that risk (see Special Populations).

Juvenile dermatomyositis (JDM)

JDM is a genuinely distinct presentation, not simply DM in a smaller body. Different from adults routine malignancy screening is not warranted in juvenile-onset IIM — a strong recommendation reflecting the much lower paraneoplastic association in children.^[6] Critically, the same anti-TIF1-gamma antibody that signals high malignancy risk in an adult does not carry that signal in a child; in JDM it is instead associated with more severe cutaneous disease, and anti-NXP2 is linked to a higher risk of calcinosis rather than malignancy.^[1] First-line therapy a combination of high-dose glucocorticoid and methotrexate from the outset, rather than glucocorticoid monotherapy, per the SHARE consensus-based recommendations.^[9] Aggressive early treatment measurably reduces the incidence of calcinosis, a complication that is far more common and clinically significant in JDM than in adult-onset disease.^[9] Routine pulmonary function testing, including diffusing capacity, is recommended in children given that ILD can be present and clinically silent.^[6]

Pregnancy

Active DM can worsen, or first present, during pregnancy, and active disease is linked to a higher risk of miscarriage, preterm birth, and intrauterine growth restriction — achieving disease control before conception is preferred whenever the clinical situation allows planning. Prednisone, azathioprine, hydroxychloroquine, and IVIG are generally considered compatible with pregnancy. Avoid methotrexate and mycophenolate mofetil, both established teratogens — discontinue with an adequate washout period before conception whenever pregnancy is planned, and manage collaboratively with maternal-fetal medicine.

Older adults and age 40 years or older at diagnosis

This is the population at highest risk of an associated malignancy (see Risk Factors and Investigations, within Assessment and Diagnosis) and needs the most vigilant risk-stratified cancer screening. As in many rheumatologic conditions treated with long-term corticosteroids, it is also a population where bone, glucose, infection, and gastrointestinal monitoring (see Monitoring and Follow-Up) deserves particular attention given age-related baseline risk.

Anti-MDA5-positive and clinically amyopathic disease

Patients in this subgroup need proactive, urgent interstitial lung disease work-up (HRCT and pulmonary function testing) at diagnosis regardless of muscle strength or symptoms, and a low threshold to treat aggressively and early if any respiratory symptom develops (see Red Flags and Choose Initial Treatment).

Renal impairment

Avoid methotrexate when eGFR is under 50 mL/min/1.73 m², per current Canadian product labelling. IVIG carries its own renal considerations and needs adequate hydration and careful dosing in reduced renal function.

Hepatic impairment

Azathioprine and methotrexate both carry hepatotoxicity risk and need closer monitoring, or an alternative agent, in significant hepatic impairment; mycophenolate mofetil's comparatively lower hepatotoxicity profile can make it the preferred alternative in this situation.

Disease-activity monitoring

Track functional strength (manual muscle testing or a validated functional index) at every visit rather than relying on CK/aldolase trends alone — enzyme levels can dissociate from clinical strength, particularly later in the disease course, and chasing a lab value rather than the patient's actual function is a recognized pitfall.

Interstitial lung disease surveillance

Repeat pulmonary function testing (including DLCO) and/or HRCT at intervals matched to baseline risk — more frequently and proactively in anti-MDA5- or antisynthetase-antibody-positive patients — and immediately with any new respiratory symptom (see Red Flags). Serum ferritin can be a useful adjunct biomarker of disease activity specifically in anti-MDA5-associated disease, correlating with treatment response in some reports.

Cancer-screening follow-up

For patients risk-stratified as high risk, continue structured screening (basic plus enhanced panels as indicated) at 1, 2, and 3 years after diagnosis, reflecting the multi-year window of elevated malignancy risk around DM onset.^[3] Reassess risk stratification, and screening status, at any point disease becomes unexpectedly treatment-refractory (see Red Flags).

Corticosteroid monitoring

• Vitamin D supplementation, adequate calcium intake, and bisphosphonate therapy for fracture prevention where indicated, alongside usual bone-density monitoring.
• Regular blood pressure and blood glucose monitoring.
• A proton pump inhibitor for corticosteroid-related gastroesophageal reflux where clinically indicated.

Steroid-sparing agent monitoring

• Methotrexate: routine CBC, liver enzymes, and renal function per usual practice; confirm eGFR before dose adjustments.
• Azathioprine: dose according to TPMT activity where available; monitor CBC and liver enzymes regularly.
• Mycophenolate mofetil: monitor CBC and for signs of infection.
• Hydroxychloroquine: baseline and annual ophthalmologic screening for retinopathy after 5 years of use, sooner with renal impairment, tamoxifen use, or pre-existing retinal disease.
• IVIG: monitor renal function and for signs of thromboembolism — both were notable adverse-event signals in the ProDERM trial.^[7]
• Rituximab: monitor B-cell counts, immunoglobulin levels, and for infection per specialist-directed protocol.

Dysphagia and calcinosis follow-up

Reassess swallowing function periodically in any patient with a history of dysphagia, even after apparent improvement.^[4] In juvenile DM, monitor for new or progressive calcinosis at routine visits (see Special Populations).

1. Inadequate response to corticosteroid monotherapy: confirm a steroid-sparing agent was started concomitantly from the outset, as recommended — if not, start one immediately rather than continuing to escalate the steroid dose alone.
2. Inadequate response to methotrexate or azathioprine after an adequate trial (allow several weeks to months, given their delayed onset of effect): switch to or add mycophenolate mofetil or tacrolimus.
3. Severe or refractory dysphagia, or rapidly progressive weakness not controlled by the above: escalate to IVIG.
4. Refractory to steroid-sparing agents and IVIG, or a myositis-antibody-positive or juvenile-onset patient: consider rituximab.
5. Anti-MDA5-associated rapidly progressive interstitial lung disease not responding to combination therapy: consider cyclophosphamide and urgent referral to a tertiary interstitial lung disease centre — this is not the point to simply wait longer on the current regimen.
6. Persistent cutaneous disease despite adequate control of the myositis: optimize hydroxychloroquine dosing, reassess photoprotection adherence, and consider topical therapy or dermatology referral rather than assuming the skin will eventually catch up to the muscle response.
7. Disease that remains unexpectedly refractory at any point in this ladder: reassess the cancer-screening status before escalating immunosuppression further — refractory DM is itself a recognized marker of an undetected malignancy (see Risk Factors and Red Flags).

• Treating a normal creatine kinase as ruling out dermatomyositis — amyopathic and hypomyopathic disease can have normal CK, aldolase, EMG, and biopsy findings despite unmistakable skin disease.
• Reassuring a patient with classic DM skin findings and normal strength that "at least the muscles aren't involved," rather than urgently pursuing anti-MDA5 testing and pulmonary work-up — clinically amyopathic disease carries a disproportionate rapidly progressive interstitial lung disease risk.
• Waiting for corticosteroid monotherapy to fail before starting a steroid-sparing agent, rather than starting one concomitantly from the outset.
• Treating the skin disease as an afterthought once the myositis responds to corticosteroids — DM's cutaneous findings are frequently more corticosteroid-resistant than the muscle disease and often need their own additional therapy (hydroxychloroquine, photoprotection).
• Deferring or skipping risk-stratified cancer screening because the patient feels well, has already started treatment, or "screening can wait until follow-up" — screening should proceed in parallel with immunosuppression, not be sequenced after it.
• Applying the same aggressive, adult-style malignancy-screening approach to a child with juvenile DM — routine cancer screening is not warranted in juvenile-onset disease, and over-testing carries its own costs and anxiety.
• Assuming an anti-TIF1-gamma-positive result carries the same high malignancy signal in a child that it does in an adult — the antibody's prognostic meaning genuinely differs by age.
• Attributing new dysphagia in a DM patient to gastroesophageal reflux and simply prescribing a proton pump inhibitor without assessing pharyngeal muscle strength and aspiration risk.
• Screening for interstitial lung disease only once a patient becomes symptomatic, rather than obtaining baseline HRCT and pulmonary function testing in every newly diagnosed patient.
• Continuing methotrexate without checking renal function, or continuing it despite eGFR falling under 50 mL/min/1.73 m².
• Chasing a normalizing CK as the sole marker of adequate treatment response while a patient's actual functional strength has not meaningfully improved.
• Escalating immunosuppression step-by-step in anti-MDA5-associated rapidly progressive interstitial lung disease as though it were routine DM, rather than recognizing it as an emergency warranting aggressive combination therapy from the outset.

How to use this table: Doses are adult doses and require patient-specific confirmation (renal/hepatic function, weight, allergy history). Pediatric dosing is weight-based per the current product monograph unless stated. Drug pricing is intentionally omitted. Select any drug name to open its complete Pharmasist Drug Database page.

Induction Corticosteroid Therapy

First-line for every newly diagnosed patient; start a steroid-sparing agent concomitantly rather than waiting for steroid failure or a full taper.

Steroid-Sparing Agents (Start Concomitantly With Induction)

Onset of effect is delayed by weeks to months — start alongside the induction corticosteroid course, not after it fails.

Cutaneous-Directed Therapy

The skin disease of DM is frequently more corticosteroid-resistant than the myositis and often needs its own additional therapy.

Refractory or Severe Disease

Reserved for disease not controlled by an adequate trial of first-choice steroid-sparing agents, or for genuinely emergent presentations.

Supportive Therapy Alongside Long-Term Corticosteroids

Standard bone- and GI-protective bundle for any patient on a prolonged corticosteroid course.