Fig. 27.1
Heliotrope rash, periorbital puffiness and malar rash in a patient with dermatomyositis
Fig. 27.2
Gottron’s papules on (a): fingers and (b): knees
Skeletal muscle weakness is typically proximal and symmetric affecting neck flexors, shoulder girdle and hip flexors. It can initially be mild and mistaken for fatigue. It is associated often with myalgia. Muscle weakness is progressive, and children develop difficulty in lifting objects overhead, combing hair, climbing stairs and getting out of bed. Neck muscle weakness causes children difficulty in lifting the head off the pillow. As weakness progresses, distal muscles also become involved resulting in a weak grip. Involvement of palatal muscles causes dysphagia and dysphonia. Swallowing difficulty manifests as coughing during eating or drinking and has to be enquired in the history. Oesophageal and respiratory muscles are also affected, and this can lead to aspiration and respiratory failure. Weakness of the respiratory muscles is a medical emergency and has to be recognised early. Children do not present with increased work of breathing or hypoxaemia but present instead with hypercarbia. Silent aspiration has been recognised in symptomatic children with JDM. Thus, swallowing dysfunction or aspiration is not restricted only to those who are severely weak. Physical examination will reveal head lag in a child after infancy and an inability to perform a sit up. Children with JDM compensate for truncal weakness by rolling on to their side instead of sitting straight up from lying. An excellent screening test to assess for lower extremity muscle weakness or pelvic girdle weakness is the Gower’s sign. Gower’s sign is considered to be positive when the following pattern appears on asking the patient to rise from lying supine or sitting on the floor:
the child rolls prone and extends arms and legs far apart
with most of the weight on extended arms, they push the body backwards
the child then extends the hip by placing hands on their knees and ‘walking’ the arms up the thighs until upright
Turning prone to rise from a supine position is the salient feature of Gower’s sign. While this is part of normal development, it is rarely seen in healthy children over 3 years of age. Gower’s sign is seen in many other conditions characterised by muscle weakness such as muscular dystrophies, inflammatory myopathy, spinal muscular atrophy and others. One third of patients can have an associated polyarticular arthritis [8, 9].
Longstanding or poorly controlled disease is associated with calcinosis and lipodystrophy. Calcinosis has been found to be present in up to 40 % of children with JDM. Calcium gets deposited as subcutaneous nodules and plaques (Fig. 27.3). These can extrude from the skin as crystals or calcific liquid causing painful ulcers [8, 9].
Fig. 27.3
X-ray of the legs showing extensive calcinosis cutis in a patient with JDM
Lipodystrophy is characterised by progressive loss of subcutaneous and visceral fat over the face and upper body and can be associated with insulin resistance. Children with severe JDM can develop vasculitis of the gastrointestinal (GI) tract characterised by crampy abdominal pain, pancreatitis and GI bleeding. It can also lead to perforation. Interstitial lung disease, pericarditis, myocarditis and conduction defects are rare complications of severe disease. Children presenting with the classic rash but no apparent muscle weakness or inflammation are said to have amyopathic JDM [8].
Diagnosis
The diagnostic criteria suggested by Peter and Bohan in 1975 are the most widely accepted. Definite diagnosis of dermatomyositis requires the presence of the characteristic rash (heliotrope rash over the eyelids with periorbital oedema, Gottron’s papules) and at least three signs of muscle inflammation and weakness (Table 27.1).
Feature | Details | |
|---|---|---|
1. | Symmetrical weakness | Usually progressive, of the limb-girdle muscles |
2. | Muscle biopsy evidence of myositis | Necrosis of type I and type II muscle fibres. Phagocytosis. Degeneration and regeneration of myofibres with variation in myofibre size with perifascicular atrophy. Endomysial, perimysial, perivascular or interstitial mononuclear inflammatory cells |
3. | Elevation of serum levels of muscle-associated enzymes | CK, LDH, aldolase, transaminases (ALT/SGPT and AST/SGOT) |
4. | Electromyographic (EMG) triad of myopathy | Short, small, low-amplitude polyphasic motor unit potentials, fibrillation potentials, even at rest, bizarre high-frequency repetitive discharges |
Definite JDM/JPM = presence of 3 out of 4 features (plus rash for JDM) | ||
Probable JDM/JPM = presence of 2 out of 4 features (plus rash for JDM) | ||
Possible JDM/JPM depends on the presence of one feature (plus rash for JDM) | ||
An international consensus survey for diagnostic criteria for JDM was suggested which included three standard criteria of skin rash, proximal muscle weakness and elevated muscle enzymes and six others, biopsy, EMG, muscle MRI, nail fold capillaroscopy, calcinosis and dysphonia.
JDM/JPM are defined with age of onset being before 16 in Europe and before 18 in America. The Bohan and Peter criteria were developed before the advent of MRI. Typical cases of JDM can be diagnosed clinically by the presence of symmetrical proximal muscle weakness and the characteristic rash. The clinical diagnosis is then confirmed by supportive investigations like muscle enzymes, EMG, MRI or muscle biopsy. With the new criteria, the requirement for electromyography and muscle biopsy has significantly reduced in the diagnosis of JDM. At diagnosis all muscle enzymes need to be tested as CPK is not necessarily elevated in all patients with JDM. Likewise, no one investigation is abnormal in all cases of JDM [6, 12, 13].
Differential Diagnosis
Other causes of weakness including neuromuscular aetiologies have to be thought of when the typical rash is absent. On clinical examination, detection of myotonia (delayed relaxation of skeletal muscle following voluntary contraction), paramyotonia or hypertrophy of muscles should suggest neuromuscular causes. Nerve pathology leads to distal weakness and sensory loss. Neuromuscular junction disorders cause patchy or diffuse weakness. In contrast, muscle involvement leads to symmetrical proximal and persistent weakness. There is minimal wasting and no sensory loss, and reflexes are usually intact [1].
Besides neurologic causes like muscular dystrophies (Duchenne and Becker’s), Myasthenia gravis and Guillain-Barre (GB) syndrome and other systemic myopathies should be considered in the differential diagnosis and excluded before diagnosing an inflammatory myopathy. These include:
- 1.
Infection-related myositis
- 2.
Endocrine cause such as hypothyroidism, hyperthyroidism, hypoparathyroidism, hyperparathyroidism, Cushing’s, adrenal insufficiency and acromegaly
- 3.
Metabolic myopathies: hypokalaemia, hyperkalaemia, hypernatraemia, hypercalcaemia, hypocalcaemia, hypophosphataemia, chronic renal failure (CRF), hypomagnesaemia and organ failure (cardiac, respiratory, hepatic failure)
- 4.
Drug or toxin-induced myopathies: glucocorticoids, lipid-lowering agents, alcohol and illicit drugs and other drugs like D-penicillamine, procainamide, L-tryptophan, amiodarone and chloroquine
- 5.
Paraneoplastic myopathy: less common in children than adults. Associated with malignancies of lung, ovary and GI tract
- 6.
Critical illness myopathy
Besides JDM, inflammation of muscles can also be seen in children with other connective tissue diseases like Juvenile idiopathic arthritis (JIA), systemic lupus erythematosus (SLE), mixed connective tissue disease (MCTD),inflammatory bowel disease (IBD) and antineutrophilic cytoplasmic antibody (ANCA)-associated vasculitides [1, 6].
Investigations
Investigations help to confirm the diagnosis of JDM, exclude systemic causes of myopathy and establish the extent of organ involvement. Therefore, blood tests should include inflammatory markers (C reactive protein and ESR), renal function tests, liver function tests, muscle enzymes, auto antibodies, thyroid function (other endocrine tests if needed), virology/infection screen and a bone profile (calcium, parathyroid hormone, vitamin D). A toxicology screen should also be considered in an appropriate setting. Of note, inflammatory markers can be normal in active muscle inflammation [1, 6].
Raised levels of muscle enzymes (creatine kinase, aldolase, aspartate aminotransferase, alanine aminotransferase and lactate dehydrogenase) in the serum indicate muscle inflammation. Alanine aminotransferase is usually elevated on initial presentation, whereas CK may be normal in 20 % of patients. Despite ongoing active disease or disease flare, muscle enzymes may be normal [1, 2]. Anaemia of chronic disease may be present. 80 % of children with JDM have ANA positivity. MSA (myositis-specific antibodies) are rarely present in children unlike adults. The anti Jo1 antibody is associated significantly with the development of interstitial lung disease. Anti-NXP2 (anti p 140/MJ) in JDM cases is associated with increased risk of calcinosis and increased muscle weakness. Anti-TIF1-gamma (anti p155/140) has been found to be associated with worse skin disease, risk of skin ulceration, lipodystrophy, and contractures [6, 13].
EMG, a part of the traditional diagnostic criteria of JDM/JPM, shows signs of denervation and myopathy as well as muscle fibre necrosis but is rarely required for diagnosis currently. Nerve conduction studies are typically normal in juvenile dermatomyositis. MRI has been validated as an indicator of disease activity and is being increasingly used in the diagnosis of JDM. MRI studies using T2-weighted images and STIR (fat-suppressed) sequences detect sites of active muscle inflammation and oedema in skin, subcutaneous tissue and myofascia. The use of MRI reduces sampling error and increases the sensitivity of muscle biopsy and EMG. Additionally quantitative parameters (fc-T2 mapping or fat-corrected T2 mapping) on MRI can also be used to further enhance its utility in diagnosis as well as in assessing chronicity [5, 14].
Musculoskeletal ultrasound (USG) is a safe, inexpensive and non-invasive imaging modality that does not need sedation. It can be used to support the clinical diagnosis of JDM and to monitor the progression of the disease. However, USG needs a significant level of expertise.
Muscle biopsy is considered the gold standard for confirming the diagnosis of JDM/JPM. However, muscle inflammation can be patchy in both muscle groups as well as in individual muscles. Muscle biopsies are scored under the four domains of inflammatory changes, vascular endothelial changes, muscle fibre abnormalities and connective tissue changes. When the clinical presentation and course is atypical, it is essential to perform a muscle biopsy to exclude muscular dystrophies and mitochondrial cytopathies.
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