Differentials
Hereditary inclusion body myositis
SIGNS / SYMPTOMS
Quadriceps more likely to be spared but not always.
May be associated with Paget’s disease and/or frontotemporal dementia.
May have positive family history.
INVESTIGATIONS
Absence of inflammation on muscle biopsy more likely in hereditary inclusion body myositis.[27]
Oculopharyngeal muscular dystrophy
SIGNS / SYMPTOMS
Clinical features include ptosis, ophthalmoplegia, and dysphagia, occasionally with proximal limb weakness.
May have positive family history.
INVESTIGATIONS
Muscle biopsy shows rimmed vacuoles and tubular filaments with an absence of inflammatory features.[89]
Late-onset distal myopathy
SIGNS / SYMPTOMS
Progressive muscular weakness and atrophy beginning in hands or feet.
INVESTIGATIONS
Muscle biopsy may show rimmed vacuoles and tubulofilamentous inclusions in several types of distal myopathy (e.g., Welander's, Udd's, Markesbery-Griggs, Laing's, distal myopathy), but there is an absence of inflammation.[87]
Amyotrophic lateral sclerosis (motor neuron disease)
SIGNS / SYMPTOMS
May mimic inclusion body myositis.
Upper motor neuron signs (not present in idiopathic inflammatory myopathies).
INVESTIGATIONS
Electromyogram shows fasciculation potentials (rare in idiopathic inflammatory myopathies), and neurogenic changes are more apparent.
Muscle biopsy shows neurogenic changes.
Myasthenia gravis
SIGNS / SYMPTOMS
Weakness more likely to fluctuate, increasing with repeated or sustained exertion.
Involvement of extra-ocular muscles is common and diplopia is a very common symptom.
Spontaneous remissions can occur.
INVESTIGATIONS
Electromyogram (EMG) typically shows abnormal decrement in repetitive nerve stimulation and increased jitter in single-fibre EMG.
Presence of antibodies to acetylcholine receptors or muscle-specific kinase.
Drug-induced myopathy
SIGNS / SYMPTOMS
History of medication associated with development of myopathy (e.g., statins, penicillamine, laxative abuse).
Withdrawal of implicated drug may reverse the myopathic damage.
As well as being potentially myotoxic, statins can also induce immune-mediated necrotising myopathy (an important IIM subtype).
INVESTIGATIONS
No differentiating tests.
Late-onset Pompe's disease
SIGNS / SYMPTOMS
More likely to develop respiratory failure (occurs in approximately one third of patients, may be the presenting symptom).
INVESTIGATIONS
At rest, electromyogram may show myotonic discharges.
Muscle biopsy tissue or cultured skin fibroblasts show a reduction of acid alpha-glucosidase activity.
Secondary metabolic myopathy
SIGNS / SYMPTOMS
Muscle weakness co-exists with systemic manifestations of specific metabolic or endocrine abnormalities, such as hypokalaemia, hypophosphataemia, hypothyroidism, hyperthyroidism, and hyperparathyroidism.
Restoration of muscle strength may occur after correction of the metabolic or endocrine condition.
INVESTIGATIONS
Abnormal blood tests indicating the presence of endocrine or metabolic diagnosis (e.g., low serum potassium or abnormal thyroid function tests).
Chronic inflammatory demyelinating polyneuropathy
SIGNS / SYMPTOMS
Weakness is usually both proximal and distal, and mildly asymmetrical.
Sensory symptoms and signs and diffuse hypo-/areflexia are prominent.
INVESTIGATIONS
Nerve conduction studies show signs of primary demyelination.
Needle electromyogram shows neurogenic pattern and no signs of myopathy.
Serum CK is typically normal.
Scleroderma
SIGNS / SYMPTOMS
Muscle weakness more likely to be low-grade if present.
INVESTIGATIONS
Mostly mild or absent elevation of serum CK.
Electromyogram shows no or little spontaneous activity.
Muscle biopsy may reveal perimysial fibrosis, scleroderma vasculopathy, and type 2 fibre atrophy with little muscle fibre destruction.[87]
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