Myopathy Flashcards
Inclusion Body Myositis
Overview
cause of myopathy
associated with cytoplasmic inclusions on muscle biopsy
Features
typically affects older males
can affect both proximal and distal muscles
characteristically affects quadriceps and finger/wrist flexors are usually more severely affected than extensor counterparts
Inclusion Body Myositis - Example Question
A 65 year old lady was referred to the neurology clinic for assessment. She reported a 3 month history of progressive weakness. She had initially noticed difficulty opening jars, but reported that over the past month she had also had difficulty walking up stairs. She did not have any pain, and reported no changes in sensation. Past medical history included osteoporosis, type 2 diabetes mellitus and hypertension.
On neurological examination there were no fasciculations, tone was normal and sensation was intact. Power was reduced in finger flexion (3/5), wrist flexion (4/5), knee extension (3/5) and hip flexion (4/5) bilaterally. Other movements were relatively spared. Upper limb reflexes were present but diminished, but the knee jerk was absent. The plantar response was flexor bilaterally. There was no tenderness over any muscle groups. Cranial nerve examination was unremarkable.
Blood results are shown below:
Haemoglobin 122 g/l White cell count 8.2 x 10^9/l Platelets 376 x 10^9/l C reactive protein 7 mg/l Erythrocyte sedimentation rate 39 mm/hr Creatine kinase 272 (24-170 U/l)
What is the most likely diagnosis?
Diabetic amyotrophy > Inclusion body myositis Motor neuron disease Myaesthenia gravis Polymyositis
Inclusion body myositis is the most common primary myopathy in this age group. It manifests as a slowly progressive weakness, usually affecting finger and wrist flexion initially. Lower limb weakness may also occur. Reflexes are usually diminished as in other myopathies. Creatine kinase levels are usually normal or only mildly raised, in contrast to polymyositis (where creatine kinase levels are usually markedly elevated). Muscles are often tender in polymyositis, and the distal muscles are usually not affected until the disease is advanced.
Motor neuron disease is an important differential to consider, but the lack of upper motor neuron signs and fasciculations goes against this. There is no description of fatigueability that would lead one to consider myasthenia gravis, and no evidence of bulbar, facial or ocular weakness. Diabetic amyotrophy is characterised by painful wasting of the proximal lower limb muscles.
Polymyositis - Overview
Overview
inflammatory disorder causing symmetrical, proximal muscle weakness
thought to be a T-cellmediated cytotoxic process directed against muscle fibres
may be idiopathic or associated with connective tissue disorders
associated with malignancy
dermatomyositis is a variant of the disease where skin manifestations are prominent, for example a purple (heliotrope) rash on the cheeks and eyelids
typically affects middle-aged, female:male 3:1
Polymyositis - Features
Features proximal muscle weakness +/- tenderness Raynaud's respiratory muscle weakness interstitial lung disease: e.g. fibrosing alveolitis or organising pneumonia dysphagia, dysphonia
Investigations
elevated creatine kinase (markedly elevated)
EMG
muscle biopsy
anti-Jo-1 antibodies are seen in pattern of disease associated with lung involvement, Raynaud’s and fever
Mx:
Polymyositis responds to immunosuppression.
Polymyositis - Example Question
A 46-year-old lady complains of proximal muscle weakness over 6 months. History is otherwise unremarkable. She has no past medical history and is on no medications. She does not drink alcohol. On examination, power is 4/5 proximally in both arms and legs, otherwise unremarkable. Blood investigations are normal except for an elevated creatinine kinase level at 900 U/L. Electromyography demonstrates myopathic features. You order a muscle biopsy to help differentiate which myopathy is present. The results show endomysial lymphocytic infiltrates that invade nonnecrotic muscle fibres. What is the most likely diagnosis?
Dermatomyositis Inclusion body myositis Systemic lupus erythromatosis Lung cancer > Polymyositis
Proximal muscle weakness is a nonspecific complaint and the elevated CK with this presentation suggests a myopathy, of which the common are inflammatory (polymyositis, dermatomyositis, and inclusion body myositis), toxic myopathies (e.g. statin- or alcohol-induced), and inherited (Duchenne/Becker muscular dystrophy, myotonic dystrophy). There is no history of toxic agents here and the history is not lifelong to suggest an inherited causes. There are no skin changes to suggest dermatomyositis. The diagnostic difficulty may be between inclusion body myositis and polymyositis. The former tends to affect wrists and fingers more than the latter. Where there is diagnostic difficulty clinically, a biopsy may help make a diagnosis:
Endomysial lymphocytic infiltrates that invade nonnecrotic muscle fibres = polymyositis
Perimysial inflammation of lymphocytes and parafascicular atrophy = dermatomyositis
Inflammatory infiltrates and inclusions within muscle fibres = inclusion body myositis.
Polymyositis responds to immunosuppression.
It is likely she has a polymyositis and coincidental carpal tunnel syndrome rather than an inclusion body myositis.
Myopathy - Differentials
Inflammatory (polymyositis, dermatomyositis, and inclusion body myositis)
Toxic myopathies (e.g. statin- or alcohol-induced)
Inherited (Duchenne/Becker muscular dystrophy, myotonic dystrophy).
Differentiating Inclusion Body Myositis from Polymyositis
Creatine kinase levels are usually normal or only mildly raised in Inclusion Body Myositis, in contrast to polymyositis where creatine kinase levels are usually markedly elevated.
Muscles are often tender in polymyositis
In Polymyositis distal muscles are usually not affected until the disease is advanced, whereas in inclusion body myositis, both proximal and distal muscles are affected from beginning. Also inclusion body myositis tends to affect wrists and fingers more.
Myopathy - Muscle Biopsies
Endomysial lymphocytic infiltrates that invade nonnecrotic muscle fibres = polymyositis
Perimysial inflammation of lymphocytes and parafascicular atrophy = dermatomyositis
Inflammatory infiltrates and inclusions within muscle fibres = inclusion body myositis.
