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N40 Neurology
Myopathies Toronto Notes 2019 Myopathies
Clinical Approach to Muscle Diseases
Myopathies are characterized by prominent symmetric proximal weakness and absent sensory changes
Good Questions to Assess Proximal Weakness
• Legs: climbing stairs, stand from sit • Arms: reach above head, wash hair
Common Medications that Cause Myopathy: steroids, statins, anti-retrovirals, thyroxine, fibrates, cyclosporine, ipecac Common Drugs that Cause Myopathy: ethanol, cocaine, heroin
Table 20. Myopathies
Inflammatory
Endocrine Toxic
Infectious Hereditary Dystrophy
Hereditary Metabolic
Hereditary Periodic Paralysis
Hereditary Mitochondrial
Etiology
Polymyositis (see Rheumatology , RH15)
Dermatomyositis (see Rheumatology , RH15)
Sarcoidosis
Inclusion body myositis
Thyroid (or) Cushing’s syndrome Parathyroid (or)
Medication
Critical illness myopathy
Parasitic, bacterial, or viral Duchenne (see Medical
Genetics, MG8) Becker
Myotonic dystrophy McArdle’s
“Channelopathy” MERRF
MELAS
Kearns Sayre
Key Clinical Features
Myalgias
Pharyngeal involvement
Myalgias Characteristic rashes Can be paraneoplastic
See Respirology, R14
Weak quadriceps and deep finger flexors
See Endocrinology, E33
Medication or toxin history
ICU patient
Hx steroids and nondepolarizing paralyzing agents
Failure to wean from ventilation
Myalgias
Inflammatory myopathy
Early onset (Duchenne and Becker)
Progressive proximal muscle weakness
Calf pseudohypertrophy
Distal myopathy Myotonia
Genetic anticipation
Exercise-related myalgias, cramping, and myoglobuminuria
Episodic weakness Normal between attacks
Myoclonus, generalized seizures, dementia, myopathy
Pediatric onset, stroke-like symptoms, episodic vomiting, dementia
Progressive ophthalmoplegia, retinal pigment degeneration, cardiac conduction abnormalities
Key Investigations
CK
Biopsy: endomysial infiltrates, necrosis
CK
Biopsy: perifascicular atrophy
ACE level
Biopsy: granulomas
CK
Biopsy: inclusion bodies
TSH
Serum cortisol Calcium panel
Toxicology screen
Biopsy: selective loss of thick myosin filaments
 myoglobin
Dystrophin analysis: absent Dystrophin analysis: abnormal
Genetic testing  lactate
serum/urinary myoglobin post-exercise
Normal,  or  K+ Biopsy: ragged red fibres
Increased lactate
MELAS = mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes; MERRF = mitochondrial encephalomyopathy with ragged red fibres ACE = acetylcholinesterase; CK = creatine kinase