Diagnosing a myopathy means answering four questions: how much muscle is being injured, whether the process is myopathic or neuropathic, which specific disease is responsible, and whether an external cause such as a drug or a hormone disturbance is driving it. Different tests answer different questions, so they are read together with the clinical pattern rather than in sequence.
Creatine kinase
Creatine kinase (CK) is the most useful blood marker. It rises with active fibre injury and falls with successful treatment, so it both supports the diagnosis and tracks response. Three caveats keep it honest: CK rises transiently after exercise or intramuscular injections, so rest and repeat measurement matter; baseline levels vary between populations; and chronically very high values point toward dystrophinopathy while normal or mildly raised values in an older weak patient favour inclusion body myositis.
Because muscle also releases transaminases and lactate dehydrogenase, isolated “liver enzyme” elevation in a weak patient should prompt CK measurement before any liver workup.
Electromyography
Electromyography (EMG) records the electrical activity of muscle and separates myopathic from neuropathic processes. A motor unit is one motor neuron with the fibres it supplies. Short, small motor unit potentials with early recruitment suggest muscle disease, while large potentials with reduced recruitment suggest reinnervation after nerve loss. Fibrillation potentials and positive sharp waves mark active irritability. In myotonic disorders the discharge waxes and wanes with a characteristic diving sound.
Muscle biopsy
Muscle biopsy identifies what is missing or misplaced: perifascicular atrophy in dermatomyositis, endomysial cytotoxic infiltrates in polymyositis, rimmed vacuoles in inclusion body myositis, ragged-red fibres in mitochondrial disease.
Genetic testing and imaging
Genetic testing has become the first confirmatory step when the phenotype points at a specific inherited defect, with biopsy reserved for unclear cases and treatment monitoring. Muscle MRI adds a noninvasive map, showing selective fatty replacement patterns that differ between dystrophies and can appear before weakness is obvious.
Reversible causes
The fourth question, whether an external cause is driving the weakness, is answered partly by simple blood tests. Thyroid disease, vitamin D deficiency and osteomalacia all produce proximal weakness, and each is treated by correcting the underlying deficiency. Measuring thyroid function and 25-hydroxyvitamin D alongside CK is therefore part of the initial workup, and invasive testing is reserved for patients in whom no toxic, metabolic or endocrine cause emerges.