Neurophysiological studies - electromyography (EMG)

What it is

Needle electrode recording of electrical activity within muscle - at rest and during voluntary contraction - to distinguish neurogenic from myopathic pathology and localise nerve/root lesions
Complements nerve conduction studies (NCS); the two are almost always performed together
Used to evaluate suspected radiculopathy, peripheral neuropathy, motor neuron disease, myopathy, and neuromuscular junction disorders

Finding	Meaning
Fibrillations/positive sharp waves (at rest)	Denervation - active axonal loss (neurogenic)
Fasciculations	Spontaneous motor unit discharge - suggests motor neuron disease when combined with other denervation features
Large-amplitude, long-duration, polyphasic units (reduced recruitment)	Chronic neurogenic pattern (reinnervation)
Small-amplitude, short-duration, polyphasic units (early/full recruitment)	Myopathic pattern

Motor neuron disease (ALS) - EMG showing widespread denervation across multiple myotomes/nerve territories, including a clinically unaffected limb is central to diagnosis (El Escorial-type criteria)
Radiculopathy - denervation restricted to muscles of a single nerve root (myotomal), sparing muscles supplied by the same peripheral nerve but a different root
Myopathy - normal insertional activity (unless an inflammatory myopathy), myopathic motor units, early recruitment

Cannot be performed on significantly anticoagulated patients without caution (bleeding/haematoma risk at needle sites) - discuss with the neurophysiologist first
Findings take ~2-3 weeks to develop after acute axonal injury (denervation changes are not immediate) - early EMG post-injury can be falsely reassuring

Motor neuron disease, radiculopathy, peripheral neuropathy, inflammatory myopathy, myasthenia gravis (repetitive nerve stimulation, single-fibre EMG)

Serial EMG can track disease progression (e.g. worsening denervation in ALS) though is more often used diagnostically than for monitoring

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