The EMG in infants with SMA (types 0 and 1) typically demonstrates prominent fibrillation potentials 7 and markedly diminished Compound Muscle Action Potentials (CMAP) amplitudes regardless of the skeletal muscle chosen for testing. The severity of the type of SMA a patient correlates with the magnitude of the EMG findings. 3 SMA does not typically involve sensory nerves 1 and thus sensory nerve conduction studies are expected to be normal. 4,6 Nerve conduction velocities may be normal until late stage disease. 4 In patients with SMA who have had sufficient disease duration, the EMG may also reveal compensatory changes such as re-innervation and an enlargement of motor unit action potential amplitudes. Generally, the EMG findings in the patient with SMA will be consistent with a denervation in the setting of a motor neuron disease, that is, there will be evidence of neuronal and axonal loss.

2,3 EMG still has a role in the diagnostic confirmation of motor neuron disease in patients with weakness and provides more immediate diagnostic findings at the bedside to guide further workup. Prior to genetic testing as the gold standard diagnostic method for SMA, electromyography (EMG) was used widely to diagnose SMA. Spinal Muscular Atrophy (SMA) is a progressive neuromuscular disease associated with typically proximal muscle weakness and atrophy due to degeneration of the anterior horn cells of the spinal cord.

Emg of spine