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.1
Prior to genetic testing as the gold standard diagnostic method for SMA, electromyography (EMG) was used widely to diagnose SMA.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.4,5
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.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.4,6 Nerve conduction velocities may be normal until late stage disease.3 SMA does not typically involve sensory nerves1 and thus sensory nerve conduction studies are expected to be normal.
The severity of the type of SMA a patient correlates with the magnitude of the EMG findings. The EMG in infants with SMA (types 0 and 1) typically demonstrates prominent fibrillation potentials7 and markedly diminished Compound Muscle Action Potentials (CMAP) amplitudes regardless of the skeletal muscle chosen for testing.8 The EMG will demonstrate poor recruitment of motor units7 with reduced Motor Unit Number Estimations (MUNEs), but the motor units themselves may appear enlarged.9
EMG findings in patients with SMA type 2 will be similar in appearance to patients with infantile onset SMA types with lower extremity muscles being more involved than upper extremity muscles.10 Fibrillation potentials may be present.9 Recruitment will be decreased with reductions in MUNEs.9 Motor units are expected to be enlarged as a compensatory mechanism for poor recruitment.3,9
Patients with later onset forms of SMA (types 3 and 4) may have slightly reduced or normal CMAP amplitudes and may not exhibit fibrillation potentials.9,11 These late onset patients will have EMGs showing mild MUNE reductions with prominently enlarged motor units reflective of chronic compensatory changes.9,11 Diagnostic EMG findings in patients with late onset SMA are best obtained in affected muscles.12
1. Prior TW, Finanger E. Spinal Muscular Atrophy. In: Adam MP, Ardinger HH, Pagon RA, et al., eds. GeneReviews((R)). Seattle (WA): University of Washington, Seattle University of Washington, Seattle. GeneReviews is a registered trademark of the University of Washington, Seattle. All rights reserved.; 1993.
2. Hausmanowa-Petrusewicz I. Role of electromyography in the diagnosis of motor neuron disorders. Neuropatologia polska. 1992;30(3-4):187-197.
3. Hausmanowa-Petrusewicz I, Karwanska A. Electromyographic findings in different forms of infantile and juvenile proximal spinal muscular atrophy. Muscle Nerve. 1986;9(1):37-46.
4. Arnold WD, Kassar D, Kissel JT. Spinal muscular atrophy: diagnosis and management in a new therapeutic era. Muscle Nerve. 2015;51(2):157-167.
5. D’Amico A, Mercuri E, Tiziano FD, Bertini E. Spinal muscular atrophy. Orphanet journal of rare diseases. 2011;6:71.
6. Farrar MA, Vucic S, Johnston HM, Kiernan MC. Corticomotoneuronal integrity and adaptation in spinal muscular atrophy. Archives of neurology. 2012;69(4):467-473.
7. Hsu CF, Chen CY, Yuh YS, Chen YH, Hsu YT, Zimmerman RA. MR findings of Werdnig-Hoffmann disease in two infants. AJNR American journal of neuroradiology. 1998;19(3):550-552.
8. MacLeod MJ, Taylor JE, Lunt PW, Mathew CG, Robb SA. Prenatal onset spinal muscular atrophy. Eur J Paediatr Neurol. 1999;3(2):65-72.
9. Arnold WD, Porensky PN, McGovern VL, et al. Electrophysiological Biomarkers in Spinal Muscular Atrophy: Preclinical Proof of Concept. Annals of clinical and translational neurology. 2014;1(1):34-44.
10. Kang PB, Gooch CL, McDermott MP, et al. The motor neuron response to SMN1 deficiency in spinal muscular atrophy. Muscle Nerve. 2014;49(5):636-644.
11. Gdynia HJ, Sperfeld AD, Flaith L, et al. Classification of phenotype characteristics in adult-onset spinal muscular atrophy. European neurology. 2007;58(3):170-176.
12. Mishra VN, Kalita J, Kesari A, Mitta B, Shankar SK, Misra UK. A clinical and genetic study of spinal muscular atrophy. Electromyography and clinical neurophysiology. 2004;44(5):307-312.