Spinal Muscular Atrophy: Electromyography

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

References

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.