Spinal Muscular Atrophy (SMA) is a progressive neuromuscular accompanied by significant typically proximal muscle weakness and atrophy due to degeneration of the anterior horn cells of the spinal cord.1
Diagnosis of SMA does not require magnetic resonance imaging (MRI), but neuroimaging of the central nervous system is typically performed as part of a differential diagnostic evaluation in patients presenting with progressive weakness.2
MRI findings in SMA will vary depending on the SMA type. Patients with the most severe forms of SMA with types 0 and 1 typically will have visible muscle atrophy (most prominent in the calf muscles) and may have more prominent subcutaneous fat with larger intramuscular fat planes.3-5 Patients with types 2, and 3 typically have a muscle atrophy with a ragged appearance or patches of MR signal abnormality within affected muscles. SMA types 2 and 3 are more likely to have fatty infiltration and increased size of the intramuscular fat planes than patients with type 0 or type 1 SMA.3-5 Patients with milder forms of type 3 SMA may have stable but reduced muscle volumes with MR signal abnormalities in muscle groups within the upper and lower extemities.6-8
MRI of the brain and spinal cord in patients with SMA may reveal cortical atrophy, increased MR signal within the globus pallidus, and an “owl’s eye” pattern of T2 hyperintensity within the anterior horn of the spinal cord correlated with the neuronal loss within this cord region.9
The MRI findings in SMA are not specific to this disease and may be seen in SMA variants like spinal and bulbar muscular atrophy10 and in other progressive neuromuscular diseases such as the muscular dystrophies.5
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. D’Amico A, Mercuri E, Tiziano FD, Bertini E. Spinal muscular atrophy. Orphanet journal of rare diseases. 2011;6:71.
3. Chan WP, Liu GC. MR imaging of primary skeletal muscle diseases in children. AJR American journal of roentgenology. 2002;179(4):989-997.
4. Liu GC, Jong YJ, Chiang CH, Yang CW. Spinal muscular atrophy: MR evaluation. Pediatric radiology. 1992;22(8):584-586.
5. Murphy WA, Totty WG, Carroll JE. MRI of normal and pathologic skeletal muscle. AJR American journal of roentgenology. 1986;146(3):565-574.
6. Sproule DM, Montgomery MJ, Punyanitya M, et al. Thigh muscle volume measured by magnetic resonance imaging is stable over a 6-month interval in spinal muscular atrophy. J Child Neurol. 2011;26(10):1252-1259.
7. Sproule DM, Punyanitya M, Shen W, et al. Muscle volume estimation by magnetic resonance imaging in spinal muscular atrophy. J Child Neurol. 2011;26(3):309-317.
8. Durmus H, Yilmaz R, Gulsen-Parman Y, et al. Muscle magnetic resonance imaging in spinal muscular atrophy type 3: Selective and progressive involvement. Muscle Nerve. 2017;55(5):651-656.
9. 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.
10. Hamano T, Mutoh T, Hirayama M, et al. Muscle MRI findings of X-linked spinal and bulbar muscular atrophy. J Neurol Sci. 2004;222(1-2):93-97.