Spinal Muscular Atrophy Workup

Author: Bryan Tsao, MD; Chief Editor: Amy Kao, MD more...

Updated: Mar 8, 2011

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Laboratory Studies
Other Tests
Procedures
Histologic Findings
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References

Laboratory Studies

Laboratory testing

The creatine kinase (CK) level is typically normal in SMA type I and normal or slightly elevated in the other types.

CSF findings are normal.

Genetic testing

Both prenatal and postnatal tests are now commercially available.

Tests for chromosome arm 5q should be performed.

The 1992 ISMAC found that the accuracy of prenatal prediction by means of chorionic villi sampling and amniocentesis was 88-99%.

Caution should be exercised when prenatal prediction is done in the presence of atypical features (see SMA variants in Physical) because these clinical variations may represent other pathogenic processes.

Other Tests

Most cases spare the cardiac system, and ECGs are normal.

Electrophysiologic studies are useful in differentiating the spinal muscular atrophies from other neurogenic and myopathic diseases.^{[30, 31]}With the exception of Kennedy and Davidenkow syndromes, sensory nerve conduction is normal in spinal muscular atrophy.

Compound motor action potentials (CMAPs) are low normal or reduced, depending on the severity of disease. In chronically weak muscles, CMAPs may be in the near-normal because of reinnervation and collateral sprouting. Motor velocities are normal. Modest slowing of motor conduction, when present, may accompany severe motor axon loss because of the loss of the fastest-conducting motor fibers.

In affected muscles, needle-electrode examination reveals widespread broad and polyphasic motor unit potentials (MUPs) firing in a reduced or rapid neurogenic recruitment pattern. Superimposed low-amplitude, short-duration, and polyphasic MUPs may be present. These configuration changes may resemble myopathic MUPs, but in the case of spinal muscular atrophy are instead due to early MUP reinnervation. Fibrillation potentials may be seen in limb and paraspinal muscles and are most striking in early or progressive spinal muscular atrophy. In late-juvenile and adult-onset forms, active motor axon loss is sparse. Fasciculation potentials are uncommon, but spontaneously firing motor unit action potentials (MUAPs) at 5-15 Hz have been described as a unique feature of SMA I and II.

Mild pseudomyotonic discharges have been observed in patients older than 6 years. However, these discharges are not specific for etiology and may be seen in chronic neurogenic disorders.

Muscle biopsy may necessary to differentiate spinal muscular atrophies from other neuromuscular disorders if genetic analysis is unrevealing. Muscle selection should be centered on clinically affected muscles but not to such a degree that degeneration renders the tissue unrecognizable.
Adequate results can be obtained with open or needle biopsy as long as the physician has adequate experience in the procedure and in processing of the tissue.
Electron microscopy can be used to evaluate for storage diseases.

Histologic Findings

Histologic findings depend on the stage and progression of disease. Initial changes include atrophy of muscle fibers with compensatory hypertrophy. This results in groups of large and small fibers (fiber-type grouping).

During the first 6-8 weeks of life, differentiating congenital fiber type disproportion and SMA may be difficult. In the chronic forms of SMA, secondary myopathic changes may be seen in addition to type grouping and may histologically resemble the muscular dystrophies.^{[32, 33]}

Classic histologic findings include the following:

Degeneration and loss of spinal motor neurons with a neurogenic pattern of muscle morphology
Occasional neuronal chromatolysis with loss of myelinated axons in both anterior and posterior roots
A disproportionate loss of myelin in the thoracic and lumbar segments (especially in the corticospinal tracts) with relative sparing of the cervical cord
Motor neurons in the brainstem, notably in the hypoglossal nucleus. (Reactive gliosis and secondary degeneration in roots and nerves are seen. However, these findings are not necessarily pathognomic for the SMAs.)

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Contributor Information and Disclosures

Author

Bryan Tsao, MD Associate Professor, Department of Neurology, Loma Linda University; Chair and Service Chief, Department of Neurology, Loma Linda University Medical Center

Bryan Tsao, MD is a member of the following medical societies: American Academy of Neurology

Disclosure: Nothing to disclose.

Coauthor(s)

Carmel Armon, MD, MSc, MHS Professor of Neurology, Tufts University School of Medicine; Chief, Division of Neurology, Baystate Medical Center

Carmel Armon, MD, MSc, MHS is a member of the following medical societies: American Academy of Neurology, American Academy of Sleep Medicine, American Association of Neuromuscular and Electrodiagnostic Medicine, American Clinical Neurophysiology Society, American College of Physicians, American Epilepsy Society, American Medical Association, American Neurological Association, American Stroke Association, Massachusetts Medical Society, Movement Disorders Society, and Sigma Xi

Disclosure: Avanir Pharmaceuticals Consulting fee Consulting

Specialty Editor Board

Robert J Baumann, MD Professor of Neurology and Pediatrics, Department of Neurology, University of Kentucky College of Medicine

Robert J Baumann, MD is a member of the following medical societies: American Academy of Neurology, American Academy of Pediatrics, and Child Neurology Society

Disclosure: Nothing to disclose.

Francisco Talavera, PharmD, PhD Senior Pharmacy Editor, eMedicine

Disclosure: eMedicine Salary Employment

Kenneth J Mack, MD, PhD Senior Associate Consultant, Department of Child and Adolescent Neurology, Mayo Clinic

Kenneth J Mack, MD, PhD is a member of the following medical societies: American Academy of Neurology, Child Neurology Society, Phi Beta Kappa, and Society for Neuroscience

Disclosure: Nothing to disclose.

Selim R Benbadis, MD Professor, Director of Comprehensive Epilepsy Program, Departments of Neurology and Neurosurgery, Tampa General Hospital, University of South Florida College of Medicine

Selim R Benbadis, MD is a member of the following medical societies: American Academy of Neurology, American Academy of Sleep Medicine, American Clinical Neurophysiology Society, American Epilepsy Society, and American Medical Association

Disclosure: UCB Pharma Honoraria Speaking, consulting; Lundbeck Honoraria Speaking, consulting; Cyberonics Honoraria Speaking, consulting; Glaxo Smith Kline Honoraria Speaking, consulting; Pfizer Honoraria Speaking, consulting; Sleepmed/DigiTrace Honoraria Speaking, consulting

Chief Editor

Amy Kao, MD Attending Neurologist, Children's National Medical Center, Washington DC

Amy Kao, MD is a member of the following medical societies: American Academy of Neurology, American Academy of Pediatrics, American Epilepsy Society, and Child Neurology Society

Disclosure: Nothing to disclose.