News & Perspective
Drugs & Diseases
CME & Education
Academy
Video
Decision Point
Edition: English

Medscape

English
Deutsch
Español
Français
Português
UKNew

Univadis

Sign Up It's Free!
English Edition

Medscape

Univadis

    X
    Univadis from Medscape

    No Results

      News & Perspective Drugs & Diseases CME & Education Academy Video Decision Point
      Drugs & Diseases > Neurology

      Spinal Muscular Atrophy Workup

      Updated: May 31, 2022
      • Author: Jeffrey Rosenfeld, MD, PhD, FAAN; Chief Editor: Stephen L Nelson, Jr, MD, PhD, FAACPDM, FAAN, FAAP, FANA  more...
      • Print
      Workup

      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.

      Homozygous SMN1 gene deletion is 95% sensitive and nearly 100% specific for the diagnosis of SMA. In patients with suspected disease and no gene deletion, SMN1 copy testing with sequencing of coding regions of SMN1 copy (if present) is suggested. [38]  Molecular testing for homozygous deletion or mutation of the SMN1 gene allows efficient and specific diagnosis. [39]

      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.

       

      Next:

      Other Tests

      Most cases spare the cardiac system, and ECGs are normal. [40]

      Electrophysiologic studies are useful in differentiating the spinal muscular atrophies from other neurogenic and myopathic diseases. [41, 42] 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.

      Previous
      Next:

      Procedures

      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.

      Previous
      Next:

      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. [43, 44]

      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.)

      Previous
      Treatment & Management
       
       
      Media Gallery
      of 0
      Find Us On
      About
      About Medscape Privacy Policy Editorial Policy Cookies Terms of Use Advertising Policy Help Center
      Membership
      Become a Member About You Professional Information Newsletters & Alerts Market Research
      App
      Medscape
      WebMD Network
      Medscape Live Events WebMD MedicineNet eMedicineHealth RxList WebMD Corporate Medscape UK
      Editions
      English Deutsch Español Français Português UK
      All material on this website is protected by copyright, Copyright © 1994-2023 by WebMD LLC. This website also contains material copyrighted by 3rd parties.