Clinician's Guide to Spinal Muscular Atrophy

Jump to:
Medical advisors icon

Meet our

Medical advisors

Edward Smith, M.D.

Edward Smith, MD

Melissa Wright, MD, PhD

Making an SMA Diagnosis

Written by Margaret Anne Rockwood | Last updated July 6th, 2026
✅ Medically reviewed by Edward Smith, MD

 

Spinal muscular atrophy (SMA) is diagnosed primarily through molecular genetic testing that demonstrates deletion or mutation of the SMN1 gene. Quantification of SMN2 copy number helps estimate disease severity.

SMA occurs when there is loss or mutation of both copies of SMN1 combined with insufficient compensation from SMN2.

Modern genetic testing can both confirm a diagnosis and provide prognostic information through analysis of SMN2 gene copy number.

A healthy person may have few or even no SMN2 copies and still be completely disease-free if SMN1 is intact. Conversely, a patient with no functional SMN1 may survive because SMN2 partially compensates.

Disease-free individuals usually possess two functional SMN1 copies (one from each parent), while SMN2 copy number varies and primarily determines disease severity rather than disease presence itself.

Initial Physical Examination & Genetic Testing

Before genetic testing is performed, several examination findings may raise suspicion for SMA:

  • symmetric proximal muscle weakness
  • muscle atrophy
  • hypotonia
  • reduced or absent deep tendon reflexes
  • tongue fasciculations
  • preserved sensation
  • normal cognition

Because SMA primarily affects lower motor neurons, weakness occurs without sensory loss, helping distinguish SMA from many neuropathic and central nervous system disorders.

Although clinical findings may strongly suggest SMA, genetic confirmation is considered the diagnostic gold standard.

  1. Genetic confirmation of SMN1deletion or mutation, which causes most cases of SMA
  2. SMN2 copy number analysis for severity prediction

Most individuals with diagnosed SMA have a homozygous deletion of exon 7 in SMN (most common) or compound heterozygous mutations involving deletion of one allele and a pathogenic variant in the other (roughly 5% of cases).

Molecular testing may include:

  • Polymerase chain reaction (PCR)
  • Multiplex ligation-dependent probe amplification (MLPA)
  • Quantitative PCR
  • Next-generation sequencing (NGS)

These methods can identify SMN1 deletions, detect pathogenic sequence variants, and determine SMN2 copy number.

Additional Analyses & Diagnostic Studies

Once SMA has been confirmed genetically, SMN2 copy number analysis provides important prognostic information.  Because SMN2 produces limited amounts of functional SMN protein, patients with more SMN2 copies generally experience milder disease.

Typical associations include:

Scroll horizontally to view all columns -->

SMN2 Copies Commonly Associated Phenotype
1-2 SMA Types 0–1
3 SMA Type 2
3-4 SMA Type 3
≥4 SMA Type 4 or other milder phenotypes

These associations are helpful but not absolute. Clinical presentation remains the most important determinant of disease type and severity.

Although genetic testing is usually sufficient to establish the diagnosis, additional studies may help support the diagnosis, assess disease severity, or exclude alternative neuromuscular disorders.

Electromyography (EMG)

Electromyography is no longer required to establish the diagnosis when genetic testing is confirmatory. However, it remains useful when the diagnosis is uncertain or alternative disorders are being considered. Typical findings in SMA include:

  • fibrillation potentials
  • positive sharp waves
  • reduced motor unit recruitment
  • chronic denervation
  • large-amplitude motor unit potentials resulting from re-innervation

These findings help distinguish SMA from primary muscle disorders, which typically demonstrate small motor unit potentials and early recruitment patterns rather than chronic denervation.

Nerve Conduction Studies (NCS)

Nerve conduction studies evaluate peripheral nerve function. Note that sensory nerve conduction studies are usually normal and that motor amplitudes may be reduced because of motor neuron loss.

These patterns help to distinguish SMA from peripheral neuropathies such as Charcot-Marie-Tooth disease, where sensory nerve abnormalities and slowed conduction velocities are often present.

Muscle Biopsy

Muscle biopsy is performed much less frequently today because of the widespread availability of genetic testing. However, it may be considered when the diagnosis remains uncertain. Typical biopsy findings include:

  • grouped atrophy
  • fiber-type grouping
  • small angular fibers
  • increased connective tissue

These changes reflect chronic denervation and re-innervation.

Creatine Kinase (CK) Testing

Creatine kinase levels are usually normal or only mildly elevated in SMA. Although CK is not diagnostic, it may help distinguish SMA from muscular dystrophies, in which CK levels are often markedly elevated.

Pulmonary and Swallowing Evaluations

Respiratory and swallowing assessments are commonly performed after diagnosis because respiratory muscle weakness and bulbar dysfunction are major causes of morbidity in SMA.

These evaluations may include:

  • pulmonary function testing
  • cough effectiveness assessment
  • sleep studies when indicated
  • swallow studies to evaluate aspiration risk

The results can help guide respiratory and nutritional management.

Newborn Screening

All 50 U.S. states and the District of Columbia now include SMA in newborn screening programs using dried blood spot testing. Screening identifies the absence of SMN1 exon 7 before symptoms develop, allowing treatment to begin during the pre-symptomatic period when outcomes are generally most favorable. Notably, some state labs do not perform the more complex testing needed to detect SMA from compound heterozygous mutations, leading to the risk of false negatives.

Evidence from newborn-screening programs has demonstrated that infants treated early are frequently able to obtain better motor outcomes than those who are not treated early.

Sources