Peds Primer: Closer Look

Spinal Muscular Atrophy (SMA)

Spinal Muscular Atrophy (SMA) is a motor neuron disease that affects the voluntary muscles used for crawling, walking, head and neck control, and swallowing. Approximately one in 6,000 infants is affected, and about one in 40 people are genetic carriers.

SMA is diagnosed with a blood test that looks for the presence of the SMN1 gene sequence. People usually have two genes called Survival Motor Neuron 1 and 2. In 95 percent of patients with SMA, there is an absence or mutation of the SMN gene sequence. The severity of the type determines if a child will be able to sit or walk independently.

Although SMA affects muscles throughout the entire body, muscles closest to the trunk of the body — shoulders, hips and back — are often the most severely affected. According to the research and support group Families of Spinal Muscular Atrophy, general symptoms and characteristics of SMA include:

  • Weakness in the legs, generally greater than in the arms
  • Difficulty feeding and swallowing
  • When respiratory muscles are affected, an increased tendency for pneumonia and other lung problems
  • Sensation and the ability to feel are not affected
  • Unusually bright and social

Patients with MSA are generally grouped into one of four categories. Only one of the categories, Type IV, is considered adult onset, with symptoms occurring after age 35. The adult form is much less common than the other types, and adults with SMA lose function slowly over time as they age. Muscles used for swallowing and respiratory function are rarely affected at this age.

Type I SMA

Type I, also referred to as Werdnig-Hoffmann Disease, is typically diagnosed at 3 to 6 months of age and is the most severe form of the disease.

Symptoms and characteristics include:

  • Poor head control: Baby cannot lift his or her head or accomplish the normal motor skills expected in early infancy.
  • Does not kick as vigorously as normal or bear weight on legs
  • Does not have the ability to sit up unsupported
  • Difficulty swallowing and feeding: Child may have trouble managing his or her own secretions.
  • Atrophy and fine tremors in the tongue
  • Weakness in the muscles between the ribs
  • Smaller chest than usual
  • Strong diaphragm muscles, resulting in breathing with stomach muscles and sunken chest
  • Lungs may not fully develop because of weak breathing


Diagnosis of Type II is typically made before 2 years of age, with the majority of cases diagnosed by 15 months.

Symptoms and characteristics include:

  • Ability to sit unsupported when placed in a seated position, but cannot come to a seated position without assistance.
  • May be able to stand at some point with the aid of bracing or standers.
  • Swallowing problems are not typical of this type, but vary from child to child.
  • Difficulty eating enough food to maintain weight and growth.
  • Tongue fasciculations and tremors in outstretched fingers are common.
  • Weak intercostals muscles and diaphragmatic breathing.
  • Difficulty maintaining normal oxygen and carbon dioxide levels.
  • Scoliosis present as the child grows, requiring the need for bracing or spinal surgery.
  • Decreased bone density leading to fractures.


Type III is also called Kugelberg-Welander or Juvenile Spinal Muscular Atrophy, and the age of onset varies from before 3 years up until adolescence.

Symptoms and characteristics include:

  • Ability to stand alone and walk, but may show difficulty doing so.
  • When walking, frequent falls, difficulty getting up from sitting on the floor or a bent-over position, possible inability to run.
  • Fine tremors visible in outstretched fingers; tongue fasciculations are less common.
  • Feeding or swallowing difficulties are uncommon.
  • Possible loss of ability to walk later in childhood, in adolescence or even in adulthood.

Each type of SMA varies from patient to patient. Experts recommend that children with SMA do as many age-appropriate activities as possible and receive assistance in achieving their potential. Assistance can include:

  • Physical or occupational therapy intervention
  • Range-of-motion activities
  • Use of seating systems
  • Use of lightweight manual chairs, power chairs or scooters
  • Use of walkers
  • Use of standing devices
  • Respiratory intervention
  • Use of water therapy
  • Maintaining a healthy diet

The prognosis for patients with SMA varies on the type and the degree of respiratory function. According to the National Institute of Neurological Disorders and Stroke, SMA patients tend to deteriorate over time, but prognoses are largely dependent on the initial severity of the diagnoses.

Sources & Resources:

Families of Spinal Muscular Atrophy
(847) 367-7620

Fight SMA/Andrew's Buddies
(804) 515-0080

March of Dimes Birth Defects Foundation
(914) 428-7100

Muscular Dystrophy Association
(520) 529-2000

Spinal Muscular Atrophy Foundation
(877) 386-3762

This article originally appeared in the August 2006 issue of Mobility Management.

Rolling Dynamics, Rolling Resistance &  Optimizing Wheeled Prosthetics