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Each child may experience symptoms differently, and the disease is divided into types based on age of onset and functional ability. There is also a range of severity within each type, and as many as 25% of individuals may not have a precise type.3

Characteristics of spinal muscular atrophy

0-6 months (infant-onset) 

Highest motor milestone achieved

UNABLE TO SIT
(“nonsitters”)

Life expectancy

≤2 YEARS4

Type

Type I
(also known as
Werdnig-Hoffmann disease)

Characteristics1,5-7

  • Poor head control
  • Weak cough
  • Weak cry
  • Progressive weakness of muscles used to chew and swallow
  • Poor muscle tone
  • “Frog-leg” posture when lying
  • Severe muscle weakness on both sides of body
  • Progressive weakness of muscles that help in breathing (intercostal muscles)

7-18 months (intermediate) 

Highest motor milestone achieved

ABLE TO SIT INDEPENDENTLY
(“sitters”)

Life expectancy

>2 YEARS
70% STILL LIVING AT AGE 25

Type

Type II
(also known as Dubowitz disease)

Characteristics1,5,6

  • Muscle weakness
  • Swallowing, coughing, and breathing problems may occur but are typically less common
  • Muscle aching and joint stiffness symptoms
  • Children may develop spinal problems such as scoliosis (curvature of the spine), which may require bracing or surgery

18 months+ (juvenile-onset) 

Highest motor milestone achieved

ABLE TO WALK INDEPENDENTLY
(“walkers”, although they may progressively lose this ability)

Life expectancy

NORMAL



Type

Type III
(also known as
Kugelberg-Welander disease)

Characteristics1,5,6

  • Scoliosis
  • Swallowing difficulty
  • Muscles in the legs are generally more severely affected than the arms
  • Muscle aching
  • Joint overuse symptoms

Late adolescence/adulthood (adult-onset) 

Highest motor milestone achieved

ALL

Life expectancy

NORMAL

Type

Type IV

Characteristics1,5

  • Physical symptoms are similar to juvenile-onset spinal muscular atrophy, with the gradual onset of weakness, tremors, and muscle twitching first noted in late teens or early adulthood


There are other diseases that are similar to spinal muscular atrophy,
but have different genetic causes. These conditions include:

  • Spinal muscular atrophy with respiratory distress (SMARD)—SMARD has similar symptoms to infant-onset spinal muscular atrophy, though it affects the upper spinal cord neurons instead of the lower motor neurons. Children with SMARD typically have low birth weight and experience symptoms within the first 3-6 months, including severe respiratory distress due to paralysis of the diaphragm.8
  • Distal hereditary motor neuropathy (sometimes referred to as Type V SMA)—Distal hereditary motor neuropathy is an extremely rare autosomal dominant genetic disease, which means that only 1 inherited copy of the abnormal gene is required for the disease to occur. Distal hereditary motor neuropathy affects nerve cells in the spinal cord and presents as weakness and wasting that starts in muscles of the upper and lower limbs and spreads later to other muscles.9-11
  • Kennedy’s disease (KD)—Unlike spinal muscular atrophy, which affects the motor neurons, KD affects both lower motor and sensory neurons and only occurs in males. KD is a disease of the X chromosome.12

For more information about spinal muscular atrophy, visit Cure SMA. To learn more about other rare diseases, please visit the following websites:


Spinal muscular atrophy is often first suspected by a parent who may notice that their child is not meeting certain milestones

Parents may observe that their child is not reaching typical physical milestones for their age, such as the ability to hold their head up, to roll over, or to sit up independently. Swallowing or feeding may also become difficult, and children may lose the ability to swallow safely without choking or inhaling food into the lungs (aspiration).9,13

While all babies develop at their own pace, the World Health Organization (WHO) offers the following general motor milestone guidelines as part of the Multicentre Growth Reference Study (MGRS):14

Motor milestones in the MGRS

  • GROSS MOTOR MILESTONE PERFORMANCE CRITERIA
  • Sitting without support Child sits up straight with head erect for ≥10 seconds. Does not use arms or hands to balance body or support position.
  • Crawling on hands and knees Child moves forward or backward on hands and knees. The stomach does not touch the supporting surface. Continuous and consecutive movements, ≥3 in a row.
  • Standing with assistance Child stands in upright position on both feet, holding on to a stable object, such as furniture. Child stands holding on for ≥10 seconds.
  • Walking with assistance Child is in upright position with back straight. Makes sideways or forward steps holding on to stable object with one or both hands. One leg moves forward while the other supports body weight. Child takes ≥5 steps.
  • Standing alone Child stands in upright position on both feet (but not on the toes) with the back straight. The legs support 100% of the weight, without holding on, for ≥10 seconds.
  • Walking alone Child takes ≥5 steps independently, with the back straight. One leg moves while the other supports most of the weight. There is no contact with a person or object.

Adapted from the WHO Multicentre Growth Reference Study.14

  • AGE OF CHILD TYPICAL MOVEMENT/PHYSICAL MILESTONES
  • 2 months
    • Can hold head up and begins to push up when lying on stomach
    • Makes smoother movements with arms and legs
  • 4 months
    • Holds head steady, unsupported
    • Pushes down on legs when feet are on a hard surface
    • May be able to roll over from stomach to back
    • Can hold a toy and swing it at dangling toys
    • Brings hands to mouth
    • When lying on stomach, pushes up to elbows
  • 6 months
    • Rolls over in both directions (front to back, back to front)
    • Begins to sit without support
    • When standing, supports weight on legs and might bounce
    • Rocks back and forth, sometimes crawling backward before moving forward
  • 9 months
    • Stands, holding on
    • Can get into sitting position
    • Sits without support
    • Pulls to stand
    • Crawls
  • 1 year
    • Gets to a sitting position without help
    • Pulls up to stand, walks holding on to furniture ("cruising")
    • May take a few steps without holding on
    • May stand alone

Adapted from the Centers for Disease Control and Prevention Milestone Checklist.13

SMA Symptoms

Parents’ reports of their children’s gross motor development tend to be reliable. Sharing observations of potential motor delays with a doctor can help to determine an appropriate care strategy.2,15,16

REFERENCES

1. Wang CH, Finkel RS, Bertini ES, et al; and Participants of the International Conference on SMA Standard of Care. Consensus statement for standard of care in spinal muscular atrophy. J Child Neurol. 2007;22(8):1027-1049. 2. Noritz GH, Murphy NA; and Neuromuscular Screening Expert Panel. Motor delays: early identification and evaluation. Pediatrics. 2013;131(6):e2016-e2027. 3. Kolb SJ, Kissel JT. Spinal muscular atrophy. Arch Neurol. 2011;68(8):979-984. 4. Markowitz JA, Singh P, Darras BT. Spinal muscular atrophy: a clinical and research update. Pediatr Neurol. 2012;46(1):1-12. 5. Prior TW, Russman BS. Spinal muscular atrophy. NCBI Bookshelf Website. Available at: http://www.ncbi.nlm.nih.gov/books/NBK1352/. Updated November 14, 2013. Accessed April 15, 2016. 6. Iannaccone ST. Modern management of spinal muscular atrophy. J Child Neurol. 2007;22(8):974-978. 7. Oskoui M, Levy G, Garland CJ, et al. The changing natural history of spinal muscular atrophy type 1. Neurology. 2007;69(20):1931-1936. 8. Darras BT, Royden Jones H Jr, Ryan MM, De Vivo DC, eds. Neuromuscular Disorders of Infancy, Childhood, and Adolescence: A Clinician’s Approach. 2nd Ed. London, UK: Elsevier; 2015. 9. Cure SMA. Understanding Spinal Muscular Atrophy (SMA). Available at: http://www.curesma.org/documents/support--care-documents/understanding-sma.pdf. Accessed January 9, 2017. 10. Online Mendelian Inheritance in Man. Neuronopathy, distal hereditary motor, type VA; HMN5A. http://www.omim.org/entry/600794. Edited January 2, 2014. Accessed April 22, 2016. 11. National Organization for Rare Diseases. Spinal muscular atrophy. https://rarediseases.org/rare-diseases/spinal-muscular-atrophy/. Updated 2012. Accessed April 17, 2016. 12. Barkhaus PE et al. Kennedy Disease. Available at: http://emedicine.medscape.com/article/1172604-overview. Updated: Jun 08, 2016. Accessed January 9, 2017. 13. Centers for Disease Control and Prevention. Developmental milestones. Available at: http://www.cdc.gov/ncbddd/actearly/milestones/. Updated January 21, 2016. Accessed April 27, 2016. 14. Wijnhoven TMA, de Onis M, Onyango AE, et al; for the WHO Multicentre Growth Reference Study Group. Assessment of gross motor development in the WHO Multricentre Growth Reference Study. Food Nutr Bull. 2004;25(1 suppl 1):S37-S45. 15. Lawton S, Hickerton C, Archibald AD, McClaren BJ, Metcalfe SA. A mixed methods exploration of families’ experiences of the diagnosis of childhood spinal muscular atrophy. Eur J Hum Genet. 2015;23(5):575-580. 16. Bartlett D, Piper M. Mothers’ difficulty in assessing the motor development of their infants born preterm: implications for intervention. Pediatr Phys Ther. 1994;6(2):55-59.