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Spinal Muscular Atrophy


Spinal muscular atrophy is a genetic disorder characterized by progressive muscular weakness.


The different types of spinal muscular atrophy may present differently but clinical symptomatology may be similar to some extent.

The following system-wise presentation is commonly in seen in SMAs:

General Appearance: Infant patients may initially present to be weak, floppy and flaccid.

Integumentary system: SMA type 1 babies may appear cyanotic upon birth.

Head and Neck: Sucking and swallowing ability is impaired.

Chest and heart: Respiratory difficulty may be presented with a hypotonic diaphragm muscle.

Abdomen: Hypotonic abdominal muscles may be observed in supple babies.

Extremities: Occasional tremors of fingers and muscular fasciculation may be observed in chronic infantile forms of SMA. Pseudohyperthrophy of gastrocnemius muscle (calf muscle) may be seen in SMA II.

Neurologic: Developmental and motor function delay are evident in SMA type 2. Late stage SMA may present with poor deep tendon reflexes.

Difficulty Walking
  • Children can walk independently but may have difficulty walking or running, rising from a chair, or climbing stairs. Other complications may include curvature of the spine, contractures, and respiratory infections.[ninds.nih.gov]
  • While children with type III usually are able to walk, most have some difficulty walking. Some may eventually need to use a wheelchair. Symptoms usually appear around 18 months of age or in early childhood.[childrenshospital.org]
  • These children show signs of clumsiness, difficulty walking, mild muscle weakness, and may be developmentally delayed. These children live long into their adult years.[chw.org]
Developmental Delay
  • These symptoms include: Developmental delays Inability to support the head Inability to sit up without assistance Breathing and swallowing difficulties which may lead to choking Spinal muscular atrophy Type 2 is less severe than type I and begins to develop[cedars-sinai.edu]
  • These children show signs of clumsiness, difficulty walking, mild muscle weakness, and may be developmentally delayed. These children live long into their adult years.[chw.org]
  • These children show signs of clumsiness, difficulty walking, mild muscle weakness, and may be developmentally delayed. These children live long into their adult years. Type IV.[stanfordchildrens.org]
Recurrent Respiratory Infections
  • However, children with muscle weakness remain at risk for recurrent respiratory infection and atelectasis.[ncbi.nlm.nih.gov]
  • Children with this form of SMA face a difficult battle and many die from recurrent respiratory infections within the first year of life. However, with new therapies, some children live into their teens or early adulthood.[ucsfbenioffchildrens.org]
  • Recurrent respiratory infections: Tongue fasciculation and ineffective swallowing may predispose patients to frequently aspirate their food.[symptoma.com]
Poor Feeding
  • Once a child is diagnosed with SMA, physical therapists and other health care professionals can reduce some of the additional complications that occur following birth, such as developmental delay, poor feeding, abnormal postures and scoliosis, loss of[moveforwardpt.com]
Unable to Stand
  • Children can sit without support but are unable to stand or walk unaided. Children also may have respiratory difficulties.[ninds.nih.gov]
Respiratory Distress
  • KEYWORDS: Distal neuropathy; IGHMBP2; Respiratory distress; SMARD1; Spinal muscular atrophy with respiratory distress[ncbi.nlm.nih.gov]
  • The most common side effects found in participants in the clinical trials on Spinraza were upper respiratory infection, lower respiratory infection and constipation.[fda.gov]
  • The most common side effects found in patients in the clinical trials were upper respiratory infection, lower respiratory infection, and constipation, the FDA said.[bostonglobe.com]
  • Constipation should be treated aggressively as it may lead to discomfort, more problems with gastric emptying, decreased appetite, and reflux. The Portal's Constipation has management information.[medicalhomeportal.org]
  • Constipation is a common problem as is being able to control excessive drooling (secretions), and getting proper nutrition and calories for proper weight gain.[smasupport.com]
  • Our pediatric gastroenterology, liver, and nutrition specialists can help manage your child’s digestive problems, from chronic abdominal pain, diarrhea, and constipation to complex conditions such as celiac disease, inflammatory bowel disease, pancreatic[dukechildrens.org]
  • Dysphagia. 2010 Sep;25(3):261-4. doi: 10.1007/s00455-009-9269-1. Epub 2010 Jan 20. Author information 1 Department of Occupational Therapy, Gangnam Severance Hospital, Yonsei University, Gangnam-gu, Seoul, 135-720, Republic of Korea.[ncbi.nlm.nih.gov]
  • Dysphagia may be present. Most children are confined to a wheelchair by age 2 to 3 yr. The disorder is often fatal in early life, frequently resulting from respiratory complications.[merckmanuals.com]
  • Patients with SMA also have significant difficulty with physical activities such as walking, crawling, and I coordination, neck and had control and dysphagia ( trouble swallowing ).[stemcellthailand.org]
Delayed Gastric Emptying
  • Gastro-Intestinal & Bowel Function Gastrointestinal problems such as reflux, delayed gastric emptying, and constipation are common. See Gastroesophageal Reflux Disease for medications that may be useful in treatment.[medicalhomeportal.org]
Abdominal Mass
  • Herein, we report a 4-month-old male infant who presented to our hospital with an abdominal mass that was diagnosed as neuroblastoma and spinal muscular atrophy type I.[ncbi.nlm.nih.gov]
Fasciculation of the Tongue
  • The child presented at two months of life with intense muscle weakness affecting predominantly proximal portions of the limbs, especially the legs, muscle hypotonia, fasciculation of the tongue, and severe respiratory muscle involvement.[ncbi.nlm.nih.gov]
Abnormal Eye Movement
  • In one patient, earlier onset before the age of 2 years was associated with a more complex clinical picture, with abnormal eye movements, progressive cognitive impairment, and a more rapid and severe course.[ncbi.nlm.nih.gov]
Subcutaneous Nodule
  • Mutations of N-acylsphingosine amidohydrolase 1 are known to separately cause Farber disease (arthritis, subcutaneous nodules, and dysphonia) or SMA with progressive myoclonic epilepsy.[ncbi.nlm.nih.gov]
Reduced Fetal Movement
  • We present a male patient with SMAX2 who presented with typical symptoms at birth, preceded by reduced fetal movements in the second and third trimesters of pregnancy.[ncbi.nlm.nih.gov]
  • There may be a history of reduced fetal movements in utero. Mortality/morbidity : median survival is 7 months - 95% die before 18 months. SMA type II Age of onset : 6-18 months. Features : developmental motor delay (delay in sitting, standing).[patient.info]
Muscular Atrophy
  • […] known as English spinal muscular atrophy rare congenital neuromuscular disorder spinal muscle atrophy survival motor neuron spinal muscular atrophy spinal muscular atrophies of childhood Spinal muscular atrophy Statements spinal-muscular-atrophy-pro[wikidata.org]
  • Sumner, Molecular Mechanisms of Spinal Muscular Atrophy, Journal of Child Neurology, 10.1177/0883073807305787, 22, 8, (979-989), (2016). Charlotte J. Sumner, Therapeutics development for spinal muscular atrophy, NeuroRX, 3, 2, (235), (2006).[doi.org]
  • Related Medscape Reference topics: Focal Muscular Atrophies Spinal Muscle Atrophy Spinal Muscular Atrophy Related Medscape resource: Resource Center Spinal Disorders Pathophysiology Spinal muscular atrophy (SMA) is caused by successive motor unit degeneration[emedicine.medscape.com]
  • Spinal muscular atrophy and the antiapoptotic role of survival of motor neuron.[ncbi.nlm.nih.gov]
Muscle Weakness
  • Some years later, his two older teenage brothers presented with proximal muscle weakness. Neurophysiology, muscle biopsy and DNA studies confirmed spinal muscular atrophy.[ncbi.nlm.nih.gov]
  • . • Individuals with SMA Type IV develop symptoms after age 21 years, with mild to moderate leg muscle weakness and other symptoms.[ninds.nih.gov]
  • In the first days of the patient's life, fractures of the right femur and right humerus were found; however, calcium-phosphate metabolism and densitometric examination were normal.[ncbi.nlm.nih.gov]
  • […] reflex any reflex action mediated through a center at the spinal cord. spinal stenosis see spinal cord compression (above). spinal tap see spinal puncture (above). spinal trauma temporary or permanent dislocation of one or more spinal vertebrae; or fracture[medical-dictionary.thefreedictionary.com]
  • Pediatric Endocrinology (see Services below for local providers) Consider referral for children who have low bone mineral density on Dexascan and have a history with fractures.[medicalhomeportal.org]
  • Owing to the similar clinical features of SMA and congenital myopathy, an electrodiagnostic study and muscle biopsy could create confusion in the correct diagnosis in some cases.[ncbi.nlm.nih.gov]
  • Differential diagnosis Differential diagnoses include SMA2, congenital muscular dystrophies, congenital myopathies, some early-onset mitochondrial disorders, and carbohydrate metabolism disorders (see these terms).[orpha.net]
  • Nasal ventilation in congenital myopathies and spinal muscular atrophies. Eur Respir Rev. 1993 ;3: 275 - 278. Google Scholar Simonds AK, Ward S., Heather S., et al.[doi.org]
  • Differential diagnosis Motor neurone disease Primary lateral sclerosis Muscular dystrophy Congenital myopathies Disorders of carbohydrate metabolism ( glycogen storage diseases ) Myasthenia gravis Poliomyelitis Investigations Blood tests Creatine kinase[patient.info]
Joint Deformity
  • Farber disease is associated with joint deformities, lipomatous skin nodules, and often is fatal by 2-3 years of age; while SMA-PME is characterized by childhood-onset motor neuron disease and progressive myoclonic epilepsy.[ncbi.nlm.nih.gov]
  • People with this condition may experience the following symptoms: Joint deformities that impede mobility Infants may be born with broken bones Kennedy’s disease is one type of X-linked spinal muscular atrophy.[cedars-sinai.edu]
  • Affected infants move less in the womb, and as a result they are often born with joint deformities (contractures). They have extremely weak muscle tone (hypotonia) at birth.[ghr.nlm.nih.gov]
  • If a child requires surgery for scoliosis or other joint deformities, intensive preoperative and postoperative physical therapy can help prevent respiratory complications and loss of strength or function. Assistive Devices.[moveforwardpt.com]
  • She presented with progressive muscle weakness, tremor, seizure, and cognitive impairment. Clinical features and electrophysiological investigations revealed a motor neuron disease and generalized epilepsy.[ncbi.nlm.nih.gov]
  • Extremities: Occasional tremors of fingers and muscular fasciculation may be observed in chronic infantile forms of SMA. Pseudohyperthrophy of gastrocnemius muscle (calf muscle) may be seen in SMA II.[symptoma.com]
  • Fine tremors of the face are present in over 90% of patients. Type 2 diabetes mellitus, hand tremor, and infertility can also occur.[encyclopedia.com]
Limb Weakness
  • As the disease progresses patients may notice limb weakness starting in the pelvis or shoulders, or weakness of the facial and tongue muscles. Symptoms of SMA-LED often develop in infancy or early childhood.[cedars-sinai.edu]
  • There is a clinical variant of SMA, known as autosomal-dominant spinal muscular atrophy characterized by a predominantly lower limb weakness and muscle wasting.[symptoma.com]
  • In addition to muscle weakness, clinical features include head lag, poor sucking and swallowing, weak cry, proximal limb weakness, and lack of reflexes. Affected children never raise their head, roll over, or walk.[encyclopedia.com]
Absent Deep Tendon Reflex
  • Other symptoms may include: Decreased or absent deep tendon reflexes, such as the relfex that occurs when you tap on your knee.[ucsfbenioffchildrens.org]
  • Infantile X-linked spinal muscular atrophy (SMAX2) is a rare form of spinal muscular atrophy manifesting as severe hypotonia, areflexia, arthrogryposis, facial weakness and cryptorchidism, and frequently accompanied by bone fractures.[ncbi.nlm.nih.gov]
  • Other symptoms include: Muscle atrophy Muscle weakness Areflexia Trouble Breathing Thin muscle mass Trouble eating or swallowing Lack of head and neck control Involuntary facial twitching Atrophic muscle changes Muscle twitching Sensory neuropathies Difficulty[stemcellthailand.org]
Unable to Walk
  • While the father first walked at the age of 19 months, the son was unable to walk at age 3 years. In both, knee and ankle reflexes were absent and sensation was intact. Serum creatine kinase levels were normal.[ncbi.nlm.nih.gov]
  • Type II SMA is characterized by onset of muscle weakness and hypotony usually after six months of age; the children are able to sit but unable to walk unaided.[doi.org]


The following workup and testing methods are implored in cases of spinal muscular atrophy:

Genetic Testing 

A genetic panel for SMN 1 sequencing and coding is amply suggested in cases of infantile SMA suspects [8]. Amniocentesis of the fetal chorionic villi has a sensitivity rating of 88% to 99% in the prenatal diagnosis of SMA. Recent studies reveal that carrier detection rating for SMA may rise to 90% [9].

Laboratory Testing 

The laboratory determination of creatine kinase (CK) level may be a helpful tool in a comprehensive SMA work up. Type 1 SMA will arbitrarily present with a normal CK level while the other forms may show CK elevations.

Electrodiagnostic Testing 

The use of an electromyography (EMG) and nerve conduction studies may be helpful in the diagnosis of SMA. Muscles which are not clinically affected will show signs of denervation with EMG while nerve conduction remains normal.


There is no available cure for SMA to date. However, there are supportive treatment options to allay the discomforts that inherently affect the SMA patients:

  • Physical therapy exercises may help delay progression of SMA leading to muscular atrophy.
  • Appliance or Equipment: This are mechanical aids like splints, bracing and spinal orthoses to help SMA patients to go about their daily chores [10].
  • Surgical correction of exaggerated scoliosis.
  • Gastrostomy placement on infants with SMA type 1.


The relative morbidity and mortality rates of SMA varies inversely with age of onset. The SMA type 1 in Werdnig-Hoffman disease carries the worst prognosis with a very high mortality rate. Life expectancy of the patient is usually unaffected with the milder forms of SMA (type 3 and type 4).

Some genetic studies point to prognosticate prenatal occurrence of SMA via amniocentesis. The findings of severe SMA by genetic studies may give some mothers the option for therapeutic abortion in some liberal countries.


The following clinical disorders are the established complications associated with spinal muscular atrophy:


Spinal muscular atrophy is transmitted via an autosomal-recessive gene from both parents, meaning the mother and the father will not convey any symptoms of the disease. The offspring will carry the SMA gene defect through a homozygous deletion type of inheritance.

The inherited genetic defects on chromosome 5q11.2-13.3 are generally associated with spinal muscular atrophy type 1 to 3 [1]. Defects in genetic SMA are not exclusive to peripheral nerves but it involves the central nervous system (CNS) as well. There is a clinical variant of SMA, known as autosomal-dominant spinal muscular atrophy characterized by a predominantly lower limb weakness and muscle wasting [2].


In the United States, SMA represent the next most common autosomal-recessive genetic disorder following cystic fibrosis. The acute form of SMA in infants occurs in 1 out of 10,000 live births, while the chronic SMAs becomes evident in 1 out of 24,000 births. The chronic infantile type of SMA accounts to 50% of all cases [3].

Spinal muscular atrophy expresses in 1 out of 10,000 live births worldwide [4]. The mortality rate of acute infantile SMA (type 1) reaches 95% with a 7 month mean survival average.

The leading cause of mortality is the complication of severe respiratory infections in SMA. There is a predilection of SMA among males especially in both infantile onset types [5].

Sex distribution
Age distribution


The familial spinal muscular atrophy is represented by two survival motor neurons (SMN1 and SMN2) [6]. In more than 95% of SMA cases there has been an evident homozygous gene disruption in SMN1 on the small arm loci of chromosome 5q.

These pathologic phenomenon is grossly brought about by spontaneous mutation and gene deletion. The missing SMN1 in the genetic defect will allow SMN2 to generate the missing protein for the spinal cord development although it can only come up with 10% of the actual protein chain length [7].


Genetic counselling for carrier parents may avert the emergence of an affected sibling. The prenatal amniocentesis of the fetal chorionic villus may help attending physicians prepare for any untoward eventualities during the baby’s delivery. Prompt diagnosis and early intervention is the key in reducing morbidity and mortality.


Spinal muscular atrophy or SMA is a hereditary disease characterized by progressive musclar weakness due to deterioration of the lower motor neuron (anterior horn) cells of the spinal cord including the motor nuclei of the central nervous system (brainstem).

The pathology is mainly on the motor neurons connecting the spinal cord and the brain to the muscles of the body. The more common types of spinal muscular atrophy include: Werdnig-Hoffman disease or SMA type 1 (infantile type of acute onset), SMA type 2 (chronic infantile type), Kugelberg-Welander disease or SMA type 3 (chronic juvenile onset), and the SMA type 4 (adult onset).

Patient Information


Spinal muscular atrophy or SMA is a hereditary disease characterized by the progressive musclar weakness due to the deterioration of the lower motor neuron (anterior horn) cells of the spinal cord including the motor nuclei of the central nervous system (brainstem).


It is due to autosomal-recessive genetic inheritance. 


Sucking and swallowing ability, tremors, weak abdominal muscles and respiratory difficulty are the usual symptoms


Patients diagnosed with spinal muscular atrophy should be submitted to rigorous pediatric neurology follow-ups for the early classification of the disease.

Treatment and follow up

The third and fourth type of SMA may be compatible to normal living till adulthood. Offspring from patients who make it to adulthood may carry the recessive gene and should be subjected to parental genetic counselling in the future.



  1. Burlet P, Burglen L, Clermont O, et al. Large scale deletions of the 5q13 region are specific to Werdnig- Hoffmann disease. J Med Genet. Apr 1996;33(4):281-3
  2. Oates EC, Reddel S, Rodriguez ML, Gandolfo LC, Bahlo M, Hawke SH, Lamandé SR, Clarke NF, North KN. Autosomal dominant congenital spinal muscular atrophy: a true form of spinal muscular atrophy caused by early loss of anterior horn cells. Brain. 2012; 135(Pt 6):1714-23 (ISSN: 1460-2156)
  3. Harding AE. Inherited neuronal atrophy and degeneration predominantly of lower motor neurons. In: Dyck PJ, Thomas PK, eds. Peripheral Neuropathy. 3rd ed. Philadelphia: WB Saunders; 1993:1051-64
  4. Pearn J. Classification of spinal muscular atrophies. Lancet. Apr 26 1980; 1(8174):919-22.
  5. Hausmanowa-Petrusewicz I, Zaremba J, Borkowska J, Szirkowiec W. Chronic proximal spinal muscular atrophy of childhood and adolescence: sex influence. J Med Genet. Dec 1984; 21(6):447-50.
  6. Martínez-Hernández R, Bernal S, Also-Rallo E, Alías L, Barceló MJ, Hereu M, Esquerda JE, Tizzano EF. Synaptic defects in type I spinal muscular atrophy in human development.J Pathol. 2013; 229(1):49-61 (ISSN: 1096-9896)
  7. Lunn MR, Wang CH. Spinal muscular atrophy. Lancet. Jun 21 2008; 371(9630):2120-33.
  8. Mercuri E, Bertini E, Iannaccone ST. Childhood spinal muscular atrophy: controversies and challenges. Lancet Neurol. May 2012; 11(5):443-52.
  9. Ben-Shachar S, Orr-Urtreger A, Bardugo E, Shomrat R, Yaron Y. Large-scale population screening for spinal muscular atrophy: clinical implications.Genet Med. 2011; 13(2):110-4 (ISSN: 1530-0366)
  10. Armon C. ALS 1996 and Beyond: New Hopes and Challenges. A manual for patients, families and friends. 3rd ed. Loma Linda, Calif: 2000:18.

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Last updated: 2019-07-11 20:44