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Updated: 6/15/2021

Spinal Muscular Atrophy

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Questions
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  • summary
    • Spinal Muscular Atrophy is a common genetic disease caused by an autosomal recessive mutation in the survival motor neuron gene. The condition presents with progressive motor weakness, scoliosis, hip dislocations, and lower extremity contractures. 
    • Diagnosis is made by DNA analysis and muscle biopsy.
    • Treatment involves a multidisciplinary approach to address motor weakness, scoliosis, and lower extremity contractures. Treatment of associated hip dislocation is observation.
  • Epidemiology
    • Incidence
      • most common genetic disease resulting in death during childhood
      • 1 in 10,000 live births
    • Anatomic location
      • progressive weakness starts proximally and moves distally
  • Etiology
    • Pathophysiology
      • caused by progressive loss of alpha-motor neurons in anterior horn of spinal cord
    • Genetics
      • inheritance
        • autosomal recessive
      • mutation
        • survival motor neuron (SMN) gene mutation
          • present in 90% of cases of SMA
          • a telomeric gene deletion
          • SMN critical to RNA metabolism and is a mediator of apoptosis
          • there are two SMN genes
          • all patients with SMA lack SMN-I protein
          • severity of disease based on number of functional copies of SMN-II
    • Associated conditions
      • orthopaedic manifestations of SMA
        • hip dislocation and subluxation
        • scoliosis
        • lower extremity contractures
  • Classification
      • Types of Spinal Muscle Atrophy
      • Type
      • Name
      • Presentation
      • Prognosis
      • Type I
      • Acute Werdnig-Hoffman disease
      • Present at < 6 months
      • Absent DTR
      • Tongue fasciculations
      • Poor, usually die by 2 yrs.
      • Type II
      • Chronic Werdnig-Hoffman disease
      • Present at 6-12 months
      • Muscle weakness worse in LE
      • Can sit but cant walk
      • May live to 5th decade
      • Type III
      • Kugelberg-Welander disease
      • Present at 2-15 years
      • Proximal weakness
      • Walk as children, wheelchair as adult
      • Normal life expectancy - may need respiratory support
  • Presentation
    • Symptoms
      • symmetric progressive weakness that is
        • more profound in lower-extremity than upper extremity
        • more profound proximally than distally
    • Physical exam
      • absent deep tendon reflexes
        • distinguishes from Duchenne's muscular dystrophy where DTR are present
      • fasciculations present
  • Imaging
    • Radiographs
      • scoliosis series
      • pelvis
  • Evaluation
    • Diagnosis based on
      • DNA analysis
      • muscle biopsy
      • prenatal diagnosis is possible
  • Treatment
    • Nonoperative
      • Nusinersen has been FDA approved for treatment of SMA. It is administered intra-thecally.
    • Operative
      • treat associated orthopaedic disorders (details below)
        • hip dislocation
        • scoliosis
        • lower extremity contractures
  • Hip Dislocation
    • Overview
      • hip subluxation and dislocation occur in 62% with type II SMA, and less frequently in Type III.
    • Treatment
      • nonoperative
        • observation alone (leave dislocated)
          • indications
            • standard of care as dislocations typically remain painless and high recurrence rate if open reduction attempted
  • Scoliosis
    • Overview
      • the development of scoliosis is almost universal
      • usually occurs by age 2 to 3 years
      • often progressive
    • Treatment
      • nonoperative
        • bracing
          • indications
            • devices may delay but not prevent surgery in children younger than ten years
      • operative
        • PSF with fusion to pelvis
          • indications
            • progressive curve
          • technique
            • address hip contractures and any other lower extremity contractures before PSF to ensure seating balance
            • to allow for intrathecal Nusinersen to be given after spine surgery, perfrom a laminectomy in the lower lumbar spine that is kept free of fusion. perform fusion of spine around this laminectomy
          • outcomes
            • for improved wheelchair sitting
            • may lead to temporary loss of upper extremity function
        • Combined PSF with anterior releases/fusion
          • indications
            • curves >100 degrees
            • very young child with high risk of crankshaft phenomenon
          • contraindications
            • pulmonary compromise
          • typically not necessary due to the high flexibility of SMA curves
  • Hip, knee, and ankle contractures
    • Overview
      • Common in the hip and knee
      • Nonambulators also develop ankle equinus
    • Treatment
      • Physical therapy
      • Surgical release is controversial as function in nonwalkers is rarely improved and recurrence is common
    • Prognosis
      • Severity of disease based on number of functional copies of SMN-II
      • Type 1 SMA has the worst prognosis, with majority of patients dying by age 2
Flashcards (10)
Cards
1 of 10
Questions (13)

(SBQ13PE.106) Survival motor neuron protein deficiency leads to:

QID: 5331

Progressive replacement of muscle tissue with fibrous and fatty tissue, causing calf pseudohypertrophy

12%

(379/3142)

Loss of anterior horn cells and progressive muscle weakness

78%

(2441/3142)

Abnormal chondroid production by chondroblasts in the proliferative zone of the physis

1%

(37/3142)

Disturbances in cell metabolism with accumulation of sphingolipids in the liver, spleen and bone marrow

3%

(106/3142)

Fetal acetylcholine receptor inhibition and nonprogressive joint contractures

5%

(158/3142)

L 2 B

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(SBQ13PE.63) Which of the following is INCORRECT regarding scoliosis in the setting of spinal muscular atrophy (SMA)?

QID: 5159

Curve flexibility tends to be higher than for idiopathic curves

30%

(728/2418)

Most curves tend to be long, C-shaped thoracolumbar curves

13%

(323/2418)

The primary surgical goal for correction is to obtain fusion in a position that will maintain sitting balance and prevent decline of pulmonary function

11%

(272/2418)

Curve magnitude correlates strongly with ambulatory status

13%

(325/2418)

Anterior release/fusion with posterior fusion are typically required for adequate surgical correction

31%

(754/2418)

L 5 B

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(OBQ13.104) A newborn infant is born with the physical exam finding shown in the video below. In addition, the child has absent deep tendon reflexes and hypotonia. Which of the following abnormalities is most likely responsible for these findings?

QID: 4739
FIGURES:

Gene mutation in SMN1

61%

(2788/4536)

Deficiency of Dystrophin protein

13%

(580/4536)

Beta-galactosidase deficiency

16%

(710/4536)

Gene mutation in COMP

5%

(235/4536)

Gene mutation in COL1A1

4%

(177/4536)

L 3 B

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(SBQ13PE.2) A 12-month-old male presents with symmetric, progressive weakness that is more profound in lower extremity than upper extremity. On physical exam, he can sit but cannot walk. He has tongue fasciculations and absent deep tendon reflexes. Serum creatine phosphokinase and aldolase levels are normal. Which of the following genetic mutations or deletions is responsible for his condition?

QID: 4915

Dystrophin gene

27%

(1521/5600)

PMP22 gene

6%

(322/5600)

Androgen receptor gene

1%

(76/5600)

SMN-1 gene

55%

(3100/5600)

Frataxin gene

10%

(540/5600)

L 4 B

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(OBQ12.233) A 3-month-old infant presents to your clinic with difficulty moving his extremities. The child had an uneventful prenatal course and birth, but since birth he has had poor head control and difficulty feeding. On physical exam, he is able to move all extremities, but he moves his upper extremities more than his lower extremities, and his hands more than his shoulders. He has no spasticity, but global weakness. The patient’s deep tendon reflexes are absent, but he has tongue fasciculations. What is the cellular pathology and prognosis for this patient?

QID: 4593

The patient has a nonprogressive injury to his brain, and he will likely require multiple orthopaedic surgeries in the future as a result of muscle imbalance

8%

(273/3263)

The patient has an absent dystrophin protein and will likely require a wheelchair by the age of 15 and will die of cardiorespiratory problems by the age of 20

16%

(525/3263)

The child will have a progressive loss of alpha-motor neurons in anterior horn of spinal cord. He will have difficulty walking, but will be able to sit independently and will likely live into the fifth decade of life

16%

(514/3263)

The child will have a progressive loss of alpha-motor neurons in anterior horn of spinal cord and will unlikely live past the age of two

58%

(1882/3263)

The child will likely go on to develop a cavus foot and hammer toes, but he should live a full healthy life

1%

(34/3263)

L 4 C

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(SAE07PE.40) A 7-year-old girl with spinal muscular atrophy (SMA) type II has popping of the left hip. Examination reveals painless subluxation of the joint in adduction with palpable reduction in abduction. Radiographs show coxa valga, subluxation of the left hip, and pelvic obliquity with elevation of the left hemipelvis. Treatment should consist of

QID: 6100

observation.

37%

(223/607)

bilateral adductor and iliopsoas releases, with nighttime abduction bracing.

9%

(57/607)

proximal femoral varus osteotomy with internal fixation.

12%

(74/607)

proximal femoral varus osteotomy with volume-reducing periacetabular osteotomy.

19%

(118/607)

proximal femoral varus osteotomy with shelf acetabular augmentation.

21%

(128/607)

N/A E

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(SBQ04PE.40) An autosomal recessive disorder caused by a homozoygous deletion in the survival motor neuron 1 (SMN1) gene would result in which of the following?

QID: 2225

Global hypotonia, difficulty swallowing, absent deep tendon reflexes, and tongue atrophy, with disease onset within the first 6 months of birth.

44%

(902/2040)

Delayed motor milestones, scoliosis, and joint contracture, with disease onset around 6-18 months of age

14%

(286/2040)

Scoliosis, hand tremors, increased lumbar lordosis, and bilateral Trendelenburg lurch, with disease onset later in life (>18 months of age).

6%

(131/2040)

Any of the above

30%

(602/2040)

None of the above

4%

(83/2040)

L 5 C

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Evidence (7)
VIDEOS & PODCASTS (4)
EXPERT COMMENTS (10)
Private Note