Spinal Muscular Atrophy

Question 1

What is Spinal Muscular Atrophy?

Answer 1

The Spinal Muscular Atrophies are a group of genetically-inherited, degenerative disorders of motor neurons. They affect all motor neurons with the exception of those controlling the extra-ocular muscles (i.e. CN III, IV, VI) and urethral and anal sphincters. Upper motor neurons in the cerebral cortex are also unaffected.

Reference: Nelson’s Paediatrics [20th Edition], p. 2996.

Question 2

How common is Spinal Muscular Atrophy?

Answer 2

Spinal Muscular Atrophy is the second commonent neuromuscular disease, affecting 10-15 per 100,000 live births.

Reference: Nelson’s Paediatrics [20th Edition], p. 2998.

Question 3

How is Spinal Muscular Atrophy classified?

Answer 3

There are 3 main classifications of SMA:

SMA Type 1 (Werdnig-Hoffmann Disease)

Infantile, Severe

~25%

SMA Type 2

Late Infantile, Slowly Progressive

~50%

SMA Type 3 (Kugelberg-Welander Disease)

Chronic

~ 25%

There are further variants in addition to these classifications.

Question 4

What causes Spinal Muscular Atrophy?

Answer 4

SMA is a genetically-inherited condition, following an autosomal recessive pattern.

The aetiology relates to continued embryonal apoptotic processes. The gene responsible for this is the survivor motor neuron gene (SMN).

Reference: Nelson’s Paediatrics [20th Edition], p. 2996.

Question 5

How does SMA Type 1 present?

Answer 5

SMA Type 1

Severe hypotonia, generalised weakness, thin muscle mass, absent tendon stretch reflexes; involvment of tongue, face, and jaw muscles; sparing of extraocular muscles and sphincters.

Unable to feed

Respiratory distress

Congenital contractures (arthrogryposis) resulting from poor tone in utero

Little movement

Note: SMA Type 1 presents from birth-6 months; most have symptoms within the first 3 months of life. Around two-thirds are floppy at birth. Around one-third was noted to have decreased movements in the womb.

Reference: Nelson’s Paediatrics [20th Edition], p. 2997; PasTest.

Question 6

How does SMA Type 2 present?

Answer 6

SMA Type 2

Early

Usually able to suck/swallow

Respiration is satisfactory in early years

Mid

Progressive weakness

Nasal speech

Swallowing problems

Late

Scoliosis

Gastro-oesophageal reflux, malnutrition, and aspiration

Reference: Nelson’s Paediatrics [20th Edition], p. 2997.

Question 7

How does SMA Type 3 present?

Answer 7

SMA Type 3

May seem normal in infancy

Progressive weakness proximally

Ambulatory

Muscular hypertrophy (might confuse with muscular dystrophies)

Fasciculations (evidence of denervation)

Myalgia

Question 8

How is Spinal Muscular Atrophy investigated?

Answer 8

Creatinine Kinase

Normal in SMA 1. May be mildly elevated in SMA 2/3

Chest X-Ray

Thin ribs in early-onset disease

Motor Nerve Conduction

Normal, except mild slowing in advanced disease

(distinguishing feature from peripheral neuropathy)

EMG

Fibrillation and signs of muscle denervation

Molecular Genetics

SMN gene on Chromosome 5 (5q11)

Lumbar Puncture

Normal

Question 9

What is the prognosis in Spinal Muscular Atrophy?

Answer 9

SMA Type 1

>65% die by 2 years of age

Most die secondary to respiratory infections

Question 10

What effect does SMA have upon intelligence?

Answer 10

Spinal Muscular Atrophy does not affect intelligence; however, given their physical impairment, efforts are often put into intellectual development.

Question 11

How is Spinal Muscular Atrophy managed?

Answer 11

There is no cure. Management is, therefore, supportive.

Orthopaedic and Physiotherapy Input

For scoliosis and contractures

Non-Invasive Ventilation