Neurology · MU Sofia

72. Spinal muscular atrophy

Explains spinal muscular atrophy as a genetic lower motor neuron disorder, covering its autosomal recessive inheritance and SMN1 gene basis, the loss of anterior horn cells, and the four clinical types from infantile to adult onset. Also outlines clinical features with sensory sparing, diagnostic tests including genetic testing and EMG, differential diagnoses, disease-modifying and supportive treatments, and prognosis by type.

Inside: 1 section · 16 flashcards · 2 min

Topic 72 of 88 in the MU Sofia syllabus for Neurology (Medicine, Year 4). The free AceMed note covers it in 1 section, about 347 words (2 min of reading), with 16 flashcards.

In this note

  1. Spinal muscular atrophy

Spinal muscular atrophy

Definition: Spinal muscular atrophy (SMA) is a group of genetic lower motor neuron disorders characterized by degeneration of anterior horn cells of the spinal cord, leading to progressive muscle weakness and atrophy.

Etiology & Genetics

  • Autosomal recessive inheritance
  • Most common cause: mutation/deletion of SMN1 gene (Survival Motor Neuron 1) on chromosome 5q
  • Reduced SMN protein → motor neuron degeneration

Pathogenesis

  • Degeneration of anterior horn cells
  • Loss of lower motor neurons → denervation of skeletal muscles
  • Muscle atrophy and weakness without sensory involvement

Classification (Clinical Types)

Type I – Werdnig–Hoffmann Disease (Severe infantile form)

  • Onset: <6 months
  • Severe hypotonia (“floppy infant”)
  • Inability to sit or crawl
  • Weak cry, poor sucking
  • Respiratory failure early

Type II – Intermediate form

  • Onset: 6–18 months
  • Can sit but cannot stand/walk
  • Progressive proximal muscle weakness
  • Scoliosis common

Type III – Kugelberg–Welander Disease (mild juvenile form)

  • Onset: after 18 months/childhood
  • Can stand and walk initially
  • Progressive proximal weakness later
  • Slow progression

Type IV – Adult form

  • Onset: adulthood
  • Mild proximal weakness
  • Slow progression

Clinical Features

Motor System: Symmetrical proximal muscle weakness (legs > arms in many cases). Muscle atrophy. Fasciculations (especially tongue in infants). Hypotonia.

Reflexes: Reduced or absent deep tendon reflexes

Sensory System: Normal sensation (important exam point).

Respiratory System: Weak respiratory muscles → recurrent infections → respiratory failure (especially Type I)

Examination Findings: “Floppy infant” posture (Type I). Weak cry. Tongue fasciculations. Muscle wasting. Hyporeflexia/areflexia. Preserved cognition

Diagnosis

  • Genetic Testing (Gold Standard): Detection of SMN1 gene deletion/mutation
  • Electromyography (EMG): Denervation potentials. Fibrillation potentials. Fasciculations
  • Nerve Conduction Studies: Normal sensory conduction. Motor abnormalities

Clinical Criteria: Progressive LMN weakness in childhood/adulthood

Differential Diagnosis

  1. 1. Muscular Dystrophy: Myopathic pattern (not neurogenic). No fasciculations
  2. 2. Cerebral Palsy: Upper motor neuron signs. Non-progressive
  3. 3. ALS: UMN + LMN signs (SMA = only LMN)
  4. 4. Peripheral Neuropathy: Sensory involvement present

Treatment

Disease-Modifying Therapy: Nusinersen (antisense oligonucleotide). Onasemnogene abeparvovec (gene therapy). Risdiplam (SMN2 splicing modifier)

Supportive Care: Physiotherapy. Respiratory support. Nutritional support. Orthopedic management (scoliosis)

Prognosis

Type I: poor prognosis (early death if untreated)

Type II: survival into childhood/adolescence

Type III: near-normal life expectancy

Type IV: mild adult form

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