Symptoms of Acute Spinal Muscular Atrophy (SMA)
Acute SMA presents with rapidly progressive bilateral proximal muscle weakness, respiratory muscle involvement with preserved diaphragm function, impaired cough with poor airway clearance, and recurrent pulmonary infections requiring immediate respiratory monitoring and support. 1, 2, 3
Clinical Presentation
Primary Neuromuscular Symptoms
- Progressive bilateral proximal muscle weakness starting in the lower extremities and ascending to involve upper extremities, trunk, and facial muscles 1, 4
- Intercostal muscle weakness with relatively spared diaphragm, creating a characteristic respiratory pattern 3
- Absent or markedly decreased deep tendon reflexes in affected limbs 5
- Muscle atrophy that is progressive and symmetrical 6, 4
Respiratory Manifestations
- Impaired cough with inability to clear airway secretions, the primary cause of acute respiratory failure 2, 3
- Recurrent pulmonary infections and aspiration due to poor airway clearance 1, 2
- Nocturnal hypoventilation progressing to daytime respiratory failure 3
- Restrictive lung disease from poor chest wall and lung growth 3
- Acute respiratory failure requiring mechanical ventilation, occurring in approximately 50% of hospitalized patients 2
Bulbar and Cranial Nerve Involvement
- Dysphagia and feeding difficulties leading to aspiration risk 1
- Dysarthria and facial weakness in more severe presentations 5
- Tongue fasciculations may be observed 4
Age-Specific Presentations
Infantile-Onset (Type 1):
- Symptom onset between 28 days and 3 months of age 1
- Inability to achieve sitting without support 1
- Severe respiratory involvement with high mortality if untreated 1, 6
- Median time from symptom onset to diagnosis approximately 3.7 months 1
Later-Onset (Type 2):
- Symptom onset after 6 months of age 4
- Ability to sit but never achieve independent ambulation 4
- Progressive respiratory muscle weakness requiring monitoring 2, 3
Critical Assessment Parameters
Immediate Respiratory Evaluation
- Measure negative inspiratory force (NIF) and vital capacity (VC) immediately upon presentation to assess for impending respiratory failure 5, 7
- Monitor for signs of respiratory distress: tachypnea, use of accessory muscles, paradoxical breathing 2, 3
- Assess peak cough flow to determine need for cough-assist techniques 3
Functional Motor Assessment
- Inability to sit without support for at least 5 seconds (Bayley Scales Item 22) indicates severe Type 1 disease 1
- Progressive loss of previously achieved motor milestones 1, 4
- Symmetrical weakness pattern distinguishing SMA from other neuromuscular conditions 5
Autonomic Function
- Preserved autonomic function distinguishes SMA from Guillain-Barré syndrome 8, 5
- Normal sensation differentiates from peripheral neuropathies 5
Key Distinguishing Features from Other Neuromuscular Disorders
Unlike Guillain-Barré Syndrome:
- SMA does not present with ascending paresthesias or sensory loss 8, 5
- SMA lacks autonomic instability (blood pressure fluctuations, heart rate abnormalities) 8, 5
- SMA has chronic/subacute onset rather than acute progression over days 8
Unlike Myasthenia Gravis:
- SMA weakness does not fluctuate with activity 5
- SMA does not typically present with isolated ocular symptoms initially 5
Unlike Inflammatory Myopathy:
- SMA lacks muscle pain or tenderness 5
- SMA does not have elevated inflammatory markers (ESR, CRP) 5, 7
Common Pitfalls
Delayed recognition of respiratory failure is the most critical error, as respiratory complications are the primary cause of mortality in SMA 1, 2, 3. Patients may appear stable with adequate oxygenation on pulse oximetry while developing progressive hypercapnia from hypoventilation 3.
Failure to initiate early airway clearance techniques when respiratory muscle strength falls below 50% of predicted leads to preventable respiratory infections and acute decompensation 3.
Underestimating the need for ICU-level monitoring in acute presentations, as 49.6% of patients with acute respiratory failure require endotracheal intubation 2.