Overview
Definition:
Spinal Muscular Atrophy (SMA) is a group of inherited neuromuscular disorders characterized by the degeneration of motor neurons in the spinal cord and brainstem
This leads to progressive muscle weakness and atrophy
While primarily a neurological condition, the severe muscle weakness and skeletal deformities, particularly scoliosis, necessitate significant surgical management for injury control and functional restoration.
Epidemiology:
SMA affects approximately 1 in 10,000 live births
It is one of the most common rare genetic diseases
SMA Type 1 (Werdnig-Hoffmann disease) is the most severe form, with onset within the first six months of life
Types 2, 3, and 4 have later onset and varying degrees of severity
The genetic defect is typically a homozygous deletion of the SMN1 gene on chromosome 5q.
Clinical Significance:
SMA poses significant challenges in patient care, requiring a multidisciplinary approach
For surgical trainees, understanding the management of severe scoliosis, contractures, and the implications of neuromuscular weakness on surgical outcomes is crucial
Effective injury control in the context of SMA focuses on preventing further skeletal deterioration and managing complications arising from immobility and weakness.
Clinical Presentation
Symptoms:
Progressive muscle weakness, primarily affecting proximal muscles
Decreased muscle tone (hypotonia)
Fasciculations of the tongue and limbs
Difficulty with movement, swallowing, and breathing
In severe forms, infants may present with joint contractures and respiratory failure
Skeletal deformities like scoliosis and hip dislocation are common due to muscle imbalance and weakness.
Signs:
Profound muscle weakness
Absent or diminished deep tendon reflexes
Fasciculations
Respiratory distress
Nutritional deficiencies
Severe, progressive scoliosis
Hip subluxation or dislocation
Joint contractures
Bell-shaped chest may be seen in infants with severe respiratory muscle weakness.
Diagnostic Criteria:
Diagnosis is based on clinical presentation and confirmed by genetic testing showing a homozygous deletion of the SMN1 gene
Electromyography (EMG) can demonstrate denervation in affected muscles
Pulmonary function tests are essential for assessing respiratory capacity.
Diagnostic Approach
History Taking:
Detailed family history for neuromuscular disorders
Age of onset of weakness
Progression of weakness
Specific functional limitations (walking, feeding, breathing)
History of falls or fractures
Previous surgical interventions
Respiratory symptoms or infections.
Physical Examination:
Comprehensive neurological examination focusing on muscle strength, tone, and reflexes
Assessment of cranial nerve function
Examination of respiratory effort and lung auscultation
Detailed musculoskeletal examination including assessment of range of motion, joint alignment, and presence of contractures
Evaluation of spinal curvature and pelvic stability.
Investigations:
Genetic testing for SMN1 deletion (gold standard)
EMG and nerve conduction studies to assess motor neuron integrity
Pulmonary function tests (spirometry, peak flow) to evaluate respiratory capacity
Plain radiographs of the spine to assess scoliosis severity and progression
Hip X-rays to evaluate for dislocations
Swallow studies (videofluoroscopy) for feeding assessment.
Differential Diagnosis:
Other motor neuron diseases (e.g., infantile spinal muscular atrophy with respiratory distress)
Congenital myopathies
Muscular dystrophies (e.g., Duchenne muscular dystrophy)
Congenital myotonic dystrophy
Spinal cord lesions
Polymyositis.
Management
Initial Management:
Multidisciplinary team approach involving neurologists, geneticists, pulmonologists, orthopaedic surgeons, physiotherapists, occupational therapists, and dietitians
Supportive care to maximize function and quality of life
Respiratory support (e.g., non-invasive ventilation, cough assist devices)
Nutritional support and feeding interventions.
Medical Management:
Emerging gene-replacement therapies (e.g., nusinersen, onasemnogene abeparvovec) have revolutionized SMA treatment by increasing SMN protein levels
These therapies aim to halt or slow disease progression but do not reverse established neuromuscular damage
Regular pulmonary toilet and vaccinations are vital
Management of infections.
Surgical Management:
Surgical interventions are primarily aimed at addressing skeletal deformities and improving function
This includes: Spinal fusion for severe scoliosis (typically >40-50 degrees or rapidly progressive) to prevent respiratory compromise and improve sitting posture
Corrective osteotomies for severe hip dislocation or contractures
Tendon lengthening procedures for contractures causing significant functional impairment
Placement of gastrostomy tubes for feeding support
Tracheostomy for severe chronic respiratory failure
Spinal instrumentation surgery requires careful preoperative optimization of respiratory function and postoperative intensive care due to risks of prolonged ventilation and infection.
Supportive Care:
Aggressive physiotherapy to maintain range of motion and prevent contractures
Occupational therapy for adaptive equipment and strategies for daily living
Nutritional counseling and support to prevent malnutrition and aspiration
Psychological support for patients and families
Regular monitoring of respiratory function and growth
Pain management.
Complications
Early Complications:
Postoperative pneumonia and other respiratory infections
Wound infections
Deep vein thrombosis (DVT) and pulmonary embolism (PE)
Mechanical failure of spinal instrumentation
Pressure sores
Anesthesia-related complications due to compromised respiratory mechanics.
Late Complications:
Failure of spinal fusion
Progression of scoliosis above or below instrumentation
Chronic respiratory insufficiency
Recurrent pneumonia
Joint contractures
Osteoporosis
Malnutrition
Psychosocial issues
Need for revision surgeries.
Prevention Strategies:
Meticulous preoperative assessment and optimization of respiratory and nutritional status
Strict sterile surgical techniques
Prophylactic antibiotics and anticoagulation
Judicious use of spinal instrumentation with appropriate rod contouring and vertebral body purchase
Aggressive postoperative physiotherapy and respiratory care
Regular follow-up imaging and clinical assessments.
Prognosis
Factors Affecting Prognosis:
Age of onset and severity of SMA (Type 1 vs
later onset)
Response to gene-replacement therapies
Degree of scoliosis and impact on pulmonary function
Presence of comorbidities
Quality of multidisciplinary supportive care
Surgical outcomes for scoliosis and other interventions.
Outcomes:
With recent therapeutic advancements, the prognosis for individuals with SMA has significantly improved, with many living longer and achieving greater functional independence
However, severe neuromuscular weakness and skeletal deformities remain challenges
Surgical interventions aim to improve quality of life and prevent life-limiting complications, but do not cure the underlying genetic disorder.
Follow Up:
Lifelong multidisciplinary follow-up is essential
This includes regular neurological assessments, pulmonary function monitoring, skeletal imaging (especially for scoliosis progression), nutritional status evaluation, and functional assessments
Adjustments to supportive care and potential surgical interventions are made based on the patient's evolving needs.
Key Points
Exam Focus:
Key genetic defect (SMN1 deletion)
Differentiating SMA types by onset and severity
Understanding the role of gene-replacement therapies (nusinersen, onasemnogene abeparvovec)
Indications for surgical intervention in SMA, particularly spinal fusion for scoliosis in the context of pulmonary compromise
Preoperative optimization and postoperative management of SMA patients undergoing major surgery
Risk stratification for surgical complications.
Clinical Pearls:
In SMA patients with scoliosis, consider surgery when curves exceed 40-50 degrees or progress rapidly, as this can severely impact lung function
Pulmonary function testing is paramount before any major surgery
Aggressive respiratory support (BiPAP, cough assist) is vital postoperatively
Recognize that fasciculations are common and not necessarily indicative of disease progression in all cases
Always consider a multidisciplinary team for holistic patient care.
Common Mistakes:
Underestimating the respiratory risks in SMA patients undergoing surgery
Delaying surgical intervention for scoliosis until pulmonary function is severely compromised
Inadequate preoperative optimization of respiratory status
Insufficient postoperative respiratory support
Focusing solely on the neurological aspect without addressing skeletal deformities and their impact on overall health.