Progressive Motor Neuron Disease
SMA Classification
Critical Must-Knows
- SMN1 Gene: Survival motor neuron gene mutation.
- Four Types: Based on age of onset and motor milestones.
- Scoliosis: Universal in Type I/II, common in Type III.
- New Therapies: Gene therapy and antisense oligonucleotides.
- Respiratory Care: Primary cause of mortality.
Clinical Pearls
- "Know the SMA types and motor milestones
- "Scoliosis is nearly universal
- "Gene therapy has changed prognosis
- "Respiratory failure is the main cause of death
Gene Therapy Era
New disease-modifying therapies have dramatically changed the prognosis for SMA.
- Nusinersen (Spinraza): Antisense oligonucleotide. Intrathecal. Increases SMN2 expression.
- Onasemnogene (Zolgensma): Gene therapy. Single IV dose. Replaces SMN1.
- Early treatment (before symptom onset) leads to best outcomes.
- Orthopaedic manifestations may still occur but are less severe with early treatment.
SMA Types
| Type | Onset | Motor Milestones | Prognosis |
|---|---|---|---|
| Less than 6 months | Never sits | Death by 2 years without treatment | |
| 6-18 months | Sits, never walks | Survives to adulthood with support | |
| Greater than 18 months | Walks (may lose ability) | Normal or near-normal lifespan | |
| Adult onset | Walks | Mild, slowly progressive |
SMA TSMA Types
| I | Infant Never sits (Werdnig-Hoffmann) |
| II | Intermediate Sits, never walks |
| III | Late Walks (Kugelberg-Welander) |
| IV | Adult Adult onset, mild |
| I | Infant Never sits (Werdnig-Hoffmann) | III | Late Walks (Kugelberg-Welander) |
| II | Intermediate Sits, never walks | IV | Adult Adult onset, mild |
Hook:I-II-III-IV: Infant, Intermediate, Late, Adult.
SHCOrthopaedic Issues
| S | Scoliosis Nearly universal |
| H | Hip Dysplasia Common in Type I/II |
| C | Contractures Hips, knees, ankles |
| S | Scoliosis Nearly universal |
| H | Hip Dysplasia Common in Type I/II |
| C | Contractures Hips, knees, ankles |
Hook:SHC - Scoliosis, Hips, Contractures.
RSGTreatment Principles
| R | Respiratory Primary focus |
| S | Scoliosis Bracing then surgery |
| G | Gene Therapy Disease-modifying |
| R | Respiratory Primary focus |
| S | Scoliosis Bracing then surgery |
| G | Gene Therapy Disease-modifying |
Hook:RSG - Respiratory, Scoliosis, Gene therapy.
Overview/Epidemiology
Spinal Muscular Atrophy (SMA) is a progressive neuromuscular disorder.
- Genetics: Autosomal recessive. Mutation/deletion in SMN1 (Survival Motor Neuron 1) gene on chromosome 5q.
- Incidence: Approximately 1 in 11,000 live births (international consensus figure). Carrier frequency roughly 1 in 40-60.
- Pathophysiology: Degeneration of anterior horn cells in the spinal cord leads to progressive weakness.
- Historical Significance: Leading genetic cause of infant death (prior to gene therapy).
Pathophysiology
Spinal Cord Pathology
- Anterior horn cells (lower motor neurons) degenerate.
- Results in denervation of skeletal muscles.
- Proximal muscles affected more than distal.
- Lower limbs typically weaker than upper limbs.
Why Scoliosis Develops
- Trunk muscle weakness leads to spinal collapse.
- Gravity + asymmetric weakness = progressive curve.
- Usually thoracolumbar, long sweeping C-curve.
- Pelvic obliquity is common.
Classification Systems
SMA Type I (Werdnig-Hoffmann Disease)
- Onset: Birth to 6 months.
- Motor Milestones: Never achieves sitting.
- Features: Severe hypotonia, frog-leg posture, paradoxical breathing.
- Prognosis: Without treatment, death by age 2 (respiratory failure).
- With Gene Therapy: Dramatically improved survival.
Clinical Assessment
History:
- Age of symptom onset.
- Motor milestones achieved and lost.
- Family history.
- Respiratory status.
Physical Exam:
- Hypotonia: Floppy infant (Type I).
- Weakness: Proximal greater than distal. Symmetric.
- Reflexes: Absent or diminished.
- Fasciculations: Tongue fasciculations are characteristic.
- Spine: Assess for scoliosis.
- Hips: Assess for dysplasia, contractures.
Investigations
Genetic Testing:
- SMN1 gene deletion/mutation: Confirmatory.
- SMN2 copy number: Prognostic (more copies = milder phenotype).
EMG/Nerve Conduction:
- Denervation pattern (rarely needed now with genetic testing).
Pulmonary Function:
- FVC (forced vital capacity) monitoring.
Imaging:
- Spine X-ray: Scoliosis assessment.
- Hip X-ray: Dysplasia, subluxation.
Management Algorithm
Medical Management
- Gene Therapy (Onasemnogene/Zolgensma): Single IV dose in infants.
- Nusinersen (Spinraza): Intrathecal injections every 4 months.
- Risdiplam (Evrysdi): Oral SMN2 splicing modifier.
- Respiratory Support: BiPAP, cough assist, suctioning.
- Nutritional Support: Gastrostomy if swallowing impaired.
Surgical Techniques
Posterior Spinal Fusion
Indications: Progressive scoliosis greater than 40-50 degrees with adequate pulmonary function.
Technique: Posterior approach. Long fusion from upper thoracic to pelvis (typically T2-pelvis). Pelvic fixation (iliac screws or S2-alar-iliac screws) is essential due to pelvic obliquity. Use of hybrid constructs or all-screw constructs.
Considerations: High perioperative risk due to respiratory compromise. Anesthesia team experienced in neuromuscular scoliosis is essential.
Complications
| Complication | Context | Management |
|---|---|---|
| Respiratory Failure | Primary cause of death | BiPAP, ventilation, cough assist |
| Progressive Scoliosis | Universal in Type I/II | Bracing, surgery |
| Hip Dysplasia | Common in Type I/II | Usually observation |
| Contractures | Hips, knees, feet | Stretching, splinting, releases |
| Perioperative Complications | High risk for scoliosis surgery | Experienced team, ICU care |
Postoperative Care
- ICU Admission: For scoliosis surgery.
- Respiratory: May require prolonged ventilation.
- Mobilization: Early sitting in wheelchair.
- Pain Management: Multimodal.
- Long-Term: Bracing discontinued after fusion. Ongoing respiratory and nutritional support.
Outcomes/Prognosis
- Type I (Untreated): Death by 2 years.
- Type I (Gene Therapy): Many achieving motor milestones, survival dramatically improved.
- Type II: Survive to adulthood with respiratory support.
- Type III/IV: Normal or near-normal lifespan.
- Scoliosis Surgery: Improves sitting balance, quality of life, may improve respiratory function.
Evidence Base
- Single IV dose of AAV9-SMN gene therapy in 15 infants with SMA type 1
- All 15 alive and event-free at 20 months vs 8% survival in a historical cohort
- High-dose cohort: CHOP INTEND rose 15.4 points by 3 months; 11 of 12 sat unassisted, 2 walked
- Transient transaminitis attenuated by prednisolone
- Randomised, double-blind, sham-controlled phase 3 trial of intrathecal nusinersen in infantile-onset SMA
- Motor-milestone response 51% (37/73) vs 0% (0/37) in controls
- Hazard ratio for death or permanent ventilation 0.53 (P=0.005)
- Trial stopped early at interim analysis for efficacy
- Open-label trial of oral risdiplam in 41 infants with type 1 SMA (2 SMN2 copies)
- At 24 months, 44% (18/41) sat without support for at least 30 seconds vs a 5% natural-history threshold
- No infant achieved standing or independent walking
- Most common serious adverse events were pneumonia (39%) and respiratory distress
- Consensus update on diagnosis, rehabilitation, orthopaedic/spinal and nutritional care
- SMA incidence approximately 1 in 11,000 live births
- Spinal management framed by functional status (non-sitter / sitter / walker)
- Recommends growth-friendly instrumentation in young children and definitive fusion to pelvis once growth is adequate
- 22 SMA patients with curves over 100 degrees and pelvic obliquity over 20 degrees, one-stage PSF with pelvic fixation
- Independent sitting improved from 22.7% preoperatively to 77.3% postoperatively (P less than 0.001)
- Mean FVC remained stable; 50% improved
- Perioperative complications in 27.3% (pneumonia, epiglottic oedema, delayed wound healing); none needed reoperation
- 26 SMA type 2 patients: 17 magnetically controlled growing rods (MCGR), 9 posterior spinal fusion (PSF)
- Major curve correction 44% (MCGR) and 55% (PSF); pelvic obliquity under 5 degrees in all at follow-up
- Pelvic fixation by T-construct (2 sacral plus 2 iliac screws); lung function unchanged by either procedure
- Low complication rate; all patients improved sitting ability
- Modified PSF skipping a thoracolumbar level to preserve intrathecal access for nusinersen
- 8 patients, mean curve correction 35.2 degrees, 4-year follow-up
- No rod breakage or loss of correction; one revision for bony overgrowth at the skipped level
- Pulmonary function before and after PSF in neuromuscular disease
- All 6 SMA patients lost vital capacity (mean loss 0.63 L)
- Contrasts with merosin-deficient dystrophy, which gained VC
Viva Scenarios
Use these scenarios to practise clinical reasoning and management decisions
The Infant with SMA Type I and Scoliosis
"8-month-old with confirmed SMA Type I. Has received gene therapy. Now developing thoracolumbar scoliosis of 30 degrees."
This child has benefited from gene therapy but is still at risk for orthopaedic complications. At 30 degrees, I would observe closely. Bracing (TLSO) can be considered for positioning but is unlikely to prevent progression. Given the young age, if the curve progresses rapidly, **growing rod constructs (e.g., MAGEC)** may be considered to allow continued growth. However, any surgery has high perioperative risk. Close collaboration with neurology and respiratory teams is essential. I would monitor closely with serial X-rays every 4-6 months.
The Adolescent with SMA Type II and Severe Scoliosis
"14-year-old with SMA Type II. Wheelchair-dependent. Thoracolumbar scoliosis of 80 degrees with pelvic obliquity. FVC 35%."
This is a challenging case. The scoliosis is severe and the pulmonary function is significantly impaired. However, surgery may still be beneficial. I would assess sitting balance and quality of life. Preoperative optimization with the respiratory team (BiPAP optimization, cough assist) is critical. Surgery: **Posterior spinal fusion from T2 to pelvis** with pelvic fixation (S2-alar-iliac or iliac screws). Expect prolonged ICU stay and potential need for postoperative ventilation. The goal is improved sitting balance and halting curve progression. I would ensure informed consent reflects the high perioperative risk.
Hip Dysplasia in SMA
"Same patient also has bilateral hip subluxation on X-ray. Is this an indication for surgery?"
In SMA, **hip dysplasia is common but surgery is rarely indicated**. The hips are often asymptomatic even when subluxated. Surgery (reconstruction) has high failure rates in this population and the functional gains are minimal since the patient is not walking. I would observe and manage with seating adaptations if there is discomfort. Pain is rare but if present, salvage options like proximal femoral resection may be considered. I would not pursue reconstructive surgery.
MCQ Practice Points
Genetics MCQ
Q: What gene is mutated in SMA? A: SMN1 (Survival Motor Neuron 1) on chromosome 5q.
Classification MCQ
Q: A child who can sit but never walks has which SMA type? A: Type II.
Prognosis MCQ
Q: What correlates with disease severity? A: SMN2 copy number - more copies = milder phenotype.
Treatment MCQ
Q: What is the mechanism of nusinersen (Spinraza)? A: Antisense oligonucleotide that increases SMN2 expression.
Scoliosis Surgery
Q: What is the extent of fusion in SMA scoliosis surgery? A: T2 to pelvis with pelvic fixation (S2-alar-iliac or iliac screws).
Hip Management
Q: How should hip dysplasia in SMA be managed? A: Observation - surgery rarely indicated due to high failure rate and minimal functional benefit.
Guidelines, Registries & Global Practice
Global epidemiology
- Incidence approximately 1 in 11,000 live births; pan-ethnic, with carrier frequency roughly 1 in 40-60.
- SMN2 copy number is the principal genetic modifier worldwide: 2 copies predict the most severe phenotype, while 3 or more copies predict milder disease.
Major guidelines, side by side
| Body / Document | Scope | Key recommendation |
|---|---|---|
| International SMA Standard of Care (Mercuri 2018) | Diagnosis, orthopaedic, rehabilitation, nutrition | Manage by functional status (non-sitter/sitter/walker); growth-friendly instrumentation early, fusion to pelvis once growth adequate |
| Newborn-screening consensus (US RUSP, many EU programmes) | Pre-symptomatic detection | Population SMN1 screening enables treatment before motor-neuron loss; best outcomes when treated pre-symptomatically |
| Cure SMA / neuromuscular society guidance | Disease-modifying therapy | Three approved DMTs (onasemnogene, nusinersen, risdiplam); choice driven by age, weight, SMN2 copies and access |
| Spine-deformity consensus (neuromuscular scoliosis) | Surgical timing | Long fusion to pelvis for sitters; preserve intrathecal access if continuing nusinersen |
Registries & access
- No single global SMA implant registry; outcome data come from national neuromuscular registries (e.g. TREAT-NMD, SMArtCARE in Europe, Cure SMA in North America) and tertiary-centre series.
- Newborn screening for SMA is now established across much of North America, Europe and parts of Asia-Pacific, but coverage remains uneven globally.
High- vs limited-resource practice variation
- High-resource settings: newborn screening, early disease-modifying therapy, MAGEC/growth-friendly constructs, intrathecal-access-preserving fusion, ICU-supported neuromuscular spine surgery.
- Limited-resource settings: diagnosis often clinical or delayed; disease-modifying drugs frequently unaffordable; management centres on respiratory support, seating, contracture care, and fusion where safe perioperative care exists.
SPINAL MUSCULAR ATROPHY
Clinical summary
GENETICS
- •SMN1 Mutation
- •Autosomal Recessive
- •SMN2 Copies = Prognosis
- •Chromosome 5q
TYPES
- •I: Never sits
- •II: Sits, never walks
- •III: Walks
- •IV: Adult onset
ORTHO ISSUES
- •Scoliosis (universal)
- •Hip Dysplasia
- •Contractures
- •Pelvic Obliquity
SCOLIOSIS SURGERY
- •T2 to Pelvis Fusion
- •Pelvic Fixation Essential
- •High Perioperative Risk
- •ICU Postop Care
MEDICAL THERAPY
- •Onasemnogene (Gene Therapy)
- •Nusinersen (Intrathecal)
- •Risdiplam (Oral)
- •Early Treatment Best
EXAM PEARLS
- •Tongue Fasciculations
- •100% Scoliosis in Type I/II
- •Hip Surgery Rarely Indicated
- •FVC Monitoring Critical
Self-Assessment Quiz
Differential Diagnosis
Distinguishing SMA from Other Causes of Childhood Weakness
| Condition | Inheritance / Cause | Discriminating Features |
|---|---|---|
| Autosomal recessive, SMN1 deletion (5q) | Tongue fasciculations, absent reflexes, normal/mildly raised CK, SMN1 testing confirmatory | |
| X-linked, dystrophin gene | Boys, markedly elevated CK, calf pseudohypertrophy, Gowers sign, preserved early reflexes | |
| Various (e.g. nemaline, central core) | Static or slowly progressive, normal/mildly raised CK, characteristic muscle biopsy/genetics | |
| Autosomal dominant, DMPK repeat (maternal) | Facial diplegia, myotonia, maternal symptoms, polyhydramnios history | |
| Tumour, syrinx, cord compression | Sensory level, asymmetry, sphincter involvement; MRI differentiates | |
| Enzyme deficiency (e.g. Pompe) | Exercise intolerance, cardiomyopathy (Pompe), specific enzyme/genetic assays |
Key Differentiators for SMA: SMN1 genetic testing is confirmatory; tongue fasciculations are characteristic; CK is normal or only mildly elevated; reflexes are absent or diminished.
Controversies & Areas of Uncertainty
- Spinal surgery in the gene-therapy era: Disease-modifying therapy is reducing curve severity in early-treated children, but it is unclear whether it will eliminate the need for fusion or merely delay it. Long-term natural history of the treated cohort is still being defined.
- Timing of fusion vs growth-friendly constructs: When to convert magnetically controlled growing rods to definitive fusion, and whether early definitive fusion in older children is preferable, remains debated; both strategies show durable correction in current series.
- Preserving intrathecal access: Nusinersen requires repeated intrathecal injection, so "skip" or fenestrated constructs and laminotomy windows are used to keep access; the optimal technique and whether oral risdiplam removes this constraint are unresolved.
- Pulmonary outcome of fusion: Fusion reliably improves sitting and halts deformity, but vital capacity often falls rather than improves after surgery — the respiratory benefit is mainly preservation, not gain.
- Lower FVC threshold for surgery: There is no universally agreed FVC cut-off; very low FVC raises perioperative risk but is not an absolute contraindication when seating and quality of life are at stake.
- Hip management: Whether asymptomatic hip subluxation/dislocation ever warrants reconstruction in non-ambulators is contested; most centres observe because reconstruction has high redislocation rates and limited functional gain.