Spinal Muscular Atrophy

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).

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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.

The topic repeatedly invokes "SMN2 copy number predicts severity," "nusinersen increases SMN2 expression," and "risdiplam is an SMN2 splicing modifier" — but these only make sense once the underlying gene biology is explicit.

Two genes, one protein

• SMN1 is the primary gene producing full-length, functional Survival Motor Neuron (SMN) protein. Homozygous loss/mutation of SMN1 causes SMA.
• SMN2 is a near-identical paralogue on the same chromosome 5q region. It differs from SMN1 by a single translationally-silent C-to-T nucleotide change within exon 7.
• That single base change disrupts an exonic splicing enhancer, so most SMN2 transcripts skip exon 7 and produce a truncated, rapidly-degraded protein (SMNΔ7). Only roughly 10 to 15 percent of SMN2 output is full-length functional SMN.

Why copy number is prognostic

• SMN2 is present in variable copy number (1 to 4+ copies). Each extra copy adds a little more full-length SMN, partially compensating for the absent SMN1.
• More SMN2 copies therefore correlate with a milder phenotype: typically 1 to 2 copies in Type 1, 3 copies in Type 2, and 3 to 4 copies in Type 3. (Copy number predicts severity at a population level but is not an absolute individual guarantee.)

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.

The surgical sections repeatedly flag "high perioperative risk," an "experienced anaesthesia team," respiratory optimisation, ICU and prolonged ventilation — but the specific principles that drive those statements deserve to be explicit, because in SMA the airway and ventilation, not the spine, are what threaten the patient.

Respiratory — the dominant concern

• Restrictive lung disease from intercostal weakness gives a low and falling forced vital capacity (FVC); a low pre-operative FVC predicts post-operative ventilatory dependence.
• Pre-operative optimisation: treat chest infection, nutritional support, and training with non-invasive ventilation (NIV/BiPAP) and mechanical cough-assist before surgery.
• Plan for elective post-operative NIV and aggressive secretion clearance rather than reactive re-intubation; protocolised extubation directly onto NIV reduces re-intubation after scoliosis fusion.

Airway, drugs and monitoring

• Bulbar weakness and impaired cough mean a real aspiration risk — careful airway protection and secretion management.
• Avoid depolarising neuromuscular blockers (suxamethonium): denervated/atrophic muscle carries up-regulated extrajunctional acetylcholine receptors and risks life-threatening hyperkalaemia.
• Expect heightened and prolonged sensitivity to non-depolarising relaxants and to opioids/sedatives — titrate carefully, use quantitative neuromuscular monitoring, and ensure full reversal.
• Long instrumented fusions carry significant blood loss; plan cross-match, cell salvage and neuromonitoring, and anticipate intensive-care recovery.

• 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.

• 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.

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

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 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.