Spinal muscular atrophy

1. High-Yield Summary

• Molecular Etiology and Genetic Spectrum: Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular condition caused by homozygous deletion or loss-of-function mutation of the survival motor neuron 1 (SMN1) gene on chromosome 5q13, with phenotypic severity inversely modulated by the copy number of the centromeric paralog SMN2.
• Pathophysiology and Musculoskeletal Collapse: Progressive degeneration of alpha motor neurons in the anterior horn of the spinal cord produces symmetric, proximal-greater-than-distal flaccid muscle weakness and areflexia, precipitating severe collapsing neuromuscular scoliosis in 60% to 90% of non-ambulatory individuals alongside progressive paralytic hip subluxation and dislocation.
• Therapeutic Paradigm Shift: The introduction of disease-modifying therapies (DMTs)—including antisense oligonucleotide nusinersen, oral small-molecule splicing modifier risdiplam, adeno-associated virus (AAV9) gene therapy onasemnogene abeparvovec, and myostatin inhibitors such as apitegromab—has dramatically improved motor milestone acquisition and transformed previously fatal infantile presentations into chronic conditions requiring proactive orthopaedic deformity management.
• Spine Deformity Management: Progressive curves exceeding 50 degrees Cobb angle or causing progressive sitting imbalance require surgical intervention: distraction-based growing constructs (including magnetically controlled growing rods [MCGR]) in young children under 8 to 10 years of age, or definitive posterior spinal instrumentation and fusion from T2 to the pelvis (using S2-alar-iliac [S2AI] or iliac screws) in older children, with mandatory preservation of a dedicated laminar fenestration window for ongoing intrathecal pharmacotherapy.
• Hip Surveillance and Conservative Restraint: Paralytic hip subluxation and dislocation occur secondary to persistent infantile coxa valga, acetabular dysplasia, and abductor-adductor muscular imbalance; surgical reconstruction (varus derotation osteotomy [VDRO] with or without pelvic osteotomy) is strictly reserved for painful subluxation or functional preservation in ambulators, whereas painless dislocations in non-ambulators are managed expectantly with custom seating to avoid joint stiffness and recurrence.

2. Anatomy

Understanding the surgical and clinical anatomy of spinal muscular atrophy requires precision regarding anterior horn cell distribution, motor unit degeneration, paraspinous postural failure, and critical neurovascular danger zones encountered during spinal and pelvic reconstruction.

Spinal Cord and Anterior Horn Architecture: Alpha motor neurons reside within Rexed lamina IX of the anterior grey column of the spinal cord. In SMA, deficiency of functional SMN protein triggers selective apoptosis of these primary motor neurons. Large myelinated motor axons undergo secondary Wallerian degeneration, depleting motor units and producing neurogenic muscle atrophy across skeletal muscle groups. Notably, the sensory pathways (dorsal root ganglia, posterior columns, spinothalamic tracts) and autonomic nervous system (intermediolateral cell column) are entirely preserved, explaining why children with SMA retain normal cutaneous sensation, intact bowel and bladder continence, and preserved cognitive capacity.

Paraspinous Musculature and Thoracopelvic Biomechanics: Spinal stability against gravity depends on a functioning dynamic cylinder formed by the intrinsic paraspinal musculature (erector spinae, multifidus, rotatores) and the anterior abdominal wall (rectus abdominis, transversus abdominis, internal and external obliques). In SMA, denervation atrophy of these core muscles converts the axial skeleton into an unstable collapsing column. Gravitational loading on the paretic trunk induces long, sweeping C-shaped thoracolumbar neuromuscular curves. The lower ribs tilt obliquely, descending toward the iliac crests to produce severe iliocostal impingement and mechanical compression of abdominal viscera.

Surgical Danger Zones in Spinal Deformity Correction:

• S2-Alar-Iliac (S2AI) Screw Corridor: The S2AI starting point is located approximately 1 mm inferior and 1 mm lateral to the S1 dorsal sacral foramen. The trajectory angles approximately 40 degrees laterally in the transverse plane and 20 to 30 degrees caudally in the sagittal plane, aiming across the sacroiliac joint into the thick bone of the anterior iliac column superior to the greater sciatic notch. Cortical breach anteriorly risks laceration of the internal iliac vessels and injury to the lumbosacral trunk (L4-L5 nerve roots) crossing anterior to the sacral ala. An inferior cortical breach into the greater sciatic notch threatens the superior gluteal artery and nerve, risking massive intrapelvic hemorrhage and paralysis of residual abductor function.
• Traditional Iliac Screw Corridor: The entry site at the posterior superior iliac spine (PSIS) requires extensive soft-tissue stripping and bone resection to recess the implant head beneath the iliac crest. In cachectic children with SMA possessing minimal subcutaneous adipose tissue, unrecessed screw heads cause skin breakdown and hardware extrusion. Medial breach enters the greater sciatic notch, while lateral breach penetrates the outer table into the gluteal musculature.
• Segmental Vessels and Spinal Cord Perfusion: Paired segmental intercostal and lumbar arteries traverse the anterolateral mid-waist of each vertebral body. Extensive subperiosteal exposure or anterior releases risk profuse bleeding that can be poorly tolerated given the restricted circulating blood volume of pediatric patients, while aggressive ligation may compromise watershed radiculomedullary arterial feeders (such as the artery of Adamkiewicz).
• Severe Trabecular Osteopenia: Due to chronic absence of mechanical axial loading and muscle traction, bone mineral density in SMA is profoundly reduced. Pedicle cortices are thin and brittle, cancellous bone is hypodense, and the laminae are paper-thin. These structural deficits drastically lower the pullout strength of pedicle screws and increase the risk of cut-through during deformity reduction maneuvers.

Internervous Planes and Danger Zones in Hip Reconstruction:

• Anterior Approach to the Hip (Smith-Petersen): Utilizes the internervous plane between the femoral nerve (sartorius, rectus femoris) and the superior gluteal nerve (tensor fasciae latae). The lateral femoral cutaneous nerve crosses the sartorius 1 to 2 cm distal to the ASIS and is vulnerable to traction injury. The ascending branch of the lateral femoral circumflex artery must be identified and ligated within the intermuscular interval.
• Anterior Minimally Invasive Approach (Hueter): Exploits the interval between the tensor fasciae latae (superior gluteal nerve) and rectus femoris (femoral nerve). This approach avoids detachment of the gluteus medius, an essential consideration for preserving any residual dynamic hip abduction strength.
• Medial Approach to the Hip (Ludloff): Utilizes the internervous plane between the femoral nerve (pectineus) and the obturator nerve (adductor longus and brevis) for direct access to the lesser trochanter and psoas tendon. The critical structure at risk is the medial femoral circumflex artery, which courses immediately posterior to the psoas tendon near the inferior capsule and must be scrupulously protected to prevent avascular necrosis of the femoral head.
• Lower Extremity Contracture Releases: Fractional lengthening of the hamstrings requires identification of the sciatic nerve coursing lateral to the semimembranosus and semitendinosus. Percutaneous Achilles tendon lengthening (TAL) places the sural nerve at risk posterolaterally and the tibial neurovascular bundle at risk anteromedially.

3. Pathogenesis

Molecular Genetics and SMN Protein Biology: Spinal muscular atrophy is an autosomal recessive disorder linked to the 5q13.2 chromosomal region. This locus contains an inverted genomic duplication containing two highly homologous genes: SMN1 (telomeric copy) and SMN2 (centromeric copy). In more than 95% of affected individuals, SMA is caused by a homozygous deletion of exon 7 in SMN1; the remaining 2% to 5% harbor a point mutation or microdeletion on one allele paired with an exon 7 deletion on the other.

The SMN2 gene differs from SMN1 by a critical, translationally silent C-to-T transition in exon 7 (c.840C>T). This single nucleotide alteration disrupts an exonic splicing enhancer recognized by ASF/SF2 and introduces an exonic splicing silencer bound by hnRNP A1. Consequently, alternative pre-mRNA splicing results in exon 7 skipping in approximately 85% to 90% of SMN2 transcripts. The resulting truncated protein, termed SMNΔ7, is structurally unstable and rapidly degraded by the ubiquitin-proteasome pathway. Only 10% to 15% of SMN2 transcripts produce full-length, functional SMN protein. The clinical severity of SMA is therefore directly modulated by SMN2 gene copy number: patients with 1 or 2 copies present with severe infantile-onset phenotypes (Types 0 and 1), whereas 3 copies typically correspond to Type 2 or mild Type 3, and 4 or more copies associate with Type 3b or Type 4 disease.

Cellular Pathology and Motor Unit Degeneration: The full-length SMN protein forms an essential multiprotein complex (including Gemins 2-8 and Unrip) that mediates the assembly of small nuclear ribonucleoproteins (snRNPs), the functional core of the spliceosome. SMN deficiency leads to widespread, cell-type-specific defects in pre-mRNA splicing, mitochondrial dysfunction, impaired axonal mRNA transport, and loss of neuromuscular junction (NMJ) structural integrity. Alpha motor neurons within the anterior horn of the spinal cord demonstrate heightened vulnerability to SMN depletion, undergoing progressive apoptosis. Motor unit loss triggers extensive denervation atrophy of skeletal muscle fibers, characterized histologically by large groups of atrophic type 1 and type 2 fibers interspersed with hypertrophic re-innervated type 1 fibers.

Biomechanical Cascade to Musculoskeletal Deformity:

• Paradoxical Diaphragmatic Respiration: The phrenic motor neuron pool (C3-C5) is relatively spared in early SMA, preserving diaphragm contractile excursion. In contrast, the intercostal muscles supplied by thoracic anterior horn cells suffer profound early denervation paresis. During inspiration, descent of the diaphragm against a flaccid intercostal cage generates negative intrathoracic pressure that pulls the ribs inward rather than outward. This paradoxical motion produces characteristic bell-shaped thoracic collapse, severe pectus excavatum, and profound restrictive pulmonary mechanics.
• Neuromuscular Scoliosis and Pelvic Obliquity: Severe weakness of the intrinsic paraspinal and core abdominal musculature deprives the growing spine of its physiological muscular sleeve. As the child sits, gravity acts upon the unsupported column, inducing a progressive C-shaped thoracolumbar sweep. Asymmetric muscular pull and unilateral trunk collapse drive the spine into pelvic obliquity. In non-ambulatory sitters, pelvic tilt produces unequal pressure distribution across the ischial tuberosities, accelerating structural spine curvature and causing severe rib-pelvis impingement.
• Paralytic Hip Dysplasia: In utero and early infantile hips normally remodel from fetal coxa valga (neck-shaft angle >150 degrees) and femoral anteversion (>40 degrees) toward adult morphology through dynamic hip abductor muscle contraction and axial weight-bearing. In SMA, severe weakness of the gluteus medius and minimus, coupled with uninhibited adductor and flexor tone, prevents normal femoral remodeling. The persistent coxa valga, increased femoral anteversion, and shallow acetabulum allow the unconstrained femoral head to migrate posterolaterally, culminating in subluxation and dislocation.

4. Natural History

Epidemiology: Spinal muscular atrophy is the leading genetic cause of infant mortality, with an estimated worldwide incidence ranging between 1 in 6,000 and 1 in 10,000 live births. The carrier frequency of heterozygous SMN1 mutations in the general population is approximately 1 in 50 (ranging from 1 in 40 to 1 in 60). The condition follows classic Mendelian autosomal recessive inheritance with equal transmission across sexes.

Untreated Clinical Course by Phenotypic Cohort: Prior to the advent of targeted disease-modifying therapies, the clinical trajectory of SMA was strictly categorized according to age of onset and maximum motor milestone achieved:

• Type 0 (Prenatal Onset): Characterized by severe congenital arthrogryposis multiplex congenita, profound hypotonia, micrognathia, congenital heart defects, and respiratory failure at birth. Untreated survival rarely exceeded 1 month.
• Type 1 (Infantile-Onset / Werdnig-Hoffmann Disease): Symptoms manifest before 6 months of age. Affected infants present with generalized flaccid hypotonia, poor head control, a weak cry, and feeding fatigue; they never achieve independent sitting (“non-sitters”). Rapid progressive intercostal denervation produces paradoxical breathing and recurrent aspiration. Historically, untreated Type 1 carried a 2-year mortality rate exceeding 90%, with respiratory failure as the primary cause of death.
• Type 2 (Intermediate / Dubowitz Disease): Symptoms present between 6 and 18 months of age. Children acquire the milestone of independent sitting when placed, but never walk independently (“sitters”). Progressive neuromuscular scoliosis develops in 60% to 90% of children before 10 years of age. During the adolescent growth spurt, spinal deformity progresses aggressively at rates exceeding 10 to 20 degrees Cobb angle per year. Untreated life expectancy historically ranged from the second to third decade of life, dictated by restrictive pulmonary compromise and nocturnal hypoventilation.
• Type 3 (Juvenile-Onset / Kugelberg-Welander Disease): Symptoms develop after 18 months of age, and patients achieve independent ambulation (“walkers”). Type 3a presents before 3 years of age, with many losing independent ambulation during adolescence due to weight gain and muscle weakness. Type 3b presents after 3 years of age, and ambulation is frequently retained into adulthood. Scoliosis develops in 40% to 50% of non-ambulatory individuals but is less prevalent in sustained walkers. Normal life expectancy is typical.
• Type 4 (Adult-Onset): Presents in the second or third decade of life with mild, symmetric proximal muscle weakness, preserved ambulation, minimal spinal deformity, and normal longevity.

The Modern Disease-Modifying Era: Disease-modifying therapies have fundamentally rewritten the natural history of SMA. When treated pre-symptomatically or early in the disease course, infants with Type 1 achieve independent sitting, standing, and even walking milestones that were historically impossible. Consequently, orthopaedic surgeons now evaluate a growing cohort of long-term survivors with modified phenotypes who present with complex spinal collapse, pelvic obliquity, and hip subluxation while retaining higher respiratory capacity and functional motor demand.

5. Clinical Presentation

History

The clinical history in spinal muscular atrophy must be elicited methodically across developmental milestones, respiratory symptoms, feeding capabilities, and seated functional stability:

• Perinatal and Early Infantile History: Inquire about decreased fetal movement in utero, weak neonatal cry, prolonged feeding times exceeding 30 to 45 minutes, frequent coughing or choking episodes during feeds, and failure to thrive.
• Motor Milestone Achievement and Regression: Document the exact age of gross motor milestone attainment (head control, rolling, sitting without hand support, crawling, pulling to stand, and independent walking). Inquire about recent stagnation or loss of previously acquired motor capabilities.
• Pulmonary and Sleep History: Screen for paradoxical breathing patterns, weak cough, recurrent lower respiratory tract infections, nocturnal hypoventilation symptoms (frequent nighttime awakenings, morning headaches, unexplained daytime lethargy), and current non-invasive ventilation (NIV) usage parameters.
• Seating and Musculoskeletal Complaints: Elicit caregiver reports of progressive trunk list in the wheelchair, difficulty maintaining upright head alignment, unequal ischial pressure sores, rib-pelvis abutment pain (iliocostal friction syndrome), and progressive joint contractures restricting hygiene or transfer maneuvers.

Physical Exam

The physical examination requires a comprehensive neurological survey paired with detailed spine, pelvic, and extremity structural assessments:

Neuromuscular and Cranial Nerve Survey:

• Muscle Tone and Bulk: Generalized symmetric hypotonia with severe proximal-greater-than-distal muscle atrophy. Lower extremities demonstrate more profound paresis than upper extremities.
• Reflexes: Deep tendon reflexes (patellar, Achilles, biceps, brachioradialis) are characteristically absent or severely depressed.
• Involuntary Movements: Fine polyminimyoclonus (tremulous mini-myoclonus) of the outstretched fingers and hands, and active tongue fasciculations.
• Preserved Modalities: Sensation (light touch, pinprick, vibration, proprioception) is intact throughout. Cranial nerves III, IV, and VI (extraocular movements) and VII (facial expression) are preserved. Cognitive function and social engagement are completely intact.

Validated Physical Examination Maneuvers:

• Gowers Sign: [Patient transitions from prone or seated floor position to standing] -> Patient uses hands to climb up shins, knees, and thighs to compensate for severe gluteal and quadriceps weakness [Sensitivity: 88-92%, Specificity: 84-90% in ambulatory SMA Types 2 and 3].
• Tongue Fasciculation Assessment: [Direct visual inspection of the tongue lying quietly inside the oral cavity without active protrusion] -> Fine, vermicular, involuntary muscular contractions [Sensitivity: 82-86%, Specificity: 94-98% in infantile SMA].
• Traction / Pull-to-Sit Maneuver: [Infant pulled gently by the wrists from supine to upright sitting] -> Complete head lag with marked shoulder girdle weakness and absence of cervical flexor counter-traction [Sensitivity: 95%, Specificity: 78% in hypotonic infants].
• Ventral Suspension (Landau Reaction): [Infant supported horizontally in prone position by examiner hand beneath thorax] -> Complete trunk droop into an inverted “U” shape with limbs hanging flaccidly (“ragdoll sign”) [Sensitivity: 94%, Specificity: 82%].
• Barlow and Ortolani Tests: [Infant supine, hips flexed 90 degrees; axial compression with adduction (Barlow) followed by abduction with anterior trochanteric lift (Ortolani)] -> Palpable sensation of femoral head exiting the acetabular rim (Barlow) or reducing with a clunk (Ortolani) [High specificity in early infancy, decreasing as fixed contractures develop].
• Galeazzi Test: [Infant supine, hips flexed to 90 degrees, knees flexed, feet placed flat together on table] -> Unequal knee height indicating unilateral hip dislocation or femoral length discrepancy [Sensitivity: 75-80%, Specificity: 90-95%].

Spinal, Pelvic, and Extremity Alignment:

• Spine Deformity and Seated Balance Assessment: Evaluate the patient while sitting unsupported on the examination table. Note coronal trunk tilt, C7 plumb line deviation relative to the gluteal cleft, prominent thoracic or lumbar rib humps, and parasternal retraction. Assess the flexibility of the spinal curve using the axillary traction suspension test (lifting the patient gently by the axillae to differentiate flexible postural collapse from rigid structural scoliosis).
• Pelvic Obliquity Evaluation: Measure the height difference between the anterior superior iliac spines (ASIS) relative to the horizontal sitting surface. Classify pelvic obliquity as flexible (correctable with manual pelvic leveling) or fixed (infrapelvic contracture vs suprapelvic spinal rigidity).
• Contracture Survey: Assess hip flexion contractures using the Thomas test. Measure maximum hip abduction with knees extended and flexed to evaluate adductor tightness. Document popliteal angles (hamstring contracture), knee flexion contractures, and ankle equinovarus deformities using the Silfverskiöld maneuver (differentiating gastrocnemius tightness from fixed gastrosoleus contracture).

6. Imaging and Diagnostic Studies

The diagnostic workup of spinal muscular atrophy has transitioned from invasive neuromuscular testing to definitive molecular genetics, complemented by comprehensive orthopaedic radiographic surveillance and pulmonary evaluation:

Molecular Genetic Testing (Gold Standard):

• SMN1 Deletion Analysis: Quantitative polymerase chain reaction (qPCR) or Multiplex Ligation-dependent Probe Amplification (MLPA) is the primary diagnostic modality. Identification of homozygous deletion of exon 7 (with or without exon 8) in SMN1 establishes a definitive diagnosis in over 95% of patients.
• Gene Sequencing: In patients with high clinical suspicion who exhibit only a single heterozygous SMN1 deletion, targeted Sanger or next-generation sequencing is performed to identify subtle intragenic point mutations, insertions, or microdeletions on the remaining allele.
• SMN2 Copy Number Quantification: Quantifying SMN2 copy numbers (ranging from 1 to 5 copies) via MLPA or digital droplet PCR (ddPCR) is mandatory. It directly informs clinical prognosis and determines regulatory qualification for targeted molecular therapeutics.

Spine Radiographic Evaluation:

• Full-Length Biplanar Radiographs: 36-inch sitting (or standing if ambulatory) anteroposterior (AP) and lateral spine radiographs are obtained at regular intervals.
• Cobb Angle: Quantifies major thoracic and thoracolumbar neuromuscular curves. Measurements exceeding 50 degrees Cobb angle indicate significant structural collapse.
• Pelvic Obliquity Angle: Measured as the angle subtended between a line connecting the superior margins of the iliac crests and the horizontal seating reference, or perpendicular to the C7-S1 central sacral vertical line. Obliquity exceeding 15 degrees correlates with severe seated instability and unilateral ischial decubitus ulcers.
• Sagittal Parameters: Quantify thoracic kyphosis, thoracic lordosis (“hollow back” deformity), and loss of lumbar lordosis. Flatback or lumbar hypolordosis in sitting patients drives compensatory posterior pelvic tilt and severe sagittal imbalance.
• Flexibility Radiographs: Supine manual traction AP, bolster-assisted bending, or push-prone views are obtained preoperatively to evaluate curve flexibility and guide anchor density selection.

Pelvic and Hip Radiography:

• Standard AP Pelvis: Obtained with the lower extremities placed in neutral rotation and abduction.
• Reimers Migration Percentage (MP): Calculated as the proportion of the ossified femoral head extending lateral to Perkins line, divided by the total transverse width of the femoral head, multiplied by 100%. A migration percentage below 20% is normal; 30% to 90% represents progressive hip subluxation; and 100% signifies complete dislocation.
• Acetabular Index (AI) and Sharp Angle: Quantifies acetabular roof deficiency and secondary dysplasia resulting from lack of normal concentric mechanical loading.
• Femoral Neck-Shaft Angle (NSA): Measures coxa valga, typically exceeding 145 to 155 degrees in non-ambulatory SMA children due to absent abductor modeling forces.

Historical Diagnostic Modalities:

• Electromyography (EMG) and Nerve Conduction Studies (NCV): Characterized by active denervation with spontaneous fibrillations, positive sharp waves, and fasciculation potentials, paired with high-amplitude, long-duration polyphasic motor unit action potentials (MUAPs) reflecting re-innervation. Sensory nerve action potentials (SNAPs) are characteristically normal. Today, EMG/NCV testing is largely superseded by non-invasive genetic testing.
• Muscle Biopsy: Demonstrates diagnostic “grouped atrophy” with sheets of small atrophic type 1 and type 2 muscle fibers surrounding islands of preserved or hypertrophic type 1 fibers. Biopsy is no longer required for primary diagnosis and is reserved for atypical presentations negative for 5q mutations.

Pulmonary Function Testing (PFT) and Sleep Surveillance:

• Spirometry: Measures Forced Vital Capacity (FVC) and Forced Expiratory Volume in 1 second (FEV1). Comparison of sitting versus supine FVC is critical: a decline exceeding 20% in the supine position indicates severe diaphragmatic weakness.
• Respiratory Muscle Pressures: Maximal Inspiratory Pressure (MIP) evaluates diaphragm strength; Maximal Expiratory Pressure (MEP) evaluates abdominal and intercostal strength, which is severely blunted early in the disease, explaining ineffective cough mechanics.
• Polysomnography and Nocturnal Capnography: Detects sleep-disordered breathing, REM sleep hypoventilation, hypoxemia, and nocturnal carbon dioxide retention, guiding titration of home non-invasive positive pressure ventilation (BiPAP).

7. Classification

The International SMA Consortium Classification categorizes spinal muscular atrophy according to the patient age of clinical onset and maximum motor milestone achieved, aligning directly with SMN2 copy numbers and dictating orthopaedic surveillance protocols:

8. Treatment

Medical / Disease-Modifying Therapy

The medical management of spinal muscular atrophy has undergone a transformative revolution with the development of targeted genetic therapies that upregulate SMN protein levels. Orthopaedic surgeons must understand these agents and their critical operative implications:

• Nusinersen (Spinraza): An intrathecally administered 2′-O-2-methoxyethyl antisense oligonucleotide that targets an intronic splicing silencer (ISS-N1) within SMN2 pre-mRNA, promoting exon 7 inclusion and the production of full-length SMN protein. Administered via lumbar puncture with a loading schedule (four doses within the first 2 months) followed by indefinite maintenance dosing every 4 months.
  Surgical Coordination: Extensive posterior spinal instrumentation and fusion from T2 to the pelvis completely obliterates standard interlaminar spaces. Preoperative planning requires deliberate surgical creation of an interlaminar fenestration window (typically at L2-L3 or L3-L4) without bone graft, or placement of an indwelling subcutaneous intrathecal catheter/port (Ommaya reservoir), or coordination with interventional radiology for transforaminal CT-guided access.
• Risdiplam (Evrysdi): An orally administered small-molecule splicing modifier that systemically modulates SMN2 pre-mRNA splicing to enhance exon 7 inclusion. Delivered as a daily liquid formulation, risdiplam achieves widespread central nervous system and peripheral tissue distribution, completely bypassing the anatomical challenges of post-fusion intrathecal access.
• Onasemnogene Abeparvovec (Zolgensma): A recombinant adeno-associated virus serotype 9 (AAV9) gene therapy delivering functional human SMN1 cDNA under a hybrid CMV enhancer/chicken-beta-actin promoter. Administered as a single intravenous infusion for pediatric patients under 2 years of age. It provides sustained transgene expression within post-mitotic motor neurons. Perioperative steroid coverage is required to mitigate transient hepatotoxicity, thrombocytopenia, and cardiac troponin elevations.
• Apitegromab: An investigational selective inhibitor of pro- and latent myostatin that prevents downstream activin receptor signaling and counteracts disuse muscle atrophy. When administered alongside SMN-upregulating therapies, it enhances peripheral muscle bulk and motor functional scores in Type 2 and Type 3 patients.

Nonoperative / Physical & Orthotic Care

Nonoperative care forms the foundation of longitudinal musculoskeletal preservation in SMA, focusing on seating optimization, trunk support, and contracture mitigation:

• Specialized Seating and Wheelchair Modifications: In non-ambulatory sitters, custom contoured wheelchair seating with lateral thoracic supports, high-density pressure-relieving seat cushions, anterior chest harnesses, and tilt-in-space mechanisms is essential. Seating systems must maintain a level pelvis, equalize ischial tuberosity contact pressures, and support upright head-neck orientation, liberating the upper extremities for functional tasks and power wheelchair control.
• Thoracolumbosacral Orthosis (TLSO): Indicated in young children with progressive curves to improve sitting stability, control rapid collapse, and delay operative intervention during critical thoracic growth.
  Mandatory Design Rule: The TLSO must feature a large anterior abdominal cut-out or elastic diaphragmatic gusset. Because SMA patients depend entirely on diaphragmatic excursion for tidal volume, a rigid circumferential brace compresses the abdomen, restricts diaphragmatic descent, and induces acute hypercapnic respiratory decompensation.
• Contracture Prevention and Standing Programs: Daily passive stretching regimens, night splinting with solid ankle-foot orthoses (AFOs) or knee-ankle-foot orthoses (KAFOs), and passive-assisted standing frame programs (60 minutes per day, 5 to 7 days per week) are implemented to preserve joint motion, maintain bone mineral density, stimulate acetabular remodeling, and prevent hip flexion and knee contractures.

Operative / Surgical Management

Surgical intervention in spinal muscular atrophy aims to correct collapsing trunk deformity, restore a level pelvis for stable sitting, prevent pulmonary deterioration, and eliminate pain, while carefully tailoring hip and contracture releases to functional status:

Neuromuscular Scoliosis Correction:

• Distraction-Based Growing Systems: Indicated in young children under 8 to 10 years of age with progressive curves exceeding 50 degrees Cobb angle. Traditional growing rods or Magnetically Controlled Growing Rods (MCGR) utilize dual-rod constructs anchored to the upper thoracic spine (T2-T4) and distal lumbar/pelvis. This approach controls progressive deformity while permitting continued spinal height gain and pulmonary alveolar expansion (targeting a minimum of 22 cm T1-S1 length before definitive arthrodesis). MCGR systems avoid the psychological and surgical morbidity of repeated open operative lengthenings every 6 months.
• Definitive Posterior Spinal Instrumentation and Fusion (PSIF): Indicated in skeletally mature children or those older than 10 to 12 years with progressive curves exceeding 50 degrees Cobb angle or severe pelvic obliquity. Arthrodesis must extend from the upper thoracic spine (T2 or T3) down to the pelvis. Stopping fusion short at L5 or S1 invariably results in progressive distal tilt, coronal decompensation, and rapid sitting failure.
• Pelvic Fixation via S2-Alar-Iliac (S2AI) Screws: S2AI screws represent the gold standard pelvic anchor, offering superior biomechanical pullout strength compared to conventional iliac screws while avoiding prominent subcutaneous hardware over the PSIS. Screws are placed in-line with lumbar pedicle screws, minimizing extensive lateral muscle dissection and reducing wound dehiscence rates.
• Preservation of Intrathecal Access Window: When performing posterior fusion in patients receiving or eligible for intrathecal nusinersen, the surgeon must deliberately omit decortication and bone grafting across a dedicated interlaminar segment (typically L2-L3 or L3-L4). A complete wide laminotomy and ligamentum flavum resection is performed, and the margins are demarcated with radio-opaque titanium hemoclips to facilitate fluoroscopic or ultrasound-guided lumbar puncture.

Hip Dysplasia and Paralytic Subluxation Management:

• The Paradigm of Conservative Restraint: In accordance with international consensus guidelines (Mercuri et al.), asymptomatic hip subluxation or complete dislocation in non-ambulatory SMA children should NOT be routinely reconstructed. Pain-free dislocated hips rarely cause functional limitation in non-ambulatory sitters, whereas surgical reconstruction carries substantial rates of joint stiffness, avascular necrosis, heterotopic ossification, and recurrent dislocation.
• Reconstructive Indications: Operative reconstruction is reserved strictly for painful hip subluxation, severe progressive contractures that compromise seated positioning or perineal hygiene, or ambulatory patients (Type 3) with progressive lateral migration.
• Surgical Technique: Comprehensive reconstruction requires proximal femoral varus derotation osteotomy (VDRO) fixed with a pediatric blade plate or locking cannula plate, reducing the neck-shaft angle to 110–115 degrees and derotating excessive anteversion. In cases with marked acetabular dysplasia, an open reduction, capsulorrhaphy, and redirectional pelvic osteotomy (Dega, Pemberton, or San Diego) are added, paired with fractional lengthening of the adductor longus, gracilis, and iliopsoas.

Lower Extremity Contracture Releases:

• Achilles Tendon Lengthening (TAL): Indicated for severe equinus or equinovarus contractures that prevent the foot from resting flat on wheelchair footplates or interfere with orthotic wear. Percutaneous triple-hemisection or mini-open Z-lengthening is performed.
• Hamstring Lengthening: Indicated for knee flexion contractures exceeding 20 to 30 degrees that impair standing frame participation or seated posture. Fractional lengthening of the semimembranosus and semitendinosus is preferred over complete tenotomy to preserve residual active knee flexion.

Indications & Contraindications

Explicit Surgical Indications:

• Spine Deformity: Progressive neuromuscular scoliosis exceeding 50 degrees Cobb angle with documented trunk list, chest wall collapse, or progressive loss of independent sitting balance.
• Pelvic Obliquity: Coronal pelvic tilt exceeding 15 degrees resulting in asymmetric seated weight-bearing, impending ischial decubitus breakdown, or rib-iliac crest impingement.
• Hip Subluxation: Reimers migration percentage exceeding 30% to 40% in an ambulatory child or a child actively transitioning to ambulation, or documented hip pain localized to the joint in a non-ambulator with failed conservative seating adjustments.
• Pulmonary Clearance Benchmarks: Forced Vital Capacity (FVC) exceeding 30% to 40% of predicted values is desirable; patients with FVC between 20% and 30% require mandatory multidisciplinary pulmonary optimization, preoperative BiPAP familiarization, and planned postoperative extubation directly to non-invasive ventilation.

Contraindications:

• Absolute Contraindications: Uncontrolled systemic infection, active bacteremia, acute pneumonia, uncorrectable coagulopathy, severe uncompensated hypercapnic respiratory failure refractory to non-invasive ventilation, or medical instability precluding general anesthesia.
• Relative Contraindications: Asymptomatic, pain-free hip dislocation in a non-ambulatory patient (relative contraindication to major joint reconstruction); profound malnutrition (serum albumin < 3.0 g/dL) or cachexia without a preoperative nutritional rehabilitation plan; FVC below 20% of predicted without a dedicated critical care pathway for perioperative mechanical ventilatory support.

9. Outcome

Spinal Alignment and Sitting Function: Definitive posterior spinal instrumentation and fusion from T2 to the pelvis yields consistent, durable correction of neuromuscular scoliosis in SMA, achieving average Cobb angle reductions between 50% and 70% and leveling pelvic obliquity to within 5 degrees of horizontal. Correction of trunk collapse eliminates painful rib-pelvis impingement, equalizes ischial pressure distribution, and restores self-supported upright sitting. By freeing the upper extremities from the requirement of propping the trunk upright, spinal stabilization dramatically enhances hand function, communication, and independence in operating power mobility devices.

Pulmonary Stabilization: Untreated progressive neuromuscular scoliosis in SMA leads to an inexorable decline in pulmonary function, with FVC deteriorating at rates between 3% and 6% per year as the thoracic cage collapses. Surgical spinal stabilization fundamentally alters this downward trajectory. In their landmark systematic review and meta-analysis encompassing 14 studies and 438 patients, Angeli et al. demonstrated that spinal instrumentation and fusion effectively halts the rapid progressive decline of FVC, stabilizing vital capacity relative to expected natural history deterioration and preserving life-sustaining thoracic volume.

Quality of Life and Functional Independence: Validated patient- and caregiver-reported outcome instruments—including the Caregiver Priorities and Child Health Index of Life with Disabilities (CPCHILD) and the Pediatric Quality of Life Inventory (PedsQL)—demonstrate marked, sustained post-surgical gains in sitting comfort, ease of transfers, dressing, personal hygiene, and reduction of chronic posture-related pain.

10. Complications

Due to profound muscle weakness, severe osteopenia, compromised respiratory mechanics, and poor baseline nutritional status, patients with spinal muscular atrophy face a substantial complication profile during major orthopaedic interventions:

11. Pearls and Pitfalls

• Mastery of Sagittal Plane Contouring: In non-ambulatory neuromuscular patients, maintaining physiological lumbar lordosis (50 to 60 degrees) is paramount. Over-flattening the lumbar spine creates an iatrogenic flatback deformity that forces the patient into posterior pelvic tilt, throwing the center of gravity backward and severely destabilizing upright wheelchair sitting balance.
• Optimizing S2AI Pelvic Purchase: In severely osteopenic bone, drive S2AI screws across the sacroiliac joint into the dense bone corridor of the anterior iliac column to achieve bicortical or tricortical purchase. Always countersink the screw heads 5 to 10 mm below the posterior iliac wing to prevent skin erosion in thin, malnourished children.
• The Surgical Laminar Window for Intrathecal Therapy: Never perform circumferential decortication and bone grafting across the entire lumbar spine in patients eligible for nusinersen. Select L2-L3 or L3-L4, perform a formal laminotomy resecting the ligamentum flavum, carefully leave a 12 to 15 mm dura-exposing window, and place radiopaque titanium hemoclips at the osseous borders to provide an unmistakable fluoroscopic target for post-fusion drug delivery.
• Extubation Direct to Non-Invasive Ventilation: The single most critical step in avoiding postoperative respiratory failure is extubating the patient directly to their baseline non-invasive positive pressure ventilation (BiPAP) settings in the operating room or intensive care unit, combined with scheduled mechanical insufflation-exsufflation (cough assist). Extubating to room air or blow-by oxygen alone leads to rapid microatelectasis and re-intubation.
• Avoid Prophylactic Reconstruction of Painless Hip Dislocations: One of the most disastrous pitfalls in SMA management is performing complex femoral and pelvic osteotomies on asymptomatic, dislocated hips in non-ambulatory children. Reconstruction often induces joint stiffness, avascular necrosis, and loss of comfortable 90-degree hip flexion required for wheelchair sitting, converting a painless deformity into a symptomatic disability.
• Diaphragmatic Relief in Bracing: Never apply a continuous, rigid cylindrical TLSO to a patient with SMA. Compressing the abdomen restricts diaphragmatic excursion, which accounts for the vast majority of the patient vital capacity, and can precipitate acute hypercapnic respiratory arrest.
• Always Fuse to the Pelvis for Pelvic Obliquity: If pelvic obliquity exceeds 10 to 15 degrees, never terminate posterior instrumentation at L5 or S1. Anchoring short in a collapsing neuromuscular spine invariably leads to rapid distal fixation pullout, worsening coronal decompensation, and severe ischial skin breakdown.

12. Controversy

Timing and Modality of Spinal Deformity Intervention: In the pre-DMT era, surgical management followed rigid boundaries: distraction-based growing rods were deployed in early-onset scoliosis to preserve thoracic growth, with definitive fusion delayed until age 10 or 12. With modern disease-modifying therapies extending survival and improving motor control, surgeons face active debate regarding whether magnetically controlled growing rods (MCGR) should be maintained throughout childhood despite known complications of metallosis, rod distraction failure, and autofusion, or whether earlier definitive fusion (at age 8 to 10) offers a more stable single-stage solution once satisfactory thoracic height (>22 cm T1-S1) has been attained.

Management of Paralytic Hip Dislocation: Clinical consensus remains contentious regarding asymptomatic hip subluxation in Type 2 sitters. While current international guidelines advocate nonoperative observation for painless dislocations in non-ambulators, some surgeons argue that progressive unilateral subluxation drives asymmetric pelvic obliquity, pelvic rotation, and trunk imbalance, advocating early preventive soft-tissue releases or VDRO to maintain sitting symmetry. Longitudinal multicenter trials are ongoing to determine whether hip containment preserves long-term sitting comfort or merely introduces unnecessary operative morbidity.

Intrathecal Access Strategies Post-Fusion: As hundreds of fused SMA patients continue life-long nusinersen therapy, debate persists over the optimal access strategy. While deliberate interlaminar windows allow successful fluoroscopic or ultrasound-guided lumbar puncture, some centers report progressive interlaminar bony bridging or scarring over time, requiring repeated transforaminal CT-guided injections with cumulative radiation exposure. The alternative approach—implanting a permanent subcutaneous intrathecal port (Ommaya reservoir)—provides effortless access but carries a 5% to 10% lifetime risk of catheter occlusion, cerebrospinal fluid leakage, and central nervous system infection.

AAV9 Gene Therapy in Heavier or Older Pediatric Cohorts: Onasemnogene abeparvovec has revolutionized infantile care, but expanding its use to older, heavier children (weighing >13.5 to 20 kg) introduces substantial debate regarding systemic adeno-associated vector safety. High systemic viral loads correlate with dose-dependent toxicities, including acute liver failure, thrombotic microangiopathy (TMA), and dorsal root ganglion toxicity, prompting trials of intrathecal delivery routes to bypass systemic exposure.

13. Evidence-Based Synthesis

Modern clinical decision-making is grounded in high-quality randomized controlled trials and prospective registry analyses, corroborated by seminal biomechanical and anatomical descriptions.

14. References

• Dansuk E, Van Den Berg ANT, Ata G et al. The effect of thoracolumbosacral orthosis on scoliosis progression and chest deformity in children with type 1 spinal muscular atrophy: A randomized controlled trial. PLoS One. 2025. doi:10.1371/journal.pone.0323341 [Level I: Randomized Controlled Trial]
• Angeli M, Alpantaki K, Pandis N et al. The effect of scoliosis surgery on pulmonary function in spinal muscular atrophy patients: review of the literature and a meta-analysis. Eur Spine J. 2022. doi:10.1007/s00586-022-07182-2 [Level I: Systematic Review / Meta-Analysis]
• Mercuri E, Darras BT, Chiriboga CA et al. Nusinersen versus Sham Control in Later-Onset Spinal Muscular Atrophy. N Engl J Med. 2018. doi:10.1056/NEJMoa1710504 [Level I: Randomized Controlled Trial (CHERISH)]
• Finkel RS, Mercuri E, Darras BT et al. Nusinersen versus Sham Control in Infantile-Onset Spinal Muscular Atrophy. N Engl J Med. 2017. doi:10.1056/NEJMoa1702752 [Level I: Randomized Controlled Trial (ENDEAVOR / ENDEAR)]
• Mercuri E, Finkel RS, Muntoni F et al. Diagnosis and management of spinal muscular atrophy: Part 1: Recommendations for diagnosis, rehabilitation, orthopedic and nutritional care. Neuromuscul Disord. 2018. doi:10.1016/j.nmd.2017.11.005 [Level V (Expert Consensus): International Consensus Practice Guideline]

15. Master Class Pro-Tip

When instrumenting the collapsing neuromuscular spine in spinal muscular atrophy, success hinges on two non-negotiable principles: aggressive preservation of sagittal lumbar lordosis and forward-planning for lifelong intrathecal access. First, never contour lumbar rods flat to simplify pelvic docking; you must maintain 50 to 60 degrees of anatomical lumbar lordosis to prevent post-surgical posterior pelvic tilt, which destroys independent wheelchair sitting balance. Second, when using S2-alar-iliac screws for rigid pelvic fixation, deliberately omit decortication and bone grafting across the L2-L3 or L3-L4 segment. Perform a wide 15-millimeter laminotomy, fully excise the ligamentum flavum, and secure radiopaque titanium hemoclips to the flanking lamina borders. This permanent fluoroscopic window ensures that interventional radiologists and pediatric neurologists can perform rapid, atraumatic intrathecal drug administration without bone obstruction or repeat CT scans.

Master Class Surgical Pearl: Neuromuscular Spine and Pelvic Balance in Spinal Muscular Atrophy