Introduction to Neuromuscular Disorders in Orthopaedics
Neuromuscular disease in children encompasses a highly heterogeneous group of conditions that affect the spinal cord, peripheral nerves, neuromuscular junctions, and the skeletal muscles themselves. For the orthopaedic surgeon, accurate diagnosis is the absolute cornerstone of management. The surgical procedures and rehabilitation protocols commonly utilized to treat static encephalopathies (such as cerebral palsy) or historical conditions (such as poliomyelitis) are frequently inappropriate—and potentially deleterious—when applied to progressive hereditary neuromuscular conditions.
The overarching goals of the orthopaedic surgeon in managing these complex patients include prescribing appropriate orthoses for lower extremity control, facilitating transfers to and from wheelchairs, preventing or correcting progressive joint contractures, and maintaining appropriate standing, sitting, and spinal postures. Treatment must be meticulously individualized, with the choice and timing of surgical procedures dictated by the specific disorder, the severity of neuromuscular involvement, the patient's ambulatory status, and the anticipated natural history of the disease.
Diagnostic Evaluation and Differentiation
Differentiating primary muscle disease (myopathy/dystrophy) from primary nerve disease (neuropathy) requires a multidisciplinary approach. The diagnosis is synthesized from a detailed clinical and family history, comprehensive physical examination, laboratory testing, genetic screening, electromyography (EMG), nerve conduction velocity (NCV) studies, and tissue biopsies.
Laboratory and Hematological Studies
Serum enzyme studies, particularly creatine kinase (CK) and aldolase, are critical first-line diagnostic tools.
* Dystrophinopathies: In conditions such as Duchenne Muscular Dystrophy (DMD), CK levels are dramatically elevated, often reaching 50 to 100 times the normal upper limit, reflecting profound and ongoing muscle fiber necrosis.
* Neuropathies and Congenital Myopathies: Conversely, in patients with spinal muscular atrophy (SMA) or congenital myopathies, CK levels may be normal or only slightly elevated (one to two times normal).
Electromyographic (EMG) and Nerve Conduction Studies
EMG and NCV studies help localize the lesion to the anterior horn cell, peripheral nerve, neuromuscular junction, or muscle fiber.
* Myopathic EMG: Characterized by short-duration, low-amplitude, polyphasic motor unit action potentials (MUAPs).
* Neuropathic EMG: Characterized by large-amplitude, long-duration MUAPs with decreased recruitment, often accompanied by fibrillation potentials and positive sharp waves at rest.
* Demyelinating vs. Axonal Neuropathy: NCV studies differentiate demyelinating conditions (markedly slowed conduction velocities) from axonal conditions (reduced action potential amplitudes with relatively preserved velocities).
Clinical Pearl: While EMG and NCV are invaluable, they are highly operator-dependent and can be poorly tolerated by young children. They should be ordered judiciously and performed by a pediatric electrodiagnostician.
Surgical Biopsy Techniques
Despite tremendous advances in molecular genetics, nerve and muscle biopsies remain essential for precise diagnosis in atypical presentations or when genetic testing is inconclusive. Atraumatic surgical technique is paramount; crush artifact renders the specimen useless for histopathological and electron microscopic evaluation.
Muscle Tissue Biopsy
The biopsy specimen must be harvested from a muscle that is clinically involved but still actively functioning. End-stage, severely fibrotic muscle will yield non-diagnostic "end-stage myopathy" results.
* Site Selection: The deltoid, vastus lateralis, or gastrocnemius muscles are preferred.
* Avoidance of Musculotendinous Junctions: The biopsy must never be taken from the region of the musculotendinous junction. The normal fibrous tissue septa in this region can be easily confused with pathological fibrosis by the neuropathologist.
* Specimen Dimensions: The harvested specimen should be approximately 10 mm in length and 3 mm in depth.
* Handling and Fixation:
* Electron Microscopy: A portion of the specimen must be fixed immediately in glutaraldehyde.
* Light Microscopy/Histochemistry: The remaining specimen must be flash-frozen in liquid nitrogen within minutes of removal.
* Contraindications: Never place a muscle biopsy specimen intended for histochemistry into saline solution or formalin, as this destroys the enzymes required for diagnostic staining.
Nerve Biopsy
When a peripheral neuropathy is suspected and genetic testing is unyielding, a sural nerve biopsy is the gold standard.
* Surgical Approach: The sural nerve is accessed laterally, positioned between the Achilles tendon and the lateral malleolus, just proximal to the level of the tibiotalar joint.
* Technique: A longitudinal incision is made. The lesser saphenous vein is identified and retracted. The sural nerve, lying adjacent to the vein, is carefully isolated using vessel loops.
* Harvest: The entire width of the nerve should be taken for a length of 3 to 4 cm to provide adequate tissue for teasing and cross-sectional analysis.
Surgical Warning: Meticulous hemostasis and strict atraumatic handling of the nerve are required. Do not use forceps directly on the nerve segment to be analyzed; handle it only by the epineurium at the extreme ends.
Muscular Dystrophies
Tremendous advances have been made in understanding the genetic basis of muscular dystrophies. Through molecular biology, chromosome locations for various abnormal genes have been identified, cloned, and sequenced.
Duchenne Muscular Dystrophy (DMD)
DMD is the most common and severe form of childhood muscular dystrophy, inherited in an X-linked recessive pattern. The gene responsible is located in the Xp21 region of the X chromosome, which codes for the protein dystrophin. The absence of dystrophin leads to sarcolemmal instability, calcium influx, and progressive myofiber necrosis.
Clinical Presentation and Physical Examination
Boys typically present between 3 and 5 years of age with frequent falls, difficulty climbing stairs, and a classic Gowers' sign (using the hands to "walk up" the legs to achieve a standing position). Pseudohypertrophy of the calves is universally present, caused by the replacement of necrotic muscle with fat and fibrous tissue.
Medical Treatment
Corticosteroid therapy (prednisone or deflazacort) is the standard of care, significantly prolonging ambulation, preserving pulmonary function, and delaying the onset of scoliosis. However, chronic steroid use introduces orthopaedic challenges, including osteopenia, increased fracture risk, and weight gain.
Orthopaedic Treatment: Lower Extremity Contractures
The primary orthopaedic goal in the ambulatory phase is to maintain walking ability for as long as possible. The predictable pattern of muscle weakness (proximal to distal) leads to a specific cascade of contractures: equinus, followed by knee flexion, and finally hip flexion contractures.
* Equinus Contractures: Initially managed with stretching and ankle-foot orthoses (AFOs) at night. When equinus exceeds 10 to 15 degrees and threatens ambulation, surgical intervention is indicated.
* Surgical Interventions: Percutaneous or open Achilles tendon lengthening (or fractional lengthening of the gastrocnemius-soleus complex) is performed. This is often combined with posterior tibialis tendon transfer through the interosseous membrane to the dorsum of the foot to correct varus forces.
* Postoperative Protocol: Immediate mobilization is critical. Prolonged immobilization in DMD leads to rapid, irreversible loss of muscle strength. Patients should be casted in a weight-bearing position and encouraged to stand within 24 to 48 hours postoperatively.
Orthopaedic Treatment: Spinal Deformity
Progressive paralytic scoliosis develops in nearly all patients with DMD once they become wheelchair-bound.
* Indications for Surgery: Posterior spinal fusion is indicated when the curve reaches 20 to 30 degrees in a non-ambulatory patient. Delaying surgery until the curve is severe (>50 degrees) is contraindicated due to the concurrent rapid decline in pulmonary function (Forced Vital Capacity [FVC]).
* Surgical Technique: Instrumentation and fusion must extend from the upper thoracic spine (T2 or T3) down to the pelvis to correct pelvic obliquity and provide a stable sitting base. Segmental pedicle screw instrumentation is the modern standard.
Becker Muscular Dystrophy (BMD)
BMD is also caused by mutations in the Xp21 gene, but the mutation allows for the production of a truncated, partially functional dystrophin protein. Consequently, the clinical course is milder. Patients typically remain ambulatory into their late teens or twenties. Orthopaedic management follows the same principles as DMD but is tailored to the slower progression of the disease.
Other Variants of Muscular Dystrophy
Emery-Dreifuss Muscular Dystrophy
Characterized by a triad of early contractures (elbow flexors, Achilles tendons, and posterior cervical muscles), slowly progressive humeroperoneal muscle weakness, and severe cardiac conduction defects.
Pitfall: The cardiac involvement in Emery-Dreifuss can be fatal. A thorough cardiological clearance, including Holter monitoring and echocardiography, is absolutely mandatory before any orthopaedic intervention, including minor contracture releases.
Limb-Girdle Muscular Dystrophy (LGMD)
A heterogeneous group of disorders affecting the proximal musculature of the shoulder and pelvic girdles. Orthopaedic management is largely supportive, focusing on maintaining joint mobility and managing late-onset spinal deformities.
Facioscapulohumeral Muscular Dystrophy (FSHD)
An autosomal dominant condition characterized by weakness of the facial and shoulder girdle muscles.
* Orthopaedic Intervention: Scapulothoracic arthrodesis is a highly effective procedure for patients with severe scapular winging. By fusing the scapula to the rib cage, the deltoid (which is typically spared) is provided with a stable fulcrum, dramatically improving active shoulder abduction and forward flexion.
Hereditary Motor and Sensory Neuropathies (HMSN)
HMSN encompasses a group of genetic disorders affecting the peripheral nervous system, the most common being Charcot-Marie-Tooth (CMT) disease.
Charcot-Marie-Tooth Disease (Peroneal Muscular Atrophy)
CMT is characterized by slowly progressive distal muscle weakness, atrophy, and sensory loss. The classic orthopaedic manifestation is the cavovarus foot deformity.
Pathomechanics of the Cavovarus Foot
The deformity is driven by specific muscle imbalances:
1. Plantarflexed First Ray: Driven by a strong peroneus longus overpowering a weak tibialis anterior.
2. Hindfoot Varus: Driven by a strong tibialis posterior overpowering a weak peroneus brevis.
3. Claw Toes: Intrinsic muscle wasting leads to hyperextension at the metatarsophalangeal (MTP) joints and flexion at the interphalangeal (IP) joints.
Clinical Evaluation: The Coleman Block Test
The Coleman block test is essential to determine if the hindfoot varus is flexible (driven by the plantarflexed first ray) or rigid (fixed structural deformity). The patient stands with the heel and lateral border of the foot on a 1-inch block, allowing the first metatarsal to drop off the edge. If the hindfoot corrects to neutral, the varus is flexible, and surgery can focus on the forefoot/midfoot. If it remains in varus, a calcaneal osteotomy is required.
Surgical Management of the Cavovarus Foot
Surgical correction must address both the bony architecture and the deforming soft-tissue forces.
* Soft Tissue Releases: Plantar fascia release (Steindler stripping) is the first step to correct the cavus.
* Bony Procedures:
* Dorsiflexion Osteotomy of the 1st Metatarsal: Corrects the rigid plantarflexed first ray.
* Lateralizing Calcaneal Osteotomy: Corrects fixed hindfoot varus and shifts the mechanical axis laterally.
* Triple Arthrodesis: Reserved as a salvage procedure for severe, rigid, end-stage deformities in older adolescents or adults, as it sacrifices hindfoot motion and increases stress on the ankle joint.
* Tendon Transfers:
* Peroneus Longus to Peroneus Brevis Transfer: Removes the deforming plantarflexion force on the first ray and augments eversion.
* Jones Transfer: Transfer of the Extensor Hallucis Longus (EHL) to the neck of the first metatarsal, with IP joint fusion, to elevate the first ray and correct the claw hallux.
Hip Dysplasia and Spinal Deformities in CMT
While less common than foot deformities, hip dysplasia can occur due to abductor weakness. Spinal deformities, particularly scoliosis and kyphoscoliosis, occur in 10% to 15% of patients with CMT. The curves often mimic adolescent idiopathic scoliosis but have a higher propensity for progression and may require earlier surgical stabilization.
Friedreich Ataxia
An autosomal recessive spinocerebellar degenerative disease. Orthopaedic manifestations include severe, progressive cavovarus foot deformities and aggressive scoliosis.
* Surgical Considerations: The scoliosis in Friedreich ataxia is notoriously difficult to manage with bracing. Early posterior spinal fusion is often required. Furthermore, hypertrophic cardiomyopathy is a hallmark of the disease; rigorous preoperative cardiac optimization is critical.
Spinal Muscular Atrophy (SMA)
SMA is an autosomal recessive disorder caused by a mutation in the SMN1 gene, leading to the degeneration of anterior horn cells in the spinal cord. It presents with profound, symmetric, proximal muscle weakness.
Orthopaedic Management in SMA
- Spinal Deformity: Early-onset, collapsing paralytic scoliosis is nearly universal in SMA Types I and II. Management historically involved custom seating systems and thoracolumbosacral orthoses (TLSO). Surgical management involves growing rod constructs (e.g., MAGEC rods) for young children to allow spinal growth, followed by definitive posterior spinal fusion to the pelvis once skeletal maturity is approached.
- Hip Subluxation/Dislocation: Paralytic hip dislocation is common. Unlike in cerebral palsy, hip reconstruction (varus derotational osteotomies and pelvic osteotomies) in non-ambulatory SMA patients is generally not recommended, as the dislocated hips are typically painless, and surgery does not improve sitting balance or quality of life, while carrying significant perioperative morbidity.
General Treatment Considerations
Regardless of the specific neuromuscular diagnosis, several overarching principles govern orthopaedic care:
1. Orthoses: Ankle-foot orthoses (AFOs) and knee-ankle-foot orthoses (KAFOs) are vital for maintaining joint alignment, preventing contractures, and prolonging ambulation. They must be lightweight and custom-molded.
2. Seating Systems: For the non-ambulatory patient, a custom-molded wheelchair seating system is essential to prevent pelvic obliquity, accommodate spinal deformity, and distribute pressure to prevent decubitus ulcers.
3. Fractures: Osteopenia is ubiquitous due to decreased weight-bearing and, in DMD, chronic corticosteroid use. Fractures must be treated with minimal immobilization. Internal fixation is often favored over prolonged casting to allow immediate mobilization and prevent the catastrophic loss of muscle strength associated with bed rest.
