Neuromuscular Disorders
Lori A. Karol
INTRODUCTION
Many neuromuscular disorders are dealt with in some detail throughout this text, and this chapter focuses on those that commonly require orthopedic attention as part of their management. Other medical and surgical issues related to the care of patients are dealt with in other chapters and are cross-referenced when appropriate.
MYELOMENINGOCELE (SPINA BIFIDA)
This congenital condition is discussed in Chapter 549. The incidence of myelomeningocele has decreased over the last 2 decades, in part due to the recommendation for prenatal folate supplementation preceding and in early pregnancy. The level of spina bifida is defined by the most caudal functioning nerve root—that is, patients with sacral level spina bifida (eg, S1 level) have functioning hip flexors, quadriceps, hamstrings, and anterior tibialis musculature, but have absent innervation of the foot intrinsic musculature and absent bowel and bladder innervation. Patients with thoracic spina bifida have no lower extremity motor or sensory function.
Treatment of the infant with myelomeningocele begins with closure of the open sac and shunting of hydrocephalus in the early neonatal time period. Orthopedic treatment begins when the child is ready developmentally to begin weight-bearing. Patients with thoracic-level spina bifida can weight-bear in standing frames but may require foot surgery to correct deformities such as talipes equinovarus, or clubfoot. Additionally, young children on occasion require hip abduction contracture release to allow them to be positioned into braces.
Upper-lumbar-level patients most often can ambulate as young children with use of hip–knee–ankle–foot orthoses (HKAFOs), but often require clubfoot release. Typically, these patients become nonambulatory in adolescence when the energy to walk with extensive bracing becomes prohibitive. Lower-lumbar-level patients have present quadriceps function and most often remain ambulatory throughout childhood. Deformity must be addressed surgically in patients with lower-lumbar- and sacral-level myelomeningocele to prevent skin breakdown and infection in the insensate foot.
Spine deformity is common in patients with myelomeningocele. Scoliosis can result from congenital malformations of the vertebrae in young children but more commonly occurs in later childhood or adolescence, presenting as a long sweeping curve with pelvic obliquity. Scoliosis is more common in patients with proximal-level spinal lesions. Bracing may be useful in young children with flexible curves, but anterior and posterior spinal fusion with instrumentation is the treatment of choice for larger curves.
Kyphosis may be present in very young children with thoracic-level myelomeningocele. Sharp angular kyphosis can interfere with the ability to sit in a wheelchair easily and may lead to chronic skin breakdown over the spine and resultant infection. Surgical treatment is technically challenging.1-6 See Chapter 536 for further discussion.
CEREBRAL PALSY
Cerebral palsy (CP) is the most common neuro-muscular disorder seen in children. It is defined as a nonprogressive (static) encephalopathy resulting from injury, ischemia, or infection of the brain in the perinatal period. Cerebral palsy is discussed in detail in Chapter 554.
Cerebral palsy can be classified either by the movement disorder present, or by the severity of involvement based on the area of the body which is affected. Spasticity is the most common movement disorder in patients with cerebral palsy. It is defined as increased tone that is worse with increasing velocity of movement, which results in limited range of motion of the joints of the extremities and can lead to contractures. Dystonia is also seen frequently in the cerebral palsy population and is defined as rigidity that is not velocity dependent and which may affect varying muscle groups. Ataxia, or pure lack of balance, is rarely seen. A mixed motor abnormality is often seen in present with CP.
The geographic classification of CP describes the area of the body affected by CP. Spastic hemiplegia is defined as ipsilateral upper and lower extremity involvement, with the upper limb typically more severely involved than the leg. A patient with spastic diplegia has bilateral lower extremity involvement, with abnormalities seen to a lesser extent in the upper extremities. Spastic quadriplegia describes the patient who has upper and lower extremity involvement, with poor trunk control, and possibly impaired head control. The cognitive level of the child is not always related to the severity of spasticity or geographic classification.
The diagnosis of CP is made by obtaining a history of prematurity, neonatal intensive care unit (NICU) hospitalization, seizures, or delayed motor milestones. Findings on physical examination include persistence of infantile reflexes, hyperreflexia in the extremities, resistance to range of motion, and clonus at the ankles. Children who are able to walk exhibit abnormalities in their gait such as toe-walking, crouch (increased flexion) at the knee, scissoring of the hips, and posturing of the arms as they walk or run.
Orthopedic treatment begins in the ambulatory child with the prescription of orthoses to assist the child when walking. AFOs (ankle–foot orthoses) are often prescribed to hold the foot in a plantigrade position and reduce toe-walking in young children with adequate range of motion. Walking aids such as walkers and crutches can help children with deficient balance.
Injection of botulinum toxin has become popular in the management of young children with CP. Botox is injected close to the neuro-muscular junction and effectively denervates the muscle, resulting in temporary weakness and reduced spasticity. It has been used most frequently in the treatment of ankle equinus due to gastrocsoleus spasticity in young children who walk on their toes. Injection into other muscles such as the hip adductors, iliopsoas, and hamstrings has been performed, but efficacy remains under investigation. The use of baclofen through an infusion pump is also currently being used to treat severe spasticity in patients with CP.7
Surgery is reserved for the school-age child who has developed contractures that interfere with functional activities. The most common surgeries performed in patients with CP are either tendoachilles lengthening or gastrocnemius fascial lengthening for ankle equinus, hamstring lengthening for excessive knee flexion, and hip flexor lengthening for crouch gait. Rotational osteotomies of the femur are frequently performed in children between the ages of 7 and 12 for persistant anteversion that results in intoeing and falling.
Surgery in nonambulatory patients is focused at the hip and spine. Hip subluxation and dislocation are frequently seen in patients with extensive neurologic involvement who are nonambulatory. Treatment consists of hip adductor and flexor release, femoral osteotomy, and pelvic osteotomy. Bilateral surgery is frequently required due to the prevalence of contralateral subluxation. Recurrent hip subluxation is not infrequent years following surgical treatment.
The patient with total body involvement and poor trunk control is also at risk for scoliosis. Bracing is usually ineffective in controlling the curve, and wheelchair modifications are frequently employed to provide comfortable sitting. When surgery is pursued, fusion of the entire spine from the second thoracic vertebra to the pelvis is most effective in obtaining correction of sitting balance. Medical complications are frequently encountered following both hip and spine surgery in this medically fragile population.8-13 See Chapter 557 for further discussion.
MUSCULAR DYSTROPHY
Duchenne muscular dystrophy (DMD) is the most frequent muscle condition seen in the pediatric orthopedic population. It is an X-linked recessive condition due to a mutation in the dystrophin gene, resulting in an absolute absence of dystrophin in muscle tissue. Myopathies are discussed in Chapter 573.
Boys typically present with complaints of abnormal gait, toe-walking, or clumsiness. The age at which DMD patients begin to walk is only slightly delayed. Symptoms are rarely apparent until age 3 to 5 years.
Physical examination reveals proximal muscle weakness. This is best visualized by the Gower’s sign, where the boy is unable to rise from a seated position on the floor without the assistance of using his arms to “walk up” the legs to achieve extension of the hips. Pseudohypertrophy of the gastrocsoleus can be seen and represents replacement of normal muscle with fibrofatty tissue. Limitation of dorsiflexion of the ankle is frequently present in young boys.
Orthopedic treatment often begins with diagnosis. Serum creatine phosphokinase (CPK) is markedly elevated in young boys with DMD. Diagnosis by DNA analysis for mutations in the dystrophin gene is confirmatory in approximately 80% of patients with DMD. Muscle biopsy with dystrophin analysis is required only in those patients in whom mutations are not identified.
Orthopedic treatment may include release of contractures of the Achilles tendon and release or transfer of the posterior tibialis to maintain a shoeable foot. Hip and knee contracture surgery is indicated in a subset of patients to prolong their ability to walk. Typically the patient loses the ability to walk between the ages of 10 and 12 years. Scoliosis develops in 90% of boys following the loss of ambulation. Early spinal fusion when the curve is mild is recommended to enable the patient to sit comfortably for the duration of his or her life. Medical treatment with steroids has gained favor recently, as research has shown a delay in loss of ambulation and fewer patients developing scoliosis.14-20 For more on muscular dystrophy, see Chapter 559.
ARTHROGRYPOSIS
Arthrogryposis includes a group of specific diagnoses, all of which produce congenital contractures and muscle weakness. Decreased fetal movement is a hallmark of the condition. The majority of patients with arthrogryposis carry the diagnosis of amyoplasia. In the neuropathic form of arthrogryposis, failure of development of the anterior horn cells of the spinal cord leads to lack of normal muscular development. The joints of the extremities do not move due to lack of muscle strength, and fibrosis of joints occurs. Classic arthrogryposis is typically not genetically transmitted, and intelligence is preserved.
There is significant variability in the severity of disease between patients. Babies with arthrogryposis frequently are born with severe clubfoot deformities, knee contractures, or irreducible hip dislocations. The upper extremities are frequently held in a position of shoulder adduction and internal rotation, elbow extension, wrist flexion, and thumb adduction. There is absence of skin creases around the joints, and the extremities appear thin and tapered due to lack of muscle development. A characteristic hemangioma is usually present on the forehead.
Genetics and neurologic referrals should be made when assessing a baby with arthrogryposis. The differential diagnosis includes such syndromes as Beal syndrome and Larsen syndrome, as well as congenital muscular dystrophies. As some of these conditions are inherited, genetic counseling is important.
Nonsurgical treatment of orthopedic deformities is typically unsuccessful in patients with arthrogryposis. If the child is felt to have the potential to walk, surgical correction of the lower extremity contractures is undertaken. Early open reduction of the hips, and clubfoot release at standing age are recommended. Knee contractures are released in patients who are able to fire their quadriceps muscle. In general, surgical treatment is very difficult due to the lack of normal joint development. Bracing is used postoperatively to reduce or delay recurrent deformity.
Treatment of the upper extremity contractures is guided by the functional needs of the child. Elbow extension contracture release or triceps transfer can assist the child in self-feeding and activities of daily living. Wrist carpectomy or fusion can place the hand in a more functional position for use.
Scoliosis may occur in adolescence. Fusion is recommended for progressive curves. The life expectancy of patients with arthrogryposis is normal.21-25
REFERENCES
See references on DVD.
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