Neuromuscular Disorders in Pediatric Orthopaedics

Paul D. Sponseller

6.1 Introduction

This chapter summarizes neuromuscular diseases that affect the skeleton as a result of pathologic conditions of the spinal cord or peripheral nervous system. These diseases may present a fully developed picture or show early subtle findings, such as a mild deviation of gait. The four tables in this chapter organize the neuromuscular diseases according to the location of pathology.

6.2 Evaluation

The elements of evaluation of neuromuscular disease include the following:

History: Prenatal, birth (gestation, weight, Apgar scores), developmental (milestones), family; functional deterioration.
Physical examination:
1. Motor strength, tone.
2. Deep tendon reflexes.
3. Cranial nerves.
4. Cerebellar signs.
Serum creatine phosphokinase (CPK):
1. Elevation directly related to amount of muscle necrosis or membrane disorder. Abnormal in Duchenne and Becker muscular dystrophies (>20 times normal in childhood and early teens), myopathies, or myositis.
2. Minimal to mild elevation in other dystrophies.
Immunocytochemistry: Directly differentiates Duchenne from Becker dystrophies and other conditions. Requires small amount of muscle tissue.
DNA mutation analysis: Requires small amount of blood or amniotic fluid, allows prenatal diagnosis.
Electromyelography (EMG): Distinguishes myopathic from neuropathic weakness.
1. Myopathic process: Polyphasic low-voltage signal; fibrillations and sharp waves.
2. Neuropathic process: Initially, brief biphasic low-voltage fibrillation.
3. Chronic process: Prolonged polyphasic fibrillation, increased amplitude.
Nerve conduction studies:
1. Abnormally slowed conduction velocities in conditions involving peripheral nerves only.
2. Normal conduction velocities in spinal muscular atrophy.
3. Normal conduction velocity (>49 m/s in arms, >39 m/s in legs) for patients over 5 years of age. Younger children have slower conduction velocity.
Muscle biopsy: Biopsy minimally involved muscle in chronic conditions and severely involved muscle in acute conditions. Vastus lateralis used for proximal myopathy should be done only in centers capable of doing special histochemistry.
1. Type I fibers: Oxidative metabolism, slow twitch.
2. Type II fibers: Anaerobic metabolism, fast twitch.
3. Myopathic process:
  1. Necrosis, phagocytosis, and inflammation.
  2. Irregularly sized fibers.
  3. Type I fiber predominance less than 50%.
4. Neuropathic process:
  1. Small group atrophy.
  2. Fiber-type grouping, angular fibers.
  3. Type II fiber predominance greater than 80%.
5. Electron microscopy (glutaraldehyde) is used to differentiate architectural changes within the congenital myopathies.
Nerve biopsy:
1. Sural nerve most commonly biopsied.
2. Guillain–Barré syndrome: Mononuclear infiltrates and focal acute demyelination.
3. Hypertrophic neuropathies: Nerve fiber loss, interstitial fibrosis, and “onion bulb” formation.
Electrocardiogram and echocardiogram:
1. Abnormal in Duchenne muscular dystrophy (sinus tachycardia and right ventricular hypertrophy), Friedreich ataxia, and myotonic dystrophy.
Genetic testing: available for many disorders; should be ordered by neurologist or neurogeneticist.

6.3 Cerebral Palsy

6.3.1 Definition

Static brain injury or lesion in prenatal, perinatal, or postnatal period (up to 2 years).

6.3.2 Causes

Prematurity, infection, hypoxia, ischemia, embolus, trauma, brain malformation.

6.3.3 Physiologic Types

Pyramidal (spastic), extrapyramidal (dystonic or athetoid), mixed.

6.3.4 Anatomic Types

Diplegic: Usually premature. Pyramidal tracts involved; magnetic resonance imaging (MRI) shows periventricular leukomalacia; cognition preserved; legs affected, most distally; equinovalgus feet most common; hips rarely subluxate; scoliosis is rare.
Hemiplegic: Focal cortical lesion; risk of seizure; cognition preserved; ipsilateral arm and leg involved. Equinovarus foot is most common; hip and spine involvement is rare. Limb shortening is usually minor.
Totally involved (quadriplegic): Usually diffuse cortical problem. Cognition may be affected; problems with spine and trunk balance; respiration and swallowing function affected. Risk of seizure. Hip subluxation, scoliosis, and limb deformities are common.
Other types: Monoplegia, asymmetric diplegia.

6.3.5 Gross Motor Functional Classification System (▶ Fig. 6.1; ▶ Fig. 6.2)

Fig. 6.1 Gross motor functional classification system. (Used with permission from Graham HK. Classifying cerebral palsy. J Pediatr Orthop 2005;25(1):128 (Fig. 1).)