The term “weakness” can refer to a general phenomenon that affects all or most of the body or may refer to a specific area, such as an extremity. The complaint can imply generalized fatigue, refusal to walk, increased clumsiness, loss of bowel or bladder function, or focal motor weakness. In infants, weakness can imply lethargy, poor feeding, or poor head control. Slowly progressive forms of weakness may be due to congenital disorders. The child may present at a time when the weakness is mild, yet progressive. Some patients may have paralysis at the time of presentation. The primary focus in this chapter is on weakness arising from neuromuscular disorders.

The pathophysiology of weakness varies with the etiology and the specific area affected. Terms that are applied to neuromuscular disorders include the following:

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  Paresis implies a complete or partial weakness.

  
  
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  Paraparesis is weakness of the lower half of the body.

  
  
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  Quadraparesis is weakness involving all limbs.

  
  
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  Hemiparesis is weakness of one side.¹

  
  
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  Quadriplegia is paralysis of all limbs; it usually results from a spinal cord lesion.

  
  
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  Hemiplegia, involving one side of the body, generally results from a lesion in the brain.

Abnormalities of the neuromuscular system are further classified as arising from an upper or lower motor neuron unit. The upper motor neuron unit arises in the motor strip of the cerebral cortex, traverses the corticospinal tract, and ends in the spinal cord adjacent to the anterior horn cell.^2,3 Upper motor neuron diseases involving the cerebral cortex or spinal cord usually present with asymmetrical weakness that is contralateral to the lesion, and are associated with hyperreflexia, increased muscle tone, and the absence of atrophy or fasciculations.^3–5 The lower motor neuron unit includes the anterior horn cells, peripheral nerve, neuromuscular junction, and muscle fibers. Lower motor neuron diseases present with symmetrical weakness that can be isolated to specific muscle groups, and are associated with decreased muscle tone and depressed reflexes. Depending on whether the disorder is acute or chronic, atrophy and fasciculations may be present (Table 55-1).^2,3,5,6

Involvement of bulbar muscles is manifested by cranial nerve findings, facial muscle weakness, and chewing or swallowing difficulties. Bulbar involvement can occur in both upper and lower motor neuron disorders.¹

In the patient presenting with weakness, it is important to distinguish between neuropathy and myopathy. Neuropathies are disorders of nerves, and tend to produce distal muscle weakness, hypesthesias or paresthesias, and decreased reflexes, especially early in the disease. The progression of weakness and sensory loss is in a stocking-and-glove distribution.^6,7 Myopathies are disorders of muscle, and can be inflammatory, infectious, congenital, or metabolic. Inflammatory myopathies usually involve proximal muscles and are often associated with muscle pain or tenderness. Reflexes become decreased late in the disease. Congenital myopathies tend to involve specific muscle groups and can present at birth with hypotonia and weakness, or in older children with a more insidious progression.^3,8

HISTORY

It is vital to distinguish between acute and chronic disorders, since this information will direct the remainder of the workup. The location of the initial weakness is elicited, and weakness is established as focal or general. Focal weakness is further characterized as predominantly proximal or distal. The rate of progression of symptoms is characterized as acute, which implies minutes to hours; subacute, meaning hours to days; and slowly progressive, which involves a prolonged period. Acute onset or rapid progression implies spinal cord compression or a vascular event involving the spinal cord or brain. Subacute progression can be due to infection, inflammation, toxin, or tumor. Slowly progressive symptoms imply a chronic or congenital disorder.³ Defining the progression of symptoms is facilitated by asking the parents questions regarding progressive difficulty in walking, recent difficulty climbing up or down stairs, or inability to go from a sitting to standing position unaided. The loss of developmental milestones implies a degenerative disorder.⁷

The patient and parents are questioned regarding symptoms preceding the onset of weakness, such as recent illness, fever, headache, neck or back pain, loss of bowel or bladder function, or trauma. Prior episodes of weakness may suggest an intermittent metabolic problem. A family history of weakness suggests a congenital disorder. A history of exposure to drugs or heavy metals suggests poisoning. A pertinent travel history is indicated, as weakness can be a manifestation of entities such as tick paralysis, or from ciguatera poisoning. A careful antenatal history is indicated to rule out a perinatal insult, as is an immunization history to evaluate the possibility of a vaccine-related complication.

PHYSICAL EXAMINATION

The physical examination begins with observation of mental status, posture, gait, and the ability of the child to get on the examining table or to sit unaided. The vital signs are assessed, with particular attention to respiratory rate and effort, as many neuromuscular disorders are associated with a risk of respiratory failure. Blood pressure and pulse are carefully monitored, as some neuromuscular disorders, such as Guillain–Barré syndrome (GBS), are associated with autonomic instability.

The patient’s general appearance is noted, with attention given to general muscular development and the presence of kyphosis, scoliosis, or lordosis, which can all suggest a congenital disorder. Lack of facial expression, snarl, or slack jaw suggests myasthenia gravis. Ptosis can be due to myasthenia or myotonic dystrophy. Inspect the muscles for wasting, fasciculation, or hypertrophy.

The neurologic examination includes an evaluation of pupillary size and reactivity and the remainder of the cranial nerves. If possible, the fundus is examined and the visual fields are assessed. One way to test facial strength is to have children blow out their cheeks and resist compression.⁸ For patients old enough to cooperate, motor strength in the extremities is evaluated and rated on a scale of 0 to 5 (Table 55-2).^1,5,7 For infants who cannot cooperate with the examination, it is possible to perform a general assessment of muscle tone and integrity by holding the baby under the arms and placing the feet on the bed. Infants with normal tone will not slide through an examiner’s hands and will actively kick both legs against the resistance of the bed. In an outstretched prone position, infants supported on the trunk should hold their head up, flex the limbs, and keep their back straight.² Older children can be asked to walk on their toes and heels. The ability to walk on the heels but not the toes suggests intraspinal pathology, although toe walking can occur with upper motor neuron disorders that cause spasticity.⁸

Deep tendon reflexes at the knees, ankles, elbows, and wrists are elicited.¹ Hyperreflexia or sustained clonus indicates an upper motor neuron lesion, whereas absent or decreased reflexes imply a problem in a lower motor distribution.² Other reflexes to be noted include the anal wink, the plantar response, and abdominal and cremasteric responses.

Sensory evaluation includes touch, pain, position, vibration, and temperature. Touch and pain are evaluated by assessing soft versus sharp stimulation and two-point discrimination. Position sense is assessed by asking the child to indicate the direction in which an examiner moves a finger or toe. Temperature sensation can be assessed by the use of a cold stethoscope or by touching the child with cold or warm water. Vibration can be tested using a tuning fork on both thumbs (interphalangeal joint) and big toes.^1,7

The sensations of touch and position-vibration do not cross in the spinal cord on their way to the brain, whereas those of pain and temperature do. An abnormality of touch and position on one side and pain and temperature on the other suggests a cord lesion. The unilateral loss of all sensations suggests a brain lesion. A stocking-and-glove distribution of sensory loss suggests a peripheral neuropathy.

LABORATORY EVALUATION

The laboratory evaluation is based on the provisional diagnosis. Generally, complete blood counts, serum electrolytes, calcium, magnesium, and phosphorus are indicated.^4,7 Elevated serum creatine kinase is nonspecific but is found in children with active inflammatory myopathies and is extremely elevated in congenital myopathies.⁹ Urine is assessed for the presence of myoglobin and, in selected cases, is used for toxicology screening.⁷

For patients with a suspected spinal cord lesion, radiographs are indicated. Even if they are negative, any patient suspected of having a developing lesion of the spinal cord requires evaluation by magnetic resonance imaging (MRI). If this is unavailable, computed tomography (CT) of the spine may be helpful.

For patients with suspected central nervous system lesions, an imaging study of the brain is indicated. Some patients may require a lumbar puncture.³

Electromyography and nerve conduction studies are indicated if lower motor neuron disease is suspected, and to distinguish neuropathic from myopathic etiologies.^2,8,10 Muscle biopsies are also of value to diagnose specific neuromuscular diseases.²

Weakness due to certain causes is common enough in the pediatric ED population to justify specific discussion.

GUILLAIN–BARRÉ SYNDROME