Weakness
The primary care physician is usually the first individual to evaluate the
patient who has acute weakness. In such patients, it is paramount to localize
the process anatomically, then consider the differential diagnosis. Acute
weakness can originate at the muscle, neuromuscular junction, peripheral
nerve, spine, brainstem, or cerebrum.
Weakness that is due to muscular disorders typically presents
proximally more than distally. The most common causes of acute weakness due
to muscular disorders are polymyositis, dermatomyositis, periodic paralysis,
McArdle disease, and electrolyte disturbances, particularly of calcium and
magnesium. Muscular dystrophy, mitochondrial myopathies, and congenital
myopathies usually do not present acutely but are common causes of chronic
weakness of muscular origin.
Acute weakness due to disorders of the neuromuscular junction include
myasthenia gravis and botulism. Loss of reflexes suggests acute
peripheral nerve processes, the most common of which are acute inflammatory
demyelinating polyneuropathy (Guillain-Barré syndrome), diphtheria,
poliomyelitis, tick paralysis, and toxic neuropathies (eg, lead poisoning).
Chronic causes include chronic inflammatory demyelinating polyneuropathy,
hereditary neuropathies, and leukodystrophy.
Spinal cord weakness is characterized by loss of power and sensation,
frequently with bowel and bladder involvement, inferior to a level of the cord.
Reflexes are increased, except sometimes after acute trauma. Transverse
myelitis, anterior spinal artery infarction, spinal cord compression from tumor,
epidural abscess, and trauma present acutely; tethered cord and hereditary
spastic paraparesis are more insidious. Cerebral and brainstem masses and trauma
frequently present acutely with hemiparesis and occasionally seizure
or cranial neuropathies, respectively.
Once the physician localizes the process, he or she can undertake a rational
diagnostic and management approach. The lower extremity weakness with loss of
reflexes exhibited by the child in the vignette is indicative of a peripheral
nerve process. Although pain in a band distribution can be observed
with transverse myelitis, herpes zoster, and spinal cord infarction, the
peripheral nerve presentation combined with facial weakness in this child is
typical of Guillain-Barré syndrome. Lumbar puncture
often reveals elevated cerebrospinal fluid protein levels with little or no
pleocytosis (albuminocytologic dissociation). Electromyography reveals a
reduction in motor unit recruitment. More importantly, nerve conduction
velocities show prolonged or absent F-wave responses. F waves measure conduction
in the proximal portion of the motor nerve and root, a typical site of
demyelination in Guillain-Barré syndrome.
Except for the facial weakness, there are no focal signs indicative of a
cerebral process that would prompt computed tomography of the head for the child
in the vignette. The absence of reflexes localizes the process of the
peripheral nerve. Electroencephalography also is used to assess patients who
have cerebral disorders such as seizures or depressed mental status. Emergent
magnetic resonance imaging of the spine is appropriate if there is suspicion of
a spinal cord process. Muscle and nerve biopsies are most useful in the
evaluation of chronic weakness from a myopathy or peripheral neuropathy.
References:
Evans OB, Vedanarayanan V. Guillain-Barre syndrome. Pediatr Rev.
1997;18:10-16
Jacobson RD. Approach to the child with weakness or clumsiness. Pediatr Clin
North Am. 1998;45:145-168
Morriss MC, Hyder DJ, Zimmerman RA. Neurodiagnostic techniques. Pediatr Rev.
1997;18:192-203
Roland EH. Muscular dystrophy. Pediatr Rev. 2000;21:233-237
Sarnat HB. Neuromuscular disorders. In: Behrman RE, Kliegman RM, Jenson HB, eds.
Nelson Textbook of Pediatrics. 16th ed. Philadelphia, Pa: WB Saunders Co;
2000:1866-1893