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Esophageal motility disorders are classified as primary when
they are one of a small number of isolated disorders of motility that
include achalasia, diffuse esophageal spasm, nutcracker esophagus,
and nonspecific esophageal motility disorders of the esophagus or
as secondary when associated with known disease
processes (Table 393-1). A basic understanding
of normal esophageal motility allows appreciation of the diagnostic
criteria for various motility disorders.
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The esophagus is a dynamic muscular conduit connecting the oropharynx
to the stomach. Two sphincters, the upper esophageal sphincter (UES) and lower
esophageal sphincter (LES), divide the esophagus from the oropharynx
and stomach respectively. The esophagus is composed of striated
muscle in the upper third and smooth muscle in the lower two thirds. Similar
to the remainder of the gastrointestinal tract, the smooth muscle
portions of the esophagus are divided into 2 layers: the innermost
arranged in a circular pattern and the outer aligned along the longitudinal
axis. Each layer is thought to perform a unique function. The circular
muscle layer collapses the lumen of the esophagus and is responsible
for generating contractile pressure, while the longitudinal layer
shortens the esophagus and may play a role in opening the LES. The
UES consists of C-shaped bundles of muscle fibers from
the cricopharyngeus muscle. The LES consists of components from
the esophagus, stomach, and skeletal muscle from the diaphragm.
At rest, the pressure within the esophagus reflects the intrathoracic
pressure and is therefore slightly negative during inspiration and
slightly positive during expiration. Closure of the UES prevents
swallowing of air, and closure of the LES prevents reflux of gastric
contents during inspiration. The pressure that is tonically maintained
by the UES varies tremendously, being almost absent during sleep
and increasing to over 100 mm Hg with emotional stress, straining,
or when the esophagus is distended or perfused with acid fluid.
The LES maintains a pressure of approximately 20 mm Hg, with values
below approximately 10 mm Hg being abnormal. LES pressure is augmented
during inspiration by contraction of the diaphragm. The LES tone
is decreased by anesthesia, morphine, diazepam, β-adrenergic
agents, dopamine, secretin, cholecystokinin, glucagon, vasoactive
inhibitory peptide, progesterone and estrogen, nitrites, nifedipine,
theophylline, intraduodenal fat, ethanol, and nicotine. Relaxation
of the LES appears to be mediated by the actions of vasoactive inhibitory
peptide and/or nitric oxide.
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Coordination of the muscles of the pharynx, UES, esophagus, and LES
are required for propulsion of food and liquid bolus to the stomach. Swallowing
induces relaxation and opening of the UES to allow transfer of food ...