Encyclopedia of Gastroenterology, Three-Volume Set, Volume 1-3

2352 Pages·2003·51.33 MB·English

by  Leonard R. Johnson

EDITOR-IN-CHIEF
L R. J
EONARD OHNSON
University of Tennessee College of Medicine, Memphis, Tennessee
ASSOCIATE EDITORS
DAVID H. ALPERS
Washington University School ofMedicine, St.Louis,Missouri
KIM E. BARRETT
University of California,San DiegoSchool ofMedicine, San Diego,California
JOHN M. CARETHERS
University of California,San DiegoSchool ofMedicine and Veterans AffairsSan Diego
Healthcare System, San Diego,California
MARK FELDMAN
University of TexasSouthwestern Medical School at Dallasand Presbyterian Hospital ofDallas,
Dallas, Texas
GREGORY J. GORES
Mayo Clinic, Rochester, Minnesota
RICHARD J. GRAND
Harvard Medical School andChildren’s HospitalBoston, Boston,Massachusetts
MARTIN F. KAGNOFF
University of California,San DiegoSchool ofMedicine, San Diego,California
RODGER A. LIDDLE
Duke University Medical Center,Durham,North Carolina
SHELLY LU
Keck School ofMedicine, University ofSouthern California,Los Angeles, California
JAMES LEE MADARA
University of ChicagoSchool ofMedicine, Chicago, Illinois
CHARLES M. MANSBACH II
University of TennesseeHealth Science Center and Veterans AffairsMedical Center,
Memphis, Tennessee
ALEXANDER R. MARGULIS
Weill Medical College, Cornell University, NewYork, NewYork
JOHN A. WILLIAMS
University of MichiganMedical School, Ann Arbor,Michigan
DOUG W. WILMORE
Harvard Medical School andBrigham andWomen’s Hospital, Boston,Massachusetts
JACKIE D. WOOD
TheOhio State University College ofMedicine andPublicHealth, Columbus, Ohio
FOREWORD
People of my age often have mixed emotions about These articles go a long way toward covering the
encyclopedias,probablybecausetheyconjureupmem- entire gamut of gastroenterology and hepatology and
ories of writing high-school term papers and reading dosoinanexpertfashion.Theycovergastrointestinal
from 26-volume encyclopedias in the library. Those and hepatic diseases, as well as syndromes, diagnostic
individuals who came of age in the computer era may andtreatmentmodalities,andphysiologicalandpatho-
not fully understand to what I am referring. As I have logicalprocesses.Wherenecessary,discussionsofsome
grownolder,Ihaverediscoveredtheusefulnessofency- ofthediseasesaredividedintoseparatearticlesdescrib-
clopedias, especially those comprising only 1, 2, or 3 ingtheconditioninpediatricpatientsandadultpatients.
volumes. My favorite for general knowledge is the The articles are easy to read, yet comprehensive, and
Columbia Encyclopedia, which was first published in usuallyencompassboththebasicscienceandtheclinical
1935 and is now in its fifth edition. I have used this aspects of that disease or process. Each article begins
encyclopedia on many occasions. For example, while with a glossary of terms that allows the uninitiated to
developing a Grand Rounds lecture on celiac disease, readthearticlebyfillingingapsinunderstanding;abrief
I discovered that the disease was described exceed- abstractthatgivesaconcise,butcomprehensive,over-
ingly well in the second century A.D. by Aretaeus the viewofthesubjectmatterfollowstheglossary.
Cappadocian. Where is Cappadocia? Is it an ancient Modern gastroenterology and gastrointestinal
city of Greece? Knowing that someone would ask me sciencelendthemselveswelltoanencyclopediaformat.
thesequestions,IconsultedtheColumbiaEncyclopedia Gastroenterology is a broad clinical field comprising
and learned that Cappadocia was the ancient Hittite issues that concern human behavior and psychology,
state located in what is now central Turkey. Recently, otherdisciplinesthatareusefulforunderstandingfunc-
I was asked to be on the thesis committee of a young tional gastrointestinal diseases, such as dyspepsia and
doctoral candidate in our Institute for Medical Huma- irritable bowel syndrome, and very technical sciences
nities.Hisdissertationproposalreferredtophilosophies that encompass endoscopy as a diagnostic and thera-
(e.g., hermeneutics and phenomenology) that I had peutic tool and often surgery as definitive treatment.
never heard of. These topics were described in the Inthemiddleofthespectrumisclassicalinternalmed-
Columbia Encyclopedia in a brief capsule format that icineasitrelatestothegastrointestinaltractandliver.
did not overwhelm my untrained and uneducated Furthermore,thebehavioral,medical,endoscopic,and
mind. Therefore,Ihavebecomeafanofsmall-volume surgicalapproachesmaydifferconsiderablyforpedia-
encyclopedias.They are quite useful. tric patientsversusadultpatients.
Why,then,hasapublishernotproducedsuchency- Asregardsbasicgastrointestinalscience,thegastro-
clopedias for the fields of medicine and the biological intestinal tract does more than simply process and
sciences?Infact,indeedonehas.Elsevierhaspublished assimilate nutrients and water through the action of
severalsuchencyclopedias,includingTheEncyclopedia its digestive enzymes and secretory and absorptive
ofCancer(nowinitssecondedition),TheEncyclopedia processes. Gastrointestinal science also encompasses
of Hormones, and The Encyclopedia of Toxicology. diverse fields such as endocrinology, immunology,
Now,under the leadershipof Editor-in-ChiefLeonard and the neurosciences. If all the endocrine cells of
R.Johnson,Ph.D.,arespectedgastrointestinalscientist, the gastrointestinal tract were combined into one
educator, editor, and author, Elsevier has published organ, it would be the largest endocrine organ in the
The Encyclopedia of Gastroenterology. Dr. Johnson has human body. Similarly, the mucosal immune cells
assembledacadreof15AssociateEditors,eachahighly and the gastrointestinal-associated lymphatic tissue
respectedexpertinoneormoreofthetopicsfeaturedin together constitute perhaps the largest organ of the
thebook.Theyhavebroughttogetherover700authorsto immune system in the body. Furthermore, this entire,
write477separatearticles,dividedintothreevolumes. complex epithelial, secretory, absorptive, endocrine,
li
lii FOREWORD
and immunological organ is controlled by its own Basic scientists and nonphysician translational
intrinsicbrainandnervoussystem,theentericnervous research scientists would certainly benefit from this
system. encyclopedia also. For instance, the mixture of basic
Gastroenterologyismadeevenmorecomplicatedby scienceandclinicalscienceinformationineacharticle
thefrequencywithwhichdiseasesofthissystemoccur is precisely what the basic scientist needs as he or she
and by its close relationship to other disciplines. Gas- writes the Introduction or Discussion sections of pub-
trointestinalcanceristhesecondmostcommontypeof lications or the Background section of grant applica-
cancer,ifmenandwomenareconsideredtogether.The tions. In addition, the encyclopedia would prove
broadfieldofnutritionborderscloselyonandisinter- quite valuable in rapidly bringing scientistsup to date
twined with gastroenterology. Finally, the intestinal inaspecificareaofgastrointestinaldisease.Inthiseraof
tract is colonized by commensal microbiota that are transgenicanimals,itisnotuncommonforthescientist
crucial for optimal health. Little is known about these whohasbeenconductingresearchinaspecificfield,for
microorganisms, but it is beginning to be understood example,immunologyorrheumatology,tocreateanew
thattheymaybeavehicleforthetreatmentorpreven- knockout mouse that presents with a gastrointestinal
tion of disease through probiotics. phenotype rather than a rheumatological phenotype.
This broad view of gastroenterology and gastro- Thus,thescientistwhohasspenthisorhercareergain-
intestinal science must have made it difficult for the inganunderstandingofrheumatologicdiseasewillneed
Editors to choose the individual articles that make up to quickly acquire a basic understanding of Crohn’s
thesethreevolumes.However,Ifindthelisttobefairly diseaseand ulcerative colitis.
completeandeacharticletobeagoodmixtureofbasic Finally, the gastroenterologist or gastrointestinal
knowledge and clinical information. If certain subject scientistcancertainlyutilizethisencyclopediaaswell.
areas are not represented as specific articles, they are It is impossible to stay abreast of all areas of gastro-
reasonablywellcoveredinotherarticlesthatareinthe enterology and gastrointestinal science and yet the
threevolumesandcanbelocatedinthesubjectindexat overlapping disciplines within gastroenterology may
the end of the thirdvolume. demandamoredetailedknowledgeofanotherwisedis-
Who will use The Encyclopedia of Gastroenterology tant field of expertise. Thus, I look forward to having
and why? I believe that the range of potential users is thesethreevolumesonmybookshelf.Theywillbehelp-
wide. For example, medical students often have diffi- fulinmyclinicalpracticeofgastroenterologyandalso
cultywithmedicaltextbooks.Eitherthetextbooksare helpfultomeasascientistandtootherresearchscientists
tooadvancedandthevariouswordsandtermsarenot inmylaboratory.
explained,makingitdifficultforthestudenttocompre- In summary, the Editors should be proud of their
hendthetext,orelsethecontenthasbeenreducedtoa contributiontotheknowledgebaseofgastroenterology.
‘‘mini’’ textbook version that often lacks substance. Theyhavefoundanicheinourfieldthathashithertonot
I believe that many of the entries in this encyclopedia been occupied. There will continue to be a need for
aregearedperfectlyformedicalstudentsnewtoclinical more elementary dictionaries and for highly detailed,
medicine.Theglossaryoftermsatthebeginningofeach advancedtextbooksandmonographs.TheEncyclopedia
articleandtheabstractsshouldbeextremelyhelpfulfor ofGastroenterologywillplayaroleinthemiddleofthis
thosewhoaremedicallynaive.Furthermore,thearticles spectrum and should be extremely valuable to a wide
arewell-craftedcombinationsofbasicscienceandclin- range of users. I believe that the three volumes will
icalscienceandthisisusefulatthemedicalstudentlevel. find their way onto the bookshelves of most medical
Physiciansfromotherdisciplineswillundoubtedly libraries, as well as those of individual practitioners
find The Encyclopedia of Gastroenterology to be a valu- of medicine and active gastrointestinal investigators.
able reference work. The explosion of medical knowl- The Encyclopedia of Gastroenterology is an extremely
edgehasmadeitdifficult,ifnotimpossible,tokeepup usefuladditionto ourfield.
with advancements in other areas of medicine. This
encyclopedia provides concise descriptions of the var-
ious gastrointestinal diseases that are easily readable, DON W. POWELL
complete, and up-to-date. This should be quite useful
Professor, Internal Medicine
fortheprimarycarephysicianoraspecialistinanother
Professor, Cellular Physiologyand Molecular Biophysics
disciplinewhoneedstoknowaboutsomespecificgas- Associate Deanfor Research
trointestinal diseaseorprocess. Interim Director,GeneralClinical ResearchCenter
PREFACE
The Encyclopedia of Gastroenterology bridges basic Many of these are review articles with comprehensive
science and clinical gastroenterology in a way that bibliographies.
shouldappealtotheexpertresearchingatopicoutside This workbeganwiththeselection ofanumberof
hisorherfieldofexpertiseaswellastostudentsandthe general areas of coverage. A group of 15 Associate
educated public. Although some articles on the basic Editors, each of whom is an expert in at least one of
medical sciences stand alone, most integrate these these areas, was subsequently recruited. The entire
topics with areas of clinical medicine. These volumes group, along with members of the Elsevier staff, then
appear at a time when general interest in, and knowl- metfortwodaysinSanDiego.Thatmeetingresultedin
edgeof,thegastrointestinaltractisexpandingatarapid the refinement of the areas of coverage, selection of
rate. Research has led to new approaches in the treat- individualarticletopicswithinthoseareas,andidenti-
ment of many gastrointestinal diseases and a plethora ficationofpotentialauthors.TheAssociateEditorsandI
of new drugs have been added to the pharmaceutical believe that we have covered the important topics of
armamentarium.Diagnosticprocedureshaveadvanced basicandclinicalgastroenterology.Eacharticleiswrit-
remarkably over the past few years, leading to an tentostandaloneasacompletesubject,sothereis,no
increased understanding of how diseases develop and doubt, a certain amount of overlap. This, however,
an improved ability to detect them. shouldmakeiteasierforthereadertolocateaspecific
The reader will find articles related to all areas of pieceofinformation.
gastroenterology.Therearearticlescoveringbasicphy- Aproductofthismagnituderepresentstheknowl-
siology, pharmacology, anatomy, immunology, and edgeandeffortsofalargenumberofindividuals.More
microbiology.Othersrelatethesebasicfieldstospecific than 600 authors contributed their expertise to the
diseases. Many of these articles are entitled with the individual articles. I am especially grateful to those
name of a disease or pathological condition. When whoproducedarticlesonshortnotice,sothatourdead-
appropriate, nutritional aspects of clinical conditions linecouldbemet.Anoutstandinggroupof15Associate
are emphasized and several articles feature aspects of Editorswasthefoundationforthisproject.Duetotheir
nutrition.Areasofparasitologyofspecialimportancein greatbreadthofknowledge,theywereabletopropose
relation to the gastrointestinal tract are also covered. topicsforarticlesandrecommendtheauthorstowrite
Separatearticlestreattopicsrelatingprimarilytopedia- them. They then recruited the authors and edited the
tric gastroenterology and numerous entries are con- completedarticles.
cerned with radiology, endoscopy,and surgery. Finally,Iacknowledgethecontributionsofthestaff
The articles are written and organized to serve as at Elsevier. The Encyclopedia of Gastroenterology was
convenient,yetcomprehensivesourcesofinformation. initiated and supported by Jasna Markovac, Sr. vice
Eachofthe477entriesbeginswithaglossaryofcross- President, Elsevier, Science and Technology. Nick
referencedtermsfollowedbyabriefabstract.Generous Panissidi,Senior Developmental Editor, was indispen-
useofprimaryandsecondaryheadingsallowsthereader sableashecontactedauthorsandkeptupwithallarticle
tolocatematerialrapidly.Tablesandfiguresemphasize submissions. Pat Gonzalez served as Production Man-
important points and concepts. Each article presents ager,andTariPaschall,Sr.PublishingEditor,andJudy
core knowledge, time-tested and generally accepted Meyer, Associate Publishing Editor, provided overall
withinthefield.Asaresult,therearenospecificrefer- management of theproject.
ences with the entries. Each contribution, however,
concludes with a list of references for further reading. LEONARD R. JOHNSON
liii
A
Abdominal Aortic Aneurysm
GORAV AILAWADIANDGILBERT R. UPCHURCH, JR.
Universityof Michigan,AnnArbor
abdominal aortic aneurysm Permanent dilation of the A 5-cm aneurysm generally carries a 25(cid:255)30% 5-year
abdominal aorta at least 50% greater than the expected rupturerisk,andincreasingaorticsizecarriessubstan-
normaldiameter. tiallyhigher risksofrupture.RupturedAAAsresultin
arteriomegaly Diffuse arterial enlargement more than 50% an overall mortality of greater than 75%, with nearly
above normal.
one-half of these patients dying prior to reaching a
dissecting aneurysm Type of aneurysm that dissects;
hospital. Risk factors for rupture include smoking,
althoughaneurysmsrarelydissect,adissectionmaylead
hypertension, chronic obstructive pulmonary disease,
toaneurysmal changes over time.
andaneurysmsize.Currentrecommendationsarecon-
ectasia Arterial dilation less than 50% of expected normal
servativemanagementuntilanAAAreaches5(cid:255)5.5cm,
diameter.
false aneurysm (pseudoaneurysm) Type of aneurysm that when the risk of rupture is greater than the risk of
involves a disruption of the arterial wall with contain- elective repair.
ment bysurrounding tissue orhematoma.
fusiform aneurysmSpindle-shaped aneurysm.
SIGNS AND SYMPTOMS
infrarenal aortic aneurysm In the anatomic classification
scheme, limitedto aortabelow therenalarteries.
ThemajorityofpatientswithAAAsareasymptomatic,
pararenal aortic aneurysm In the anatomic classification
andonlyapproximately50%ofAAAsaredetectableon
scheme,comprisingajuxtarenalaneurysm(near,butnot
physicalexam.Manyarediscoveredincidentallyduring
involving, the renal artery orifices) or a suprarenal
workup for unrelated problems. The classic triad of a
aneurysm (involving the renal arteries but not the
rupturedAAAincludesapalpablepulsatileabdominal
superior mesenteric artery).
saccular aneurysmEccentrically shaped aneurysm. mass, hypotension, andabdominalor back pain.
thoracoabdominal aneurysm In the anatomic classification
scheme, involving the suprarenal mesenteric vessels;
SCREENING AND DIAGNOSIS
may also involve the descending thoracic aorta in the
chest. Ultrasoundis an excellent screening test in an asymp-
true aneurysm Type of aneurysm that involves all three tomaticpatientsuspectedofhavinganAAA.Inprepara-
layers ofarterial wall.
tion for AAA repair, patients often undergo an
abdominalcomputertomography(CT)scanwithintra-
Abdominal aortic aneurysms are a disease primarily of
venouscontrasttodelineatetheextentoftheaneurysm.
elderly, Caucasian males; rupture risk correlates with
In addition, a CT scan with three-dimensional recon-
aneurysm size. Elective repair is generally undertaken
structionsmayhelptodeterminethefeasibilityofendo-
in patients with an abdominal aortic aneurysm of
vascular AAA repair (Fig. 1). Patients with significant
5(cid:255)5.5cm. Unexplained abdominal or back pain should
azotemia may undergo magnetic resonance angiogra-
raisesuspicionforaorticrupture.Aorticrupturecarriesa
phytoavoidtheriskofcontrastnephropathy.Anaor-
significantmortality.
togram is performed when concurrent renal or
mesenteric disease is suspected.
NATURAL HISTORY AAA REPAIR
Abdominalaorticaneurysms(AAAs)arethe15thlead- Charles DuBost performed the first aortic aneurysm
ingcauseofdeathintheUnitedStates.AAAshavea4:1 repair in 1951 using an aortic homograft. With their
male:female predominance and are 3.5 times more contributions, DeBakey, Cooley, and Crawford
common in Caucasians than in African-Americans. improved outcomes of modern AAA repair. Despite
Nearly 90% of aortic aneurysms are infrarenal. improved survival with elective AAA repair, emergent
EncyclopediaofGastroenterology 1 Copyright2004,Elsevier(USA).Allrightsreserved.
2 ABDOMINAL AORTIC ANEURYSM
specifictothegastrointestinalsystemincludeischemic
colitis and aortoduodenal fistula. Early postoperative
signs of abdominal pain, distension, or bloody stools
shouldpromptimmediateevaluationwithflexiblesig-
moidoscopy. Aortoduodenal fistula is a late complica-
tionfollowingAAArepairandismainlyduetoerosion
of the proximal aortic suture line into the duodenum.
Hematemesis or hematochezia should prompt upper
endoscopy in any patient with an AAA or a history of
AAArepair.
See Also the Following Articles
ComputedTomography(CT)(cid:15)Ultrasonography
FIGURE 1 Three-dimensional CT scan of aorta in a patient
beingevaluatedforendovascularAAArepair,showingtheprox-
Further Reading
imalaneurysmextent,involvementoftheiliacarteries,andother
anatomiclandmarksimportantduringrepairofanAAA,suchas
Cronenwett, J. L., Murphy, T. F., Zelenock, G. B., Whitehouse,
excessiveaorticthrombus(T)orcalcification.
W. M., Jr., Lindenauer, S. M., Graham, L. M., Quint, L. E.,
Silver, T. M., and Stanley, J. C. (1985). Actuarial analysis of
rupturedAAArepairisstillassociatedwithahighmor- variable associated with rupture of small abdominal aortic
tality rate. Endovascular treatment with aortic stent aneurysms.Surgery98,472(cid:255)483.
Dimick,J.B.,Stanley,J.C.,Axelrod,D.A.,Kazmers,A.,Henke,P.
grafts, first performed in 1991, has become an option
K., Jacobs, L. A., Wakefield, T. W., Greenfield, L. J., and
inpatients who meet specific anatomic criteria.
Upchurch, G. R., Jr. (2002). Variation in mortality after
abdominalaorticaneurysmectomyintheUnitedStates:Impact
ofhospitalvolume,gender,andage.Ann.Surg.235,579(cid:255)585.
POSTOPERATIVE COMPLICATIONS
Katz, D. J., Stanley, J. C., and Zelenock, G. B. (1994). Operative
mortality rates for intact and ruptured abdominal aortic
Myocardialinfarctionandpulmonaryfailuremayoccur
aneurysms in Michigan: An eleven-year statewide experience.
following AAA repair. Postoperative complications J.Vasc.Surg.19,804(cid:255)817.
Achalasia
JACKIE D. WOOD
TheOhio State University Collegeof Medicineand PublicHealth
achalasiaFailureofthesmoothmuscleofthedigestivetract large intestine. An obvious consequence of achalasia
torelax. inanyofthesesphinctersisobstructionoftheforward
gastrointestinal sphincter A ring of circular muscle that passageoflumenalcontentsfromonecompartmentto
contracts continuously and closes the lumen of the another.
alimentarycanal.
lower esophageal sphincter A ring of circular muscle that
closes the orifice between the esophagus and the
stomach. PHYSIOLOGY
sphincter of Oddi A ring of circular muscle that closes the
entryofthe bileduct into thesmall intestine. The specialized physiology of the sphincteric circular
muscle coat accounts for the ability of the muscle to
The term achalasia is derived from the Greek word sustain contraction. The contractile apparatus of
chalasis, which in English translates to relaxation. these smooth muscles consists of the two proteins
Achalasiaisdefinedasfailureofrelaxationofthesmooth actinandmyosin.Contractionoccursduringformation
muscle in any region of the digestive tract. It is most of cross-bridges between actin and myosin filaments.
commonly applied to describe failure of relaxation in Contractiletensionismaintainedbya‘‘catch’’mechan-
thevarioussphinctersinthegut.Sphinctersareregions ism that latches the cross-bridges in place without
where the circular muscle coat normally persists in a expenditure of additional energy. Input from the ner-
continuous state of contraction that produces a ring- vous system leads to uncoupling of the cross-bridges,
likeclosureofthelumen.Sphinctersfunctiontoprevent relaxation of contractile tension, and opening of the
thebackfluxoflumenalcontentsfromonedigestivecom- sphincter. The nervous input involves the release of
partment to another. Prevention of the backward move-
chemicalneurotransmittersatneuromuscularjunctions.
ment of the acidic contents of the stomach into the
Sphincteric muscles are innervated by motor neu-
esophagusisanexampleofsphinctericfunction.
rons in the enteric nervous system. Most of the
motor neurons to the sphincters are inhibitory
A nervous mechanism relaxes the sphincters with motor neurons. Firing of nerve impulses by the inhi-
appropriate timing to open the lumenal orifice and bitory motor neurons releases inhibitory neurotrans-
permit passage from one compartment to the next. mitters at their junctions with the smooth muscle
Relaxation is transient, with contraction and closure fibers of the sphincter. Two important inhibitory
occurring after passage of the material through the neurotransmitters are vasoactive intestinal polypep-
sphincter. During a swallow, the lower esophageal tide and nitric oxide. These neurotransmitters act
sphincter relaxes to permit passage into the stomach. to inhibit contraction of the smooth muscle and
Emptying of the stomach occurs during transient open the sphincter. Decisions as to when to open
relaxation of the sphincter located at the junction a sphincter are made by integrative neural networks
with the small intestine. Delivery of bile from the located either in the central nervous system or in the
gallbladdertothesmallintestineoccursduringrelaxa- gut itself. The neural networks control the firing of
tionofthesphincterofOddilocatedattheopeningof the inhibitory motor neurons. The inhibitory motor
the bile duct into the small intestine. Passage of con- neurons are silent most of the time and the contrac-
tents from the small intestine into the large intestine tile behavior inherent in the muscle holds the
takes place during relaxation and opening of the sphincter in a closed state. Activation of the inhibi-
sphincter that separates the two dissimilar compart- tory motor neurons releases contractile tension in the
ments. The internal anal sphincter relaxes to permit muscle and the sphincter opens. Contractile tension
passage of feces during defecation and then closes to redevelops and the sphincter closes coincident with
prevent inopportune release of the contents of the cessation of motor neuronal firing.
EncyclopediaofGastroenterology 3 Copyright2004,Elsevier(USA).Allrightsreserved.
4 ACHALASIA
PATHOGENESIS the surfaces of enteric neurons that are similar to
antigensexpressedbythetumorcells.Testsinpatients
Sphincteric physiology predictsthatlossoftheinhibi-
withloweresophagealsphincterachalasiaindicatethat
tory motor innervation will result in achalasia. The
asignificantproportionofthesepatientshavecirculat-
inherent contractility of the sphincteric musculature
ingantibodiesthatreactwithentericneurons.Achalasia
keeps the sphincter closed and opening cannot occur
inChagas’diseaseissimilartoparaneoplasticsyndome
whentheinhibitoryinnervationismissing.Thepatho-
inthatantigenicepitopesexpressedbytheblood-borne
physiologic result is an obstruction to passage of the
parasite Trypanosoma cruzi are sufficiently similar to
lumenalcontentsthroughthesphincter.Materialaccu-
antigens on enteric neurons that an immune response
mulatesanddilatesthedigestivecanalproximaltothe
toentericneuronsfollowstheresponsetotheparasite.
sphincterbecause propulsivemotility generallymoves
Congenital absence of enteric neurons, including
lumenalcontentsinthedirectionfrommouthtoanus.
inhibitorymotorneurons,occursinHirschsprung’sdis-
Achalasiainthesphincterbetweenesophagusandsto-
ease.MutationsinRetandendothelingenespreventthe
mach(i.e.,theloweresophagealsphincter)canleadtoa
fetal development of the enteric nervous system in a
gross dilation of the esophagus described as a mega-
variablelengthoflargeintestineincludingtheinternal
esophagus. Likewise, achalasia in the internal anal
analsphincter.Theinternalanalsphincterisachalastic
sphincter obstructs the passage of feces and can lead
and presents an obstruction to the passage of feces. A
to a megacolon. Achalasia in the sphincter of Oddi
megacolon develops as feces accumulates proximal to
obstructs the delivery of bile to the small intestine
theneurallydeficient segment.
and can lead to overdistension of the biliary tree that
is experienced as pain in the right upper abdominal
quadrant. See Also the Following Articles
Lossoftheinhibitoryinnervationoccursinparallel
withgeneralizeddystrophyintheentericnervoussys- AnalSphincter(cid:15)Chagas’Disease(cid:15)ParaneoplasticSyndrome
temthatmaybeacquiredorcongenital.Themostcom-
monlyacquiredformreflectsanautoimmuneattackon
Further Reading
the enteric nervous system that may be related to the
presence of a tumor elsewhere in the body, may be Castell, D., and Richter, J. (1999). ‘‘The Esophagus,’’ 3rd Ed.
related to an infectious agent, or may be idiopathic in LippincottWilliams&Wilkins,Philadelphia.
nature.Autoimmuneattackontheentericnervoussys- Verne, G., Sallustio, J., and Eaker, E. (1997). Anti-myenteric
neuronal antibodies in patients with achalasia: A prospective
temoccursinassociationwithsmall cell carcinomaof
study.Digest.Dis.Sci.42,307(cid:255)313.
the lung. This is called paraneoplastic syndrome and
Wood, J., Alpers, D., and Andrews, P. (1999). Fundamentals of
occurs when the immune system attacks antigens on neurogastroenterology.Gut45,1(cid:255)44.
Aging
MAKAU LEE
Universityof Mississippi MedicalCenter
aging Postmaturational changes occurring between middle thinning of the tongue, involving both mucosa and
ageandoldage,resultinginreductionofthefunctional muscles, and weakening of muscles of the mouth and
capacityofthephysiologicalsystemsandincreaseofthe pharynx)andfunctionalchanges(suchasalterationsin
vulnerabilityof anorganism tochallenges and diseases. pharyngealsensation,proprioception,andtasteacuity;
digestivesystemOrgansystemresponsiblefordigestionand
discoordination of masticatory muscles, resulting in
absorption.
prolongedswallowing;andalteredperistalticresponse
afterdeglutition).Manometricstudieshaveshownthat
Aging, the postmaturational changes occurring between
upperesophagealpressureisdecreasedanduppereso-
middleageandoldage,reducesthefunctionalcapacityof
phagealsphincterrelaxationafterdeglutitionisdelayed
thephysiologicalsystemsandincreasesthevulnerability
in the elderly. Salivary output and flow remain
of an organism to challenges and diseases, thereby
unchanged inthe elderly.
increasingthelikelihoodofdeath.
Clinically, these age-related changes in oro-
pharyngeal structures may affect the elderly patient’s
PHYSIOLOGY OF AGING ability to swallow liquids and may lead to reduced
food intake. Reduced food intake and malnutrition
Therearenumeroustheoriesofaging;andtheycanbe
intheelderlycanalsooccurforvariousreasons,includ-
classified into two main categories: the ‘‘programmed’’
ing endogenous or medication-induced depres-
theories, which view aging as the result of a predeter-
sion, anorexia, social isolation, physical handicaps
mined genetic blueprint, and the ‘‘wear and tear’’ the-
that interfere with the elderly person’s ability to
ories,whichviewagingastheconsequenceofcontinual
prepare food, dental problems or ill-fitting dentures,
stress and injuries. Although none of the theories of
and forgetfulness and failure to eat as the result of
agingcansingularlyexplaintheagingprocess,available
diseases of the central nervous system. Furthermore,
dataindicatethattheultimatemechanismofagingisthat
structural lesions (such as Zenker’s diverticulum) and
of molecular changes: all of the changes involve both
upperesophagealsphincterdysfunction(suchascrico-
informational molecules (such as DNA and RNA) and
pharyngeal achalasia) occur more frequently in the
structuralmolecules(suchaslipids,carbohydrates,and
elderly.
proteins) in a process that is determined genetically
andepigenetically.
Both clinical and basic investigations have demon- AGING AND THE ESOPHAGUS
strated that aging is associated with specific physiolo-
Manometric studies have demonstrated minor to mild
gical and structural changes in the digestive system.
changesinesophagealmotilitywithaging.Forinstance,
These age-related changes in the digestive system
there are reductions in contractile velocity and ampli-
maycontributetothedevelopmentofvariousdigestive
tude,increasesinsynchronouscontractions,polyphasic
disordersthataremorecommonamongtheelderly.In
waves and tertiary contractions, upward displacement
theclinicalsetting,whensymptomsandsignsoccurin
of lower esophageal sphincter, incomplete lower eso-
elderly patients, the physician must differentiate age-
phagealsphincterrelaxation,andfailureofcontractions
associated physiological changes from the conse-
after deglutition in the distal esophagus. Radiographic
quences ofdiseases.
evaluations frequently reveal dilatation of the esopha-
gusandincreasesinsynchronousandtertiarycontrac-
AGING AND OROPHARYNGEAL
tions in the elderly. Presbyesophagus (or corkscrew
STRUCTURES
esophagus),whichdenotestheincreaseintertiarycon-
Age-related physiological changes in oropharyngeal tractions detected radiographically, has no known
structures include structural alterations (such as pathophysiologic consequence. Available data suggest
EncyclopediaofGastroenterology 5 Copyright2004,Elsevier(USA).Allrightsreserved.