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perceivedeasily.Direct-current anodicstimulation,ascendingin2-dB steps,isthenappliedtoregions
innervated by three gustatory nerves (greater petrosal, glossopharyngeal, and chorda tympani nerves,
lateralized) using 5-mm-diameter stainless steel electrodes. To assess the chorda tympani nerve, the
measured point is atthe edgeofthe tongue, ~2 cm awayfromthe centerofthetonguetip. Inthearea
innervatedbytheglossopharyngealnerve,stimuliarepresentedonthecircumvallatepapillae.Ifpatients
showastronggagreflex,itispossibletoswitchtothefoliatepapillaenearthepalatinearch.Forthearea
innervatedbythegreaterpetrosalnerveonthepalate,wemeasureatapoint1cmabovethearchofthe
palateand1cmlateraltothecenter(Fig.14.3).
Fig.14.2a,bElectrogustometerRIONTR-06modelusedforresearchanddiagnosisinJapan.
aElectrogustometer.
bElectrogustometerinuse.
TheTR-06electrogustometerallowsthepresentationoflow-currentstimuliofdifferentdurations(0.5,
1.0,and1.5 seconds).Theduration is setat 0.5 secondsinourclinic. Itwas reportedbyLoucksand
Doty
40
thatthedurationofthestimulusinfluencesgustatorythresholds.However,becauseofhabituation
oradaptationtothestimulus,alongerdurationdidnotnecessarilyleadtogreatersensitivity.Inaddition,
tastesensationadaptstorepeatedstimulation,so1.5-secondintervalsareneeded.Thenormalvaluesof
electrictasteare0±8,4±14,and10±22dB,inregionsofthechordatympaninerve,glossopharyngeal
nerve,andgreaterpetrosalnerve,respectively.Itisnormalforthedifferencebetweenleftandrightsides
tobeunder6dBforeacharea.Whentheelectricalstimulusisover20dBattheanteriortongue,patients
canperceivealgesthesiaonthetonguesurfaceordiffusedintotheneck.Asmentionedabove,EGMalso
stimulatesthetrigeminalsystem.However,thetastethresholdisdifferentfromthetrigeminalthreshold.
We needtoconfirmwhichpatientscanperceivedegustationor algesthesia.The thresholdofelectrical
tasteincreaseswithage.Nakazatoetal
41
reportedthatthethresholdsofregionsinnervatedbythechorda
tympani andglossopharyngealnerves inpeoplewerehigher in those over60 years old, andpeople in
their70sor olderdemonstratedasignificantlyhigherelectricaltastethresholdinregionsinnervatedby
thegreaterpetrosalnerve.Theauthorsreportedthatthethresholdsofelectricaltastewereunaffectedby
smokingordentures.Inaddition,thresholdsinwomenarelowerthanthoseinmen.
Fig.14.3Thelocationfortheassessmentofeachtastenerve(quotedfromref.34).N. petrosussuperf.major,greater superficialpetrosal
nerve;N.glossopharyng.,glossopharyngealnerve.
SomatosensoryTest
Tasteandintraoralsomatosensationarecloselyrelated.Tastereceptorsofthetonguepapillaeareclose
to mechanoreceptors, nociceptors, and thermoreceptors. Some taste stimuli may also induce tactile,
thermalsensations, suchas tingling andburning.
42
Some tastants, suchas saltsandacids, may activate
somatosensoryreceptors,andcanproduceburningandtingling.
43
The capsaicinthreshold testhasbeenestablishedto measuresomatosensoryperceptioninhumans.
8
Thistestusescapsaicin-impregnatedfilterpaperstripssimilartotastestrips.
32
Thefilterpaperstripsare
impregnatedwithfiveconcentrationsofcapsaicin(0.0001to1%).Thestripsareplacedmidlineonthe
tonguetipforwhole-mouthtesting,forwhichthemouthisclosed,oronthetongueedgeofbothsidesfor
lateralized testing, for which the tongue is extended. Thresholds related to sensory perception and
intensityaremeasured.Thetestappearsapplicabletobasicresearch,
22,27
butsofarnotforclinicaluse.
Therearenoage-orgender-relatednormativedataforthetest.Thresholdsdependmainlyonfoodhabits,
inparticularingestionofspicyorhotfood.
Diagnosisoftastedisturbanceincludesquestionnaireadministrationandpsychophysicaltastetesting
(whole-mouthtastetestandregionaltasteandsomatosensorytests).
ObjectiveMeasurements
GustatoryEvent-relatedPotentials
Gustatory-evoked potentials elicited by gaseous stimuli (acetic acid for sour; chloroform for sweet;
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thujoneforbitter;ammoniumchlorideforsalty)werefirstreportedin1985.
44
Now,commercialdevices
are available that allow objective assessment of taste-eliciting gustatory event-related potentials in
humans. The custom-made gustometer (gustometer 002, Burghart Messtechnik, Wedel, Germany) (Fig.
14.4)enablesthepresentationofuptofivedifferenttastants(highlyconcentratedsolutionsofsalt,citric
acid,sugar,quinine,orglutamate).Acontinuouswaterflowavoidsthermalormechanicalstimulation.An
initialstudyhasdemonstratedtheusefulnessofthismethodinclinicalpractice,butasyetonlythetaste
qualitysour(citricacid)hasbeenused.
45
Furtherresearchisneededtoestablishalltastequalities.
MagneticResonanceImaging
The neural processes oftasteperceptionandtasteintensity maybevisualizedwithfunctionalmagnetic
resonanceimaging(fMRI).Sofar,twostudieshavebeenpublishedthatinvestigatedneuralrepresentation
oftasteintensityaftersweetandbitterstimulation
46
andaftersweetandsaltystimulation.
47
Severalbrain
regionsareinvolvedinthedifferentiationofsweetandbittertasteintensities:thecerebellum,putamen,
pons,amygdala,andmiddleinsula.
46
SimilarresultswerereportedbySpetteretal
47
Theyfoundthattaste
intensityisrepresentedinthemiddleinsula,butamygdalaactivationwasonlyfoundbystimulationwith
saltiness. Sweet taste intensity was not represented in the amygdala. So far, fMRI has been used to
explorescientificquestions.Inthefuture,fMRImaybecomeapotentialtoolforobjectivetastetestingin
clinicalpractice.
Fig. 14.4 Gustometer for clinicalassessment of gustatoryfunctionusing event-relatedpotentials (gustometer 002, BurghartMesstechnik,
Wedel,Germany);insetshowsthenozzleelementforstimuluspresentation.
InVivoImagingoftheTasteBuds
Attemptstovisualizetheperipheralgustatoryorganinvivowerefirstpublishedin2004.
48
Theauthors
usedaconfocallaserscanningmicroscope(Fig.14.5)toidentifyasinglefungiformpapillaonthehuman
tongue
49
andtomeasurethevolumeofthetastebudsovertime.
50
To facilitate serial investigations of the same papilla, reference papillae are defined. These are
fungiformpapillaewithacharacteristicshapeorarrangementofsurfacetastepores(Fig.14.6).
Preliminarystudies demonstrated differences in taste bud volume over time in healthy humans.
49,50
Volumetric data of taste buds from a patient with hypogeusia and with side-differing measurementsin
gustatorytestingseemtodifferfromthoseinhealthysubjects.
51
Furtherstudiesareneededtoestablishthe
potentialofthisnovelinvivomethodfordiagnosisoftastedisturbances.
Inthefuture,objectivemethodsmaybecomeapotentialtooltoassesstasteprocessingobjectively
andtovisualizeperipheraltasteorgansandneuralprocessesoftasteperception.
CaseReports
Case1
A40-year-oldwomanunderwenttympanoplastyforcholesteatomaontherightside.Herchordatympani
nerve (CTN) was transected during the surgery. This nerve confers general sensations on the tongue
surface.
26,27
Aftersurgery,thepatientcomplainedoftastedisorderandtonguenumbnessintheregionof
rightCTNinnervation.HertastefunctionsbeforeandaftersurgeryareshowninTable14.2.Althoughthe
symptomsbegantodisappear2weeksaftersurgery,theelevatedEGMthresholdcontinuedformorethan
2years.
Saitoetal
52
reportedthatanelevatedEGM thresholdinpatientswithCTNinjuryis not necessarily
relatedtotheirsymptoms.TheyalsoreportedthatCTNregenerationwasobservedin42.3%ofpatients
with a transectedCTN.In anotherstudy, thesymptomsdisappearedmorethan2 years aftersurgeryin
88.9%ofthepatientswith a transectedCTN,regardless ofthe absenceofEGMthresholdrecovery in
54.5%.
16
Subjectivetasteisexpressedasawhole-mouthtastesensation.Inhibitionofglossopharyngeal
nerve functionis released, which compensatesfor CTNdamage.Therefore,a discrepancy inrecovery
betweenthesymptomsandEGMthresholdisoftenobserved.EGMisavailableforlocalizingthesiteof
anatomicaldeficitfollowingdamagetotheCTN.
Case2
A 55-year-old woman had a severe motor vehicle accident. Head magnetic resonance imaging and
computedtomography(CT) showed bilateral frontal brainandcerebelluminjury and occipitalfracture
(Fig.14.7).AlthoughtherewasnotemporalbonefractureonheadCT,shehadfacialpalsyontheright
side.Owingtotheaggravationofcerebralhematomaandedema,sheunderwentcraniotomytoremovethe
hematoma2hoursaftertheaccident.Shepresentedwithageusiaandanosmia4monthsaftertheaccident.
Althoughthe impaireddetection thresholdwas mild,identificationthresholdimpairmentwas severeon
olfactoryfunctiontesting.Hertastetestingresultsatthefirstexaminationand2yearslaterareshownin
Table14.3.Atthefirstexamination,herEGMthresholdwasapproximatelywithinnormallimitstomild
impairment,exceptontherightanteriorside,whichwasinnervatedbythedamagedfacialnerveatthat
time. However, a severely impaired perception of taste was observed on FPD testing. There were
discrepanciesbetweendetectionandidentificationthresholdsinbothsmellandtastetesting.Reiteretal
53
reported that detection threshold levels were useful in evaluating the receptor cell function, whereas
identification scores might be more specific for brain injury. They also reported that spontaneous
recoveryfrompost-traumaticageusiaoccurredmorefrequentlythanthatfrompost-traumaticanosmia.
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Fig. 14.5 Clinical setting for in vivo examination of the gustatory organs: parabolic Plexiglas disc. Inset: contact element of the water
immersionobjectivelens:1,parabolicPlexiglasdisc;2,contactelementofthewaterimmersionobjectivelens.(FromJustetal.
54
)
Fig.14.6a,bLaserscanningphotomicrographsoffungi-formpapillae.
aEn-faceconfocallaserscanningphotomicrographofasinglebutterfly-shapedfungiformpapillawithsixtastepores(labeled1–7)visibleon
thesurface(400×400μm).
bCrosssectionofconfocallaserscanningphotomicrographofafungiformpapilla.Theheightofthetastebudis~40μm.
The patient’s ageusia and anosmia were treated with nasal steroid drops and oral zinc sulfate,
respectively. Although anosmia did not recover atall,ageusia was mostlyimprovedinthis case. The
patient’sfacialpalsyalsorecoveredcompletely.
A marked difference between the two kinds of taste test is observed in patients with several
encephalopathies,forexample,stroke,cerebraltumor,headtrauma,andothercerebralconditions.These
conditionsmayaffectcognitivegustatoryfunctions,butdetectionfunctionisoftenpreserved.Thisfinding
of dissociation can also indicate early damageto the turnover of taste receptors. Electrical taste can
sometimesbeperceivedevenwhenFPDtestresultsareinconclusive.Theoretically,anelectriccurrentis
astrongerstimulusthanchemicaltastant.Asinthiscase,tastefunctiontendstorecover.Therefore,both
qualitativeandquantitativetastetestingmethods,suchasEGMandFPDtests,areneededtoconfirmthe
diagnosis.
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Fig.14.7a,bFindingsfromT1-weightedaxial(a)andT2-weightedsagittal(b)MRIimmediatelyaftertheincidentinCase2.Cerebraledema
andcontusionsareseeninthefrontallobe.
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15TasteDisorders
BasileN.Landis,JosefG.Heckmann
Summary
Tastedisorders are frequentlyencountered inspecializedoutpatient clinics. However, they occur
lessoftenthanolfactorydisorders.Mostpatientswithtastedisorderssufferconsiderably,whichcan
present as gustatory loss, or decreased or changed gustatory perception.Within this chapter, we
presentthedifferentcauses,diagnosticprocedures,andpossibletherapiesfortastedisorders.
Introduction
Talkingabouttastealwaysrequiresclarificationbetweenthemedicaltermtasteand“taste”asitisused
indailylanguage.Tasteusedineverydaylanguagemeansthesensationsoneexperiencesduringeatingor
drinking. This is composed of texture, retronasal smell, and “pure” taste inputs. The physiological
definitionof taste is restricted to the five qualities sweet, sour, salty, bitter, and umami (the taste of
glutamate).Asaconsequence,mostpeopleconfuseolfactionandgustation. Forexample, most patients
witholfactorydysfunctioncomplainofsmellandtastealteration,whiletestingrevealsthatolfactionbut
notthefivebasictastesareaffected.
1
Inclinicalpractice,patientsrarelypresentwithtastedisordersrestrictedtothefivebasictastes.The
reason for this low incidence is not clear. One explanation could be that many taste disorders are
transient,andpatientsonlyseekmedical advice forlong-lasting orpermanent tastedisorders.Another
explanationcouldbethatmanypatientswithtastedisordershaveconcomitant,moretroublesomehealth
problems(e.g.,facial paresis, stroke) andforgetto mentionthe tastedisorder.Tasteassessmentis far
fromaroutineprocedure,andthelowincidenceoftastedeficitsmaysimplyreflectthefactthatthissense
is rarelytested.Finally, tasteisrepresentedbythree cranial nerves per side, makingtaste innervation
redundant.This couldleadtoahigherresistanceoftastetotraumaandinjury,thus explaining thefew
tastedisordersobservedinclinical practice. Anintactsenseoftasteissignificant intheregulationof
nourishment, the recognition ofpoisonous androtten food,andeven for theexperience of the hedonic
aspectsofacommonmeal.
Recentroutinetastetestinginalargesampleofthegeneralpopulationshowedthatmeasurabletaste
impairmentismorefrequentthanpreviouslythought.Inacohortof761healthyindividualsaged5to89
years,reducedtastefunctionwasmeasuredin5%,despitethefactthattheseindividualsconsideredtheir
tastecapacitytobenormal.Mostofthesecaseswerehypogeusia;ageusiawasobservedrarely.
2
While
taking the patient’s history, it is therefore important to ask specific questions such as, “Do you have
problems tasting sweet, sour, salty, or bitter flavors?” The isolated question, “Do you have taste
problems?”istoovagueandwillbeansweredtoooftenwithno,evenwhentasteproblemsexist.
3
Taste problems can be an important sign or symptom in daily clinical practice, as several case
vignettesreport.Severetastedisorderscanoccurafterlaryngoscopy,
4
stroke,
5
temporal bonefracture,
6
dissectionofthebrain-supplyingarteries,
7
inceliacdisease,
8
orevenasaparaneoplasticsymptom.
9
Such
severe taste disorders can lead to anorexia, weight loss, and impairment of well-being. An isolated
impairment of gustatory sensitivity to salt was recently identified as a risk factor for arterial
hypertension.
10
Thereisconfusionbetweensmellandtastesymptoms.Tastedisorderscanbethefirstsymptomof
underlyingpathologies.
TypicalClinicalFindings
Tastedisordersdonothavea distinctpathognomonicclinical finding that can be identifiedeasily.The
clinicalpresentationsareratherheterogeneous,andherewetrytodescribethedifferenttypesofgustatory
disordersthatcanbepresentandhowtheycanbediagnosed.
PatientHistory
Thepatient’shistory,asinmanymedicalproblems,iskeytounderstandingthedisorder.Whenitcomesto
tastedisorders,itmustberememberedthatpatientsdonotalwaysratetheirgustatoryfunctioncorrectly.
11
Sometimespatientsdonotevennoticetastedeficiency.
12
Thus,askingabouttastefunctionismandatory
butunreliable, and becomes meaningful onlywith concurrent testing. When taking a history, emphasis
shouldbeputonspecificquestionsabouttastingthedifferentqualities,productionofsaliva,swallowing
andchewingcapacity,orofacialpain,formerinfectionsorsurgeryintheheadandneckregion(especially
the middle ear), diseasesofthe salivaryglands,hearing orvestibularimpairment,andoral hygiene.In
addition, drug intake (and when this intake began), comorbidities (e.g., diabetes, thyroid dysfunction,
Siccasyndrome[Sjögrendisease],cancer),andformerradiochemotherapyshouldbeascertained.
13
ClassificationofTasteDisorders
Tastedisorders canbeclinicallyclassifiedasquantitativeorqualitativedisorders.Themostimportant
quantitativetastedisordersareageusia,acompletelossoftaste,andhypogeusia,adiminishedtaste.The
most important qualitative taste disorders are dysgeusia and parageusia, characterized by a distorted
senseoftasteoranongoingbadtasteevenintheabsenceofatastestimulant.Thedysgeusiatasteisoften
described as bitter, sour, or metallic. Phantogeusia is a gustatory hallucination and is encountered in
patients with epilepsy or psychotic disorders. Difficulty identifying a tastant that has been properly
perceivedistermedtasteagnosia.Itisassumedthattheanteromedialtemporallobehasanimportantrole
inrecognizingtastequality.
14
Despitethedichotomousclassificationinclinicalpractice,quantitativeand
qualitativetastedisorders oftencoexist.The main differencebetweenqualitativeandquantitative taste
disordersisthatthelattercanbetestedobjectivelyandassessedpsychophysically,whereasqualitative
disorderscannotyetbeassessedbyanyavailabletechnique.Qualitativetastedisordersarecomparable
totinnitusinhearing:afrequent,bothersomecomplaintthatmayoccurwithorwithouthypoacousisand
cannot be measured. Qualitative taste disorders (dysgeusia, phantogeusia, parageusia) are much more
frequentlyencounteredinaclinicalsettingthanquantitativetastedisorders(Table15.1).Analternative
waytoclassifytastedisordersistodistinguishbetweencentralorperipheraltastedisorders(Fig. 15.1)
according to the site of the lesion. Finally, taste disorders can be classified according to their cause
(Table15.2).Congenitalageusiahasneverbeenreportedtothebestofourknowledge(unlikecongenital
anosmia,whichisfrequent
15
).Selectivetastedeficienciessuchasageusiaforasingletastequalityseem
tobe rare(exceptforbitter).Onlyafewreportsoftheseexist,anditis notclearifthisis a resultof
under-reportingortheparticularbiologicalimportanceofanintacttastefunctionpreventingsuchisolated
andcongenitalageusia.
16,17
Table 15.1Clinicalclassificationoftastedisorders(dysgeusia)
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