Tiirkish Neiirosiirgery 11: 79 - 86, 2001 Ziyai: Approaclres /0 Ihe Bmiiisleiii

Posterior and Lateralto the Brainstem

Anterior,Approaches

(Ön, Arka ve Yandan Beyin Sapina Yaklasimlar)

IBRAHIM M. ZIY AL, TUNÇALP ÖZGEN

Hacettepe University SchooI of Medicine, Department of Neurosurgery, Ankara

Received : 26.2.2001 ~ Accepted : 22.6.2001

Abstract: TIie aim of this study was to demonstrate posterior,postero-IateraI, IateraI, antero-IateraI and anteriorapproaches to the brainstem, and to discuss the indicationsfor each approach. Twenty cadaver head specimens (40 sidesin total) were used biIateralIy to demonstrate differentapproaches to the brainstem. The posterior approaches wcresupracerebeiiar, combined supra- and infratentoriaI­transsinus, subtonsiIIar-transcerebelIomedulIary, andmidiine transvermian approaches. The postero-IateraIapproaches were retrosigmoid, trans-sigmoid, and extremeIateraI-transcondyIar approaches. From the IateraI direction,subtemporaI-transzygomatic and petrosaI approaches werestudied. From the antero-IateraI aspect of the skull, \Veperformed orbitozygomatic-transsyIvian approach, and theanterior approach studied was transmaxiIIary-transoraIapproach. Three-hundred and sixty degree visualization ofthe brainstem was achieved with all these approaches .. Theinstrument distance required to reach the brainstem via theanterior and posterior approaches was greater than thatrequired with the antero-IateraI, IateraL, and postero-IateraIapproaches. A variety of neurosurgicaI approaches are usedto access the brainstem. Good knowIedge of the surgicaIanatomy and of the indications for each approach areessential. However, safeIy gaining access to the brainstemis aIways challenging, and further research is needed in thisarea.

Key Word s: Brain stem, skull base, surgicaI approach

Özet: Bu çalisma, posterior, postero-IateraI, IateraI, antero­IateraI ve anteriordan beyin sapina cerrahi yakIasimIariortaya koymak ve cerrahi endikasyonIarini tartismakamaci iIe gerçekIestiriIdi. Çalismada iki tarafli oIarak 20kadavra spesimeni kullaniIdi. Posteriordan:Supraserebellar, kombine supra -ve infratentoryal­transsinüs, subtonsiIler-transserebellomedüller, orta hattransvermian; postero-Ia terald en: retrosigmoid,transsigmoid, uzak IateraI-transkondiIer; Iateralden:subtemporal-transzigomatik, petrazaI; antero-lateralden:orbitozigomatik-transsyIvian; ve anteriordan:transmaksiiier-transoraI yakIasimIar uygulandi. TümyakIasimIar kullanilarak kafatasinin her yönünden beyinsapina 360 derece yaklasim sagIandi. Anterior ve posterioryakIasimIarda, antero-IateraI, IateraI ve postero-IateralyakIasimIarla karslIastirilinca, beyin sapina uIasabiImekiçin daha uzun enstrüman mesafesine ihtiyaç oIdugugözIendi. Sonuç oIarak, beyin sapina ulasmak amaci iIefarkli nörosirürjik yakIasimIar uyguIanabilir. HeryakIasim için cerrahi anatominin iyi biIinmesi veendikasyonun dogru konuImasi gereklidir. BununIabirlikte, beyin sapina güvenli bir sekiIde giris yolIari halasorguIanmakta ve daha iIeri çalismaIari gerektirmektedir.

Anahtar Kelimeler: Beyin sapi, cerrahi yaklasim, kafatabani

INTRODUCTION

A variety of skuIl base approaches are used toaccess the surface of the brainstem (2, 13, 14, 24). Tominimize morbidity and mortality, it is crucial to gain

maximum exposure via the most direct route. Thedecision regarding which approach should be usedmust alsa take into account the neurosurgeon'sknowledge of the anatomy involved, and whetherhe or she is prepared to perform the technique.

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Tiirkish Neiirosiirgery 11: 79 - 86, 2001

However, even when the surgeon has all thenecessary anatomical information, it is always achallenge to access the brainstem safely (2, 6, 10,22,24). This report describes the important anatomy inseveral approaches to the brainstem, and discussesthe various strategies for removing lesions in thebrainstem.

MA TERIAL AND METHOD

Twenty cadaver head specimens (a total of 40si des) injected with microfil were used todemonstrate several approaches to the brain sternsurface.

SURGICAL APPROACHES TO THE

BRAINSTEM SURFACE

Approaches to the brainstem from theposterior, postero-Iateral, lateral, antero-Iateral andanterior aspects of the skull enable the surgeon toreach lesions in a variety of locations (Table 1)(Figure 1).

Ziyai: Approiichcs lo Ihe Brniiislriii

Table 1: Anterior, posterior and lateral approachesto the brainstem

Anterior, Posterior and Lateral Approachesto the Brainstem

PosteriorSu pracerebellarCombined Supra and lnfratentorial­Transsinus

Subtonsillary Transcerebeiiomed u lIa ryMidline Transvermian

Postero-Lateral

RetrosigmoidTranssigmoidExtreme Lateral, Transcondylar

La.le.i:al

Subtemporal TranszygomaticPetrosal

Partial LabyrinthectomyRetrolabyrin th ineTransla byrin thine

Antero-lateral

Orbitozygoma tic- TranssylvianAnterior

Transmaxillary- TransoralTransbasal

ri O'ii LV II! "I',

Figure 1: Surgica! approaches to the braInstein

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Tiirkisii Neiirosiirgery 11: 79 - 86, 2001 Ziyai: Apprilaciie, lo iii~ Bmil1sl~llI

Postera-Lnteml Approaches:For ponto-medullary lesions that present in the

middle or inferior cerebellar peduneles, or in theolivary area, a far-lateral retrosigmoid approach isoptimaL. Extreme-Iateral transcondylar orretrocondylar approaches (16, 27) can be used toreach the lateral and antero-lateral surface of the

medulla, and the cervico-medullary junction (Figure4A and B). A combined retro- and pre-sigmoidapproach (26) with sectioning of a non-dominant,well collatera1Ized sigmoid sinus (the transsigmoidapproach) may be useful for accessing some lesions(Figure 5A and B). However, when the lesion islocated in the cervico-medullary area and is anteriorin the midline, the exposure provided may still beinadequate.

Lnteral Appraaelies:The petrosal approach (1, 20, 26)

(retrolabyrinthine, partial labyrinthine ortranslabyrinthine) can be used to expose lesi on s ofthe pons that are situated anteriorly andanterolaterally. This technique is also used to exposelesions on the lateral aspect of the midbrain (Figure6A and B).

in the lateral part of the fourth ventriele, the bestmethod is the subtonsillar-transcerebellomedullaryapproach (Figure 3) (2, 11,22,23). In this technique,the cerebellar tonsils are mobilized and the cerebello­

medullary fissure is opened, but there is no splittingof the cerebellar vermis. In some cases, it may benecessary to resect one tonsil (23).

Al1tera-Lateral Appraaches:Lesions located close to the anterior surface of

the midbrain are best removed via fronto-temporal,orbito-zygomatic (9, 18), pterional-transsylvian(21)or anterior subtemporal approaches (Figure 7).Thetranssylvian approach is preferred for lesions thatare antero-medial to the cerebral peduneles, and thetranstentorial subtemporal approach is best for thoselocated anterolaterally at the level of the medial(spinal) lemniscus and lateral geniculate body. Thesixth cranial nerve in the ambient cistern is the onlyclearly identifable structure on the surface of themesencephalon.

Aii terIor Approaelies:

Both transmaxillary and transoral approaches(12,25) can be used to access lesions of the medulJa

and pons that are located anteriorly and on themidline (Figure 8A and B). Due to the high risk ofcerebrospinal fluid (CSF) leakage and infection, these

ir.

Figure 2: The superior and inferior colliculi after acombined su pra /infra ten torial-transsinusapproach (BV: basal vein of Rosenthal, GV:Galen's vein, PG: pinea! gland, SC: superiorcollicu!us, IC: inferior colliculus)

Figure 3: In the subtonsillary transcerebellomedullaryapproach, the cerebello-medullary fissure wasopened, the right tonsil was resected, and the lefttonsil was retracted. This wide exposure allowedvisualization of the distal segments of the PICA,the lateral recess of the fourth ventricle and theposterior part of the brainstem (U: uyula, T:tonsi!, IV: fourth ventric1e, pica: posterior inferiorcerebellar artery, M: medulla)

PosterIor Approaches:Posterior approaches to the brainstem are used

when the lesi on is situated in the collicular plate, orclose to the pial surface of the fourth ventricle. Forlesions of the dorsal mesencephalon and collicularplate, the supracerebellar infratentorial approach (6,19,21), the occipital transtentorial approach (7, 8), orthe combined supra- and infratentorial transsinusapproach (Figure 2) may be used (17,28). For lesionsat the midline or in the fourth yen triele, a

transvermian approach (4, 15) is necessary. However,excessive splitting of the vermis may lead topermanent truncal ataxia. For lesions of the ponto­medullary junction and the medulla that are located

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approaches should only be used when no otheroptian is available. Careful dural dosure or repair,and protracted lumbar CSF drainage are necessaryto promote healing. When attempting to reach theanterior brainstem, the transbasal approach (5) isanather optional corridor (Figure 9A and B).

Figure 4A and B: Extreme lateral transcondylar orretrocondylar approaches can be used to reach thelateral and antero-Iateral surfaces of the medulla,and the cervico-medullary junction (C2g: C2ganglion, va: vertebral artery, OC: occipitalcondyle, eid: ci dorsal root, clv: ci ventral root, M:medulla, IX-X-XI and XII: lower cranial nerves).

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Figure SA and B: A combined retro- and presigmoidapproach with the sectioning of a non-dominant,well collateralized sigmoid sinus (the trans­sigmoid approach) may be useful for accessingsome lesions. The figures of a !aterally placedspecimen's right side show presigmoid andretrosigmoid areas without (A) and with (B)sectioning of the transverse sinus. The relationshipof lower cranial nerves with the brainstem andtheir exit through the juguler foramen and thehypoglossal cana! is demonstrated on Figure SB.(SS: sigmoid sinus, P: pons, V: trigemina! nerve,VII: fadal nerve, VIII: vestibulocochlear nen·e, aica:anterior inferior cerebellar artery, M: medulla, ix­x-xi and XII: lower cranial nerves).

Turkish Neiirosurgery 11: 79 - 86, 20U1

Figure 6A and B: A petrosal approach (retrolabyrinthine,partiallabyrinthine or trans-Iabyrinthine) can beused to expose lesions involving the pons. FigureA shows the labyrinth and Figure B shows thewide exposure achieved af ter removal oflabyrinth (FSM: mastoid segment of the facialnerve, SSC-LSC and PSC: superior, lateral andposterior semicircular canals, P: pons, V:trigeminal nerve, VI: abducens nerve VII: facialnerve, VIII: vestibulocochlear nerve, BA: basilarartery, aica: anterior inferior cerebellar artery).

RESULTS

In each of the different approaches, we achieved360-degree visualization of the brainstem. Theinstrument distance required in the anterior andposterior approaches was greater than that requiredin the antero-Iateral, lateral and postero-Iateralapproaches. Table 2 summarizes the strategies andzones that have been investigated for accessinglesions in the brainstem.

DISCUSSION

Treatment of brainstem lesions requires twomajor steps. First, the surgeon must select the

Ziyai: Appraadies lo the Bmiiislrl1l

Figure 7: Lesions located on or near the anterior surfaceof the midbrain are best removed by fronto­temporal, orbito-zygomatic, transsyh'ian oranterior subtemporal approaches. The figureshows the combination of the orbito-zygomaticand subtemporal approaches at the righ side ofthe specimen. Anterior part of the tentorium iscut and removed. The relationship of cranialnerves III and V with the brainstem aredemonstrated.( Cranial nerves: III, IV, V and V1­V2-V3, P: pons, pea: posterior cerebral artery) .

approach that will best expose the affected area.Several important anatomical structures will beencountered in any of the technigues. To avoiddamaging these structures, and to be able to visualizethe appropriate surface of the brainstem, the path tothe lesion should be as direct as possible, and shouldallow maximum exposure of the area of interest. Asnoted above, we found that the antero-Iateral, lateral

and postero-Iateral approaches were more direct thanthe anterior and posterior approaches. Even amongvariations of any given approach, skin-brainstemdistances may di ffer. For example, of the variousanterior approaches, the transbasal approachinvolves a longer skin - brainstem distance than thetransoral approach. However, the transbasalapproach for brainstem lesions is very complicatedand, although it is theoretically possible, this methodis rarely necessary.

The second step is removal of the lesion, whichmay be deep or superficial in the brainstem. if thelesion is on the surface, tissue manipulation is easierand safer. Hovewer, it is difficult to expose deep­seated lesions without causing morbidity. Regardingbrainstem anatomy, it is generally considered thatmotor tracts remain in the anterior portion; sensorytracts, cranial nerves and the vestibular nuclei occupythe posterior and lateral aspects; and the tegmentum

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Tiirkish Neiirosiirgay 11: 79 - 86, 2001 Ziyai: Appraaeli!'s lo ili" Braiii"leiii

Figure 8A and B: A: Using the transoral approach, after removal of the CL vertebral arch (red dotted line) and the odontoidprocess (black dotted line), the transvers ligament (TL) is exposed (FM: foramen magnum, LM: lateral mass of Cl,B: body of C2). B: The transoral approach can be used for lesions of the medulla and pons that are located anteriorlyin the midline. The figure demonstrates the pons and the meC;ulla after opening of the eliyus dura. (P: pons, M:medulla, BA: basilar ar tery, VA: vertebral artery, AICA: anterior inferior cerebellar artery, ASA: anterior spinalartery, VI: abducens nerve, XI: accessory nerve) .

Figure 9A and B: A: Af ter a bifrantal craniotomy and orbital osteotomy, and having passed through the ethmoid andsphenoid sinuses, the eliyus is drilled anteriorly and the eliyus dura is exposed (transbasal approach) (CLD:eliyus dura, ST: sella turcica, c: carotid artery, O: optic nerve). B: The eliyus dura is then opened and the basilarartery (B), vertebral arteries (V), medulla (M) and pons (P) are exposed.

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and the medial longitudinal fasciculus occupy themiddle and posterior portions. The majority of thebrainstem vasculature enters and exits through theanterior and lateral surfaces, and damage to thesevessels may have severe long-term effects.

Even though several reports have focused onthis second step (2, 3, 10, 24), there is still cantroversyabout how to minimize the brainstem damage, andmore research is needed. Cantore et.a!. divided thebrainstem into six distinct areas around the reticular

formation, which is located roughly at the center ofthe structure. The six divisions were the dorsal

medulla, ventral medulla, dorsal pons, ventral pons,dorsal mesencephalon, and ventral mesencephalon(3). Kyoshima et.a!. reported safe entry via sites inthe floor of the fourth ventricle. These zones

preserved the nuclei of the pairs of cranial nerves VIand VII, as well as the mediallongitudinal fasciculus.The authors described two triangular sites, andnamed these zones the supra- and infrafacialtriangles (lO). Ericolo and Turazzi suggested entrythrough the floor of the fourth ventricle through themedian sulcus at apoint between the abducens andthe onilomotor nuclei, as long as the two mediallongitudinal fasciculi had no crossing fibers at thatlevel (2). A summary of the findings regarding

Table 2: Methods of entering various parts of thebrainstem (2, 3, 10,24)

Part ofthe StrategyandEntryZoneBrain

Stern

DarsalPosterior medullotomy

Medulla The paramedianoblique route n

Ventral Medtillathelevelofthennteriorlatera

suIcus between the radides of thXII.cranialnervesandofC

cervical root. Tra ns-fourth-ven tricu la rapproac

Dorsal Pons(suprafacialandinfrafacia

triangles) In front of the exit point of theVentral Ponsand/or Vii-Viii cranial nerve pairs

Medianintercollicularapproac1

DorsalTrans-thirdventricularapproac

Mesencephalon(mesencephalic fIoor of thethir

ventric1e) Windowborderedabovebyth

posterior cerebral artery, below bVentralthesuperiorcerebellarartery

Mesencephalonmedially by the emergence of II

cranial nerve and the basilar arterand laterally by the pyramidal trac

Ziyai: Approac!ics to ili... Hmiiislciii

options for brainstem entry sites is presented inTable 2.

In conclusion, this report describes severalapproaches that can be used to reach the brainstemsurface; however, as mentioned, gaining entry to thebrainstem is always challenging, and furtherinvestigation is needed to determine whichtechniques are safest.

Acknowledge1l1eiit: The nuthors wish to thniik

fennifer Pryll for prepnring the illustrntiolls.

Correspondence: Ibrahim M. Ziyai, MD.Hacettepe Üniversitesi Tip Fakültesi,Nörosirürji Anabilim Dali,Samanpazari, AnkaraTel: 3123052627Fax: 312311 1131

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