Whereas the frontotemporal approaches are relativelysystematized in the sense that the craniotomies proceedfrom defined burr hole sites,76,78 the lateral suboccipital ap-proaches are performed from various and nonstandardizedinitial burr hole placements. Because the transverse andsigmoid sinuses are the natural limits of these exposures,the knowledge of their cranial topography constitutes themain factor in the planning of these posterior approaches.

The lambdoid, occipitomastoid, and parietomastoid su-tures are easily recognizable structures on the external cran-ial surface.35,42,47,49,60–63,69–72 For this reason we initially inves-tigated their relationships with the transverse and sigmoidsinuses, and based on these findings, we evaluated the topo-

graphic profiles of strategic burr hole sites related to thesesutures (Fig. 1).

MATERIALS AND METHODSThis study was conducted in two stages, in each of which 50

paired temporoparietooccipital regions in 25 adult human driedskulls were studied through observations and measurements.57

In the first part, the anatomical relationships of the lambdoid,occipitomastoid, and parietomastoid sutures to the transverse andsigmoid sinuses were studied in 25 adult human uncataloged skulls,originally from India, at the Theodore Gildred Microsurgical Edu-cation Center of the Department of Neurological Surgery of the Un-iversity of Florida. The calvariae of the skulls were removed abovethe lambda and the superior orbital ridges, the transverse and sig-moid sinuses superior and inferior margins were drawn on the outer

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Suboccipital burr holes and craniectomies

GUILHERME C. RIBAS, M.D., ALBERT L. RHOTON JR., M.D., OSWALDO R. CRUZ, M.D.,AND DAVID PEACE, M.S.

Department of Neurosurgery, University of Florida, Gainesville, Florida; and Departments ofNeurosurgery and Surgery (Clinical Anatomy Discipline), University of São Paulo Medical School,São Paulo, Brazil

Object. The goal of this study was to delimit the external cranial projection of the transverse and sigmoid sinuses,and to establish initial strategic systematized burr hole sites for lateral infratentorial suboccipital approaches based onexternal cranial landmarks particularly related to the lambdoid, occipitomastoid, and parietomastoid sutures.

Methods. The external cranial projection of the transverse and sigmoid sinuses was studied through their externaloutlining obtained with the aid of multiple small perforations made from inside to outside along the inner margins ofthe sinuses of 50 paired temporoparietooccipital regions in 25 dried adult human skulls. The burr hole placement wasstudied by evaluating the supratentorial, over-the-sinuses, and infratentorial components of 1-cm-diameter openingsmade at strategic sites identified in the initial part of the study, which was performed in another 50 paired tem-poroparietooccipital regions.

The asterion and the midpoint of the inion–asterion line were found to be particularly related to the inferior half ofthe transverse sinus; the transverse and sigmoid sinuses’ transition occurs 1 cm anteriorly to the asterion across theparietomastoid suture, and the most superior part of the sigmoid sinus is located anteriorly to the occipitomastoidsuture, with its posterior margin crossing this suture posteriorly to the most superior aspect of the mastoid process,which is located at the most superior level of the mastoid notch. Burr holes made at the midpoint of the inion–asteri-on line, at the asterion, 1 cm anterior to the asterion, just inferiorly to the parietomastoid suture, and over the occipito-mastoid suture at the most superior level of the mastoid notch are appropriate to expose the inferior half of the trans-verse sinus at its midpoint, the inferior half of the transverse sinus at its most lateral aspect, the transverse and sigmoidsinuses’ transition, and the posterior margin of the basal aspect of the sigmoid sinus, respectively.

Conclusions. These findings allow an estimation of the transverse and sigmoid sinuses’ external cranial projection.The asterion and the most posterior part of the parietomastoid suture constitute a suitable initial burr hole site at whichto perform an upper or asterional suboccipital craniectomy to expose the superior aspect of the cerebellopontine angle(CPA). The occipitomastoid suture at the most superior aspect of the mastoid notch constitutes an adequate initial burrhole site at which to perform a basal suboccipital craniectomy to expose the lower portion of the CPA. The sites canbe used together as initial burr hole sites to perform wide suboccipital exposures, because they already constitute nat-ural infratentorial lateral limits.

KEY WORDS • cranial suture • transverse sinus • sigmoid sinus • burr hole •craniotomy • suboccipital approach • cerebellopontine angle • posterior fossa

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Abbreviation used in this paper: CPA = cerebellopontine angle.

surface of the skulls after multiple small perforations were made onits bone landmarks from inside to outside, and the main points to bestudied were identified.

The second part of the study was done at the Anatomical Museumof the Biomedical Sciences Institute of the University of São Paulothrough observations and measurements obtained in 25 adult humanskulls cataloged according to their race, sex, and age (Table 1).Thecalvariae had already been removed in these skulls, and the points tobe studied were lightly marked with a pencil.

The burr hole study was conducted by evaluation of the posteri-or fossa, the area over the sinuses, and supratentorial components of1-cm-diameter burr holes placed at strategic sites related to the cran-ial sutures. The burr holes were plotted with the aid of a circular de-vice adapted to a compass, and the measurements of the height ofeach burr hole’s topographic components provided the data to elab-orate its topographic profile.

An extensive statistical analysis was done to compare the resultsamong the different sides, sexes, and races.

RESULTS

Anatomical Relationships of the Lambdoid, Occipitomastoid,and Parietomastoid Sutures With the Transverse and SigmoidSinuses

Anatomical Relationships of the Lambdoid and Occipitomas-toid Sutures With the Transverse Sinus. The relationships ofthe lambdoid and occipitomastoid sutures with the trans-verse sinus were evaluated through the asterion position

(Fig. 2). The analysis of its distance to the superior andinferior margins of the transverse sinus, and of the sinusheight at this point, led us to conclude that the asterion isparticularly related to the inferior half of the transversesinus. The topography of the transverse sinus segment pos-terior to the lambdoid and occipitomastoid sutures wasevaluated through the disposition of the inion–asterion linein relation to the sinus (Fig. 3).

The analysis of the distances to both margins from themidpoint of the inion–asterion line, and the sinus height atthat point, showed that its position in relation to the trans-verse sinus is similar to the disposition of the asterion,being also related to the inferior aspect of the sinus sulcus(Fig. 4).

The topography of the part of the transverse sinus ante-

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Fig. 1. Photographs of dried adult human skulls showing the external cranial surface (left) and the internal cranial surface (right).

TABLE 1Identification of the 25 catalogued skulls studied

Feature Value

raceAfrican-American 7Caucasian 9mixed 9

sexF 11M 14

age (yrs)min 18.0max 60.0mean 33.9median 34.0

Fig. 2. Schematic drawing showing the variation of the asteri-on’s position in relation to the transverse sinus (mean values, tak-ing into account variations between races). Values in all schematicdrawings except Fig. 3 are given in centimeters, and commasdenote decimal points.

Fig. 3. Schematic drawing showing the disposition of theinion–asterion line in relation to the transverse sinus.

rior to the lambdoid and occipitomastoid sutures was eval-uated through analysis of the disposition of its most antero-superior and anteroinferior points in relation to the asteri-on. These points, which were equivalents to the superiorand inferior aspects, respectively, of the transverse sinus/sigmoid sinus transition, presented the disposition depictedin Fig. 5 in relation to the asterion.

Anatomical Relationships of the Parietomastoid SutureWith the Transverse Sinus. These relationships were initial-ly evaluated through analysis of the transverse sinus’ mostanterosuperior and its most anteroinferior points in relationto this suture. Whereas the position of the point corre-sponding to the superior aspect of the sinuses’ transitionvaried from 0.1 to 0.3 cm above the parietomastoid suture,the point corresponding to the inferior aspect of the sinus-es’ transition was situated below the suture at a mean dis-tance of 0.8 cm, regardless of the race, sex, and side of theskulls (Fig. 6).

The easily identifiable parietomastoid suture/squamoussuture meeting point was particularly studied in terms of itsposition in relation to the superior margin of the transversesinus or to the middle fossa floor/tentorium, and wasshown to be related with this level (Fig. 7).

Anatomical Relationships of the Occipitomastoid SutureWith the Sigmoid Sinus. These relationships were evaluatedby studying the topography of the occipitomastoid suture/sigmoid sinus posterior margin crossing point. The analy-sis of the distances of this crossing point to the asterion andto the jugular foramen identified its position along the oc-cipitomastoid suture (Fig. 8).

This point was also shown to be closely related to the in-ion–mastoid tip line level and to the posterior aspect of themastoid notch (Figs. 9 and 10).

Study of Burr Hole Sites Related to Lambdoid, Occipito-mastoid, and Parietomastoid Sutures

The results presented here were determined throughevaluation of the 1-cm burr holes’ mean topographic pro-files in six openings made in three different groups thatwere studied for specific purposes.

Burr Holes at the Asterion and at the Midpoint of the In-ion–Asterion Line. As shown in Fig. 11, these results provethat both burr holes are adequate to expose the transversesinus, with the second one being more appropriate to ex-pose both the sinus and the posterior fossa compartment.

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Fig. 4. Schematic drawing showing variation in the position ofthe midpoint of the inion–asterion line in relation to the transversesinus (mean values, taking into account variations between races).

Fig. 5. Schematic drawing showing the disposition of the mostanterosuperior and anteroinferior points in the asterion and trans-verse sinus (T.S.; mean values according to race).

Fig. 6. Schematic drawing showing variations in the position ofthe most anterosuperior and anteroinferior points in the transversesinus in relation to the parietomastoid suture (mean values, takinginto account variations between races).

Fig. 7. Schematic drawing showing variations in the position ofthe parietomastoid suture/squamous suture meeting point in rela-tion to the transverse sinus superior margin/middle fossa floor level(mean values, taking into account variations between races).

Fig. 8. Schematic drawing showing variations in the position ofthe occipitomastoid suture/sigmoid sinus posterior margin crossingpoint along the occipitomastoid suture (mean values, taking intoaccount variations between races).

Burr Holes Anterior to the Asterion. As shown in Fig. 12,these two burr holes were also evaluated for their potentialto expose the inferior aspect of the transverse sinus/sig-moid sinus transition, and the latter proved to be more ap-propriate for this purpose, yielding results not dependenton race, sex, or side of the skull (Table 2).

Burr Holes Over the Occipitomastoid Suture. As shown inFig. 13, the comparison of their topographic profiles showsthat both burr hole sites are appropriate to expose the sig-moid sinus posterior margin and the posterior fossa com-partment.

DISCUSSION

The suboccipital approach to the CPA used nowadays isstill based on the unilateral craniectomy described by Dan-dy8,9 in 1929 and in 1934, which was later made through astraight lateral incision as proposed by Adson1 in 1941, andthen extended laterally and inferiorly as was already em-phasized by Bucy7 in 1951. The microsurgical techniquesintroduced by Rand and Kurze51 in 1965 enabled the devel-opment of acoustic tumor exposure, particularly by drillingof the posterior meatal wall.

Although they obtain very similar CPA exposures, themost experienced neurosurgeons in this field perform theircraniectomies differently according to their own experi-

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Fig. 9. Schematic drawing showing variations in the position ofthe occipitomastoid suture/sigmoid sinus posterior margin crossingpoint in relation to the level of the inion–mastoid tip line (mean val-ues, taking into account variations between races).

TABLE 2Exposure of the inferior aspect

of the transverse sinus/sigmoid sinus transition*

Exposure (%)

Site Yes No

burr hole 3 34 66burr hole 4 84 16

* Exposure of the inferior aspect by the 1-cm burr hole with its superiorbase on the asterion and on the parietomastoid suture, and by the 1-cm burrhole centered 1 cm anteriorly to the asterion and with its superior base onthe parietomastoid suture.

Fig. 10. Schematic drawing showing variations in the positionof the occipitomastoid suture/sigmoid sinus posterior margin cross-ing point in relation to the posterior and superior aspect of the mas-toid notch (mean values, taking into account variations betweenraces).

Fig. 11. Schematic drawings showing topographic profiles ofthe 1-cm-diameter burr hole centered on the asterion (1), and cen-tered on the midpoint of the inion–asterion line (2).

Fig. 12. Schematic drawings showing topographic profiles ofthe 1-cm-diameter burr hole with its superior base on the asterionand on the parietomastoid suture (3), and centered 1 cm anteriorlyto the asterion and with its superior base on the parietomastoidsuture (4).

ence,37,38,44,45,52,53,55,56,58,59,67,68,76,77,79,80 placing the initial burrholes randomly or over nonspecific anatomical sites.

Malis37 placed his initial burr hole behind the occipito-mastoid suture, between the asterion and the posterior as-pect of the mastoid notch, hence below the transverse sinusand posterior to the sigmoid sinus according to our find-ings. The lateral suboccipital craniotomy described by Yas-argil and colleagues76,77,80 is done from “two infratentorialburr holes located on the superior nuchal line and one su-pratentorial burr hole placed 2 to 3 centimeters above thisline.”76 Considering our results pertinent to the inion–aste-rion line, which is higher than the superior nuchal line,6 andthe mean topographic profile of the burr hole centered 1 cmabove its midpoint, the highest burr hole proposed by Yas-argil might not be supratentorial in some circumstances.Ojemann and Martuza46 described this approach as a “cran-iectomy that exposes the lateral two thirds of the cerebellarhemisphere and the transverse and sigmoid sinuses medialmargins.” Samii and Draf58,59 and Rhoton52,53,56 also empha-sized the importance of the lateral extension, without point-ing to specific landmarks for the burr holes and craniecto-my placements.

Our study was based on the assumption that there is aprecise relationship between the location and the volume ofthe venous sinuses and the markings on the skull, whichhas already been confirmed by previous authors.43,73–75 Theeasy identification of these markings on the internal sur-face, and of the sutures on the external surface of the skull,made this study feasible. The main conclusions reachedfrom the statistical analysis of the anatomical relationshipsof the lambdoid, occipitomastoid, and parietomastoid su-tures in the transverse and sigmoid sinuses part of the studywere as follows: 1) the asterion and the midpoint of theinion–asterion line are particularly related to the inferiorhalf of the transverse sinus; 2) the superior and inferiorpoints of the transverse and sigmoid sinus junction are sit-uated above and below, respectively, the posterior part of

the parietomastoid suture; 3) the meeting point of the pari-etomastoid and squamous sutures is located at the level ofthe posterior part of the middle fossa floor; and 4) theoccipitomastoid suture and the crossing point of the sig-moid sinus posterior margin are situated at the level of thesuperior and posterior aspect of the mastoid notch, and alsocorrespond to the level of the intersection point of theoccipitomastoid suture with the imaginary line of the in-ion–mastoid tip.

In the study of burr hole sites there were no significantdifferences regarding the topographic profiles of the sexesand sides of the skull. The main conclusions reached in thispart of the study were as follows: first, burr holes centeredat the asterion (1) and at the midpoint of the inion–asterionline (2) are adequate to expose the transverse sinus; second,burr holes with their superior base on the asterion and onthe parietomastoid suture (3) and centered 1 cm anterior tothe asterion with their superior base on the parietomastoidsuture (4) are sufficient to expose the inferior aspect of thetransverse sinus next to the transverse sinus/sigmoid sinustransition, with the latter site more often involving the infe-rior aspect of the sinuses’ transition; and third, burr holesover the occipitomastoid suture at the posterior level of themastoid notch (5) and centered on the intersection point ofthe occipitomastoid suture/inion–mastoid tip line (6) aresuitable to expose the posterior aspect of the sigmoid sinusand the posterior fossa compartment.57

The results of this study allowed the delineation of thetransverse and the sigmoid sinuses externally based on theidentification of external bone landmarks, and providedthe groundwork for the part of the study relating to burrhole sites. That part of the study was conducted to estab-lish systematized lateral suboccipital approaches to exposethe superior aspect, the inferior aspect, and wide access tothe CPA.

Delineation of the Transverse and Sigmoid Sinuses

Allowing an error range of 0.5 cm, the results of thisstudy enabled the transverse and sigmoid sinuses’ marginsto be progressively traced on the skull’s outer surface fromthe suture identification. Because the asterion is specifical-ly related to the inferior half of the transverse sinus, its iden-tification, together with the knowledge of the sinus heightat this point (1 cm), permit its inferior and superior marginsto be delineated at the level of the asterion. The occipito-mastoid suture and crossing point of the sigmoid sinus’posterior margin can be identified along the suture at thelevel of the most superior aspect of the mastoid notch, and/or at the level of this suture’s point of intersection with theimaginary line of the inion–mastoid tip, and allow the pos-terior margin of the sigmoid sinus to be outlined. Theknowledge of the relationship of the inion–asterion line andits midpoint with the inferior aspect of the proximal portionof the transverse sinus permits its delineation, and by usingparallelism the superior margins can then be completed(Fig. 14).

ASTERIONAL SUBOCCIPITAL CRANIECTOMY

A circumscribed craniectomy systematically startedthrough a burr hole centered on the asterion, and extendedmainly along the parietomastoid suture is particularly ap-

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Fig. 13. Schematic drawings showing topographic profiles ofthe 1-cm-diameter burr hole over the occipitomastoid suture at thelevel of the posterior aspect of the mastoid notch (5), and centeredon the intersection point of the occipitomastoid suture/inion–mas-toid tip line (6).

propriate for the approach to the upper portion of the CPA(Figs. 15 and 16), because it provides exposure of the infe-rior aspect of the distal portion of the transverse sinus andits transition into the sigmoid sinus. The extension of thebone removal depends on the surgical plan and on theexperience of the surgeon.

The cranial approach required for posterior fossa neuro-vascular decompression of the trigeminal nerve constitutesthe prototype of a high lateral suboccipital craniectomy, be-cause the cerebellum has to be predominantly retractedalong the tentorial aspect of its anterolateral margin30,41 dueto the trigeminal nerve/vascular relationships14,18,19,41,54 andto avoid stretching the facial nerve.

Although varying in size and shape, the craniectomiesperformed nowadays for this purpose are very similar to theone described by Dandy8 in 1929 when he proposed thepartial section of the trigeminal sensory root for the treat-ment of tic douloureux, emphasizing the importance of theinferior aspect of the transverse sinus/sigmoid sinus transi-tion exposure to approach the fifth cranial nerve. Jannet-ta,20,24,29,31,32 who popularized and further developed the neu-rovascular decompression technique originally describedby Gardner and Miklos11 in 1959 and Rand and Kurze50 in1981, performed this procedure through a high suboccipi-tal craniectomy that measured 1.5 to 2.3 3 3 cm,21 expos-ing the distal part of the transverse sinus. According toJannetta,31 the sigmoid sinus has to be exposed only indolichocephalic patients. Rhoton55,56 described his approachfor this purpose as a 4-cm suboccipital craniectomy, andFukushima did it through a 1- to 1.5-cm-diameter boneremoval situated “above the superior nuchal line and belowthe asterion.” (Fukushima, personal communication, 1989)

It is interesting to note that the Gray’s Anatomy text-book,72 despite being strictly anatomical, describes the aste-rion region as “a point for trephining over the transversesinus.”

The burr hole centered 1 cm anteriorly to the asterionand with its superior base on the parietomastoid suture con-stitutes another option to initiate an exposure of the upperpart of the angle, because this burr hole generally alreadyexposes the inferior aspect of the transverse sinus/sigmoidsinus transition. This burr hole site is approximately equiv-alent to the one located 3 to 5 mm below the posteriorextremity of the supramastoid crest proposed by Hakubaand colleagues15–17 to expose the junction of the transverse,sigmoid, and superior petrosal sinuses.

According to our findings, the burr hole site located“medially and inferiorly to the asterion” as proposed by Al-Mefty3 to expose the posterior fossa at the level of thetransverse sinus/sigmoid sinus junction may be too posteri-or and inferior for its purpose. The initial burr hole for asuboccipital craniectomy placed at the superior nuchal line/occipitomastoid suture intersection point as proposed byGuthrie, et al.,13 might also be inferior to the transverse andposterior to the sigmoid sinus. Day, et al.,10 also describedthe asterion over the transverse sinus and posterior to itstransition to sigmoid sinus.

BASAL SUBOCCIPITAL CRANIECTOMY

Because exposure of the proximal aspects of the facialand vestibulocochlear nerves and related vessels36,39,

41,48 requires the opening of the cerebellopontine and cere-bellomedullary cisterns40 to allow retraction of the cerebel-lum petrosal surface and flocculus, the suboccipital ap-proach for neurovascular decompression of the facial nerveis ideally made through a more basal craniectomy.4,5,22,23,25,33

For this purpose, and for the exploration of the vestibulo-

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Fig. 14. Photograph showing delineation of the margins of thetransverse and sigmoid sinuses and their main points based onidentification of the lambdoid, occipitomastoid, and parietomastoidsutures.

Fig. 15. Schematic drawing showing the area of bone removalfrom the asterion to expose the inferior aspect of the transversesinus/sigmoid sinus transition.

cochlear, glossopharyngeal, and vagus nerves, Jannetta andcolleagues21–23,25–28,33,34 proposed a 2.5- to 4-cm-diameter ba-sal and lateral craniectomy.

A basal suboccipital craniectomy can be systematicallystarted through a burr hole placed over the occipitomastoidsuture at the level of the most posterior aspect of the mas-toid notch, and/or at the intersection point of the occipi-tomastoid suture/inion–mastoid process tip line, becausethese burr holes already expose both the posterior fossacompartment and the sigmoid sinus. The extension of thecraniectomy will also depend on each particular case, buteven very circumscribed bone removals started at this pointwill provide an adequate exposure of the posterior aspect ofthe internal meatus, of the facial–vestibulocochlear nervecomplex and related vessels, and of the glossopharyngeal,vagus, and accessory nerves and related structures (Figs. 17and 18). The surgeries for removal of small acoustic tumorsand for the aforementioned nerves’ neurovascular decom-pression could be systematically done with such a crani-ectomy.

Fukushima (personal communication, 1989) suggestedthat this procedure be accomplished through a 1- to 1.5-cm-diameter bone removal situated “one finger below thesuperior nuchal line,” which is approximately equivalent tothe initial burr hole proposed here. The basal suboccipitalcraniectomy itself is equivalent to the “minima” posterior

approach described by Bremond, et al.,6 as a 3-cm craniec-tomy located in the angle between the superior nuchal lineand a line tangential to the occipitomastoid margin, and tothe retrosigmoid approach proposed by Sterkers,66 whichconsisted of a 3-cm bone removal situated between thesuperior and inferior nuchal lines.

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Fig. 16. Photographs showing asterional suboccipital craniectomy steps (A–C) and microsurgical exposure (D) of the superior aspect ofthe CPA.

Fig. 17. Schematic drawing showing the initial burr hole siteover the occipitomastoid suture at the level of the posterior aspectof the mastoid notch and/or at the intersection point of the occipito-mastoid suture/inion–mastoid tip.

WIDE SUBOCCIPITAL CRANIECTOMY

More extensive suboccipital craniectomies, as are usual-ly required for the surgical treatment of CPA tumors, couldbe systematically initiated through the two burr holes justmentioned for the upper and lower approaches to the CPA:with its center on the asterion (1), and over the occipito-mastoid suture at the most posterior aspect of the mastoidnotch (2), because they generally already expose the inferi-or margin of the transverse sinus and the posterior marginof the sigmoid sinus, which constitute the natural limits ofthis approach. From these two initial burr holes the craniec-tomy can be extended as much as necessary (Figs. 19–21).Gardner, et al.,11 proposed extending a suboccipital craniec-tomy inferiorly along the occipitomastoid suture for sur-gery of glomus jugulare tumors, which is adequate consid-ering the relationships of the inferior part of this suture withthe sigmoid sinus. Descriptions of suboccipital bone re-

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Fig. 18. Photographs showing basal suboccipitalcraniectomy steps (A–C), and microsurgical exposure(D) of the inferior aspect of the CPA.

Fig. 19. Photograph showing the initial burr hole sites proposedfor wide lateral suboccipital exposures.

moval in more recent texts often relate the transverse andsigmoid sinuses with the asterion and with the occipito-mastoid suture behind the mastoid process, respectively, intheir illustrations, but usually without giving further ana-tomical details.2,3,64,65

CONCLUSIONS

The asterion and the most posterior part of the parieto-mastoid suture are related to the most lateral aspect of theinferior margin of the transverse sinus and with the transi-tion between the transverse and sigmoid sinuses, and henceconstitute proper sites at which to start and to delimit theexposure of the superior aspect of the CPA. The occipito-mastoid suture at the level of the mastoid notch is particu-larly related to the posterior margin of the sigmoid sinus,and hence constitutes a proper initial burr hole site at whichto perform a basal suboccipital craniectomy to expose thelower portion of the CPA. Both can be used as initial burrhole sites to perform wide suboccipital exposures, becausethey already constitute natural infratentorial lateral limits.

References

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Fig. 20. Photographs showing a wide CPA craniectomy (A) and a microsurgical view (B) of the angle.

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Fig. 21. A and B: Preoperative (A) and postoperative (B) magnetic resonance images of an acoustic tumor located at the right CPA andresected in a 50-year-old man. C–H: Intraoperative photographs showing the anatomy and approaches. Intraoperative identification of theasterion, lambdoid, occipitomastoid, and parietomastoid sutures, with the patient in the sitting position (C). Initial burrholes placed just ante-riorly to the asterion to expose the transition between the transverse and sigmoid sinuses (1), and over the occipitomastoid suture just poste-riorly to the mastoid process at the most posterior level of the mastoid notch (2), which lies just posteriorly to the posterior margin of the sig-moid sinus (D). Wide suboccipital craniectomy site (E). Dural opening (F). Exposure of the tumor (G). Skull base view of the facial nerveafter removal of the tumor (H).

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Manuscript received June 9, 2005.Accepted in final form July 18, 2005.Address reprint requests to: Guilherme C. Ribas M.D., R Eduar-

do Monteiro 567, Sao Paulo 05614-120 Brazil. email: guilherme@ribas.med.br.

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