Childs Nerv Syst (2004) 20:554–566DOI 10.1007/s00381-004-0991-6 S P E C I A L A N N U A L I S S U E

Marion WalkerSamuel R. Browd

Craniopagus twins: embryology, classification,surgical anatomy, and separation

Received: 16 March 2004Published online: 23 July 2004� Springer-Verlag 2004

M. Walker ()) · S. R. BrowdDivision of Pediatric Neurosurgery,Department of Neurosurgery,Health Sciences Center and PrimaryChildren’s Medical Center,University of Utah,Salt Lake City, UT, USAe-mail: marion.walker@hsc.utah.edu

Abstract Introduction: With recentadvances in brain imaging and neu-rosurgical techniques, there has beena renewed interest in the surgicalseparation of craniopagus twins.Successful separation in recent cases,along with widespread publicity, hasattracted craniopagus twins from allover the world to be referred to pe-diatric neurosurgical centers forevaluation and consideration for sur-gical separation. Separation of bloodsupply: It has become apparent,however, that the most critical deci-sions in surgical planning are relatedto separation of the blood supply tothe conjoined brains. In fact, in cra-niopagus twins that survive pregnan-

cy or the first few days of life, there isusually little shared brain tissue. Theshared blood supply is far and awaythe more critical issue. It is very dif-ficult to successfully separate crani-opagus twins in one surgical proce-dure. Staged separation, with gradualre-routing of the shared blood supply,has been a successful alternative.Case studies and discussion: Wediscuss here our experience withthree sets of craniopagus twins andour approach to staged separation.

Keywords Craniopagus twins ·Cerebral vasculature · Embryology ·Sagittal sinus

Introduction and historical perspectives

The physical union of twins is one of the oldest recordedbirth defects and has led to curiosity and ridiculethroughout the ages. The quotation attributed to the Bid-denden Maids “As we came together, we will also gotogether” was recorded in the early 12th century [8] andSebastian Munster appears to be the first to describe thecraniopagus deformity in the collection Cosmographiauniversalis, published in the 16th century [18] (Fig. 1). Asanatomical and surgical knowledge advanced, the conceptof separating conjoined twins became a possibility. At-tempts at separation began after the turn of the 20thcentury and coincided with the birth of the neurosurgicalsubspecialty [7].

Baldwin et al. summarized the early history of surgicalseparation of craniopagus twins [1]. The first attempt wasmade in 1928 by Cameron, but unfortunately led to theintraoperative death of both twins. In 1932, Leiter re-

ported cephalopagus twins in whom surgical separationwas performed on the 8th day of life by sacrificing one ofthe twins. The surviving infant later died as well. In 1949,Barbosa reported the separation of craniopagus twins whosurvived only 3 h after the operation. Baldwin notes thatGrossman et al. made the first partially successful sepa-ration in 1953, with one twin surviving for 1 month andthe other surviving with marked mental retardation. Voriset al. reported the first successful separation with long-term survival of both twins in 1957. One twin was re-ported as normal following surgery and the second childhad a left hemiparesis and mental retardation. Finally,O’Connell in 1962 described his experience with two setsof twins. Only one child of the pair survived surgery ineach of the cases. Based on the poor initial success duringattempted separation, Baldwin noted that “the followingproblems in the treatment of head-conjoined twins de-serve special consideration:

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1. Risks associated with surgical separation2. Sequelae (of surgery)3. Long-term management of large cranial and dural

defects in a growing child” [1]

These concerns remain today in the light of modernadvances in neurosurgical care.

Definitions, epidemiology, and embryology

Conjoined twins are classified by the proposed site ofunion and divided into two groups depending upon theaspect of the embryonic disc involved. The ventral sub-groups are joined over a single yolk sac with a sharedabdomen and umbilicus and include those joined ros-trally, caudally, and laterally. The dorsal subgroup isconjoined in the neural tube, each with a separate abdo-men and umbilical cord [15]. The embryology of con-joined twins is quite complex and the terminology appliedis multiplicative and extensive. Generally, the terminol-ogy describing the various types of conjoined twins canbe subdivided into eight types:

1. Omphalopagus—fixed at the umbilicus2. Thoracopagus—chest3. Cephalopagus—head4. Ischiopagus—hip5. Craniopagus—helmet6. Rachipagus—spine7. Pygopagus—rump8. Parapagus—side (Fig. 2; reprinted from [15, 16])

Overall, conjoined twins represent a rare congenitalanomaly occurring at a frequency of approximately 10–20per million births in the United States [9, 6]. Among thisgroup, cephalic conjoining, or craniopagus twinning,represents one of the rarest congenital anomalies, occur-ring in 2–6% of all conjoined twins, with an overall oc-currence rate of 0.6 per million births [14]. Additionally,it is noted that conjoined twins are always geneticallyidentical and share the same sex. Females are mostcommonly affected, with a ratio of female/male of 4:1.No association of race, age, parity, maternal age, heredityor environmental influences has been reported.

The exact nature of how conjoined twins develop re-mains unclear. Older theorists suggested that conjoinedtwins develop as a result of the failed fusion of a singlefertilized ovum, with the abnormality occurring near the

Fig. 1 Craniopagus twins depicted by Samuel Munster in the early12th century (Cosmographia universalis)

Fig. 2 Types of twins according to Spencer. O omphalopagus, T thoracopagus, Ce cephalopagus, I ischiopagus, Pa parapagus, Dipparapagus diprosopus, Dic parapagus dicephalus, Cr craniopagus, R rachipagus, Py pygopagus [15, 16]

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end of the 2nd week of gestation [18]. A newer hypoth-esis, presented by Spencer in 2000, suggests that cranialfusion occurs between two separate embryos prior to theend of the 4th week of gestation when the cranial neu-ropore remains open [16]. Spencer discusses that fissionof the developing embryo is unlikely to result in con-joined twins, but secondary fusion of two originallyseparate monovular embryonic discs could theoreticallyexplain each of the eight types of conjoined twins dis-cussed earlier. Spencer further speculated that intact skinwill not fuse to intact skin, including the ectoderm of theembryo. Two embryonic discs could only unite in loca-tions where the ectoderm is normally absent, normallydestined to fuse, or is broken down.

Spencer suggests that craniopagus, rachipagus, andpygopagus twins result from the fusion of the primitiveneural folds of two separate, dorsally oriented embryonicdiscs, and the union can occur only after the ectoderm atthe summit of the fold is disrupted to allow the neural andsurface ectodermal layers to separate from one another.This group of twins is united in the cranium and/or theaxial skeleton, the meninges, and not infrequently, thecerebral cortex or spinal cord [15]. Spencer [15] notes:

The union in craniopagus may occur on any portion ofthe calvarium, but does not include either the face orthe foramen magnum. The juncture, rarely symmetri-cal, may involve the entire diameter of the head oronly a portion thereof, and there is infinite variation inboth axial and rotational orientation. The vertebralaxes may form a straight line or any angle in anyplane, and the twins may face either the same or ex-actly opposite directions or any point in between. Allof these factors will affect the subsequent develop-ment, distortion and displacement of the brain, themeninges, and the vascular system, as well as thepossibility of survival of the twins after separation.

While the incidence of craniopagus twinning is low,several authors have attempted to classify the numerousphenotypic variations seen in clinical practice. O’Connelldeveloped an exhaustive classification schema in hispublication in 1976 Craniopagus twins. Surgical anatomyand embryology and their implications [12]. He initiallyclassified craniopagus twins as partial or total. Partialcraniopagus was defined as having limited surface areainvolvement, with either intact crania or cranial defectsand contact of the dura mater with possible fusion withinthe defect. The abnormality extends no deeper than theleptomeninges and is “simply a defect of the cerebralcoverings....” Total craniopagus twins are defined assharing an extensive surface area with widely connectedcranial cavities. Among total craniopagus twins, Bucholzet al. developed four general subclassifications: frontal,parietal, temporoparietal, and occipital (Fig. 3) [2]. Bu-cholz describes frontal craniopagus twins as facing each

other with the axis of the bodies forming an acute angle.Temporoparietal craniopagi are joined immediately abovethe external auditory meatus and occipital craniopagustwins are joined immediately over or at the occipitalprotuberance, causing the infants to face away from eachother. The fourth variant, parietal craniopagus, occurswhen twins fuse at the vertex with the axis of the twinsforming an obtuse angle. O’Connell refers to this con-figuration as vertical craniopagus and suggests thatamong vertical craniopagi, three subtypes exist that eachhave varying degrees of rotation of one head upon theother and that these variations present with different de-formities of the brain and abnormalities of the circulation[12]. O’Connell’s Type I vertical craniopagus has bothtwins facing in the same direction. Type II twins havefaces on opposite sides of the conjoined skull, the longaxis of one head being rotated relative to that of the otherthrough an angle of 140� or more. Type III twins have an

Fig. 3 Bucholz et al.’s subclassification of total craniopagus twins:frontal, parietal, temporoparietal, and occipital

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intermediate angle of rotation of the long axis of one headon that of the other (Figs. 4, 5, 6).

The classification of different craniopagus phenotypeshas important implications during surgical planning. Au-thors who have written about the separation of craniop-agus twins note that the morphology of the fusion relatesto the underlying abnormalities of cortical developmentand associated anomalous or shared development of ar-terial and venous anatomy. It is known that craniopagustwins can demonstrate fused or interdigitated regions ofcortex, neurovascular anomalies of both arterial and ve-nous anatomy, as well as shared osseous and soft tissues.Paramount among these concerns is a unifying conceptamong authors regarding the development of a commonsuperior sagittal sinus and the importance of this sharedvenous drainage at the time of attempted surgical sepa-ration. The importance of a shared venous system is acommon theme, regardless of the type of craniopagusfusion, and we will discuss how this anatomical variantbecomes a critical factor in determining the survival andlong-term morbidity of craniopagus twins.

The anomalous neurovascular development induced incraniopagus twins has been a matter of great interestamong the authors who have attempted surgical separa-tion. Many authors have attempted to describe the em-bryology and subsequent development of craniopagus

twins in an attempt to better understand the challengesfaced at the time of surgical separation. O’Connell de-veloped several hypotheses regarding fetal developmentand “growth forces” that influence calvarial, cortical, andvascular development, leading to the noted variations invertical craniopagus presentation. The importance of thevenous anatomy was addressed by O’Connell and he wasone of the first authors to suggest that the venous anatomymay be among the most difficult issues to deal withduring separation. O’Connell described the vascular ab-normalities he encountered at operation and during post-mortem examination of Types II and III twins. He de-scribed large veins converging on a vascular channel thatproved to lie along the line of contact of the cerebralcortex. The peripheral venous channel was described as acircumferential sinus in the base of a dural fold that couldbe “representative of the falx cerebri but lying betweenthe cerebra of the two children instead of between thehemispheres of each one’s cerebrum.”

O’Connell proceeded to discuss the developmentalmechanisms for a shared superior sagittal sinus in thecontext of normal superior sagittal sinus development, anexcerpt follows [12]:

Fig. 4 O’Connell’s classification: type I vertical craniopagus

Fig. 5 O’Connell’s classification: type II craniopagus

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The normal superior sagittal sinus arises from a plexusof veins which is situated in the sagittal plane betweenthe developing cerebra. This venous plexus lies in awell-marked layer of embryonic tissue (ectomeninx)which later forms the dura mater, that portion betweenthe cerebral hemispheres giving rise to the falx cerebriand that between cerebrum and cerebellum to thetentorium cerebelli, the two being continuous beneaththe posterior cerebrum. As the cerebrum develops su-periorly and posteriorly, the ectomeninx develops withit and an extension of the venous plexus is carriedposteriorly within it. This plexus is continuous withanother similar one in the embryonic tissue betweenthe cerebrum and cerebellum from which the lateralsinus will develop. Thus, with the growth posteriorlyof the developing cerebral hemispheres, the falx ce-rebri with the sinuses within it develops progressivelyfrom anterior to posterior. All the veins from the su-perior part of the lateral surface of the cerebrum draininto this superior sagittal sinus. The sinus drains intothe transverse sinus lying in the membrane betweencerebrum and cerebellum. This is the residuum of thethree dural plexuses which drained into the internaljugular vein. With the growth of the cerebrum poste-

riorly, it is tilted in the same direction through 90degrees to gain the position of the adult lateral sinus,the portion distal to the bend becoming the sigmoidsinus. As a result of the same posterior development ofthe cerebrum the ectomeninx between it and the cer-ebellum is compressed to form the tentorium cerebelli.

In total vertical craniopagus there is no ectomeninx atthe site of union and no development of skull, duramater, or falx cerebri can occur in an area which israpidly increasing in extent as a result of cerebral de-velopment. There can, therefore, be no sagittal venousplexus and a superior (or inferior) sagittal sinus cannotdevelop. However, at the line of apposition of thecerebral hemispheres of each child, a shallow infoldingof the ectomeninx occurs between the brains, andcontains a plexus in its base, formed by venous trib-utaries from each brain. This represents the superiorsagittal sinus but is instead a circumferential one.

(Fig. 7).As we will discuss, dealing with a common sagittal

sinus or a “circumferential sinus” is often the mosttechnically challenging part of any craniopagus twinseparation and it is clearly discussed by most authors whohave attempted the procedure as a common factor in twinmorbidity and mortality, both during the procedure and inthe postoperative period. Various technical approaches fordealing with the aberrant venous anatomy have been de-scribed and the techniques vary based on whether theseparation occurs in a staged or non-staged manner.While the venous anatomy is regarded as one of the most

Fig. 6 O’Connell’s classification: type III craniopagus

Fig. 7 Circumferential venous sinus of total vertical craniopagus

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challenging issues faced during separation, many otheranatomical abnormalities must be dealt with during sep-aration, each critical in their own right to the success ofthe procedure and the long-term outcome of surgery.

Issues related to separation

Clearly, separation of craniopagus twins has long beenregarded as a monumental task with high rates of mor-bidity and mortality. The most recent literature review,published in 1987 by Bucholz et al. determined the pe-rioperative mortality over the prior 10 years was 36%,compared with 61% for twins separated before 1974 [2].Clearly, much improvement had occurred in surgical andanesthetic techniques, but several areas of concern wereraised by the author regarding factors that predispose to adifficult separation. Todorov et al. in their 1974 article oncraniopagus twins concisely summarized the multiplechallenges faced during separation [17].

The majority of complete craniopagus twins have onlydura separating their brains and connections betweenopposed cortex may exist. It is suggested that the presenceof shared brain tissue does not necessarily affect survivalafter separation per se, but it clearly can impact uponneurologic morbidity. Bucholz et al. found that all caseswith profound neurologic deficits had cerebral connec-tions prior to surgery [2]. Additionally, they found cere-bral connections to be most common among frontal andtemporoparietal fusions. Modern imaging techniques uti-lizing MRI and 3D modeling allow for anatomic fusionsto be evaluated. This in turn allows for rational plains ofdissection to be determined prior to surgery.

Bucholz et al. emphasized the notion that early sepa-ration provides the greatest opportunity for normal neu-rologic development. However, he also reported that in-fants who were separated immediately during the post-natal period sustained the highest rates of mortalityamong all groups [2]. Currently, most authors suggestwaiting until the twins are several months of age afterwhich time the substantial blood loss encountered duringsurgery can be better tolerated [4]. The concept of sepa-ration during a staged versus non-staged procedure canimpact on the issue of blood loss and the ability of theinfants to tolerate surgery. Staged separation allows fordivision of brain tissue at an early age without encoun-tering the substantial blood loss commonly dealt withduring non-staged procedures.

Another critical issue raised is the difficulty in dealingwith scalp and skull defects. Early attempts at separationwere technically challenging both from the standpoint ofaberrant intracranial anatomy but also regarding the af-termath of separation and the need to reconstruct duralclosures, the cranial vault and overlying scalp. Modernneurosurgical and reconstructive plastic surgery tech-niques have made this portion of the separation less

problematic. The use of tissue expanders preoperativelyallows for large scalp defects to be covered and moderntechniques allow for free flap rotation if additional cov-erage is required after separation. The intraoperative useof autologous dural patch grafts such as fascia lata orxenogenous products such as bovine pericardium hasmade closure of large dural defects technically feasible.Additionally, the use of ventricular shunting has reducedthe incidence of postsurgical CSF leaks. This in turn hasreduced the incidence of postoperative infections, in-cluding ventriculitis and meningitis, which were commonmorbidities seen in early surgical series.

The dilemma of a shared sagittal sinus

As previously introduced, the aberrant development of thearteriovenous system is among the greatest challengesfaced during separation. While arterial collaterals andbridging arterialized vessels are common, it is the sharedvenous anatomy that is the most difficult to address andhas the most profound impact on survival and morbidity.Anomalous venous drainage has been apparent in allcases of total craniopagus twins [10]. In discussing thesurgical dilemma of surgical separation, O’Connell dis-cussed the importance of the cerebral venous systemstating, “in the case of the venous drainage, the changeshave the highest importance. The superior sagittal sinuswill be absent in both twins in most cases and [will] bereplaced by a circumferential sinus which unites the lat-eral sinuses in the two children” [12]. Bucholz and col-leagues reiterated this concern, suggesting that reducedoperative mortality and morbidity was a partial result ofbetter control of hemorrhage from the venous sinuses [2].

Our experience has been that the shared venousdrainage is among the most important factors dictatingoutcome after surgery and most authors discuss thatseparation often requires donation of the shared sagittalsinus to one of the two twins. In cases where twins areseparated during a single procedure, it is the twin whoreceives the common sinus who inevitably has the lowermorbidity and mortality. How the shared sagittal sinus isdealt with is among the most important factors in deter-mining whether successful surgical outcome will be ob-tained for both twins.

Current practice regarding the surgical management ofa shared venous system has taken two divergent paths,each predicated on a different philosophy regarding thetiming and method of venous separation. The majority oftreating surgeons have proposed division and subsequentreconstruction of sinus remnants during planned separa-tion or when emergent separation is warranted [3, 4, 11].Alternatively, the separation of craniopagus twins can beperformed via a staged approach. Drummond et al. werethe first to publish their account of a staged proceduredeveloped to address the conundrum of a shared superior

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sagittal sinus [5]. The Baragwanath craniopagus twinswere joined in the posterior parietal region and found toshare the posterior third of the superior sagittal sinus. Thetorcula also drained the straight sinus and a commonlateral sinus. The neurosurgical team decided to slowlyocclude the single posterior third of the common sinusover several days. At the first stage, the junction of theshared sagittal sinus was exposed and a screw clamp wasapplied with the screw exteriorized through the skin flap.The screw was gradually closed over 1 week “allowingcollateral veins to dilate and take over the drainage oftheir respective brains.” At the second operation, theclamp was removed and the posterior third of the sagittalsinus was found to be thrombosed as it was ligated anddivided. The third stage comprised the final separationand it was noted that postoperatively there was no brainedema or swelling. Both twins survived the separation;however, long-term outcomes and any neurologic mor-bidities were unfortunately not addressed in the article.

Our experience with separating craniopagus twins hasled to the belief that the shared venous drainage is themost critical factor in dictating overall survival of bothtwins and impacts on the postoperative morbidity andcognitive functioning. While a single surgery for separa-tion has been successful in many cases, we have found astaged approach provides promising results, with thesurvival of both twins in our series of three sets of totalcraniopagus twins operated on between 1985 and 1996.

As first demonstrated by Drummond and colleagues,techniques to divert venous flow may be used to promotethe development of venous collaterals selectively in oneof the twins. By selectively altering the venous drainagein one twin, it may be possible to lessen or completelynegate the dependence of that twin on direct venousoutflow to the common sagittal sinus. Thus, our approachhas been to slowly promote deep venous drainage in onetwin in the hope of donating the common sagittal sinus tothe other twin at the time of final separation.

Instead of using a device to progressively constrictvenous flow within the dominant venous outflow tract, wehave opted to ligate venous branches as they enter thecommon sagittal sinus over multiple surgeries. Duringpreoperative planning, we have attempted to define thevenous anatomy and characteristics of the venous outflowfrom each twin. Based on these findings, we have selectedone twin to be the recipient of the common sagittal sinus.The twin who will lose the sagittal sinus at the time ofseparation subsequently undergoes serial ligating and di-vision of bridging veins that meet the shared superiorsagittal sinus. The procedure to separate one twin fromthe common sinus occurs over many operative sessions inthe hope of reducing the risk of venous infarction and/orvenous hypertension. Conceptually, it is anticipated thatserial ligation will promote development of the deep ve-nous system allowing for complete separation from the

common sagittal sinus without incurring the devastatingconsequences of venous infarcts or venous hypertension.

The separation of craniopagus twins is a complex un-dertaking that requires the reconstruction of many struc-tures including neurovascular, dural, calvarial, and cuta-neous entities. Each structure encountered and dealt withcan lead to a variety of complications that impacts on theoverall outcome of separation, including the immediatesurvival of the twins and long-term morbidity and mor-tality. The few surgeons who have taken on the challengeof separation are intimately aware of the complexity ofthese cases and the pitfalls encountered along the way.While no perfect solution exists to address these issues,we support the concept of staging the separation for thepreviously noted issues, some of which are theoretical andothers practical.

A case study of craniopagus separation

By way of example, our most recent case of craniopagus twinspresented in 1996, undergoing separation that same year using theaforementioned techniques. This case involved 5-month-old femalecraniopagus twins born in Honduras on 23rd September, 1995. Atthe time of presentation here the twins were feeding appropriatelyand had no overt hemodynamic or neurologic deficits. At the re-quest of the family, the neurosurgical team proceeded with a work-up to determine if surgical intervention was feasible.

The twins shown in Fig. 8 were of the O’Connell type I verticalcraniopagus. The twins were fused at the vertex and both faced thesame direction. Following an exhaustive general medical evalua-tion, which noted no other abnormalities, we obtained CT, MRI,and angiographic studies to analyze the complex anatomy. A dis-

Fig. 8 Type 1 vertical craniopagus twins

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cussion of the findings and the utility of each imaging techniquewill be described.

Evaluation of CT imaging proved most useful in defining thebony anatomy of the calvarium. Each child had a separate metopicsuture extending into a shared anterior fontanel. An infrafontanelwas seen between the shared anterior fontanels. Each child had acoronal suture that was continuous with the coronal suture of theother, without evidence of discontinuity or synostosis. Each childhad a separate lambdoid suture and posterior fontanel. The poste-rior portions of the sagittal sutures extended from each child’sposterior fontanel obliquely to meet at the level of the waist be-tween the fused calvaria. No orbital or maxillofacial defects werepresent.

Magnetic resonance imaging provided both an understanding ofthe brain anatomy and an initial view of the arterial and venousanatomy. The left hemisphere of twin A and the right hemisphere oftwin B were well formed and not opposed. However, the righthemisphere of twin A and the left hemisphere of twin B was in-terdigitated with each other over several centimeters (Fig. 9). Asmall cleft was present between the interdigitated segments. Therewas no evidence of cortical gray matter bridging between the twins.

The arterial anatomy from the MR study demonstrated normal,well-formed carotid and vertebral arteries and the proximal intra-cranial branches were intact. There was some distortion of the ar-terial anatomy because of the distorted hemispheres,but arterialperfusion was apparently intact (Fig. 10).

As anticipated, the venous anatomy was the most complex.Both twins had an individual sagittal sinus in the anterior inter-hemispheric fissure, which then joined at the level of the coronalsuture. A common sagittal sinus then coursed in a curvilinear C-

shaped fashion from anterior to posterior above twin A’s lefthemisphere and twin B’s right hemisphere. The common superiorsagittal sinus extended posteriorly and curved behind the mass ofthe opposed hemispheres dividing at the lambdoidal suture in sucha way that there were separate distal sagittal sinuses, separate tor-cula, and separate transverse sinuses (Fig. 11). Importantly, therewere many large cortical veins that surrounded the interdigitatedbrain. These veins drained into the common sagittal sinus.

Cerebral angiography was performed to further characterize thecomplex arterial and venous anatomy. Of importance, it was notedon the cerebral angiogram that there was some cross filling of thearterial channels between twin A’s right hemisphere and twin B’sleft hemisphere. There was bidirectional contrast filling throughsome arterial channels, one of which appeared to be an angularbranch of the middle cerebral artery and another branch anterior tothe central sulcus. The contrast flow was preferentially from twin Bto twin A (Fig. 12).

Fig. 9 Magnetic resonance image showing hemispheral relation-ships of the twins

Fig. 10 Magnetic resonance angiogram showing relatively normalarterial anatomy

Fig. 11 Magnetic resonance venous anatomy showing commonsagittal sinus dividing into separate distal sagittal sinuses, torculas,and transverse sinuses

Fig. 12 Reconstruction of angiographic findings showing crossfilling of arterial channels

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The venous anatomy seen with the cerebral angiogram provedto be more useful than the MR venogram. It was noted that eachtwin had a well-formed anterior superior sagittal sinus that ex-tended in the interhemispheric fissure to an anterior venous con-fluence (Fig. 13). A large common venous channel extended fromthe confluence posteriorly on the convex side of the joined headscurving posteriorly where the venous channel divided and extendedas separate channels in each twin. It was noted that multipledraining veins were coming into the anterior and posterior ends ofthe common venous channel (Fig. 14). Additionally, there appearedto be a second parallel channel inferior to the common sagittal sinusin twin A, representing either a second sagittal sinus or a largeaccessory cortical venous channel. The accessory venous channelalso received multiple draining veins from the adjacent cerebralhemisphere. Twin B demonstrated a large circumferential veincoursing on the opposite side of the head and appearing to cir-cumnavigate the large interdigitated cortical area. This channelextended from the anterior to the posterior portion of the commonsagittal sinus.

Finally, it was noted that the twins had well-formed deep ve-nous systems. However, the posterior straight sinus was abnormal,coursing superiorly to the region of the posterior venous conflu-ence, possibly representing an abnormal falcine sinus.

After much deliberation and based on the enormous amount ofpreoperative information, it was felt that a successful separationcould be performed. Based on our prior experience, review of thereported literature, and discussions with colleagues, we proceededto develop a surgical plan that focused on addressing the complexvenous anatomy. As with prior attempts, we have felt that it isdifficult to perform a single operation and provide adequate venousdrainage for the twin who loses the shared sagittal sinus. The dif-ficulty with a single surgery is centered around the need to re-construct a sagittal sinus de novo in one of the two children. Our

contention has been that a staged procedure carried out over mul-tiple sessions over several months has the ability to promote deepvenous drainage in one twin facilitating complete donorship of thesagittal sinus to the other twin at the time of separation. By inter-mittently sectioning draining veins that enter the common sinus inone twin, we believe it is possible to redirect the venous outflow tothe deep venous system. This concept provides the rationale for amulti-staged procedure.

The goals of surgery were then to produce this result whileaddressing the other key issues of separation, including sharedneuronal tissue, shared bridging arteries, and the complex problemof bony and soft tissue reconstruction after separation. Prior to thefirst surgery we consulted with our colleagues in anesthesiologyand plastic surgery at our institution and each member of the sur-gical team provided a focused strategy for dealing with each of theunique issues facing the separation. Anesthesiology provided twoseparate teams for intubation and support for each twin during themultiple surgeries. The plastic surgery team developed a plan fortissue expansion and post-separation reconstruction of the cranialvault of each twin and reconstruction of scalp defects. The oper-ating room team provided important guidance regarding fieldpreparation and techniques for sterile draping that would take intoaccount our need to rotate the patients during surgery. After muchconsultation and debate, we began the separation in March 1997.

The first stage of separation dealt with opening a bifrontal flapand identifying the cleft between the two brains. Both twins had ananterior fontanel and we used this to strip away the dura and turn abifrontal craniotomy. We subsequently opened the dura and iden-tified a cleft that was clearly the line demarcating the division ofthe twins’ hemispheres. Frameless stereotaxy was employed duringthe procedure and we used the stereotactic system to confirm thelocation of the interhemispheric fissure. Under magnified vision weconfirmed that the twins’ hemispheres were separate, without union

Fig. 13 Venous anatomy showing anterior confluence of sagittalsinuses

Fig. 14 Venous angiography showing large common venouschannel dividing into separate channels for each twin posteriorly

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of brain parenchyma. We then turned our attention to multiple areaswhere shared vessels were identified. These included both arterialand venous vessels, which were of small caliber, typically less than0.5 mm, in diameter. We proceeded to identify several vessels thatwere present on the preoperative cerebral angiogram and decidedthat one artery originating from the middle cerebral distributioncould be taken safely. After this, several medium to large veinswere addressed. Our technique is to first place a temporary vascularclip across the venous structure slated for division and observe thebrain parenchyma for signs of venous congestion or hypertension.Allowing for adequate time for these changes to evolve, we failedto see any deleterious effects from occluding several large veinsand they were subsequently sacrificed (Fig. 15). This procedurewas repeated on several occasions during the first surgery in anattempt to promote deep venous drainage in twin B. At the time ofclosure, we placed a small layer of silastic sheeting between the twohemispheres to reduce adhesions and scarring between surgicalprocedures. It was also decided at the first surgery to leave the boneflaps out and freeze them for use during the reconstruction. It wasfelt that replacing the bone flap on several occasions would lead toa decrement in the structural integrity of the bone.

The twins returned to the operating theater 1 week later forfurther exploration, during which time the large dural venous sinuswas identified as it was seen overlying the right hemisphere of twinA. The dura was opened over the parietal and occipital lobes andfurther division of venous and arterial bridging segments wereperformed. The division was carried down to the level of themidline or falx. Dural substitute was utilized for closure during thisportion of the procedure and the craniotomy was replaced withplates and screws. At the conclusion of the second procedure, theplastic surgeons placed tissue expanders in the parietal and occip-ital region for use during the later stages of reconstruction (Fig. 16).

Two surgeries were carried out due to complications over thenext 2 weeks for evacuation of intracerebral hematoma and inser-tion of an extraventricular drain for hydrocephalus. The next stagedprocedure was the first attempt at directly addressing the sharedsagittal sinus.

Approximately 1 month from the initial surgery, the twins werebrought to the operative theater where a fronto-parietal craniotomywas performed. The dura was opened on twin B’s side of thesagittal sinus. The hemisphere was easily identified as it cameadjacent to the sinus. Any remaining bridging veins were coagu-lated and divided at this time. It was noted under direct visualiza-tion that the confluence of the two separate frontal sinuses cametogether at approximately the level of the coronal suture. This pointwas identified and the frontal portion of the sinus for twin B was

divided and oversewn. The frontal portion of the sinus from twin Awas left intact and remained continuous with the main portion ofthe shared sagittal sinus. It was estimated at the time of closure thatapproximately one half of twin B’s hemisphere had been discon-nected from the superior sagittal sinus.

Several weeks later the twins returned to the operating theaterfor further exploration and division of bridging veins. During thefourth surgery for separation, the final bridging veins were dividedleaving twin B with no venous inflow to the common sagittal sinus.Approximately 3 months had transpired between the first surgicalexploration and the final division of twin B’s bridging veins. It wasalso found during the surgery at 3 months that the twins had an areaof fused parenchyma in the posterior parietal–occipital regionmeasuring approximately 2–2.5 cm in diameter. The shared braintissue was coagulated and divided. During this surgery, it was de-cided to preserve the posterior confluence until the time of the finalseparation. Over the next 3 months, multiple surgeries occurred,including re-operation for wound infection and shunt malfunction.During this period, the plastic surgery team also replaced andmanipulated tissue expanders in anticipation of the final separation.

The final separation occurred just over 6 months from the initialsurgery. Two anesthesia teams and two neurosurgical teams werepresent for the final separation. Two operating room tables wereplaced head to head and the twins were draped in such a fashion asto allow for 360� of rotation. As noted, there remained one largevenous connection at the posterior confluence. The connection oftwin B to the sinus at the confluence was approached at the be-ginning of the procedure. The outflow tract of the common sagittalsinus to twin B was ligated without incident (Fig. 17). Interestingly,at the time of final ligation of the dural sinus, the blood pressure in

Fig. 15 Occlusion of venous channels

Fig. 16 Position of tissue expanders

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both twins normalized, leading to optimal cardiac function in bothtwins.

At this point attention was turned toward the final stages ofseparation. During earlier surgeries, silastic sheeting had beenplaced after areas of cortex or vasculature were separated. The useof silastic sheeting proved to be a valuable adjunct facilitating theseparation at the final surgery. It was noted that a few remainingareas of attachment were evident at the depths of the falx cerebriand in the frontal region. These divisions led to the final andcomplete separation of the twins. Closure of each twin includedreconstruction of dural defects using dural substitute. The tissueexpanders placed by plastic surgery were removed and adequatescalp expansion had occurred facilitating complete closure of thecranial vault in both twins.

Following separation, the twins remained hospitalized for sev-eral months. During this period of time the twins returned to theoperating room on several occasions to deal with scalp recon-struction and shunt malfunction. At the time of discharge, twin A(who received the sagittal sinus) had symmetric motor strength anda slightly disconjugate gaze, which had improved dramaticallysince the separation. The wounds were well healed and she re-mained on phenobarbital. Twin B was symmetric in motor strengthbut delayed in development compared with twin A.

Discussion

The history of craniopagus twin separation holds manylessons, which we have attempted to incorporate into ourcurrent strategy for separation. It has been noted by mostprior authors that the presence of a shared superior sag-ittal sinus or circumferential sinus is among the mostchallenging obstacles to a successful separation. Thedifficulty lies in deciding how to address this issue. Ourstrategy has been to perform staged procedures to pro-

mote deep venous drainage in one twin with the prede-termined goal of donating the shared sinus completely tothe other twin. The counterargument is to reduce themorbidity of repeated surgeries by performing the sepa-ration during one procedure during which the shared sinusis addressed by ligation and/or reconstruction, with onetwin receiving the sinus and the other twin undergoingsome form of reconstruction of the superior sagittal sinusde novo. Reconstruction does not address the problem ofthe bridging veins that must be sacrificed during the re-construction of the sinus.

Two recent reports discuss separation during a singleoperative session; however, the overall outcomes fromthese surgeries were suboptimal. Khan and Saberi discussthe separation of O’Connell type 2 twins, with the deathof both twins during surgery secondary to blood loss inone twin during dissection of tissue planes and the secondtwin during separation of the common sinus [11]. Ca-meron et al. discuss the use of cardiopulmonary bypassand hypothermic circulatory arrest during the separationof occipital craniopagus twins at Johns Hopkins in the late1980s [3]. The authors note that the neurosurgical teamdivided the sinus under circulatory arrest, reconstructedthe sagittal sinus of one twin with a pericardial patch, andthe second twin’s sinus was closed primarily. Of note, thetwins received 60 units of blood products during theprocedure and required inotropic support for unstablehemodynamics after cardiopulmonary bypass. The twinwho required reconstruction of the sinus suffered an an-oxic event secondary to aspiration at 5 months and wasseverely debilitated. The twin who received the sharedsinus was developmentally delayed, but retained normalneurologic function. These two recent reports demonstratethe difficulty in proceeding with a single surgery forseparation and illustrate the high risk of mortality and/orhemodynamic instability secondary to hemorrhage at thetime the shared arteriovenous anatomy is separated.

The importance of the shared sagittal sinus and its re-lationship to outcome was addressed by Bucholz et al. [2].They reviewed all prior reports describing the separationof craniopagus twins, starting with a report by Cameron in1928 forward. Based on these studies, Bucholz et al. de-veloped a “mean venous scale,” which was derived todescribe the venous drainage and determine the relation-ship between the complexity of venous drainage and theoutcome following separation. Outcome was assessed on afour-point scale (1=normal, 2=focal neurologic deficit,3=severe neurologic deficit, 4=death). The mean venousscale was found to be highly correlated with overall out-come with a correlation coefficient of 0.70 for outcomeand 0.62 for survival (P<0.0001; Table 1).

As noted, hemorrhage at the time of surgery is mostlyproblematic at the sagittal sinus and among shared venousoutflow tracts. Regardless of whether the separation isstaged or not, any attempt at separation of craniopagustwins must deal with the issue of a common sagittal sinus.

Fig. 17 Ligation of the outflow tract of the common sagittal sinusto twin B

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A critical part of the decision then becomes decidingin the preoperative period which twin will receive thecommon sinus and which twin will need the sinus re-constructed or, in the case of staged procedures, whichtwin will receive the common sinus and which twin willundergo serial ligation of draining veins to promote thedevelopment of deep venous drainage. Among papersreviewed, this decision is commonly correlated with theoutcome of the twins. Inevitably, the twin who receivesthe shared sinus has a better neurologic outcome than thetwin who has to undergo de novo reconstruction of thesagittal sinus. We contend that the ability to slowlychange the venous outflow dynamics may be a more re-liable method of preventing postsurgical morbidity in thetwin who is not selected to receive the common sinus andmay lead to improved neurologic outcome.

Clearly, there is associated risk in either scenario. Oc-clusion of draining veins can lead to venous hypertensionand cortical infarctions in the same fashion as they occurin normal children and adults. The consequences of largevenous infarctions can be profound, causing local tissuedamage and cerebral edema. Isolation from the sagittalsinus must occur slowly or these devastating complica-tions can occur. Over time, the isolation of the cortex fromthe sagittal sinus leads to the development of collateralvenous outflow into the deep draining veins that normallyservice the deep cerebral white matter and basal ganglia.The deep white cerebral white matter and basal gangliadrain centrally into the subependymal veins of the lateralventricles, eventually exiting via the vein of Galen to thestraight sinus (Fig. 18) [13].

Finally, the rationale for performing separation in astaged fashion is further supported by the literature reviewperformed by Bucholz et al. [2]. They reported a signif-icantly lower death rate when separation was performedin a staged fashion (p<0.03) and suggested “a carefullyplanned staged procedure...should have a high successrate in separating craniopagus twins...” [2]. While ourgroup favors the use of the staged approach to promotedeep venous drainage and to avoid needing to reconstructthe sagittal sinus, Bucholz et al. favor the staged proce-dure to allow for, among other issues, decreased bloodloss at the time of surgery. Bucholz notes,

...blood loss was the major limiting factor to continuedsurgery, and this loss was minimized by performing astaged dissection and preserving the cleavage planebetween operations with silicone sheeting. Stagedseparation is also beneficial in terms of allowing the

Table 1 Mean venous scale and outcome (adapted from [2]).Outcome scale: 1 normal, 2 focal neurologic deficit, 3 severeneurologic deficit, 4 dead. Mean venous scale: 1 no venousdrainage interrupted, 2 only cortical veins divided, 3 major duralsinuses encountered and ligated

Outcome scale Mean venous scale

1 1.5€0.692 2.0€0.633 2.6€0.584 2.8€0.50Total 2.4€0.79

Fig. 18a, b Deep venous drainage. a Anatomic drawing of themedullary and subependymal veins. The medullary veins drain thedeep white matter into the subependymal veins. The subependymalveins outline the lateral vehicle. 1 septal vein, 2 anterior caudateveins, 3 terminal vein, 4 thalamostriate vein, 5 direct lateral vein, 6internal cerebral vein, 7 vein of Galen, 8 interior ventricular vein, 9basal vein of Rosenthal, 10 inferior sagittal sinus (cut off), 11straight sinus, 12 medial atrial vein, 13 medullary veins. b Anatomicdrawing of the subependymal veins as seen from above. 1 septalvein, 2 anterior caudate vein, 3 terminal vein, 4 internal cerebralvein, 5 basal vein of Rosenthal, 6 inferior ventricular vein, 7 lateralatrial vein, 8 vein of Galen. Adapted with permission from [19]

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References

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use of skin expanders and in allowing the division ofthe brain tissues early in development [2]

Conclusions

The common sagittal sinus found in craniopagus twins isamong the most challenging of neurosurgical dilemmasand has enormous impact on the morbidity and mortalityseen following craniopagus twin separation. While twoviable options exist regarding separation, namely stagedversus non-staged separation, we feel that the stagedseparation with promotion of deep venous drainage isadvantageous and leads to better postsurgical outcomes.Regardless, the avoidance of complications in this patientpopulation is paramount and even under the best cir-cumstances many issues predispose these patients tocomplications that can lead to profound morbidities andoccasional mortality. Thanks to new developments inplastic techniques, the ability to close the dura and cranial

vault have produced marked improvement in the rates ofmortality from infection leading to sepsis. Advances inimaging, including high resolution MRI, MR angiogra-phy, 3D CT modeling, and conventional angiographyhave allowed new insight into the complex arteriovenousanatomy present in craniopagus twins and has facilitatedpreoperative planning. Intraoperative guidance withplatforms such as Stealth have made intraoperative nav-igation and localization more precise and modern anes-thetic techniques provide for cerebral protection duringsurgery, allowing for treatment of profuse intraoperativebleeding and hypovolemia, and provide surgeons withoptions such as cardiopulmonary bypass and hypothermiccirculatory arrest. While the separation of craniopagustwins remains one the most challenging endeavors un-dertaken in neurosurgery, continued medical advancesprovide new techniques and insights into the managementof this complex issue. By providing the option of sepa-ration to patients and families, the hope of a normal ex-istence becomes possible.