Historical Perspective

Because of their rarity and the profound curiosity they elicit, conjoined twins were often represented in ancient literature and art. Historical narratives suggest that although conjoined twins were sometimes revered, persecution and exploitation were more common among those who survived infancy.

The first documented illustration of craniopagus twins dates from the 15th century, describing a set born in Bavaria, Germany, in 1491 as a “warning from God” (Fig. 180-1). A famous case of craniopagus parasiticus was reported by Sir Everard Home¹ (1756-1832) in his collected works as the “Boys of Bengal,” a set of twins born in India in the 1770s (Fig. 180-2). In the early 19th century it became quite popular to exhibit conjoined twins in traveling road shows and circuses as “freaks” of nature and even to distribute postcards and autographed pictures of them (Fig. 180-3).

FIGURE 180-1 Early image of conjoined twins from Cosmographia universalis by Sebastian Münster (1488-1552), a Basel edition printed by Sebastian Heinrich-Petric in 1550.

(Courtesy of James T. Goodrich.)

FIGURE 180-2 Preserved skull of the Boys of Bengal.

(Courtesy of the Hunterian Museum Collection.)

FIGURE 180-3 The McCarther twins. These photographs were given out at events. The bottom photo contains the contact information for their press agent.

(Courtesy of James T. Goodrich.)

Several sets of craniopagus twins have lived into adulthood, although more than 90% of craniopagus twins die by age 10 years, regardless of attempts at surgical separation. The first successful craniopagus surgery in which one twin lived involved Roger and Rodney Brodies, separated in stages by Oscar Sugar in 1952-1953. Although Roger died a month after surgery, Rodney lived to the age of 11 and died of complications of hydrocephalus.

Classification and Demographics

In his fundamental work on teratology, Forster ² introduced the term craniopagus to describe twins joined at the head. Craniopagus twins are defined by a union of the calvaria only; unions involving the foramen magnum, skull base, vertebrae, or face are classified separately (e.g., cephalopagus, parapagus, rachipagus, diprosopus) (Fig. 180-4).³ In craniopagus twinning, the union is rarely symmetrical and may involve any portion of the head. An infinite number of configurations can occur, based on the attachment location and the degree of individual rotation of one head on the other. Variations in the degree of conjoining or sharing of underlying structures such as the meninges, venous sinuses, and cortex add to the individual nature of each case.

FIGURE 180-4 Variations of the union in craniopagus twins.

(Redrawn from Spencer R. Conjoined Twins: Developmental Malformations and Clinical Implications. Baltimore: Johns Hopkins University Press; 2003).

Several authors, beginning with O’Connell,⁴ have attempted to classify the numerous phenotypic variations seen in craniopagus twins. According to O’Connell’s classification schema, partial craniopagus twins have limited surface area affected, with intact crania or minimal cranial defects. Total craniopagus twins share an extensive surface area with widely connected cranial cavities. O’Connell⁴ subclassified “vertical” craniopagus twins—”parietal craniopagus,” according to the classification of Bucholz and associates⁵ (Fig. 180-5)—into types I through III, based on the degree of rotation of one on the other. Stone and Goodrich,⁶ in an effort to evaluate outcomes among different types of twins, used the partial and total divisions described by O’Connell and also defined two main subtypes: angular and vertical. Angular craniopagus twins have an intertwin longitudinal angle less than 140 degrees, regardless of axial rotation. Vertical craniopagus twins display a continuous longitudinal cranium and are further subdivided based on the degree of intertwin axial facial rotation: type I, rotation less than 40 degrees (formerly O’Connell type I); type II, rotation between 140 and 180 degrees (O’Connell type II); and type III (“intermediate”), rotation between 40 and less than 140 degrees (O’Connell type III).⁶

FIGURE 180-5 Examples of the four general subclassifications of craniopagus twins (top to bottom): frontal, parietal, temporoparietal, and occipital.

(Redrawn from Bucholz RD, Yoon KW, Shively RE. Temporoparietal craniopagus. Case report and review of the literature. J Neurosurg. 1987;66:72-79.)

Craniopagus twins are extremely uncommon, occurring once in every 0.6 to 2.5 million live births (Table 180-1).⁶^,⁷ The cause of the conjoining of twins is unclear; both “incomplete fission” and “fusion” hypotheses have been proposed by embryologists.³^,⁸ There are no known environmental or genetic factors. Interestingly, the female-to-male ratio is nearly 4 : 1, although this has not been correlated with any cause of the condition.

TABLE 180-1 Demographic and Embryologic Characteristics of Craniopagus Twins

INCIDENCE

Conjoined twins: 10 to 20 per million births in the United States

Craniopagus twins: 0.6 per million births in the United States

GENETICS

Identical (monozygotic)

Female-to-male ratio 4 : 1

CAUSE

No known genetic or environmental causative factors

Fusion or incomplete fission

MORTALITY

Conjoined twins: 40% stillborn; 33% die in perinatal period

An extensive review of the literature from 1919 through 2006 notes 64 well-documented cases of craniopagus with 41 separation attempts—29 performed as single-stage separations and 12 as multistaged separations.⁹

Surgical Separation

Risk Stratification

The decision to separate craniopagus twins is obviously fraught with serious considerations. Stone and Goodrich ⁶ proposed their classification system in part to better understand the outcomes after surgical separation. It is now recognized that shared dural venous sinuses present significant difficulties when proceeding with separation, but numerous other risk factors must be assessed when evaluating craniopagus twins for separation. Critical to the surgical decision-making process is an understanding of the degree of shared scalp, calvaria, and dura (independent dural envelopes or incomplete dural separation); the amount of separation, interdigitation, or fusion of cortex or deeper structures; the extent of shared arterial connections and cross-flow; the extent of shared or common venous sinuses and drainage; the presence of paired or separate venous outflow; the presence or absence of independent deep venous drainage; and the presence of common or separate ventricular systems and hydrocephalus.¹⁰ Because each of these factors has serious implications for the surgical strategy, we proposed a scheme by which individual cases can be evaluated to determine the surgical risk (Table 180-2).¹⁰ Although each case involves many unique considerations, this scheme, which assigns point values based on the aforementioned criteria, can be used to gain a preliminary understanding of the degree of risk associated with a separation attempt; a higher score is indicative of a more difficult separation.

TABLE 180-2 Surgical Risk Stratification

CHARACTERISTIC	SCORE*
Scalp
Minor surface area shared (<10 cm²)	1
Major surface area shared (>10 cm²)	2
Calvaria
Minor surface area shared (<10 cm²)	1
Major surface area shared (>10 cm²)	2
Dura Mater
Independent dural envelope surrounds cortex	1
Dura shared along one or more planes	2
Neural Tissue
Completely separate	1
Interdigitated but not fused	2
Minor areas of fusion (<5 cm² total surface area)	3
Major areas of fusion or involvement of eloquent cortex (>5 cm² total surface area)	4
Arterial Connections
None	1
Minor feeding branches (M4, A4, P4)	2
Distal branches (M3, A3, P3)	3
Proximal branches (M2, A2, P2)	4
Major vascular trunks (M1, A1, P1, ICA)	5
Venous Connections
None	1
Separation of major sinuses with minor shared draining veins	2
Shared along anterior of superior sagittal sinus and/or shared distal (transverse/sigmoid)	3
Shared along posterior of superior sagittal sinus without involvement of the torcula	4
Shared along posterior of superior sagittal sinus with involvement of the torcula	5
Deep Venous Drainage
Present	1
Absent	2
Cerebrospinal Fluid (Ventricular Anatomy)
Separate ventricular systems	1
Shared ventricular system	2
Venous Outflow
Ipsilateral	1
Contralateral (crossed)	2
Arterial Flow
Ipsilateral	1
Contralateral (crossed)	2