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CHAPTER I

INTRODUCTION

Twin–twin transfusion syndrome (TTTS) is thought to result from an

unbalanced sharing of blood between two fetuses via placental vascular anastomoses.

The negative corollary is evident from the fact that the disease does not occur in

dichorionic pregnancies, as vascular anastomoses do not develop in such placentas. As a

result of the unbalanced blood exchange, one fetus receives too much blood (the

recipient twin, or recipient), and one fetus loses too much blood (the donor twin, or 

donor). The unbalanced blood sharing triggers a series of pathophysiological changesthat characterie the natural history and outcome of the disease.!

"lacental vascular anastomoses provide the basis for intertwin transfusion

(#donor$ to #recipient$) and failure to compensate for the circulatory imbalance set up

 by deep unidirectional arteriovenous anastomoses, as the result of a paucity of 

superficial bidirectional arterioarterial anastomoses, has been proposed as the

 pathogenic mechanism.%

Two&thirds of all monoygotic twin gestations are of monochorionic placentation.

'ost monochorionic placentas have vascular anastomoses, but only –!* of 

monochorionic pregnancies develop TTTS.!,%,+

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CHAPTER II

LITERATURE REVIEW

TWIN–TWIN TRANSFUSION SYNDROME

I. DEFINITION and INCIDENCE

Twin&to&twin transfusion is a complication uniue to monochorionic twin

 pregnancies. Although previously -nown by such terms as feto–fetal transfusion

syndrome, autotransfusion syndrome, intrauterine parabiotic syndrome, twin–twin

transfusion syndrome (TTTS) is now commonly used. The incidence of TTTS

ranges between !.* and /.0* of twin pregnancies and between * and !* of monochorionic twins, although the incidence may be higher and may be influenced

 by criteria used to establish the diagnosis of TTTS.!,%,1,

II. DIAGNOSIS

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The diagnosis of TTTS was originally based on the identification of anemia of one

twin and polycythemia of the other with a growth&restricted donor twin, birthweight

discordance of greater than %*, or a hemoglobin difference of 2 g3dl.!

The obstetrical ultrasonographic criteria used included confirming monochorionic

 placental mass using absence of the twin&pea- sign, oligo& or anhydramnios in the

donor using amniotic fluid poc-et measurements (defined as a vertical fluid poc-et

4% cm or an amniotic fluid index 4 cm), and the absence of fetal bladder,

 polyhydramnios (defined by a vertical poc-et 25 cm or an amniotic fluid index 2%

cm) in the recipient and signs of significant fetal cardiac decompensation, cardiac

hypertrophy, tricuspid insufficiency and hydrops.!,%,1

III. STAGING

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Twin&twin transfusion syndrome (TTTS) is diagnosed prenatally by ultrasound. The

diagnosis reuires % criteria6 (!) the presence of a monochorionic diamniotic

('78A) pregnancy9 and (%) the presence of oligohydramnios (defined as a

maximal vertical poc-et :';"< of % cm) in one sac, and of polyhydramnios (a

';" of 5 cm) in the other sac ';" of % cm and 5 cm represent the th and 0th

 percentiles for amniotic fluid measurements, respectively, and the presence of 

 both is used to define stage = TTTS.

=f there is a sub>ective difference in amniotic fluid in the % sacs that fails to meet

these criteria, progression to TTTS occurs in !* of cases. Although growth

discordance (usually defined as %*) and intrauterine growth restriction (=?@)

(estimated fetal weight!* for gestational age) often complicate TTTS, growthdiscordance itself or =?@ itself are not diagnostic criteria. The differential

diagnosis may include selective =?@, or possibly an anomaly in ! twin causing

amniotic fluid abnormality. Twin anemia& polycythemia seuence (TA"S) has

 been recently described in '78A gestations, and is defined as the presence of 

anemia in the donor and polycythemia in the recipient, diagnosed antenatally by

middle cerebral artery ('7A)–pea- systolic velocity ("S;) !. multiples of 

median in the donor and '7A "S; !. multiples of median in the recipient, in the

absence of oligohydramnion or polyhydramnios. Stage = may progress to a

nonvisualied fetal bladder in the donor (stage ==), and absent or reversed end&

diastolic flow in the umbilical artery of donor or recipient twins may subseuently

develop (stage ===) followed by hydrops (stage =;).!,%

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CHAPTER III

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The donor- re!"!en# $D-R% &ore' !n (!(o endo&o"! e(!dene

#o &)""or# #he h*"o#he&!& o+ a ne# #ran&+er o+ ,ood +ro

donor #o re!"!en# !n #/!n-#/!n #ran&+)&!on &*ndroe

I. DONOR-RECIPIENT $D-R% SCORE

=n obligatory TTTS, the placental vascular design is deterministic or fatalistic. The

faulty placental vascular design consists of the presence of either more numerous or 

larger arteriovenous (A;) anastomoses from donor to recipient. As a result, more

 blood flows from donor to recipient. To investigate this hypothesis, we assessed thesie, number, and direction of arteriovenous anastomoses. 8emonstration of faulty

 placental vascular design6 The 8 score 8etailed endoscopic analysis of the sie,

number, and direction of A; anastomoses in TTTS placentas indeed suggests larger 

or more numerous A; anastomoses from donor to recipient. =n a study we conducted

on % TTTS patients with pure A; anastomoses identified endoscopically during

laser surgery, the number of vascular communications was established in two ways6 !

B as a single communication, if only an artery and a vein participated

B as multiple communications, if prolific branching precluded an accurate estimate of

the number of vessels.

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Cor single communications, the vessel sie used for the calculations was that of the

largest participating vessel. Cor areas with multiple communications, the vessel sie

used was that of the largest vessel immediately prior to the branching. ;essel sie

was sub>ectively classified as hair, small, medium, or large (Cigure 1.!). A relative

weight of D! (small), D!.+ (medium), or D!.0 (large) was assigned for the different

vessel sies, based on visual observations. !,1,

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The direction of flow of A; communications, whether from donor (A;8) or from

recipient (A;8) was established by tracing bac- the artery to the corresponding

fetus of origin, -nowing that arteries cross over veins. Cour additional patients had

superficial arterioarterial (AA) or venovenous (;;) anastomoses. Eecause the

direction of flow in AA or ;; communications could not be established, these

 patients were excluded from the analysis. The net direction of blood flow in each

case was determined by performing an algebraic sum of the A; communications

using "oiseuilleFs law for capillary flow6!,/,

where is flow rate, 7 is a constant to account for unit conversion, r is the radius of 

the capillary, p is pressure, E is the volume factor of the fluid, m is viscosity, and G is

the length of the capillary. Although differences in blood pressure, fluid viscosity, and

vessel length could be present, the prevailing factor in the euation is the radius of 

the blood vessel, elevated to the +th power. An algebraic sum of the weighted flow in

each case (HA;8s – HA;8s) yielded a donor recipient (8) score (Cigure

1.%).!,/,

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Table 1.! shows the number, sie, and direction of the communications and the

corresponding 8 score for each individual case in the % patients studied. The mean

number of communications was +.! (range !–5). 7ommunications were bidirectional

(i.e. both A;8s and A;8s were present in the same placenta) in 0* of cases. =n

% patients, a single unidirectional A;8 was found. Iighty percent of cases had a

negative 8 score, suggesting indeed a net exchange of blood from donor to

recipient. Two patients had a positive 8 score, suggesting an apparent net flow of 

 blood from recipient to donor. This most li-ely represents the relative imprecision of 

vessel sie assessment. !

To test the validity of the conclusion with regard to the sensitivity of the

measurement, we tabulated the 8 score for varying ranges of relative vessel sie.Table 1.% shows different scenarios for the medium& and large&sied vessels

considered (!.%–!./ for the medium, and !.–%.! for the large9 Cigure 1.1). As can be

seen, the minimum percent of negative 8s in the worst scenario is *. Actually,

even if with extreme relative sies, such as D!, D!, and D! for small, medium,

and large vessels, the results of the 8 score remain unchanged. This means that the

-ey element in the assessment of the sie of the anastomosis is the sub>ective

ualification of the vessel as small, medium, or large. 'is>udgment of the relative

vessel sies within the same patient is unli-ely, as one can continuously compare

their sies during surgery. !,+,/,

8ifferences in sie assignment between patients do not alter the results, since the 8 

score is performed individually. Jur subseuent assessment of the sie, number, and

direction of A; anastomoses in a total of + TTTS patients has shown the presence

of at least one A;8 in all cases. =n no instance were there communications only

from recipient to donor (A;8s). Sixteen cases (+*) had communications only from

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donor to recipient (A;8s). The endoscopic analysis of the number, sie, and

direction of A; anastomoses suggests that a vascular anatomical basis may indeed be

responsible for the development of TTTS in a subset of patients. Although the actual

amount of blood exchange can only be estimated from this assessment, a larger 

number or larger sie of A;8s suggests that, indeed, a net flow of blood may ta-e

 place from donor to recipient twin and be responsible for the syndrome.!,/,

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II. IN-VIVO STUDIES

The description of placental vascular anastomoses from fetoscopies during laser 

 photocoagulation of the chorionic plate may be used to study the placental

architecture. This method is sensitive because of the endoscopic magnification of 

the image and is controlled because the placental surface remains with a white scar 

at the coagulation point. The problem is that technically difficult fetoscopies as

anterior placentas or turbid amniotic fluid may give false negative for anastomoses

incidence of tricuspid regurgitation increased with stage (Cigure 1.5), but has no

 prognostic value..!

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CHAPTER IV

CONCLUSION

B Twin–twin transfusion syndrome (TTTS) is thought to result from an unbalanced

sharing of blood between two fetuses via placental vascular anastomoses.

B As a result of the unbalanced blood exchange, one fetus receives too much blood (the

recipient twin, or recipient), and one fetus loses too much blood (the donor twin, or 

donor).

B "lacental vascular anastomoses provide the basis for intertwin transfusion (#donor$

to #recipient$) and failure to compensate for the circulatory imbalance set up by deep

unidirectional arteriovenous anastomoses, as the result of a paucity of superficial

 bidirectional arterioarterial anastomoses, has been proposed as the pathogenic

mechanism.% 

B 8emonstration of faulty placental vascular design6 The 8 score 8etailed endoscopic

analysis of the sie, number, and direction of A; anastomoses in TTTS placentas

indeed suggests larger or more numerous A; anastomoses from donor to recipient

B The -ey element in the assessment of the sie of the anastomosis  is the sub>ective

ualification of the vessel as small, medium, or large.

B The endoscopic analysis of the number, sie, and direction of A; anastomoses

suggests that a vascular anatomical basis may indeed be responsible for the

development of TTTS

B Although the actual amount of blood exchange can only be estimated from this

assessment, a larger number or larger sie of A;8s suggests that, indeed, a net flow

of blood may ta-e place from donor to recipient twin and be responsible for the

syndrome.

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