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Twin Anemia Polycythemia Sequence (TAPS)

Although the definition of TAPS excludes TTTS, both conditions may coexist (TTTS + TAPS) in approximately 2.5% of all TTTS patients.

The condition is generically defined as the presence of large hemoglobin differences between two monochorionic twins, in the absence of classic amniotic fluid volume discordances typical of twin-twin transfusion syndrome (TTTS).

What is TAPS?

Twin Anemia Polycythemia Sequence (TAPS) is a relatively newly described complication of monochorionic twins. The condition is generically defined as the presence of large hemoglobin differences between two monochorionic twins, in the absence of classic amniotic fluid volume discordances typical of twin-twin transfusion syndrome (TTTS). In other words, the fetuses have large differences in their hemoglobin/hematocrit values, but do not have polyhydramnios > 8 cm and oligohydramnios < 2 cm in the amniotic fluid. TAPS may occur spontaneously in approximately 3.5% of all monochorionic twins, but may also result iatrogenically after laser therapy in up to 16% of patients. Although the definition of TAPS excludes TTTS, both conditions may coexist (TTTS + TAPS) in approximately 2.5% of all TTTS patients. Along with TTTS and SIUGR (selective intrauterine growth restriction), TAPS is recognized today as a distinct complication of monochorionic twins.

What is the cause of TAPS?

The cause of TAPS is not well understood. In theory, TAPS is though to occur from a chronic transfer of blood between the two fetuses through very small caliber arteriovenous anastomoses. The small size of the vessels prevents large blood volume shifts, such as those seen in TTTS, but allows for the chronic development of anemia in one twin and polycythemia in the other. Although indeed cases of TAPS may be seen in the presence of such anastomoses, the actual pathophysiologic mechanism is difficult to understand. Furthermore, TAPS may also occur in the presence of larger caliber anastomoses or even in the total absence of placental vascular anastomoses, which makes understanding the etiolog of TAPS even more difficult.

How can TAPS be diagnosed?

TAPS can be diagnosed post-natally or antenatally. Post-natally, a hemoglogin difference of 8g/dL has been arbitrarily chosen as the definition for TAPS. Antenatally, this Hb difference can be expected to exist if the middle cerebral artery peak systolic velocity (MCA-PSV) of one twin is consistent with fetal anemia (MCA-PSV greater than 1.5 multiples of the median, -MOM, and the other twin is polycythemic (MCA-PSV <1.0 MOM). A new definition of TAPS using a delta MCA-PSV MOM of >0.5 has been proposed, but it’s not universally accepted.

Are there degrees of TAPS?

Similar to the Quintero staging system for TTTS, a staging system for TAPS has been proposed. The staging system using the MCA-PSV MOM of >1.5/<1.0 definition for TAPS is as follows:

  • Stage 1: MCA-PSV > 1.5/<1.0 MoM, without signs of fetal compromise
  • Stage 2: MCA-PSV >1.7/<0.8 MoM, without signs of fetal compromise
  • Stage 3: As Stage 1 or 2, with cardiac compromise of donor *
  • Stage 4: Hydrops of donor
  • Stage 5: Intrauterine demise of one or both fetuses preceded by TAPS

The postnatal staging of TAPS is as follows:

  • Stage 1: Hb difference > 8 g/dL
  • Stage 2: Hb difference > 11 g/dL
  • Stage 3: Hb difference > 14 g/dL
  • Stage 4: Hb difference > 17 g/dL
  • Stage 5: Hb difference > 20 g/dL

How can TAPS be treated?

Depending on the gestational age at which TAPS is diagnosed, delivery may be an option. If the diagnosis is made far from viability, different treatment options have been proposed, including:

  • Expectant management. Depending on the Stage of TAPS, e.g., Stage I or possibly Stage II, expectant management may be an option. In more advanced stages (e.g, Stage III-IV), delivery, if possible, or antenatal treatment may be required. Expectant management may result in non-progression or progression of the condition. If the condition remains stable, the patient may be followed until an appropriate gestational age for delivery has been achieved. If the condition progresses, the patient may need to be delivered or offered antenatal treatment if far from viability.
  • Antenatal treatment. Antenatal treatment of TAPS has centered on two options:
    • Intrauterine transfusion or exchange transfusion. This option consists of transfusing the anemic baby and removing blood from the polycythemic baby. Technically, this may prove to be challenging, and ultimately, it is not effective, as the underlying cause of the condition persists. In our opinion, this option should be reserved for extreme situations where neither delivery nor definitive treatment can be offered, and should not be considered as a first-line form of therapy.
    • Laser therapy. Similar to the treatment of TTTS, laser occlusion of the placental vascular anastomoses results in complete resolution of the condition. Our group favors this form of treatment for patients with any Stage of TAPS, but particularly for those in Stages III or IV or earlier stages remote from viability.

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