Definition of polyhydramnios and oligohydramnios

TTTS. Definition of polyhydramnios and oligohydramnios

Polyhydramnios and oligohydramnios are defined as sonographic estimates of amniotic fluid volumes above and below the 95th percentile for gestational age, respectively. Ultrasound assessment of amniotic fluid volume has been either subjective, or semiquantitative using the maximum vertical pocket (MVP), amniotic fluid index (AFI), or the 2-dimensional pocket (2D). 

The MVP is defined as the largest vertical pocket of amniotic fluid without the presence of umbilical cord or other fetal parts. The AFI is the sum of the 4 MVPs taken in each quadrant with the transducer aligned in the sagittal plane. The 2D diameter is the product of the MVP times the maximum transverse diameter in any particular quadrant. 

Both the MVP and the AFI have been shown to behave similarly in singletons, dichorionic twins, and smaller in twins than in singletons.14 The decision to use the MVP instead of the AFI in terms of the sonographic definition of TTTS is based on several grounds. First, the AFI is known to increase, with gestational age until approximately 33 weeks15,16 (Figure 7.1), whereas the MVP remains relatively stable throughout the second trimester (Figure 7.2). 

Secondly, the AFI is impractical in twins, because of the varying location of the dividing membrane. Lastly, the uterine fundus may not have surpassed the umbilicus in some patients, which hinders the designation of the four quadrants for the AFI. Some disagreement has existed as to which MVP cut-off values should be used in the definition of oligohydramnios and polyhydramnios. Most authors agree on ?2 cm for the definition of oligohydramnios, and ?8 cm for the definition of polyhydramnios. 

Oligohydramnios defined as ?2 cm corresponds to the 5th percentile in MVP estimates, and changes little throughout gestation.16–19 The 95th percentile corresponds to an MVP of approximately 8 cm.14 Because of the known inaccuracy of amniotic fluid volume estimates, regardless of the sonographic technique used, the 2 and 8 cm cut-offs should be viewed as the minimum criteria to diagnose TTTS. 

The importance of adhering to a standard definition of TTTS cannot be overemphasized because of the grave implications of treating non-TTTS patients or not treating bona fide TTTS patients. Non-descriptive terms such as poly/oli, or twinpolyhydramnios- oligohydramnios-sequence (TOPS), used in the past, are slowly being abandoned. While there has been universal acceptance of an MVP of ?2 cm as part of the definition of TTTS, some authors have argued that using <1 cm is a more stringent criterion. 

We have looked at this question in our own patient population. If one agrees that stage III and stage IV TTTS patients represent the extremes of the spectrum in terms of disease severity, the use of a 1 cm cut-off would declassify 27% of patients as having TTTS (Figures 7.3a,b). Some authors have suggested using ?10 cm for the definition of polyhydramnios particularly after 20 weeks. Such decision would declassify 38% stage III and 8.3% of stage IV TTTS patients. 

Using a cut off of 20 weeks, 22% of patients would be under diagnosed using an MVP of >10 cm. Although the MVP rises slightly with gestational age before 16 weeks of gestation, the use of an MVP <8 cm is unwarranted, as it is likely to result in significant overlap with the normal population (Figures 7.4a,b).