Relationship between L/T ratio and LHR in the prenatal assessment of pulmonary hypoplasia in congenital diaphragmatic hernia

The lung to thorax transverse area ratio (L/T ratio) and the lung area to head circumference ratio (LHR) have been widely used for the assessment of pulmonary hypoplasia in fetal congenital diaphragmatic hernia (CDH). The aim of this study was to evaluate the relationship between the L/T ratio and the LHR, and to clarify the characteristics of these two indicators as prognostic predictors by means of retrospective concurrent measurements from the same subjects with prenatally diagnosed fetal CDH. The medical records of 55 fetuses who had undergone a prenatal evaluation of isolated CDH from 1988 to 2006 were studied. The L/T ratio and the LHR were determined as the early values (earliest measurement performed earlier than 33 weeks of gestation) and as the late values (latest measurement performed later than 34 weeks of gestation) and analyzed, as well as the clinical data. Of the 55 infants, 13 died resulting in a 76.4% survival rate. A correlation expressed in the linear equation [(LHR) = 14.4 × (L/T ratio) − 0.11] was recognized between the early L/T ratio and the early LHR. All cases with an early L/T ratio of less than 0.08, or with an early LHR less than 1.2, died. Of the 13 cases, 5 with an early L/T ratio not lower than 0.08, but less than 0.13, died. Of the 17 cases, 4 with an early LHR not lower than 1.2, but less than 2.0, died. All cases with an early L/T ratio not lower than 0.13, or with an early LHR not lower than 2.0, survived. In 24 cases, the late values, which were measured at an interval of more than 4 weeks, were compared with the early values. Although the L/T ratio was consistent, the LHR increased in the late value compared to the early value. A good linear correlation was recognized between the L/T ratio and the LHR in the early phase of gestation, and the cutoff point of the prognostic prediction was determined in both indicators. In contrast to the L/T ratio, a definite cutoff point throughout the gestation may not be available in the LHR, because there is a natural increase of the LHR in the late phase of gestation.