ORIGINAL ARTICLE Year : 2019  Volume : 8  Issue : 1  Page : 510 Fetal transcerebellar diameter to abdominal circumference ratio (TCD/AC) and to femur length ratio (TCD/FL) in the assessment of normal fetal growth Karumanchi Ashok Hussain^{1}, Silpa Kadiyala^{1}, Amancharla Yadagiri Lakshmi^{1}, Manchala Hindumathi^{2}, ^{1} Department of Radiodiagnosis, Sri Venkateswara Institute of Medical Sciences, Tirupati, Andhra Pradesh, India ^{2} Department of Obstetrics, Sri Venkateswara Institute of Medical Sciences, Tirupati, Andhra Pradesh, India Correspondence Address: Context: Intrauterine growth retardation (IUGR) is a major risk factor for perinatal mortality and morbidity, and its early detection is useful in deciding frequency of antenatal surveillance, optimal delivery timing, and early neonatal management to avoid perinatal mortality and morbidity. Most of the parameters for detection of IUGR are gestational age dependent. Transcerebellar diameter to abdominal circumference ratio (TCD/AC) is a gestational age independent parameter. IUGR due to uteroplacental insufficiency can present as isolated femur length reduction. So, transcerebellar diameter to femur length ratio (TCD/FL) can be useful in early detection of IUGR. Aims: To evaluate the accuracy of TCD/AC and utility of TCD/FL in normal pregnancy at varying periods of gestation and to derive a cutoff value for assessment of fetal growth. Materials and Methods: In this prospective study, 200 patients with cumulative ultrasound age (CUA) between 1440 weeks. Routine ultrasound parameters and transcerebellar diameter were measured. TCD/AC and TCD/FL was calculated in all patients and for individual subgroups formed according to CUA. Statistical Analysis: Mean and standard deviation, 3^{rd}, 50^{th} and 97^{th} percentile for TCD/AC and TCD/FL was determined for all patients to determine age specific cut off values. Relationship between continuous variables is studied using correlation and simple regression. Results: In our study, TCD with AC and FL show strong correlation throughout pregnancy. TCD/AC dispersed normally with mean value of 14.013 and SD of 0.738 and cut off value is 15.49. TCD/FL is dispersed normally throughout pregnancy with mean value 64.592 and SD of 3.998. TCD/AC, TCD/FL in subgroups also shows similar mean and SD as compared to entire study population. Conclusion: A cut off value of mean +/ SD (15.49), (72.588) for TCD/AC and TCD/FL could be used as a growth parameter for detection and determine the severity of IUGR.
Introduction Intrauterine growth retardation (IUGR) is a major risk factor for perinatal mortality and morbidity.[1],[2],[3],[4] Early detection of IUGR is useful in deciding frequency of antenatal surveillance, optimal delivery timing and early neonatal management to avoid perinatal mortality and morbidity.[2],[5] Ultrasonography (USG) remains cornerstone for evaluation of fetal growth.[5],[6] Most commonly used growth parameters include biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC) and femur length (FL).[6] However, these parameters are gestational age dependent, which limits their utility at extremes of growth.[6],[7] Several studies have reported FL to AC ratio (FL/AC) and transcerebellar diameter to AC ratio (TCD/AC) as age independent growth parameters.[6],[7],[8],[9],[10],[11],[12],[13] FL/AC ratio is useful in early detection of asymmetrical IUGR.[6],[8] However, utility in symmetrical IUGR is questionable as both FL and AC are affected.[13] TCD/AC is constant through 14 to 42 weeks with cut off values ranging from 15.4 to 15.98 in different studies.[6],[9],[12],[13] In our study, we aim to analyze if its value remains constant throughout the gestational age and determine the cutoff value in our sample population. We intend to introduce a new parameter TCD/FL and observe its reliability throughout the pregnancy. If it remains constant, our objective is to formulate a cutoff value for diagnostic purpose. Materials and Methods The study was conducted prospectively on 200 subjects in Department of RadioDiagnosis, SVIMS, Tirupati a tertiary care teaching hospital in the time frame of Sep 2017 to Oct 2018 after obtaining approval from the Institutional thesis Approval and Ethics Committees. Inclusion Criteria were a) Singleton live pregnancy with cumulative ultrasound age (CUA) between 1440 weeks b) Fetal biometric parameters within normal range. Exclusion Criteria were a) Foetus with congenital anomaly on USG scan b.) Sonological evidence of IUGR c) Multiple gestations d) women with hypertension, sickle cell hemoglobinopathy, diabetes mellitus e) foetal macrosomia's and f) women with prior history of abnormal babies, stillbirths. Method of collection of data In our institute antenatal ultrasound is done as a part of routine examination and for specific examination. In first trimester routine ultrasound is primarily performed for evaluation of gestational sac, detection of fetal viability, dating, uterine, cervix, cul de sac and adnexal evaluation and measurement of nuchal translucency (as a part of screening protocol for aneuploidy). In second and third trimester, routinely antenatal USG is done primarily for detection of fetal anomaly, anatomical survey, placental localization, follow up of fetal biometry, fetal presentation and amniotic fluid measurement. Limited and detailed evaluation is done for specific indications. This study was restricted to those with POG between 1440 weeks with normal fetal biometric parameters within two standard deviations. Measurements were obtained by using a G.E. VOLUSON 730 PRO ultrasound machine in Realtime 2D image with 35.0 MHz abdominal curvilinear transducer. BPD and HC is measured in true axial section at the level of thalamus and cavum septum pellucidum. On rotating the transducer, slightly below this thalamic plane, posterior fossa with characteristic butterflyshaped cerebellum is visualized. Transverse cerebellar diameter is measured placing caliper in outer margin to outer margin fashion [Figures 1(ad)]. AC was measured along outside of skin line on a true transverse plane at the level at which following structures are visible junction of the umbilical vein and left portal vein as J shaped anechoic structure and fetal stomach. The transverse section should be circular rather than oval, kidneys and umbilical cord insertion should not be visible. Femur length was measured as horizontal position as possible perpendicular to ultrasound beam as possible. Only ossified diaphyseal length measured. Epiphysis was not included in measurement. Full length of femur was kept in single screen so that posterior acoustic shadow can be seen. Statistical analysis Data was compiled in preset format and analyzed with standard statistical formulae. Mean and standard deviation for TCD/AC and TCD/FL was determined for all patients. Data is shown as mean ± SD. Later, study population is again divided into six subgroups based on POG. Data on categorical variables was presented as number and percentage. Mean and standard deviation of each group for TCD/AC and TCD/FL was calculated. The 3rd, 50th and 97th percentile of entire study population and subgroups was measured for TCD/AC and TCD/FL to determine age specific cut off values. The relationship between continuous variables is studied using correlation and simple regression. Results having P < 0.05 are considered significant. Results TCD/AC and TCD/FL was calculated for each participant which included patients whose POG was ranging from 14 wks to 40 wks. CUA was calculated by biometric examination, which included BPD, HC, AC and FL. Distribution of patients according to CUA was shown in [Table 1].{Table 1} The study population predominantly included ANC cases coming for routine antenatal checkup, bulk of which are comprised of patients for second trimester anomaly scan. In our institute, second trimester anomaly scan is performed between 1820 wks. Hence, more than half of the patients were between 1822 weeks. TCD and AC showed strong correlation from 14 wks to 40 wks (r = 0.987, P value <0.0001). Similarly TCD and FL showed strong correlation between 14 to 40 wks (r = 0.983, P value <0.0001). The relationships are studied by linear regression which can be used for prediction of AC and FL basing on TCD shown in [Figure 2]a and [Figure 2]b. In both cases the linear model is strong and statistically significant.{Figure 2} The mean and standard deviation was calculated for both TCD/AC and TCD/FL [Table 2]. Mean and standard deviation for TCD/AC and TCD/FL was further calculated for the sub groups [Table 3] and [Table 4]. TCD/AC is distributed normally in entire study population with mean (+/ 2SD) measuring 14.01 (+/1.476) and is also normal in subgroups.{Table 2}{Table 3}{Table 4} TCD/FL distributed normally in entire study population with mean (+/ 2SD) measuring 64.59 (+/ 7.99). All the subgroups from 18 wks to 40 wks show normal distribution. More dispersion is seen in subgroup between 14 wks to 17w6d than entire study population and other groups with mean (+/ 2SD) measuring 67.02 (+/ 5.28). 3rd, 50th and 97th percentile of TCD/AC and TCD/FL for study population and subgroups displayed in [Table 5] and [Table 6].{Table 5}{Table 6} Discussion IUGR due to uteroplacental insufficiency or asphyxia leads to centralisation of fetal blood flow with sparing of brain at the expense of other body parts.[14] Various primate models have proven that the blood flow to cerebellum is maintained even in acute asphyxia.[15] In IUGR, cerebellum is least affected and TCD remains one of the most reliable parameter for measurement of true gestational age.[5],[15] IUGR leads to early depletion of hepatic glycogen and subcutaneous fat. This leads to early decrease in abdominal circumference.[8] Hence, abdominal circumference is considered sensitive parameter for early detection of IUGR.[8],[13],[16],[17] However, accurate date of LMP or prior 1st trimester dating scan is necessary for prediction of IUGR.[13] TCD/AC takes into account both least and most affected growth parameters, so even hypothetically TCD/AC is an ideal parameter for detection of IUGR. In their study of 700 patients, Meyer et al. concluded that correlation between TCD and AC is high between 14 to 40 wks.[7] The study also concluded that TCD/AC remains constant between 14 to 40 weeks and has high sensitivity in detection of IUGR. TCD/AC has been confirmed as a good age independent parameter for detection of IUGR by several other studies. In our study, TCD and AC showed strong correlation. TCD/AC values dispersed normally with mean value of 14.01 and SD of 0.74 with cutoff value of 15.49 which agrees with 97th percentile of 15.53 and are comparable with earlier studies. TCD/AC and cut off values of other studies with their SD for IUGR is displayed in [Table 7] and [Table 8].{Table 7}{Table 8} Meyer et al.[7] in their study have represented 10th, 50th and 90th percentile of study population and by each week from 14 to 40 wks. In our study, difference in distribution may be attributed to difference in racial profile of study population. FL is the most commonly used growth parameter among all long bones. Villar et al.[20] concluded in early asymmetrical IUGR, which is usually caused by uteroplacental insufficiency is associated with sparing of head and CRL. However, later Weisz et al.[21] associated isolated short femur length with IUGR and SGA. Tardos et al.[22] and Zalel et al.[23] also described association between isolated short femur lengths with SGA likely to be caused by uteroplacental insufficiency. Since, TCD is least affected parameter in IUGR, TCD/FL may be useful in detection of IUGR presenting initially as isolated FL reduction. AC is affected early in IUGR due to uteroplacental insufficiency, FL is affected later and TCD is least affected. So, TCD/FL may be used to assess the severity of IUGR along with TCD/AC. In our study, TCD and FL show strong correlation. TCD/FL is distributed normally in entire study population with mean (+/ 2SD) measuring 64.592 (+/ 7.996). This agrees with a 97th percentiles of 73.54. Among subgroups between 1840 wks mean and standard deviation remain comparable. Between CUA of 14 wks to 17w6d, mean (+/2SD) is 67.2 (+/ 10.56). Higher dispersion of values is seen at this CUA range. So, TCD/FL needs to be evaluated further in larger population with inclusion of participant with IUGR for its utility as a parameter for detection and assessment of severity of IUGR. The limitations of this study are as already stated more than half of the participants belonged to CUA 1822 wks with limited representation from 1417w6d and more than 34 wks. Hence, results may be affected by selection bias. We have not included any participant with IUGR and their follow up till birth. So, further studies with larger study population with a significant subset comprising of pregnancy IUGR need to be carried out to further validate the reliability and clinical utility of TCD/FL ratio. In conclusion, the results of our study demonstrated that strong correlation between TCD and AC throughout the pregnancy with constant TCD/AC with a cut off value of 15.49 could be used as a growth parameter for detection of IUGR. TCD and FL also show strong correlation throughout the pregnancy with constant TCD/FL especially between 18 to 40 wks with cut off of 72.588 and may specially be useful in detection of subset and severity of IUGR which present as isolated femur length shortening. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest. References


