Early and accurate assessment of brain damage in neonatal hypoxic-ischemic encephalopathy (HIE) can be useful to predict its long term consequences. A meta-analysis concluded that deep gray matter Lactate (Lac)/ N-acethylaspartate (NAA) and Lac/Creatine (Cr) have superior prognostic accuracy than conventional or diffusion-weighted MRI ¹. However, ratios can change by alteration of either of the metabolites involved, thus limiting understanding of how metabolism disturbances are associated with neuro-impairment. Our purpose was to investigate the prognostic ability of absolute metabolite concentrations (NAA, Cr, Choline(Cho) and Lac) of the deep gray matter in neonates with HIE.

Patients and clinical data: 44 of 48 near term neonates (at least 35 gestational weeks) with HIE (Sarnat grade II or III) treated in a single NICU unit in Japan from October 2009 to January 2014 were included (4 infants were excluded: 1 with congenital heart disease, 3 lost to follow-up). All underwent MR examinations within 2 weeks of birth: 16 had early exams at 24-96 hours, 41 had late exams at 7-14 days, and 13 had exams at both time points. Neurodevelopmental outcome was assessed at 18–24 months corrected age by determining the developmental quotient (DQ) using the Kyoto Scale of Psychological Development (KSPD), and functional disability according to the Gross Motor Function Classification System (GMFCS). Neurodevelopmental impairment (NDI) was defined as a GMFCS grade of level 2–5 or a DQ <70, adverse outcome as death or survival with NDI, and favorable outcome as survival without NDI.

MR Spectroscopy: 1H-MRS were acquired in the deep gray matter [Fig.1] on 3T clinical scanners (MAGNETIM Verio, Siemens) using 12 or 32 channel head coils. The acquisition parameters were as follows: PRESS sequence with water suppression pulse; TE/TR: 30/5000 ms; number of excitations: 4 to 16; spectral bandwidth: 2000Hz; number of points: 1024. For the signal quantification, reference scans without water suppression were also acquired. The total acquisition time was about 5 min. The data processing was performed using LCModel [Fig.1]. Metabolite concentrations were automatically calculated by comparing the proton peaks of the metabolites without water suppression in the same voxel. The reference water concentration was assumed to be 48.9M ².

Statistics: Fisher’s exact test and Mann-Whitney-Wilcoxon test was used to compare background characteristics and absolute metabolite concentrations of infants with adverse and favorable outcomes. For each metabolite, receiver operator characteristics (ROC) curves were calculated using logistic regression, from which area under curve (AUC), and cut-off with optimum sensitivity and positivity were determined.

8 had adverse and 36 had favorable outcomes. Gestational age, birth weight, male/female ratio, Apgar score at 5 and 10 minutes, pH of the umbilical blood, proportion of infants given hypothermia treatment, and age (in days) MR was taken, did not differ significantly between the two groups [Table 1]. Measurement’s median and prognostic utilities of quantitated absolute metabolites concentrations by outcome are shown in Tables 2 and 3. NAA, Cho and Cr concentrations showed excellent prognostic values (AUC 0.96 to 0.99) with the adverse outcomes having significantly lower measurements compared to those with favorable outcomes in all measurement timing, while Lac was less efficient (AUC 0.74). NAA and Cr concentrations measured at early examinations revealed perfect prognostic values (AUC 1.00). Figure 2 shows distributions of absolute concentration of 4 metabolites by age at measurement. All measurements of infants with adverse outcome revealed lower NAA, Cho and Cr concentrations than the cutoff values, regardless of measurement timing.It is understandable that low absolute NAA concentration predict adverse outcomes. However, it is surprising that those of Cho and Cr revealed such high AUC values. Moreover absolute concentration of Cr had a little higher AUC than that of NAA. We speculate brain energy level and cell membranous proliferation of infants with adverse outcome would become lower shortly after the injury from HIE. It means some MRS evaluations by the ratio to assume such metabolites a denominator would have a problem. Absolute concentrations were well quantified by short echo PRESS sequence with higher signal noise ratios except for Lac which was overlapped with the peaks of lipid/macromolecules in short echo ¹H-MRS. However, visual detection of elevated lactate with typical twin peaks was easy and we could use it clinically as a biomarker for poor outcome.Absolute low Cr and NAA concentration data quantified by 1H-MRS with short echo sequence within 2 weeks after birth clearly predict adverse long-term prognosis and particularly outstanding in the measurements at 24-96 hours. Absolute Cr and NAA concentration within 2 weeks can be excellent biomarkers of infants suffered with neonatal HIE.