The results of this study indicate that therapeutic hypothermia for NE is associated with improved brain metabolism and preserved brain microstructure in the deep gray nuclei. Therapeutic hypothermia is associated with higher ADC values in the basal ganglia and thalamus and lower Lac:NAA in the basal ganglia. Due to the strong association between injury to the deep gray nuclei and outcome, we attempted to evaluate differences in microstructure and metabolism between treated and nontreated neonates with predominant deep gray nuclear injury but were unable to demonstrate statistically significant differences between the groups. Finally, treated neonates with normal qualitative imaging findings had metabolic ratios similar to those in healthy neonates without brain injury; for reasons that are not yet established, their ADC values in the thalamus were slightly higher than those of healthy neonates.
Serial imaging studies of neonates with encephalopathy who did not receive hypothermia have characterized sequential changes in ADC, FA, and metabolic ratios.15-18
As shown in the initial study, soon (when imaged at 2–5 days) after the latent period (which lasts 6–15 hours), there is a drop in both ADC and FA values thought to be secondary to cytotoxic edema and metabolic stress related to secondary energy failure, which may dramatically affect water diffusion. This is followed by a period of pseudonormalization of ADC values, which occurs around the second week (7–10 days) after birth.15-18
Analysis of metabolic parameters reveals decreased NAA:Cho and increased Lac:NAA ratios.14,19
These changes are presumed to be secondary to mitochondrial dysfunction, decreased intracellular energy, neuronal loss with cell membrane turnover, and possibly astrocytic injury.25
In the hypothermia-treated neonates, we did not observe the expected decrease in ADC and NAA or the expected increase of lactate in the deep gray nuclei.
While the precise mechanism of neuroprotection conferred by hypothermia is not fully understood, it is likely multifactorial with different mechanisms specific for each phase of cerebral injury. The possible mechanisms were presented in a recent review by Drury et al.26
This review highlights the possibility that the degree of neuroprotection may be related to the timing of initiation of therapy. Hypothermia may have a discrete impact on each of the stages that follow a hypoxicischemic insult (acute insult, reperfusion [30–60 minutes postinsult], latent period [6–15 hours postinsult], and secondary energy failure [6 hours to >3 days]). In the present study, hypothermia was initiated early, with most neonates reaching the target temperature at <6 hours after birth, but the precise timing of treatment initiation related to the acute insult is not known.
Despite this uncertainty, the data support the hypothesis that hypothermia results in recovery of oxidative metabolism (less lactate detected in treated neonates), decreased cytotoxic edema (ADC levels near normal), and decreased neuronal injury (more normal NAA) as measured by these advanced imaging techniques. The clinical significance of and the reasons for higher ADC values within the cerebrum following hypothermia therapy are unknown. Possibly the findings are transient and related to hypothermia therapy itself, though the subjects were imaged when they were normothermic. Another possibility would be that the hypothermia merely delays secondary energy failure; however, prior studies have shown that therapeutic hypothermia is effective in improving outcome in the short-term (18 months of age),1
indicating that the effects are not merely temporary. Finally, we cannot entirely exclude the possibility that the increase in ADCs may be technical and related to the increased number of directions of data acquisition in the hypothermia-treated neonates compared with controls.27,28
This explanation seems unlikely, however, because ADC values tend to decrease by approximately 5% with an increasing number of directions from 6 to 30, but we noted an increase in ADC values. A study of serial DTI and MR spectroscopy during therapy, immediately after therapy, and several days after rewarming may better elucidate the impact of hypothermia on DTI and MR spectroscopy parameters.
There are few studies in human neonates that report associations between hypothermia and brain microstructure or metabolism. The data presented here differ from the published studies in objectives, timing of imaging, and study population. One small study, comparing 3 hypothermia-treated neonates with 3 nontreated and 4 healthy controls, showed that mean diffusivity in the putamina and thalami of treated neonates was similar to that seen in control infants, but the timing of imaging was quite variable, with some as early as the first week of life and others as late as 7 weeks after birth, making a direct comparison with our study difficult.29
A second study of a larger cohort (n
= 47) imaged at 5–12 days of age evaluated the relationship among ADC values, T1 and T2 signal intensity ratios, and outcomes at discharge and at 9 months of age.30
All subjects in this study received hypothermia, so there was no comparison between treated and nontreated neonates. The authors reported no difference in ADC values between neonates with a normal or mild deficit and those with a severe deficit (death or abnormal consciousness, tone, hearing, vision, absent gag/suck/feeding autonomy) when evaluated at discharge or 9 months of age. They did find that T2 intensity ratios were independently associated with outcome but were not better at predicting outcome than qualitative measures. The inability of ADC values to differentiate among neonates with different outcomes was likely related to the timing of the MRI because the scans were obtained at a median age of 7 days, likely during the phase of pseudonormalization of the ADC values. Finally, in the most recent single-center study, 10 of 81 study subjects were treated with hypothermia.20
As a secondary aim, the authors compared the ADC values and Lac:NAA ratios between hypothermia-treated and nontreated neonates. ADC values in the basal ganglia were similar in treated and nontreated neonates with a favorable long-term outcome. A direct comparison of the ADC values and metabolic ratios between treatment groups was not performed. The authors also demonstrated that when imaging is performed during the first week of life, adding ADC or Lac:NAA ratios to qualitative scoring resulted in better prediction of outcome than qualitative scoring alone.
This study has several limitations. Major limitations include the small sample size, especially in the nontreated group with NE and in the number of subjects with predominant injury to the deep gray nuclei, as well as a lack of long-term neurodevelopmental follow-up data. This small sample size may have limited the ability to detect a difference between the groups. Follow-up data in this cohort will be required to understand the very important question of clinical relevance of DTI and MR spectroscopy values after hypothermia; this answer will determine whether the MR imaging metrics will be useful as early biomarkers of treatment efficacy. Another important limitation is the timing of MR imaging. ADC values are known to evolve with time, yet nontreated subjects were imaged slightly earlier than treated subjects, possibly before pseudonormalization and healthy neonates were imaged later than the other 2 groups. This difference in the timing of MR imaging studies, in addition to the treatment-related factors, may contribute to the difference in ADC values. This difference was accounted for by adjusting for the time of scanning and by including an interaction factor between treatment and age at scanning because both have an impact on the ADC values that are measured.17,18
The difference persisted after adjustment, making it less likely that the results are due to differences in the timing of imaging. Serial imaging during the first week of life may provide a better understanding of the trajectory of the measured values and how they differ from those of healthy infants.
Demonstrating that hypothermia treatment of encephalopathic neonates correlates with normal ADC values and 1H-MR spectroscopy ratios reassures the clinician that injury to the cerebral microstructure and metabolism has been ameliorated. While we did not evaluate the association between DTI and 1H-MR spectroscopy with outcome, having normal conventional imaging and normal diffusion or metabolic parameters after therapy is important preliminary and prognostic information. We plan to correlate with medium-term (4-year) outcome when the children in our study reach that age.