Neonates admitted to the Intensive Care Nursery at UCSF were enrolled in an ongoing study to determine whether MRI findings can serve as a predictor of outcome following HIE from 2004 to present.
Our therapeutic hypothermia program started in November 2007. Institutional eligibility criteria for therapeutic hypothermia include the following: 1) Birth at greater than or equal to 36 weeks post-menstrual age, 2) The presence of one or more of the following: an APGAR score of less than five at ten minutes of life, a history of prolonged resuscitation at birth, the presence of severe acidosis defined as a cord pH or any arterial or venous pH of less than 7.0 within 60 minutes of birth, or a base deficit of greater than -12 from cord blood or any arterial blood gas within 60 minutes of life, and 3) The presence of moderate-severe encephalopathy identified by the attending neonatologist or pediatric neurologist. Encephalopathy was defined as abnormal mental status ranging from a hyperalert state to comatose, with associated abnormalities of tone, abnormal neonatal or deep tendon reflexes, the presence of clinical seizures, or an abnormal background pattern or seizures as identified by amplitude integrated electroencephalography.
Per protocol, cooling was initiated within 6 hours of life. For neonates born outside our center, passive cooling was performed during transport from referring institutions. Whole body cooling was achieved with a blanket cooling device (Cincinnati Subzero Blanketrol III), regulated by the infant’s core temperature measured with a rectal probe. Neonates were maintained at 33.5C for 72 hours and monitored with aEEG and continuous video-EEG for the duration of the cooling, and for 24 hours after rewarming. During cooling, all neonates were sedated with morphine administered as a continuous infusion to minimize shivering.
Of the 72 neonates referred to UCSF for evaluation of encephalopathy and possible therapeutic hypothermia, 5 were excluded for late referral (after the first 6 hours of life), and 5 infants were excluded because they did not have moderate encephalopathy, resulting in 62 infants started on therapeutic hypothermia; of these, 46 completed the full 72 hours and 35 were enrolled in our MRI study. Reasons for early termination of hypothermia and non-enrollment are shown in the . Rewarming prior to completion of therapy, and redirection of care to comfort measures was at the discretion of the attending neonatologist and pediatric neurologist.
Flow diagram depicting study subject selection
Non-treated neonates were selected from our MRI study cohort based on eligibility for hypothermia using our institution-specific criteria. Between June 2004 and initiation of our therapeutic hypothermia program (November 2007), 20 of 38 infants enrolled in our MRI study also met our hypothermia criteria, and were selected as controls. An additional five infants admitted after 2007 that were ineligible for therapeutic hypothermia based on time of referral (> 6 hours of life) were enrolled and categorized as non-treated infants. The increase in number of neonates eligible for our MRI study after 2007 is related to increased referrals to our center for therapeutic hypothermia. The Committee on Human Research at UCSF approved the research protocol. Parents of eligible neonates were approached and written informed consent was obtained.
Clinical data were collected by trained neonatal research nurses, and included information regarding prenatal, perinatal, and postnatal variables. Variables included sex, gestational age, birthweight, mode of delivery, APGAR score at 1, 5, 10 minutes of life, pH, and base deficit from the umbilical artery or first newborn blood gas within 1 hour of life, a resuscitation score reflecting the amount of resuscitation required at birth: 1 = no intervention, 2 = blow-by oxygen, 3 = endotracheal suctioning, 4 = bag-mask positive pressure ventilation, 5 = endotracheal intubation with positive pressure ventilation, 6 = endotracheal intubation with ventilation and medication, and the need for chest compressions. Perinatal data including maternal report of decreased fetal movements, fetal distress during labor (variable decelerations, late decelerations, fetal bradycardia, fetal tachycardia, lack of fetal heart rate variability), and perinatal sentinel events (placental abruption, uterine rupture, umbilical cord accident, neonatal anemia/hypovolemia) were also collected. An encephalopathy score, assessing mental status, ability to feed, need for respiratory support, tone, reflexes, and the presence of seizures was assigned on day of life 1-3 [17
]. A detailed neurologic exam was performed within 24 hour of imaging and a neuromotor score (0-6) was assigned based on abnormalities of tone, reflexes and power and cranial nerve dysfunction [18
Magnetic Resonance Imaging
Neonates were transported to the MRI scanner in MR compatible incubator, accompanied by a neonatology fellow/attending and a team of trained research nurses. Imaging was performed with a dedicated neonatal head coil, [19
]. Scans were obtained at a median of 5 days of life in the hypothermia patients and 4 days of life in the non-treated patients. The difference in the timing of scans was due to several factors including treatment with hypothermia, as scans were not performed during cooling. A series of standard MR sequences were performed for clinical assessment of the neonatal patient on a 1.5T GE scanner (GE Healthcare, Milwaukee, USA) that include 1) T1 weighted sagittal and axial spin-echo images with TR/TE of 500/11, 4mm thickness, 1 excitation, 192×256 encoding matrix; 2) T2 weighted axial dual echo, spin-echo with TR of 3sec, TE of 60 and 120ms, 192×256 encoding matrix, 4mm thickness. DWI was performed using a spin echo EPI diffusion sequence with TE/TR 99/7000ms, FOV 180mm, 128×128, 3mm slice thickness (no skip), b value of 700 s/m2
, 6 directions, and 3 averages; some infants had data obtained in 30 directions. The total examination time was approximately 1 hour.
A neuroradiologist blinded to the clinical course reviewed the MRI images. We used a previously validated MR scoring system for acute and subacute signal abnormalities, and the extent of injury in the basal ganglia/thalamus (BG/T) region (scored from 0 to 4) and watershed (WS) region (scored from 0 to 5), () [5
]. Each sequence (T1, T2 and DWI) was assigned a BG/T and WS score. The scores represent the extent of injury observed, and were used to categorize images into two additional outcome measures. 1) Predominant pattern of injury - normal, WS predominant, or BG/T predominant injury. The WS pattern was assigned when the WS region scores were higher than the BG/T scores. The BG/T pattern was assigned when the BG/T scores were higher or as high as the WS region scores. Neonates with total brain injury (maximum BG/T and WS scores) were assigned to the BG/T pattern. 2) A dichotomous outcome of normal-mild injury (normal imaging or WS score of ≤ 2 or BG/T score of ≤ 1) vs. moderate-severe brain injury (WS score of ≥ 3 or BG/T score of ≥ 2) was modeled after a similar classification scheme found to be predictive of outcome in the TOBY trial [11
MRI Brain Injury Scoring System
Statistical analysis was performed using Stata software version 9.2 (Stata Corporation, College Station, Texas). Clinical variables were compared using either Chi-square or Fisher exact test for categorical variable, Student T-test for continuous variables, or Wilcoxon rank-sum for non-parametric continuous variables. Wilcoxon rank-sum was used to assess the difference in the extent of injury (comparing the maximal score assigned to the WS and BG/T regions) and the predominant pattern of injury between the treated and non-treated infants. The relative risks of normal imaging and basal ganglia predominant injury in treated infants were calculated. Logistic regression was used to assess the association between therapeutic hypothermia and moderate-severe brain injury. Clinical factors that differed between the two groups (at p < 0.05) and were associated with the outcome (time of MRI scan and initial level of encephalopathy) were included in the regression model. A P value of ≤ 0.05 was considered to be significant.