We found 13 patients, 9 females (60%) and 4 males (40%). In all cases the middle cerebral artery territory was involved, 77% in the left hemisphere. 3 cases showed bilateral involvement.
As aforementioned, imaging plays a fundamental role in the evaluation of NAIS patients, so Radiologists must be familiar with imaging protocols and NAIS radiological findings (figures 1 & 2).
A. US
Ultrasound is the first-line imaging modality because of its lack of ionizing radiation, portability, availability, and economic factors [7, 8]. Microcurved and phased-array high frequency probes should be used (5-18 MHz) [1, 2]. Nevertheless, head US is less sensitive than MRI to detect small cortical infarcts and depict its distribution and extension [1, 2, 6] and US findings may be less conspicuous in early evaluation [6]. Most neonatal ischemic strokes appear as a wedge-shaped focal increase in echogenicity in the supply region of an artery, typically the middle cerebral artery [8] (figures 3 to 8), whereas hemorrhage would be hyperechogenic to the choroid plexus. We recommend a systematic evaluation of the posterior fossa through posterolateral fontanelle.
The corresponding arterial inflow can be visualized and measured with Color Doppler and spectral Doppler US (figures 3 to 8). The ipsilateral affected great vessels may show a decreased resistive index (RI) due to vasodilation and increased diastolic flow (figures 4 to 6).
Normal RI associates better clinical outcome [2]. Using Duplex and spectral Doppler increases the sensitivity of US [2].
Head US was the first-line study in all our patients, showing some degree of pathological findings in 10 of the patients (77% initially). In the other 3 cases, follow-up US ended-up showing abnormalities. 7 patients (53%) had an abnormal Doppler US study.
B. MRI
MRI allows a much complete evaluation of the patient and is the modality of choice evaluating NAIS due to its superior sensitivity and specificity, and its contribution to clarifying the diagnosis and establishing a prognosis. Our standardized MRI protocol includes the following sequences: T1-W, T2-W, DWI, SWI, and MR arteriography (MRA) and MR venography (MRV). Images are acquired usually at 1.5 or 3T. Pharmacological sedation of the newborn is avoided in most cases by feeding them just before the acquisition and swaddling them appropriately [8].
Restricted diffusivity of the infarcted area is found within hours of the insult, so DWI is the most sensitive modality in acute NAIS evaluation (fig. 3 to 9).
Pseudonormalization of DWI occurs by day 4 to 21 [2]. By day 2,
T2-W images show the “missing cortex sign” (fig. 3)
due to the increased signal intensity of the edematous cortex ribbon being isointense to unmyelinated white matter and, by the seventh day, the cortical ribbon shows very low signal intensity [1, 2]. T1-W images show an increased signal intensity of the cortex (7 days- 1 month) . FLAIR images are not useful in the NAIS evaluation (high signal of infarction upon the high signal intensity of unmyelinated WM) [1]. SWI helps to depict hemorrhage or clotted blood
(figure 8). MRA and MRV help to identify the cause of the infarction, as they show abnormalities in 62% of patients (occlusion, reduced flow)
(fig. 7 & 8), and can discard other etiologies such as venous thrombosis [1] Time-of-flight (TOF) MRA allows evaluating vascular permeability without needing intravenous contrast administration.
MRI evaluation may also show acute Wallerian degeneration as restricted diffusion (DWI) and increased signal intensity (T2-W) affecting white matter tracts dependent on the infarcted area (fig. 5 & 9).
This damage is secondary to the excitotoxicity caused by glutamate release. Wallerian degeneration may predict persistent neurologic impairment [1].
All of our cases had typical acute arterial stroke MRI findings. Six of the patients showed some degree of hemorrhagic transformation. MRA of one patient showed occlusion of left proximal MCA (fig. 8),
and another patient showed a doubtful occlusion of M2 segment in left MCA. In this last case, Doppler US found turbulent flow in the MCA with very low RI
(fig. 7).
C. CT
CT remains relegated to an emergency setting without MRI availability or when the newborn is too unstable to perform an MRI [1]. Low dose CT is recommended (100 kV and 200 mAS) and pharmacological sedation is also avoided. CT findings of IAS are similar to those in adulthood: sulcal effacement, loss of grey-white matter differentiation, or wedge-shaped area of hypoattenuation in an arterial distribution, depending on the time of onset. We must acknowledge that, in infants, physiologically hypoattenuating cortical areas, located in the posterior temporal and occipital lobes, exist, hindering the CT assessment [1], and that unmyelinated brain makes subtle hypoattenuation less conspicuous [6].
Only one of our patients had a CT scan performed before the MRI, as hemorrhagic infarct could not be excluded with US (fig. 8) and urgent neurosurgical intervention was probable.