Prenatal MR Imaging Detection of Deep Medullary Vein Involvement in Fetal Brain Damage

Discussion

The reported cases show how fetal cardiac failure may be associated with DMV congestion, perivenous hemorrhage, and various patterns of brain edema. The peculiar features of such intracranial alterations may be related to the relatively immature fetal venous parenchymal system.¹

Medullary veins draining from cerebral WM are divided into superficial and deep groups. The DMV group consists of relatively long venous channels originating in the subcortical WM, running centripetally toward the ventricular surface and draining via the subependymal veins. DMVs reach the ventricular wall, becoming large and fewer in number and, finally, converging at specific points (mainly the lateral corner of the ventricles) with a typical fan-shaped pattern. These points are localized along a line that runs above the caudate nucleus and, posteriorly, bifurcates along its tail and the roof of the occipital horn.³ Some authors described a third group of medullary veins (the intracerebral anastomotic or transcerebral veins), which cross the entire hemisphere, reaching the subependymal veins.^4,5 Normal DMVs are difficult to identify on MR imaging, and few reports of their pathologic aspects have been reported in neonates.⁶ Our 3 cases had parenchymal WM lesions with distribution in the DMV territory in addition to cytotoxic or interstitial-vasogenic brain edema and other cerebral and extracerebral signs of venous hypertension. It is known that generalized brain swelling could be part of a near-death condition; however, regarding our 3 cases, fetal death was more likely due to cardiac failure rather than to brain stem herniation or compression. Lesions were always located in the frontoparietal periventricular WM; no lesions were identified in the temporo-occipital WM, probably because of the possible higher frequency of transcerebral medullary veins in these areas, which might represent a protective factor. Intraparenchymal hemorrhage had been previously described in association with PVL as the result of bleeding into tissue already damaged by ischemia.⁷ In our 3 cases, lesions were partially hemorrhagic, with characteristic wedge-shaped distribution, suggesting the presence of small venous infarctions.

A recent report on fetal cases with severe congenital heart defects showed that diffusivity in the periatrial WM and thalamus was higher compared with that in controls, suggesting that brain development is affected in utero, probably because of cerebral perfusion changes.⁸ In our 3 cases, all having congenital or acquired cardiopathy, ADC in bihemispheric WM was also abnormal (increased in 2 cases and reduced in 1 case). However, it is not possible to establish how much of this change is due to cardiopathy itself, and how much, to the consequent severe cardiac failure, which, nevertheless, seemed to be the main factor. Our cases are probably very different from the large cohort of fetuses with congenital cardiac anomalies recently presented by Limperopoulos at al,⁹ which showed significant reduction of brain volume growth with respect to controls. In such a chronic scenario, in which heart failure and central venous congestion were probably not present, the mechanisms producing brain development impairment should have been different from our DMV-associated ones, resulting from a condition of prolonged hypoperfusion and oxygen/glucose deprivation.

The fetal brain is known to be particularly susceptible to hypoperfusion, oxidative stress, and consequent hypoxic-ischemic injury because of arterial immaturity and lack of autoregulation. Such factors can lead to the typical condition of PVL, with WM damage and volume loss mainly affecting the posterior periventricular regions.¹⁰ From this limited preliminary series, it appears that DMV could be involved in the development of atypically located PVL (ie, deep frontal lobes) and hemorrhagic necrosis. All of our cases were related to circulatory failure that may cause central venous hypertension. Our cases had severe cardiac failure because intrauterine death occurred some days after imaging. One hypothesis is that venous hypertension could induce DMV engorgement and, hence, small venous infarctions in DMV territory; we suppose that, in some cases, it could be the main factor that leads to leukomalacia. Therefore, hypoxic-ischemic injury might have a role, but, as previously suggested by some authors,⁶ it is probably not the initial and only event.