martedì 5 dicembre 2006

Fetal hydrocephalus


The cerebellar hemispheres (Figure 1), vermis and cisterna magna (Figure 1) are normal.
There is hydrocephalus, manifested by the marked dilatation of lateral ventricles, a midline falx (Figure 2 and Figure 3), and thinning of the cerebral cortical mantle (Figure 2 and Figure 3).
The fourth ventricle (Figure 4) is normal, therefore, the obstruction is suspected to be at the level of the Acqueduct of Sylvius (Figure 4). No obstructing lesions are identified.
Aqueductal Stenosis is more common in males as seen in this case (Figure 5)

Diagnosis: Fetal hydrocephalus

Fetal MRI is used as a problem-solving tool in obstetrics with a wide variety of indications when ultrasound findings are equivocal or need further evaluation.
MRI in the first trimester is a relative contraindication. MRI poses no known risk to the fetus in the second and third trimester. Intravenous gadolinium is not approved for use in pregnancy.
Ventriculomegaly is the most common CNS abnormality identified on prenatal US and also the most common indication for fetal MRI. 70-84% of fetuses with ventriculomegaly show associated structural or chromosomal anomalies.

Ultrasound is the primary technique for fetal imaging for its proven utility, widespread availability and relative low cost. However, there are pitfalls in the evaluation of the brain and spine with US. Discrimination of normal and abnormal appearance of the CNS on US is based on the ability to produce high resolution images of the cerebrum, cerebellum, and spine. Maternal obesity, oligohydramnios, suboptimal fetal lie may cause inability to obtain adequate US images. MRI is less affected by these factors and over the past decade has emerged as a clinically useful supplement to US where US findings have been inconclusive or insufficient to guide treatment choices.

Indications for fetal MRI

Neurologic indications:
- Ventriculomegaly
- Agenesis of the corpus callosum
- Posterior fossa abnormalities
- Malformations of cerebral cortical development - neuronal migrational anomalies such as schizencephaly, lissencephaly, polymicrogyria, and gray matter heterotopia.

Nonneurologic indications:
- Congenital diaphragmatic hernia
- Pulmonary sequestration
- Congenital cystic adenomatoid malformation
- Airway obstruction
- Volumetric mesurments of individual fetal organs

Safety concerns arise for both mother and fetus and the FDA guidelines require labeling of MRI devices to indicate the the safety in pregnancy has not yet been established. Maternal safety concerns are addressed by standard MRI screening while fetal safety concerns are related to teratogenesis and acoustic damage.

In general, MRI in the first trimester is a relative contraindication owing to the higher risk of mutation during the period of organogenesis. MRI poses no known risk to the fetus in the second and third trimester. Intravenous gadolinium is not approved for use in pregnancy as it has been shown to cross the placenta and appears within the fetal bladder. The half life of the drug in the fetal circulation and its effect on the developing human fetus are unknown.

Fetal ventriculomegaly and hydrocephalus

As in the case exemplified here, ventriculomegaly is the most common CNS abnormality identified on prenatal US. Ventriculomegaly is defined as an atrial width >10 mm measured in an axial plane at the level of the thalami and at the posterior margin of the glomus of the choroid plexus. The atrial diameter of the lateral ventricles remain relatively constant in size from 15-35 weeks gestation while the surrounding brain continues to grow, thus the lateral ventricles appear proportionately larger earlier in gestation. From 70-84% of fetuses with ventriculomegaly show associated structural or chromosomal anomalies. Structural abnormalities include agenesis of the corpus callosum, Dandy-Walker syndrome, holoprosencephaly, cortical malformations, porencephaly, and intracranial hemorrhage.

In patients with mild ventriculomegaly (atrial width of 10-15 mm) with no chromosomal or structural abnormalities, the frequency of neurodevelopmental abnormality ranges from 0-36% compared with 84% in patients with additional abnormalities.

Congenital hydrocephalus, as in the case exemplified above, is most commonly (43%) caused by occlusive malformations of the Aqueduct of Sylvius. Etiologies for Aqueductal stenosis are developmental (forking, narrowing, transverse septum{X-linked resessive}), infectious (toxoplasmosis, CMV, syphilis, mumps, influenza), and very rarely, neoplastic (glioma, pinealoma, meningioma). It occurs more commonly in males (M: F = 2:1). Thumb deformities occur in 16% and there is a mortality of 11-30%.

On imaging, Aqueductal stenosis causes enlargement of the lateral and third ventricles with a normal fourth ventricle. These may become markedly dilated with absent septum pellucidum with compression of the central white matter and cortex. At a later stage, the cerebral cortex becomes atrophic and paper thin.

In-utero procedures such as ventriculoamniotic shunts and cephalocentesis have high morbidity and mortality and outcomes are generally worse than when shunting procedures are done in the neonatal and infancy period.

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