hydrocephaly – aqueductal stenosis
description
hydrocephalus is defined as an excessive amount of cerebrospinal fluid (csf) in the cranial cavities. csf is mainly produced in the choroid plexuses situated in the lateral ventricles, flowing from either side into the third ventricle via the foramen of monro and then through the sylvian aqueduct to reach the fourth ventricle, located within the brainstem. csf then exits the ventricular system through the foramina of luschka and magendie into the cisterna magna and subarachnoid space to be reabsorbed by the granulations of pacchioni located along the superior sagittal sinus. although the above definition can be satisfied in situations where there is an apparent increase in ventricular size due to destruction or maldevelopment of adjacent cerebral tissue (secondary to porencephaly or agenesis of the corpus callosum respectively), known as hydrocephaly ex vacuo, it is typically and more practically applied where any of the following criteria coexist: – overproduction of csf (as in the rare choroid plexus papilloma) – obstruction of csf flow (due to stenosis of the sylvian aqueduct) – impairment of csf resorption by the arachnoid granules the cause is unknown in over one third of cases. obstructive hydrocephalus is by the far the most common type and is usually associated with spina bifida, the arnold chiari type ii malformation, aqueduct stenosis, and the dandy-walker malformation. other obstructive causes include intracranial tumors, intracranial haemorrhage or cysts blocking the foramina of monro, luschka or magendie.
diagnosis
ventriculomegaly is the most typical feature. the enlargement is often so obvious that measurements are unnecessary, but an abnormal increase in the size of the lateral ventricles measured at the atrium in the transverse plane (the lateral atrial diameter) is the most useful objective ultrasonographic diagnostic aid. any measurement over 10mm is abnormal, the normal range lying between 7 to 10 mm from fifteen to thirty five weeks gestation. examination of the choroid within the lateral ventricles may also be helpful. the choroid plexus always lies in the gravitationally dependent position until it rests against the lateral ventricular wall. if the choroid plexus is seen to fill the lateral ventricular body in its transverse dimension ventriculomegaly is not present. in true ventriculomegaly the choroid plexus will be seen dangling downwards and will be angulated from the mid-line. occasionally the poor echogenicity of the brain parenchyma in this region may be mistaken for cerebrospinal fluid surrounding the atrial choroid and a misdiagnosis of hydrocephaly may be made. this mistake will not be made if close attention is paid to the position of the choroid plexus. as the diagnosis may be difficult to make in some cases, especially between eighteen and twenty four weeks gestation, serial scans become important particularly with progressively obstructive lesions. it should be emphasised that a normal head size does not exclude hydrocephaly. if lateral ventriculomegaly is found, then a careful assessment of spinal and intracranial anatomy should be undertaken to ascertain the extent of the condition and its underlying cause.
differential diagnosis
once a diagnosis of hydrocephalus has been made a search should be carried out for the cause. this may include intrauterine viral infection (torch), trisomies 13 or 18, the dandy walker malformation, arachnoid cyst, agenesis of the corpus callosum, holoprosencephaly, intraventricular haemorrhage, vein of galen aneurysm, meckel-gruber syndrome, choroid plexus papillomas or cysts, lissencephaly, hydranencephaly, porencephaly, schizencephaly, spina bifida and encephalocele (with arnold chiari malformation causing obstruction). it is therefore important to have a plan of action once ventriculomegaly has been found. even in expert hands however it is not always possible to differentiate obstructive from non-obstructive (communicating) hydrocephalus. a useful approach to establishing the cause of fetal hydrocephalus is found in spirt et al (1990). if ventriculomegaly is found first check the choroid plexus to rule out a large cyst or echogenic mass. if no mass is seen choroid plexus papilloma is ruled out. next evaluate the posterior fossa in the transcerebellar plane. the cisterna magna, lying posterior to the cerebellum, is readily seen. the normal depth is 5 mm, and any measurement over 10 mm is abnormal; the differential includes dandy-walker malformation or subarachnoid cyst. if the posterior fossa is normal but the third and lateral ventricles are enlarged aqueduct stenosis is likely. attention is now paid to the cranium. if the head circumference is decreased in the presence of ventriculomegaly the possibility of cytomegalovirus infection as a cause should be considered. a bony defect will be seen if a cephalocele exists; the ‘lemon’ sign of the arnold-chiari malformation will be seen with spina bifida. the spine itself should be evaluated to rule out defects before proceeding with a careful morphologic scan of the other fetal systems. fetal karyotype and echocardiography are essential in cases where ventriculomegaly is found particularly where additional malformations coexist. the outcome of fetal ventriculomegaly will depend on the presence of associated malformations. review of the literature suggests that there is an 80% chance of survival and a 50% chance of normal development with isolated ventriculomegaly.
sonographic features
transverse atrial diameter greater than 10 mm.
choroid plexus not filling atrium.
dangling choroid plexus (due to increased csf in ventricle).
enlarged third ventricle and normal fourth ventricle – but this is not consistant (therefore making specific diagnosis difficult).
rule out spinal defect.
look for associated lesions.
associated syndromes
- aminopterin
- aminopterin-like
- amniocentesis
- aniridia-renal anomalies
- anophthalmia
- anophthalmia-cleft lip/palate
- aqueduct stenosis
- basal cell nevus
- bijlsma
- bone fragility
- campomelia-ankyloglossia
- cataract
- centromere instability-immunodeficiency
- cerebroarthrodigital
- charge association
- chromosome
- ciliary akinesia/dyskinesia
- clefting-corneal opacity
- crouzon
- daish
- de hauwere
- epidermal nevus syndrome
- fine
- fraser
- goldenhar
- habel
- hajdu-cheney
- hanhart
- heptacarpo-octatarso-dactyly
- hydrocephalus
- hydrocephalus
- hydrolethalus
- jeune thoracic dysplasia
- masa
- mulibrey nanism
- neurocutaneous melanosis
- neurofibromatosis
- ochoa: urofacial syndrome
- osteogenesis imperfecta, lethal type
- osteoglophonic dwarfism
- oto-palato-digital ii
- palmer-pagon
- petersí-plus
- pfeiffer
- pseudomarfanism
- renal cysts
- rhizomelic chondrodysplasia-thrombocytopenia
- rogers
- schinzel-giedion
- sengers
- thakker-donnai
- tibial aplasia-ectrodactyly
- trigonocephaly
- van biervliet
- vater
- vacterl
- waaler
- walker-warburg
- weaver
- winter
- winter-wigglesworth
- x linked-unusual facies-callosal agenesis
references
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