arteriovenous malformations

it appears that fetal arteriovenous malformations may develop in or enlarge to a detectable size over a short period of time, usually in the third trimester. to date, no vein of galen aneurysm has been reported prior to 30 weeks gestation. the sonographic findings in this disorder are typical. in the transthalamic plane, a round or ovoid hypoechoic mass is seen posterior to the third ventricle. a tubular extension of the mass, representing a dilated straight sinus, can be traced posteriorly to the enlarged torcular of herophilus (confluence of dural sinuses) which has a triangular shape. the combination of a dilated vein of galen, straight sinus and torcular results in a key-hole shaped sonolucency. doppler examination of the abnormality may show arterial and/or venous flow, depending on the area sampled. the posterior cerebral artery and its branches are the most commonly involved feeders to this malformation, resulting in turbulence at the confluence of arterial and venous streams. occasionally this may be seen without resorting to doppler scanning. ventriculomegaly may be observed secondary to aqueductal compression. the disorder is associated with a poor outcome, but the infant’s chance of survival can be maximized by planning delivery at a tertiary care centre appropriately equipped to manage the affected newborn. heart failure, rather than hydrocephalus, is the major finding in symptomatic newborns. the antenatal appearance of the heart is variable, but generalized cardiomegaly or right ventricular enlargement is the rule. coarctation of the aorta and transposition of the great vessels occur with increased frequency in infants with this abnormality.