Vein of Galen aneurysm is also referred to as a varix of the vein of Galen or vein of Galen malformation. This anomaly is a complex arteriovenous malformation affecting the vein of Galen and the cerebral arteries. The vein of Galen is a single, midline structure formed by the convergence of the two internal cerebral veins and the basal veins of Rosenthal posterior to the splenium of the corpus callosum; the vein courses posteriorly to empty into the straight sinus. During embryologic development, cerebral arteries and veins cross in close proximity to each other; fistulous connections may exist because only a few cell layers separate these vessels (Padget, 1956). These fistulas persist because of an arteriovenous pressure gradient. The size and number of arteriovenous fistulous connections determine the eventual size of a vein of Galen aneurysm.

Aneurysm of the vein of Galen was first described in 1937 by Jaeger et al. (1937). The first precise anatomical definitions of these vascular malformations were described in 1960 (Litvak et al., 1960). Malformations of the vein of Galen most probably arise early in embryogenesis in the 20 -to 40-mm fetus, when arteries and veins are still simple endothelial tubes (Padget, 1956). Following an anatomic analysis of 23 cases of vein of Galen aneurysm, it was concluded that the venous sac most probably represents persistence of the embryonic median prosencephalic vein of Markowski, not the vein of Galen per se (Raybaud et al., 1989). Even though there is plausible evidence that the aneurysmal sac is the persistent embryonic median prosencephalic vein of Markowski rather than the true vein of Galen, this author concluded that it was reasonable to retain the generally accepted nomenclature of vein of Galen aneurysm, but to restrict its use to cases in which the arteriovenous fistulas are within the wall of the venous sac. Vein of Galen aneurysms may be defined as direct arteriovenous fistulas situated between choroidal and/or quadrigeminal arteries and an overlying single median venous sac.

The size of the aneurysm of the vein of Galen determines its clinical presentation. When the aneurysm is large, as much as 50% to 60% of the cardiac output may be shunted through the lesion (Cumming, 1980). This arteriovenous shunt may result in high-output congestive heart failure, and these patients tend to present with hydrops in utero, or with cardiac failure in early neonatal life.

Other cases of vein of Galen aneurysm are not associated with cardiac failure and may not present until the first year of life. Hydrocephalus may also occur in association with a large vein of Galen aneurysm, although the cause is uncertain. Possible mechanisms for the development of hydrocephalus include compression of the sylvian aqueduct by the aneurysmal mass and defective cerebrospinal fluid resorption resulting from intracranial venous hypertension (Gold et al., 1964; Diebler et al., 1981).

Cerebral damage including cerebral infarction, periventricular leukomalacia, and hemorrhagic infarction may also occur in association with aneurysm of the vein of Galen (Norman and Becker, 1974). Suggested mechanisms by which such cerebral parenchymal injury may occur include

1. 
   

   a steal-induced ischemic phenomenon from overlying abnormal vessels;

   
   
2. 
   

   cerebral ischemia due to compromised perfusion from congestive heart failure;

   
   
3. 
   

   hemorrhagic infarction from thrombosis of the dilated vein of Galen;

   
   
4. 
   

   atrophy resulting from compression of adjacent structures by the aneurysm;

   
   
5. 
   

   alteration of flow occurring as a result of surgical therapy (Gold et al., 1964; Norman and Becker, 1974).

Vein of Galen aneurysms have been classified into four subtypes according to the severity of the lesion and the age of the patient at the onset of symptoms (Amacher and Shillito, 1973). Four groups have been described:

1. 
   

   Neonates with severe heart failure and cranial bruit.

   
   
2. 
   

   Children with mild heart failure and cranial bruit.

   
   
3. 
   

   Children younger than 1 year of age with cranial bruit and hydrocephalus.

   
   
4. 
   

   Patients with headaches and syncope.

Thrombosed vein of Galen aneurysm may also represent a fifth subtype (Beltramello et al., 1991). Congestive heart failure does not seem to be a feature of thrombosed vein of Galen aneurysm, as it is possible that occlusion of the aneurysm by the thrombosis may protect patients from this complication. In all cases of vein of Galen aneurysm thrombosis described to date, the initial presenting symptoms were attributed to hydrocephalus (Beltramello et al., 1991). Twenty-one cases of vein of Galen aneurysm thrombosis have been reported in the literature (Beltramello et al., 1991). Of these, 12 cases were diagnosed in patients younger than 1 year of age, 4 in patients between 1 and 14 years of age, and 5 in adults. Neuroradiology findings in vein of Galen aneurysm thrombosis include intracranial calcifications in the region of the vein of Galen in 50% of cases and a midline spherical mass, shown by computed tomographic (CT) scanning, that does not fill angiographically (Beltramello et al., 1991).

The true incidence of vein of Galen aneurysm is unknown, but it is an extremely rare abnormality. One of the largest reported series has described 43 cases that had been referred for endovascular treatment, together with a review of 335 additional cases previously reported in the literature (Zerah et al., 1992). More recently, there have been more than 30 cases of prenatal aneurysm of the vein of Galen diagnosed through the use of ultrasound and pulsed Doppler examination (Suma et al., 1991; Rodesch et al., 1994; Pilu et al., 1997; Has et al., 2003; Paternoster et al., 2003). The largest single prenatal experience described 18 cases of vein of Galen aneurysm diagnosed in the third trimester (Rodesch et al., 1994).

The typical prenatal sonographic finding is that of a cystic or tubular mass in the midline of the brain just above and behind the thalamus (Figure 22-1). A draining vessel may be seen extending posteriorly in the direction of the straight sinus. Around this central cystic mass, heterogeneous cystic areas may be present that represent dilated blood vessels (Figure 22-2) (Comstock and Kirk, 1991). Turbulent venous or arterial flow may be demonstrated using color Doppler examination within the draining vessel (Figure 22-1) (Jeanty et al., 1990; Sepulveda et al., 1995). Occasionally, turbulent flow may be seen within the cystic mass even without the aid of Doppler sonography (Comstock and Kirk, 1991).

Magnetic resonance imaging (MRI) appears to be an important adjunctive tool in patients suspected to have vein of Galen aneurysm prenatally. MRI can be used to confirm the diagnosis and to demonstrate the anatomical structure of the anomaly (Kurihara et al., 2001) (Figure 22-3). Both MRI and 3D ultrasound may be useful because they allow a better understanding of the spatial orientation and the course of the vessels and they may help guide the postnatal management (Heling et al., 2000; Lee et al., 2000; Has et al., 2003)

Appearance of the cerebral ventricles is variable. Hydrocephalus may be seen prenatally and seems to be unrelated to the size of the aneurysmal dilatation (Comstock and Kirk, 1991). The antenatal appearance of the heart is also variable, but generalized cardiomegaly or right ventricular enlargement is commonly noted (Comstock and Kirk, 1991). Enlargement of the heart should not be ascribed simply to presumed high-output cardiac failure until targeted fetal echocardiography is performed. Coarctation of the aorta and transposition of the great vessels occur with increased frequency in infants with vein of Galen aneurysm (Figure 22-3) and may also be responsible for cardiomegaly (Warkany, 1971; Watson et al., 1976). Massive hydrops has been reported as early as 31 weeks of gestation (Hirsch et al., 1983; Reiter et al., 1986). In addition, there have been autopsy reports of prenatal brain ischemia, periventricular leukomalacia, and cortical brain atrophy associated with vein of Galen aneurysm (Hirsch et al., 1983; Reiter et al., 1986; Baenziger et al., 1993).