Vein of Galen and Non-Galenic Cerebral Arteriovenous Fistulas




(1)
Nuffield Department of Surgical Sciences, Oxford University, Oxford, UK

 




Preamble

The subjects of this tutorial are important as they introduce the student to direct arteriovenous fistulas of the brain. The term ‘vein of Galen malformation’ (or vein of Galen aneurysmal malformation) has replaced two older terms: aneurysm of the vein of Galen and aneurysmal dilatation of the vein of Galen. This is because the original terms have been misunderstood and frequently used incorrectly in the past. Both terms have specific anatomical definitions and until quite recently were used somewhat indiscriminately for a range of angioarchitectures involving arteriovenous shunts to an enlarged deep venous collecting system.

The history of the definitions we now use and the scholarship that underpins them exemplifies how difficult it is to identify angiographic patterns and formulate therapeutic rules in low volume conditions. We should always remember this when we make a diagnosis, suggest a prognosis and formulate a treatment plan for individual patients. Equally, advances in medicine depend on our willingness to question the assumptions of our predecessors.

This chapter will therefore cover the features that distinguish the vein of Galen malformation from arteriovenous fistulas and malformations draining to the vein of Galen, with which they were confused in the past. It is also a chance to revisit the embryology of the cerebral circulation. Finally, cerebral arteriovenous fistulas that drain to other cerebral veins, i.e. non-Galenic, are included because they affect predominantly paediatric patients and because they contrast the vein of Galen malformation with lesions that pose similar challenges in their endovascular treatment.


12.1 Arteriovenous Fistulas and Malformations Involving the Vein of Galen


These are intradural lesions, which mostly occur in infants and children. These are generally high-flow lesions with overlap in their mode of presentation and the neurological disturbances they cause. The incidence is difficult to gauge, but they represent no more than 15% of paediatric patients seen in large neurovascular centres. Because they are rare, it is generally agreed that neonates and infants are best managed in specialist centres, so numerically they are likely to represent a very small component of the practice of most endovascular therapists.

There are three main types:



  • Vein of Galen malformation (VOGM). This lesion is defined as an arteriovenous fistula draining to the embryological precursor to the vein of Galen (VOG), i.e. the median prosencephalic vein of Markowski. The vessels involved are situated in the subarachnoid space.


  • Cerebral arteriovenous fistula (CAVF). These comprise arteriovenous fistulas supplied by cerebral arteries which initially drain to pial veins lying in the subpial space. Some then drain to an enlarged VOG and some to superficial cortical veins, i.e. non-Galenic CAVF. They are characterised by a single pedicle or a small number of pedicles draining to a single fistula.


  • Brain arteriovenous malformations (BAVM). This is a BAVM involving deep parenchymal structures and draining to an enlarged VOG. They may present at any age and will not be considered in any detail in this tutorial.


12.1.1 History of the Definition of the Vein of Galen Malformation


The literature testifies to considerable confusion about the difference between the VOGM and the finding of a dilated (aneurysmal) VOG secondary to an AVM or arteriovenous fistula because of their similar clinical findings. The solution and the angiographic definition of the VOGM was provided by Raybaud, Strother and Hald in 1989 [1] (see below), who were the first to recognise that the dilated midline vessel seen in the VOGM was in fact a persistent median vein of the prosencephalon or vein of Markowski. Before going on, it is worth revisiting the embryology of the cerebral vasculature.


12.1.2 Embryology


As was described in Tutorial 1, prior to the choroidal vascular stage, the neural tube is nourished by amniotic fluid. After closure, the neural tube is initially supplied by surface vessels of the meninx primitiva, but as the cerebral vesicles develop at the rostral end of the neural tube, an increasing demand for blood supply is met by choroidal vessels (choroidal stage).

The choroidal and quadrigeminal arteries are present by the 5th week and supply the developing choroid plexus. The plexus of the primary prosencephalic vesicle develops along the junction of its division into diencephalon and telencephalon secondary vesicles. The expansion of the telencephalic choroid plexus is accompanied by differentiation of a dorsal vein on the ventral surface of the diencephalon to drain the choroid plexus. This was called the ‘vena medianna prosencephali’ by Hochstetter [2]. He identified that it was transient and only evident between the 8th and 11th week (8–50-mm stage), i.e. at the time the pallium was growing. I will refer to it as the median prosencephalic vein (MPV) (see Fig. 12.1).

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Fig. 12.1
Median prosencephalic vein of Markowski shown in an artist’s impression. Its afferent arteries are the choroidal arteries and the anterior cerebral artery (ACA) (Reproduced from Raybaud et al. [1] with permission)

Because the choroid plexus develops before vessels have penetrated the neural parenchyma, the MPV is the first vein to drain an intracerebral structure. At this stage, the choroid plexus is supplied largely by choroidal branches of the anterior cerebral artery. By the 12th week, the MPV regresses and its distal remnant joins with the paired internal cerebral veins to form the vein of Galen. It is assumed that the efferent veins of the MPV (and rostral VOG) persist with it, as one possible configuration (choroidal type) of VOGM.

The formation of the dural sinuses is complex and results from the fusion of multiple separate venous plexi. A transient falcine sinus connecting the VOG to the superior sagittal sinus has frequently been observed in the foetus and is often present with the VOGM. It is occasionally present in neonates and infants as a normal variant, but the embryology is unclear.


12.1.3 Early Descriptions of the Vein of Galen Malformation


Early descriptions of the VOGM failed to appreciate that the cause lay in a regression failure of this embryology. Enlargement of the ‘VOG’ was first described by Steinheil in 1895 [3] as a varix aneurysm, and the condition became known as the aneurysm of the vein of Galen.

Litvak et al. in 1960 [4] were the first to recognise the difference between the lesion associated with the MPV and other causes. They described primary and secondary aneurysms of the VOG, distinguishing midline arteriovenous fistulas from AVMs as causes for the dilated VOG. Gold et al. in 1964 [5] provided a clear description of the modes of presentation of what they called the vein of Galen malformation. They recognised three clinical types: (a) neonates presenting with congestive heart failure, (b) infants presenting with hydrocephalus and seizures and (c) older children and adults presenting with headaches and spontaneous intracranial haemorrhage. Amacher and Shilitto [6] added a fourth group: infants and young children presenting with increasing head size and mild congestive heart failure.

In 1988, Yasargil [7] described a classification of the angiographic findings in patients with arteriovenous shunts to the VOG. This distinguished four types:



  • Type I: pure fistulas with a direct connection from an enlarged artery to the VOG. Any nidus occurred only at the ampula of the VOG. The whole lesion is extra-axial.


  • Type II: an AVM of transmesencephalic and transdiencephalic thalamoperforator arteries with branches to normal brain tissue. This lesion is both intra- and extra-axial.


  • Type III: mixed type I and type II.


  • Type IV: brain AVM with draining veins proximal to and draining into the VOG.

In 1989, Raybaud et al. [1] published their paper advancing a coherent argument that aneurysmal dilation of the VOG was due to persistence of the rostral component of the MPV based on a detailed analysis of 23 cases with only 12 complete sets of angiograms. They argued that an embryonic insult between 6 and 11 weeks caused the condition in neonates and infants. They showed that the fistula arose from subarachnoid choroidal and quadrigeminal (collicular) arteries and that normal deep venous drainage of the brain was diverted away from the dilated VOG (because no normal connection was ever established between the developing internal cerebral veins and the persistent MPV). They identified several abnormalities of the venous drainage from the MPV: principally absence of the straight sinus in 50% of their cases, stenosis and absence of the sigmoid sinus, persistence of the falcine sinus and an anomaly they described as a falcine loop. However, they proposed that the term aneurysmal VOG be retained (Fig. 12.2).

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Fig. 12.2
Vein of Galen malformation drawn from two anatomical dissections. Two fistula sites are shown; an anterior fistula receiving choroidal vessels [anterior cerebral artery (ACA) and choroidal branches (Chor A) of the posterior cerebral artery (PCA)] and an inferior fistula receiving vessels from an arterio-arterial collection of collicular arteries (Collic A) [branches of superior cerebellar arteries (Sup C A), PCA and posteromedial choroidal arteries] (Reproduced from Raybaud et al. [1] with permission)


12.1.4 Current Descriptions of the Vein of Galen Malformation


The classification of aneurysmal VOGs was simplified by Berenstein and Lasjaunias [8] who proposed separating malformations draining to the persistent MPV into mural and choroidal types and that the term vein of Galen malformation (VOGM) be used only for arteriovenous shunts to the MPV.

The two VOGM types were defined as:



  • Choroidal type: Comprised of multiple fistulas in the cistern of the velum interpositum1 which communicate with the anterior MPV. They are subarachnoid in position and therefore extra-axial. They drain to the anterior aspect of the MPV and represent a persistent component of the embryonic choroidal vascular pattern. This is the usual pattern in neonates presenting with heart failure.


  • Mural type: In which the fistula occurs in the wall of the MPV with a small number of feeding arteries, principally derived from the choroidal and quadrageminal arteries (collicular arteries). This type of fistula pattern is seen in children usually presenting in infancy with macrocephaly and failure to thrive.

The features common to both types are drainage to the MPV, no connection with the normal deep venous drainage of the brain and an arterial supply from the posterior choroidal arteries (Raybaud et al. found it difficult to distinguish posterior medial from posterior lateral choroidal arteries, Fig. 12.2).


12.1.5 Distinguishing Features of Choroidal and Mural Types of Vein of Galen Malformation


Relative distinguishing features are the earlier presentation in the neonatal period (i.e. within 28 days of birth) of the choroidal VOGM. It is always associated with heart failure, a falcine sinus (together with absence of the straight sinus) and usually with patent sigmoid sinuses. Babies presenting with congestive heart failure have abnormalities of CSF absorption and failure to thrive.

The mural VOGM tends to present later in infancy (i.e. 6 months to 3 years) with symptoms and signs of increasing head size, cardiac failure (of lesser severity than neonates with choroidal VOGM), failure to thrive and hydrocephalus. Falcine sinuses may or may not be present and obstructions of the sigmoid sinuses are more commonly seen, in which case drainage of the VOGM is redirected anteriorly via the basal veins, to the cavernous sinuses, orbital veins or transosseous scalp veins. The cavernous sinuses are normally rudimentary at birth and develop over the first 18 months of life. Without them, drainage from the VOGM is redirected to these alternative routes.

The VOGM arterial feeders were summarised by Brunelle [10] as 100% posterior choroidal arteries (medial and lateral), 69% anterior cerebral artery (trigonal branch of the pericallosal artery) and 30% ‘transmesencephalic’ (midline arteries arising directly from the basilar artery (BA). The relative contributions of common arterial feeders to the two types distinguished by Berenstein and Lasjaunias [8] are summarised in Table 12.1. Less commonly, either type receives contributions from anterior choroidal or lenticulostriate arteries and rarely from the middle cerebral artery.


Table 12.1
Arterial feeders to VOGM



















Choroidal type

Mural type

Posterior choroidal arteries

Posterior choroidal arteries

Pericallosal artery

Collicular (quadrageminal) arteries

Thalamoperforator (subependymal branches) arteries
 

All other fistulas associated with aneurysmal dilatation of the VOG do not involve a persistent MPV and therefore develop subsequent to the embryonic period and after the development of a normal pattern of deep venous drainage to (and from) the VOG. In this tutorial, they will be considered together with cerebral arteriovenous (non-Galenic) fistulas that arise elsewhere in the brain and also usually present in children and young adults.


12.2 Vein of Galen Malformation


It is now time to leave aside the discussion of terminology and consider the clinical problems posed by the phenotype of the individual patient.


12.2.1 Presentation


The presenting symptoms in a series of 232 patients (predominantly children) were reviewed by Johnston et al. [11] and are summarised in Table 12.2. Only 9% were adults and approximately 50% presented with congestive heart failure. There is a modest male predominance in the incidence of the VOGM (1.7:1).


Table 12.2
Presenting features of the VOGM































Symptom and signsa

n =

Congestive heart failure

110 (47%)

Hydrocephalus

44 (19%)

Bruit

57 (25%)

Focal neurological deficit

37 (16%)

Seizures

26 (11%)

Haemorrhage

25 (11%)

Total

232


aSymptoms and signs at presentation in a review of reported cases [11]

Symptoms due to the high-flow arteriovenous shunts are:


  1. (a)


    Congestive heart failure (CHF)

     




  • High output cardiac failure secondary to arteriovenous shunting commonly occurs soon after birth (with loss of the low resistance placental circulation). Neonates with choroidal rather than mural types of VOGM are usually affected. The severity of heart failure does not relate to the size of the shunt, arterial supply or venous drainage pattern, or the presence of outflow restrictions. It may be associated with persistent right-to-left cardiac shunting and requires emergency medical treatment. Failure of medical treatment is an indication for emergency endovascular treatment. Associated atrial septal defect or persistent ductus arteriosus is common and may need treatment to reduce strain on the right side of the heart. The prognosis is worse if CHF is present at birth or in neonates under 2 weeks of age. CHF is usually less severe when it first occurs in older children.



  1. (b)


    Failure-to-thrive and developmental delay

     




  • Neurological dysfunction is attributed to poor or reduced cerebral perfusion caused by a combination of CHF and hydrocephalus. Signs of neurological dysfunction are difficult to assess in infancy and they manifest cerebral dysfunction as failure to thrive or delayed achievement of developmental milestones. Venous hypertension affecting the hypothalamus and pituitary has been implicated in the process as well as hydrocephalus causing a reduction in cerebral tissue perfusion.



  1. (c)


    Increasing head size and hydrocephalus

     




  • Hydrocephalus occurs with enlargement of all the CSF spaces secondary to raised intracranial venous pressure and poor CSF absorption, i.e. communicating hydrocephalus. Aqueduct compression by the enlarged vein of Galen is a secondary and less important cause (because ventricular shunting of CSF is notoriously unhelpful). A degree of venous outflow obstruction caused by atresia or absence of venous sinuses, i.e. straight sinus and sigmoid sinuses, may contribute to raise venous pressure. If the cavernous sinuses are underdeveloped and this route to the pterygoid plexus is unavailable, orbital and transosseous scalp veins enlarge. If the cavernous sinuses have developed, engorged scalp veins are less evident. Obstructions in the venous sinuses are less frequent in neonates, and enlarged head veins and cephalomegaly are signs more often seen in infants. Spontaneous thrombosis of draining veins can occur at any age and exacerbate venous hypertension and hydrocephalus.

Aug 17, 2017 | Posted by in NEUROSURGERY | Comments Off on Vein of Galen and Non-Galenic Cerebral Arteriovenous Fistulas

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