Vein of Galen Malformations

First described in 1895 by Steinheil, the various vein of Galen malformations have come to be classified in both clinical and angiographic terms. The majority of these rare malformations are high-flow lesions that can lead to serious cardiovascular and neurological sequelae. The advent of neuroimaging techniques in conjunction with interventional technology have led to higher survival rates and less morbidity than conventional treatments of the past.

Epidemiology

Vein of Galen malformations are rare vascular anomalies in children, and carry a high risk for progressive cardiovascular and central nervous system deterioration. About 30% of vein of Galen malformations present in neonates. Knowledge about their pathogenesis and pathophysiology is limited. Yasargil classified vein of Galen malformations based on angiographic findings, while Lasjaunias relies upon the source

Table 2. Two systems of classification for vein of Galen malformations

System

Features

Yasargil Type I: Simplest malformation with pure cisternal component and single or few feeders from pericallosal and posterior cerebellar arteries.

Type II: Fed by thalmoperforating and posterior cerebellar arteries.

Type III: Mixed pattern of supply from pericallosal, thalmoperforating and posterior cerebellar arteries.

Type IV: Parenchymal malformation Type II and III lesions are more common, have higher flows and more contributions.

Lasjaunias Mural type: Receive blood supply from collicular and posterior choroidal vessels. Usually present in infants with developmental delay and hydrocephalus.

Choroidal type: Large number of bilateral feeding vessels from choroidal, pericallosal and thalamoperforating arteries.

of the feeding vessels to classify these lesions (Table 2). Vein of Galen malformations also are categorized into the following grades:

Grade 1: Degree of ectasia of straight sinus is proportional to that of vein of Galen—both only minimally enlarged.

Grade 2: Vein of Galen is more dilated than straight sinus; both structures are moderately increased in size.

Grade 3: Marked dilation of both structures.

Grade 4: Prominent enlargement of vein of Galen with normal, stenotic, or absent straight sinuses.

Pathology

The great cerebral vein (of Galen) is the largest of the deep cerebral veins, but does not exceed 2 cm in length. It is a short, centrally located structure a few millimeters above the pineal gland and is the collecting vessel for a large group of veins coming from the deep, medially located areas of the diencephalon, basal ganglia and midbrain. The galenic system drains the medial deep thalamic nuclei, medial occipital and temporal lobes, and the superior cerebellar surface. It is formed by the joining of the two internal cerebral veins with the basal veins of Rosenthal, extends beneath the splenium of the corpus callosum, penetrates the junction of the falx and the tentorium, and becomes the straight sinus when joined with the inferior saggital sinus. Sacrifice of this structure in humans and animals has been performed without untoward consequences.

The vein of Galen is formed embryonically at the age of 3 months when its embryological precursor, the median prosencephalic vein of Markowski, joins with internal cerebral veins and basal veins posteriorly to form the vein of Galen. Remnants of primitive arteriovenous connections that persist in this location are hypothesized to account for the various malformations. Since the vein of Galen is not

attached by dural structures, it can dilate enormously, much more so than the more restricted straight sinus. The venous wall becomes thick and tough in response to increased flow, pressure and turbulent flow. This is in constrast to arterial vessels, whose walls become attenuated in response to the same factors.

Clinical Features Neonates

Neonates with a vein of Galen malformation can develop progressive and medically unresponsive high output cardiac failure. These children show signs of progressive high output preload heart failure with tachypnea, high jugular venous oxygen tension, decreased urine output, hepatosplenomegaly and metabolic acidosis, often immediately after birth. Other physical signs include a machine-like pancardiac murmur with palpable thrills over neck and chest and a loud cranial bruit of virtually constant intensity. Pulmonary hypertension and a right-to-left shunt through a patent ductus arteriosus can be present. The high flow/low resistance intracranial shunt leads to cardiac and neural ischemia with as much as 80% to 90% of the infant's cardiac output passing through the galenic fistula. In utero cardiac ventricular enlargement contributes to early cardiac decompensation post delivery. Rapid and effective therapy is required, with reduction of the size of the intracranial shunt in order to reverse or stabilize the abnormal hemodynamics.

Infants

Infants often have mild cardiac enlargement that does not typically require treatment. They present with an enlarged head circumference, pansystolic intracranial bruit, prominent veins around eyes and forehead, and a full fontanelle. Mild hydro-cephalus is thought to be secondary to venous hypertension from high flow shunting, and not from aqueductal stenosis.

Older Children

These children often have deep midline shunts. Typical symptoms include chronic headaches, learning disabilities, seizures and, rarely, SAH.

Imaging

Transcranial ultrasound is an effective way to diagnose malformations in utero. CT with or without contrast demonstrates parenchymal calcifications from long-standing ischemia secondary to venous hypertension. MRI is very useful to delineate the malformation in relationship to normal brain structures. MR angiography can define the type of malformation present. Transarterial angiography is the definitive study required to define the complex vascular anatomy, the major arterial supply, and assist with prognosis and planning for the best therapeutic approach (Fig. 1).

Treatment

The appropriate approach to treatment of vein of Galen malformations is early and accurate diagnosis, as well as early, aggressive stabilization of the cardiovascular function. Some lesions are benign in their clinical course, however this is rare.

Figure 1. A vein of Galen malformation seen on the axial T-1 weighted MRI image (A), and on the cerebral angiogram, lateral projection (B). In the latter image, posterior is to the left; the vertebral artery can be seen at the bottom of the image leading directly to a large dilated venous structure that fills rapidly.

Figure 1. A vein of Galen malformation seen on the axial T-1 weighted MRI image (A), and on the cerebral angiogram, lateral projection (B). In the latter image, posterior is to the left; the vertebral artery can be seen at the bottom of the image leading directly to a large dilated venous structure that fills rapidly.

Medical therapies in the neonate with high-output cardiac failure are supportive and often unsuccessful in stabilizing the patient. In milder forms of the disease, medical therapy is important for stabilizing the patient in preparation for definitive treatment. Most often these lesions cause progressive neurological defects. In general, early, aggressive treatment is better than conservative care.

Historically, the primary treatment was direct surgical ligation of the lesion. Transvenous and/or transarterial endovascular intervention has been much more effective in complete obliteration of these lesions. Today, surgery for vein of Galen malformations is reserved for cases that include hydrocephalus and access for endovascular therapies. Surgery also is considered for Type I malformations with simple feeders for exposure and clipping. Type I, II and III malformations can be treated with interventional procedures. Type IV lesions should be treated as a true AVM of the brain parenchyma.

Outcome

Survival rates range from 70% to 80% across all types, with a 50% survival rate in neonates with heart failure. Combined endovascular/surgical treatments have improved outcomes, but morbidity and mortality of this disease remains high. Outcome is poor for patients whose initial scans show pronounced hydrocephalus or cerebral atrophy. The single best therapy has yet to be determined, and, thus, it is likely that a multimodal approach will yield the best outcomes in the future. A subgroup of patients treated for malformations with persistent fistula seem to do well, but the long-term outlook is not defined. Patients in this subgroup might be candidates for stereotactic radiosurgery.

Moyamoya

General

Moyamoya is derived from a Japanese word meaning "haze" or "puff of smoke," which describes the angiographic appearance of this chronic occlusive cerebrovascu-lar disease. Moyamoya disease is characterized by the progressive narrowing of one, but usually both, of the proximal internal carotid arteries (ICA) and their branches, typically at the level of the siphon and extending to the circle of Willis (Chapter 2, Fig. 9), with compensatory formation of an anastomotic collateral network of proximal penetrating arteries at the base of the brain (moyamoya vessels).

Epidemiology

Moyamoya disease occurs mostly among the Japanese; while ethnic Chinese and Koreans rank second. Half of all reported cases occur in Japan, where the prevalence is 3.16 per 100,000 people. The overall incidence is 0.35 per 100,000 people. Intracranial bleeds are rare in Japanese pediatric cases, are more frequent in Koreans, and very frequent in Chinese pediatric patients with moyamoya. The female to male ratio is 1:8, and 10% of patients demonstrate a family history of the disease. While the age of onset varies, it most frequently occurs in young females. Symptom onset peaks at between 10 and 14 years of age.

Pathology

Pathological changes include vessel wall thinning, intimal thickening, dilation of arterioles, medial necrosis and discontinuity of the internal elastic membrane of some vessels are seen. Although the cause is unknown, moyamoya disease is found in association with HLA B51. Beta-fibroblast growth factor (FGF) is implicated in the disease process and is abundant in cerebrospinal fluid (CSF). Immunohistochemi-cal staining for beta-FGF is pronounced in the meninges. The progression of pathological changes follows an atypical pattern:

1. Narrowing of carotid bifurcation followed by appearance of moyamoya vessels.

2. Moyamoya vessels expand.

3. Collaterals develop between external carotid and ophthalmic arteries.

4. Occlusive phenomenon involves the posterior communicating artery.

5. Cerebral circulation becomes dependent on flow from branches of the external carotid artery and posterior circulation.

Intimal thickening begins at the distal end of the carotids, slowly progressing and encroaching on the anterior circle of Willis. The intima is thickened by fibrous tissue, and proliferation of elastic fibers is seen within the internal elastic membrane, causing it to become tortuous. An inflammatory process is absent in the vessel wall. Intimal thickening may begin unilaterally, but always becomes bilateral. The posterior circulation may become involved as the disease progresses, however, the vertebrobasilar system rarely becomes involved.

Secondary lesions, in the form of collaterals, occur as a response to stenosis and decreased blood flow. Two types exist: intracerebral collaterals and collaterals from external carotid circulation. In the early phase, intracerebral collaterals are a functional net-like array of vessels at the base of the brain; pre-existing leptomeningeal arteries become dilated on the surface of the brain, giving a hyperemic appearance. These vessels receive blood from the posterior circulation. Later, transdural collaterals form from the external carotid system and become the primary source of flow. This leads to decrease in moyamoya vessels at the brain base.

The second type of collaterals (external carotid) take longer to develop. The time lag before collaterals develop causes episodic ischemia. Consequent decrease in cerebral blood flow gives rise to transient ischemic attacks, usually beginning before age 10. After collaterals mature, a period of quiescence follows until the second peak , which occurs when patients are in their 40s and develop ischemia or intracerebral hemorrhage. Abnormal basal collaterals are fed by branches of the ICA in the early stage of disease, and are then fed by the posterior cerebral artery in the later stage of disease (extend into parietal and temporal lobes). There are two types of aneurysms in moyamoya: true saccular, which never disappear on an angiogram; and pseudoaneurysm, which may disappear on an follow-up angiogram. These aneu-rysms occur with equal frequency. Clipping is complicated if a aneurysm is surrounded by collaterals.

Clinical Features

Moyamoya disease presents with motor and sensory symptoms of varying severity, ranging from transient ischemic attacks to fixed neurological deficits. Ischemia occurs most commonly in watershed zones, and resulting motor deficits are easiest to notice and most frequent at onset. Posterior watershed ischemia yields visual field deficits that most patients do not notice. Attacks can be precipitated by crying or fever. It is believed that hyperventilation leads to hypocapnia and further constriction of cerebral vessels. The side of attack and extremity involved are not always fixed. Some patients will present with only headaches and seizures. Headaches often occur in the morning with nausea, and respond well to calcium-channel blockers. Disease progression is either episodic or fulminant, with the episodic course attributed to delay in development of collaterals following vessel occlusion. Ischemia is more common in children than adults. Involuntary choreiform movements occur in 3% to 6% of patients and are not seen in sleep. These movements can be initial and recurrent, and are triggered by excitement and crying. SAH is the most common symptom leading to diagnosis in adults.

Electroencephalography (EEG) shows changes during and after hyperventilation ("rebuild-up phenomenon"). Hyperventilation causes a build-up of high-voltage slow waves that disappear when hyperventilation stops; 20 to 60 seconds later there is a return of high-voltage slow waves. The first wave is attributed to vasoconstriction. The second wave is attributed to ischemia, secondary to "steal" from moyamoya vessels to the now dilated cortical vessels. MR and conventional cerebral angiography will demonstrate the haze of moyamoya vessels. A baseline IQ test is recommended.

Associated diseases include neurofibromatosis, sickle cell anemia, Fanconi's anemia, Down's syndrome, connective tissue diseases, craniopharyngioma and optic glioma. These can occur following CNS infection, such as tuberculosis meningitis or leptospirosis, intracranial radiation and renal artery stenosis with hypertension.

Treatment

Medical therapy for the disease has not proved effective. Aspirin, however, is important for decreasing the number of transient ischemic attacks (TIAs).

Surgical Procedures

The general surgical goal is to increase the number of transdural collaterals that are already part of the moyamoya process. The procedures include:

1. Superficial temporal artery to middle cerebral artery bypass to increase blood flow to the hemispheres. Not a popular option because the MCA needs to be temporarily occluded.

2. Encephalomyosynangiosis (EMS) where the inner surface of the temporalis muscle is laid in apposition to the brain surface. Collateral formation is induced by ischemic brain. It is often complicated by transient focal seizures and chronic subdural hematomas. The procedure is not popular because of disfigurement.

3. EDAS (encephalo-duro-arteriosynangiosis). The intact superficial temporal artery with a length of attached galea aponeurotica is placed on a dural opening in apposition to the brain. Delayed cerebral angiography demonstrates enlargement of the donor vessel, increased transdural collateral formation, decreased moyamoya vessels and increased MCA blood flow. Clinical status and EEG improves, but there is no change in intellectual functioning. The benefits of this procedure include the absence of temporary occlusion of MCA, and the ability to plan craniotomy to avoid disruption of the existing collaterals. Anesthesiologists should be made aware that a hyperventilating patient can constrict already maximally dilated vessels and worsen neurological deficits.

Outcome

Preoperative factors that lead to good postoperative IQ following EDAS:

1. TIAs with no hypodense lesions on CT or fixed neurological deficits.

3. EDAS done ages 4-14 years.

4. Preoperative IQ >70.

5. EDAS done within 6 years of onset.

The prognosis, related to age at onset of symptoms, is poor if under the patient is under 6 years of age. Angiographically, prognosis is related to the extent of the occlusive process. Bilateral strokes occurring at a young age invariably lead to developmental delay. Most intracerebral hemorrhages are intracerebral or intraventricu-lar in location.

Was this article helpful?

0 0

Post a comment