Arteriovenous Malformation

Arteriovenous malformations (AVMs) refer to congenital vascular malformations characterized by tangled masses of arteries and veins that develop in any part of the body. Spinal and cranial AVMs pose a substantial risk for hemorrhage.

The disease is related to the following processes:  congenital and has an incidence of about  1 / 100.000.

Overview

Arteriovenous malformations (AVMs) are congenital vascular malformations characterized by tangled masses of arteries and veins that develop in any part of the body. Spinal and cranial AVMs pose a substantial risk for hemorrhage. Malformation of blood vessels is often related to abnormal development of brain tissue. Etiology of the disorder is not clear, but is considered to be multifactorial. Angiogenesis and genetics are presumed to play vital role in the development of malformed vascular structures. These are rare disorders and is found mostly in young adults. Morbidity rate for the disorder ranges from 30% to 50%. About 10% to 15% of the patients may have life threatening situations as complications of AVM. A multidisciplinary approach is used in the treatment including surgery and endovascular techniques.

Etiology

The actual cause of AVM is not clear. Congenital lesions resulting from a malfunction in the embryonal capillary maturation process is considered to be one of the most important cause of the vascular disorder. The vessel tangle, or nidus, is often connected by one or more fistulae. In a nidus, the arteries directly drain into the veins [1]. A possible mutation in the germline that affect the angiogenesis is proposed to be an important etiologic factor in the development of AVM. Studies also suggest that apoptotic cell death and vascular modeling as possible factors affecting development and maintenance of vascular abnormalities.

Genetic mutation in chromosome 7 is associated with the formation of multiple lesions and is found to occur in families. This mutation is more prevalent in Hispanic American population of the Southwest. Another theory also suggests the involvement of angiopathic reaction following a cerebral ischemia or hemorrhage in the development AVMs [2].

Epidemiology

Incidence of AVM in different parts of the world is not known. As the disease is rare and is mostly asymptomatic, detection rate for symptomatic lesions is taken as reliable estimate of prevalence rate [3]. One of the prospective studies from New York Islands show the detection rate of AVM to be about 1.34 in 100,000 persons [4]. Reports from countries like Australia, Scotland and Sweden show the detection rate to be ranging from 0.89 to 1.24 in 100,000 persons. In Scotland, the prevalence of AVM is reported to be about 18 per 100,000. In US, approximately 300,000 persons are presumed to be affected by AVM. Approximately 78% of these patients remain asymptomatic. Mortality due to any of the AVM is around 10-15% while different types of morbidity are reported from 30-50% of the patients.

AVM is associated with intracranial hemorrhage in 2-4%. Hemorrhage is expected to recur within a year after the initial bleeding, in around 7-33% of the patients [5]. About 15-40% of the patients may develop seizures. Although the condition is presumed to be congenital, onset of symptoms is most commonly noted in young adults. Hemorrhage and seizure, two most common manifestations of AVM, is seen in young children and also in adults above the age of 40 years. About 66% of the adults with AVM show different degrees of learning disabilities.

Sex distribution
Age distribution

Pathophysiology

Without an intervening capillary network in the nidus, arteries drain directly into the veins. The arterial walls are characterized by abnormal muscularis and the vessels show a variety of flow-related signs such as dilation and aneurysms due to the high velocity blood flow. Structural abnormalities of the vessels make them susceptible to rupture and hemorrhage may result.

Cranial AVM may result in neurological dysfunction due to subarachnoid hemorrhage, or a hemorrhage in the intraventricular space or parenchyma [6]. About 15-40% of the patients with cranial AVM may have seizures that lead to neurological deficits. In some patients, neurological dysfunction may be a progressive feature that occurs over months or years. This is presumed to be related to the siphoning of blood from neighboring tissues. Neurological dysfunction may also arise from enlargement of nidus or venous hypertension. Large sized AVMs may damage the tissues of brain and spinal cord by their presence. These malformations constrict the flow of cerebrospinal fluid resulting in its accumulation. Cerebrospinal fluid buildup increases the pressure on neurological structures leading to further damage.

Prognosis

Prognosis of AVM depends on the risk of hemorrhage and thus assessment of the risk is important in deciding the treatment plan. Prognosis is good for AVMs that are of moderate size and are superficially located in the brain. Interventional treatment may not be recommended for these cases. The risk of hemorrhage is elevated in young adults. Hemorrhage due to AVM gives a good outcome when compared to hemorrhage due to other causes. This may be due to the fact that reorganization of the brain is better in young adults.

Presentation

Seizure or hemorrhage is often the first clinical manifestation of AVM. The seizures are usually focal, either simple or partial complex, or secondarily generalized. About two thirds of the patients may have learning disability when they are young, but this may not be prominent in adults. History of the patient may also show headaches, which later takes the form of migraine. Focal neurologic deficit may be noted in some.

Some of the common symptoms of AVMs include:

  • Muscle weakness
  • Paralysis
  • Lack of coordination
  • Dizziness
  • Visual disturbances
  • Numbness, tingling or pain
  • Confusion
  • Memory deficits
  • Hallucinations

Spinal AVMs also present acutely only after hemorrhage. They may have severe headaches, meningismus, or photophobia.

Workup

A number of traditional and imaging techniques are used to locate and define AVMs. High-quality imaging techniques like MRI and CT scanning play a key role in the diagnosis of these malformations. Although CT scan provides an easy identification of the hemorrhage, only large AVMs can be located with the help of this scanning. For initial diagnosis, MRI is more accepted. If hemorrhage has occurred, a follow-up MRI may be needed as blood obscures the image. Angiogram may also be recommended in these cases. Magnetic resonance angiography enables to locate and assess AVMs that are moderately sized, but may be inadequate to find the detailed structure of arteries and veins, and also vessel aneurysms. In most of the cases aneurysms are present in the region away from the nidus. But those present within the nidus are more susceptible to rupture.

Angiogram is an essential part of diagnosis, particularly for hemodynamic assessment. This technique is also essential for deciding on the most appropriate treatment plan. Super selective angiography with a femoral artery puncture helps in measuring the pressure within the nidus.

Laboratory studies are generally not recommended for diagnosis or evaluation of AVMs. In case of subarachnoid hemorrhage, lumbar puncture helps to detect the presence of blood in spinal fluid. Transcranial Doppler is a non-invasive method to evaluate and locate cerebral AVMs. It is also a safe method for monitoring the brain after gamma knife surgery [7].

Treatment

Treatment of AVMs is based on the risk of hemorrhage associated with the condition. History of hemorrhage, size of malformation, deep venous drainage and high pressure within the nidus, all increase the risk of hemorrhage. Since there is a high probability of recurrence, patients who have hemorrhage should be given specific treatment to prevent recurrence. Earlier, patients with AVM who did not have hemorrhage were also subjected to invasive treatment to rule out the possibility of future hemorrhages. This practice is not much accepted because of the low rate of hemorrhage in patients who do not have hemorrhage as a manifestation.

Patients with one or more of the risk factors for rupture should be treated specifically, while those who do not have any high-risk factors are treated for the symptoms. Treatment usually include anticonvulsants for seizure and analgesic for headache, when present. Standard anticonvulsant therapy is suggested for improving seizures. A sudden onset of severe headache is usually a sign of brain hemorrhage and immediate assessment is needed. For headaches, without hemorrhage, analgesics are used. If focal neurological symptoms are present, serotonin is recommended. Glucocorticoids are suggested for improving neurological functions. These help in reducing vasogenic edema.

Endovascular embolization, surgical resection and focal beam radiation are the invasive techniques used in the treatment of this condition [9]. Decision to undergo surgery is based on the Spetzler-Martin grading scale where AVMs range from grade I (size smaller than 3 cm) to grade III (size larger than 6 cm). If the AVM is located in a functionally critical part of the brain it is assigned grade IV, while it is grade V if it has a deep venous drainage. Surgery is suggested for grade I and II, and if surgery has an increased risk due to the location of AVM, radiation therapy is the treatment choice [8]. Surgical resection is found to be very effective with smaller lesions [10]. For AVMs of larger size (grade III), embolization followed by surgical extirpation is the preferred treatment modality. A multimodal approach with embolization, radiosurgery and surgery is used in grade IV and V. Age of the patient, diffused nature of the AVM, presentation of hemorrhage, and blood supply to the nidus are considered before deciding on the treatment plan.

Prevention

Since it is a congenital condition and is presumed to be linked to chromosome 7, prevention is not possible. Hypertension is one of the risk factors for hemorrhage and it would be wise to avoid this condition. Careful and continuous monitoring is important in preventing hemorrhages in patients with AVM.

Patient Information

Arteriovenous malformation (AVM) is a vascular disorder characterized by a tangle of blood vessels. This malformation may occur in any part of the body, but is more common in brain or spine. The actual cause of the condition is not yet known. AVMs are more common among men and is found to run in a family. Prevalence of this condition is also not worked out completely. AVMs are rare and is found to affect less than 1% of the population.

AVM may remain asymptomatic in many people and may be found during a scan indicated for some other health issue. The most common symptoms, when present, are bleeding in the brain and seizure. These tangles of blood vessels may increase the pressure on the surrounding tissues of brain and spinal cord, resulting in headache, vision defects, weakness and other neurological symptoms. Stroke like symptoms are also common when blood flow from the neighboring tissues are directed towards the affected area. AVMs may also result in complications like weakened blood vessels and brain damage. Weakening of blood vessels leads to hemorrhage, which in turn affects the surrounding tissues. AVMs may enlarge in size with age. This may prevent the flow of protective fluid in different areas of brain resulting in fluid accumulation.

Cerebral angiography, CT scan and MRI are the most common techniques used in the diagnosis of AVMs. Images from these techniques help to locate the malformation and the flow of blood through them. This enables to decide on the appropriate treatment for the condition. Treatment of AVM depends on the location, size and symptoms. AVMs in the brain may need either surgery or radiation therapy. Surgery is recommended if there was a hemorrhage earlier and want to prevent further bleeding. Outcome of treatment depend on the location of malformation and severity of bleeding. Most of the patients have a quick and good recovery after surgery. Endovascular techniques also give a good prognosis.

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References

  1. Weinsheimer S, Kim H, Pawlikowska L, Chen Y, Lawton MT, Sidney S, et al. EPHB4 gene polymorphisms and risk of intracranial hemorrhage in patients with brain arteriovenous malformations.Circ Cardiovasc Genet. 2009;2(5):476-82.
  2. Kim H, Su H, Weinsheimer S, Pawlikowska L, Young WL. Brain arteriovenous malformation pathogenesis: a response-to-injury paradigm. Acta Neurochir Suppl. 2011;111:83-92.
  3. Berman MF, Sciacca RR, Pile-Spellman J. The epidemiology of brain arteriovenous malformations. Neurosurg. 2000;47(2):389-396. 
  4. Stapf C, Mast H, Sciacca RR, Berenstein A, Nelson PK, Gobin YP, et al. The New York Islands AVM Study: design, study progress, and initial results. Stroke. 2003;34(5):e29-33.
  5. Mast H, Young WL, Koennecke HC. Risk of spontaneous haemorrhage after diagnosis of cerebral arteriovenous malformation. Lancet. 1997;350(9084):1065-1068.
  6. Laakso A, Dashti R, Juvela S, Niemelä M, Hernesniemi J. Natural history of arteriovenous malformations: presentation, risk of hemorrhage and mortality. Acta Neurochir Suppl. 2010;107:65-9.
  7. Park SH, Hwang SK. Transcranial Doppler study of cerebral arteriovenous malformations after gamma knife radiosurgery. J Clin Neurosci. 2009;16(3):378-384.
  8. Ogilvy CS, Stieg PE, Awad I, Brown RD Jr, Kondziolka D, Rosenwasser R. AHA Scientific Statement: Recommendations for the management of intracranial arteriovenous malformations: a statement for healthcare professionals from a special writing group of the Stroke Council, American Stroke Association. Stroke. 2001;32(6):1458-71.
  9. Sahlein DH, Mora P, Becske T, Nelson PK. Nidal embolization of brain arteriovenous malformations: rates of cure, partial embolization, and clinical outcome. J Neurosurg. Apr 27 2012;117(1):65-77.
  10. Hernesniemi J, Romani R, Lehecka M, Isarakul P, Dashti R, Celik O, et al. Present state of microneurosurgery of cerebral arteriovenous malformations. Acta Neurochir Suppl. 2010;107:71-6.

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