ARTERIOVENOUS MALFORMATIONS (AVMs)

WHAT IS AN ARTERIOVENOUS MALFORMATION?

Sketch of brain AVM
An arteriovenous malformation is a tangle of arteries and veins that develop during the embryonic or foetal period. They occur in the brain or spinal cord and affect 1% of the population, both sexes and all racial groups being affected equally.
Sketch of brain AVM
 

WHAT ARE THE SYMPTOMS?

Most patients experience few or no symptoms at all. Often the malformation is discovered by accident while performing a CT or MRI scan for another reason (car accident, headache, seizures, brain haemorrhage…) or at an autopsy. The haemorrhages are usually small, in fact microscopic, and remain unnoticed, but sometimes they are large and can cause severe injuries or even death.

Seizures can be partial or generalised and may be accompanied or not of loss of consciousness, whereas headaches can vary greatly in frequency, duration and intensity, sometimes mimicking a migraine. The distribution of the headache is not specific to the site where the malformation sits but can affect any part of the head.

AVMs can also cause a range of neurological symptoms, such as paralysis or loss of coordination in one or more members, gait disturbances, difficulties to perform tasks that require planning, nausea, visual disorders, lack of coordination of eye movement, speech problems, abnormal sensations such as numbness or tingling or spontaneous pain, memory loss, mental confusion, hallucinations or dementia. In children and adolescents the malformations may cause them to be behind in school.

One of the most typical symptoms of AVMs is the presence of thrills, which sometimes the patient can hear, and some others only the doctor can hear with the help to a device called a stethoscope. This sound is similar to that produced by a stream of water flowing at high speed through a narrow pipe.

All these symptoms can occur at any age but are usually manifested in the 2nd to 4th decades of life. Pregnancy can cause a worsening of symptoms because it is accompanied by increased blood volume and blood pressure.

Galen’s vein aneurysm is a malformation at the centre of the brain. The blood passes directly from arteries to veins in such quantities that the patient shows heart failure since birth. It is frequently associated with hydrocephalus and mental retardation due to brain damage during pregnancy. This brain damage is irreversible even if the malformation is treated and causes severe mental retardation.

HOW DO AVMs DAMAGE THE BRAIN AND SPINAL CORD?

In the brain, damage occurs through 4 mechanisms:

  • Theft of blood to neighbouring nerve tissue, since most of the blood is going to the AVM.

  • Brain haemorrhage.
  • Compression of surrounding nervous tissue, for malformations tend to increase in size over the years.
  • Making the flow of cerebrospinal fluid (CSF) more difficult, by mechanically blocking the passage of CSF and/or by increasing the blood pressure in the veins that drain the brain.

Usually, arteries and veins are connected by capillaries. Capillaries are a mesh of fine vessels that slow blood flow, and it is in them where the exchange of nutrients and gases (oxygen, carbon dioxide, …) takes place. In AVMs this intermediate capillary network is missing, so the arterial blood passes directly into the veins. This means that veins receive blood with high, arterial blood pressure, which they are not prepared to endure, and so they start to dilate. The wall of the veins with blood at arterial pressure becomes thinner and can eventually rupture, causing brain haemorrhages. Such haemorrhages are often microscopic, causing limited damage and few symptoms. However, the iron deposits derived from haemorrhagic blood induces the appearance of seizures.

Most AVMs show signs of bleeding in the past, and even in absence of significant bleeding or blood theft of neighbouring nervous tissue, AVMs can damage the brain or spinal cord simply because of their size.

WHERE TEND AVMs TO BE FORMED?

AVMs can form almost anywhere in the brain or spinal cord.

AVMs of the spinal cord are classified in 2 types:

  • Dural AVMs in which the lesion lies in the coverings of the spinal cord (duramater).

  • Spinal cord AVMs in which the lesion sits directly on the spinal cord.
Spinal cord dural AVM

Spinal cord dural AVM

Angiography image of spinal cord dural AVM
Spinal cord dural AVM
Spinal cord dural AVM
Angiography image of spinal cord dural AVM
Spinal cord AVM
Spinal cord AVM
Spinal cord AVM
Spinal cord AVM

Spinal nervous tissue is damaged by:

  • Blood theft caused by malformation

  • Venous pressure increase in the spinal cord
  • Haemorrhages

Spinal AVMs can cause sensory or motor disorders with loss of motility of one or more limbs, as well as pain due to pressure on spinal nerve roots.

WHAT ARE THE RISKS OF AVMs?

The greatest risk that AVMs pose is the possibility of bleeding, with an annual incidence of 2-4%. Most haemorrhages are small and so are unnoticed, but some haemorrhages are larger and can cause severe neurological damage or even death. Haemorrhage is more likely in small AVMs with one point of venous drainage and in pregnant women, especially due to the increased volume of blood throughout pregnancy and due to the increase in blood pressure during childbirth.

AVMs that have bled are 9 times more likely to bleed again than those that have not done so, especially in the first year after the haemorrhage.

WHAT OTHER TYPES OF VASCULAR MALFORMATIONS MAY AFFECT THE CENTRAL NERVOUS SYSTEM?

In addition to AVMs, there are other vascular malformations:

  • Cavernomas

  • Telangiectasias
  • Venous malformations

Just like AVMs, these malformations can be formed nearly anywhere in the central nervous system (CNS). However, these malformations have usually low blood flow, because they only involve one type of abnormal blood vessel, whereas AVMs involve both arteries and veins, and these malformations have a very low risk of haemorrhage. In general, they tend to cause less neurological damage, so the treatment does not need to be as aggressive as with AVMs.

Cavernomas

Sketch of a cavernoma
Cavernomas are giant capillaries with a range of sizes (a few millimetres to several centimetres) and very thin walls, filled with blood that flows very slowly and without pressure. Often a patient will have multiple cavernomas at the same time, and since some cavernomas have a familial incidence, performing brain MRI scans of the patient’s family is advisable. They rarely bleed, and, if they do, the haemorrhages are small and so the risk of death is remote. However, the haemorrhagic blood will cause iron deposits to build up in surrounding nervous tissue, thus also causing seizures. To stop the seizures, the cavernoma, together with the iron deposits, must be removed.
Sketch of a cavernoma
 

In addition, removal is strongly recommended if the cavernoma sits in the spinal cord or brainstem, where repeated micro-haemorrhages may lead to severe neurological damage.

Capillary telangiectasia

Brain telangiectasia
Capillary telangiectasia consists of groups of dilated capillaries similar to those seen in the legs of many people. They rarely cause damage to neighbouring tissue and haemorrhages are exceptional.
Brain telangiectasia
 

Venous malformations

Brain venous malformation
Venous malformations involve hypertrophied veins draining blood from neighbouring nervous tissue without interfering with its function. They rarely bleed and the vast majority of them are asymptomatic and benign. They cannot be removed because doing so usually damages the nervous tissue on which they sit.
Brain venous malformation
 

CAUSES OF VASCULAR LESIONS

Vascular lesions result from developmental defects in the embryonic and/or foetal period. In some cases, they may be related to genetic mutations, e.g. cavernomas may be due to a mutation on chromosome 7, which is especially frequent in South Americans living in the US. Other times, cavernomas are acquired lesions that appear after injury (trauma, radiation, …) on the CNS.

Certain types of vascular malformations of the central nervous system are part of complex syndromes, such as Rendu-Osler-Weber, Sturge-Weber, Klippel-Trenaunay, Parkes-Weber and Wyburn-Mason.

HOW ARE THESE VASCULAR MALFORMATIONS DIAGNOSED?

The most common test is angiography, which is carried out by injecting into the arteries a substance, known as a contrast, that can be seen using X-rays. Next, some X-ray scans are performed. These will show arteries and veins, as well as any vascular malformations. The problems of angiography are that it is an invasive technique, because an artery in the groin has to be pricked to inject the contrast, and that some vascular malformations cannot be seen with angiography, such as cavernomas and telangiectasias.

Angiography for diagnosis and treatment of a brain AVM
Angiography image of a brain AVM
Angiography for diagnosis and treatment of a brain AVM
Angiography image of a brain AVM

CT scans are especially useful to see whether or not there are haemorrhages, but the malformations themselves are sometimes not seen.

MRI, however, does show all vascular malformations in addition to any haemorrhages and the consequences thereof (iron deposits at the sites where there has been bleeding). MRI will allow an assessment to be made as to whether there is hydrocephalus (accumulation of CSF) or not and the mass effect on the neighbouring nervous tissue. MRI angiography does not involve a contrast and will show more vascular malformations than angiography, but the quality of the image is inferior to that of angiography.

HOW ARE THESE VASCULAR MALFORMATIONS TREATED?

Medications can relieve symptoms such as headache, but cannot cure the vascular malformations themselves. The treatment options for them are:

  • Surgical removal

  • Embolisation
  • Radiosurgery

The choice as to which option(s) to implement will depend on the type, size and location of the vascular malformation in question.

Venous malformations and telangiectasias rarely require treatment, and, moreover, surgical removal could lead to unnecessary damage to nervous tissue.

Cavernomas can be removed if required, but only if their location allows so. When they cannot be removed due to their location, the alternative treatment is radiosurgery.

AVM treatment decisions require a meticulous assessment of each particular case, carefully outweighing the possible risks and benefits of each treatment option. The natural history of that specific AVM, i.e. what would happen if it was not treated, is impossible to predict, but we know that each year 2-4% of AVMs bleed and that the incidence of haemorrhage increases by a factor of 9 if there has already been previous bleeding. Each haemorrhage carries the risk of causing neurological sequelae, more or less severe, and even death.

Surgical removal involves accessing the malformation to eliminate it completely, for it there is any left, no matter how small it is, the risk of haemorrhage will remain. This removal is especially recommended if the malformation is located in at the surface of the brain or spinal cord and they are not excessively big (not more than 3cm in diameter). Malformations located in deep areas of the brain, such as the thalamus or hypothalamus, are, however, difficult to remove without leaving neurological sequelae.

Craniotomy for the removal of a brain AVM
Surgical removal of a brain AVM
Craniotomy for the removal of a brain AVM
Surgical removal of a brain AVM

Endovascular embolisation involves puncturing an artery in the groin and introducing a catheter, which is guided through the arterial network until the tip is located within the AVM. A substance, which will solidify, obstructing the malformation, is then injected. In any case, for the embolisation to have a curative effect, the malformation must be blocked completely: if there is any left the risk of haemorrhage will remain. Embolisation only manages to block completely 30% of cases, but in the rest it is useful to reduce the size of the malformation and allow it to be completely blocked by surgery or radiosurgery.

Embolisation of a brain AVM
Embolisation of a brain AVM with particles
Embolisation of a brain AVM with glue
Embolisation of a brain AVM
Embolisation of a brain AVM with particles
Embolisation of a brain AVM with glue
Radiosurgery to treat AVMs

Radiosurgery delivers ionising radiation in small bundles that all converge at the malformation. The radiation damages the walls of the vessels involved in the malformation and cause their obstruction due to the thickening of their walls. Several months are, thus, needed for the radiation to have an effect; in fact, between 12 and 24 months are usually required.

Radiosurgery is usually preceded by embolisation or surgery, especially in cases of large malformations where combined treatment can reduce the risks and increase effectiveness, but in any case, radiosurgery will require several months to occlude the malformation, and during this time the risk of haemorrhage will persist.

Radiosurgery to treat AVMs
 

Although both embolisation and radiosurgery are less aggressive than surgery, they are not completely devoid of risk. It is the combination of these 3 techniques that has allowed the treatment of malformations which, due to their size, were considered incurable in the past.

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