What is Arteriovenous Malformation: Types, Symptoms, Treatment, Pathophysiology, Complications, Prognosis
The heart and the blood vessels constitute the circulatory system. The arteries, veins and capillaries constitute the vascular system of our body. The arteries carry pure blood from the heart to the tissues and organs; while the veins carry impure blood from the tissues to the lungs. The capillaries connect the high pressure arteries to the low pressure veins. This helps in slowing down the blood flow and permit exchange of oxygen and nutrients. In case of absence of capillaries, the normal blood circulation is disrupted. This condition is called arteriovenous malformation. It is a complicated problem and needs to be understood in detail.
What is Arteriovenous Malformations?
Arteriovenous malformations (AVMs) refer to the connection of arteries and veins without capillaries between them. It results in abnormalities which lead to tangling of arteries and veins. This in turn interferes with the normal blood circulation.
Arteriovenous malformations are congenital and develop at birth. Overall, they are rare in occurrence. They can occur anywhere in the body including brain, spinal cord, heart, abdomen, lung, kidney, iris, shoulders and extremities of arms. When they occur in heart it results in heart failure.
Types of Arteriovenous Malformations
The different types of arteriovenous malformations are:
- Cavernoma: This occurs when there is an abnormal clustering of enlarged capillaries that doesn't connect with any significant arteries or veins.
- Venous Malformations: It involves clustering of enlarged veins with no connecting arteries.
- Capillary Telangiectasia: This is similar to cavernoma having abnormally enlarged areas in capillaries, resembling the spoke of wheels and with no connected arteries.
- Dural Arteriovenous Fistula: This happens when there is a direct connection between more than one arteries and veins into a sinus.
The arteriovenous malformations can occur at any place in the body. The commonest sites of occurrence are:
- Brain Arteriovenous Malformations: These occur on the cortical surface, deep into the brainstem, thalamus or basal ganglia and within the dural space. The presence of artery and vein tangle leads to reduction in the amount of oxygen reaching the nerve tissues, causing bleeding into surrounding tissues and by compressing parts of the brain or spinal cord resulting in damages in the brain or spinal cord.
- Dural Arteriovenous Malformations: These occur when the veins of the brain drain into the venous sinuses, before it leaves the skull and travels back to the heart.
- Spinal Cord Arteriovenous Malformations: These arise on the extramedullary surface or within intramedullary portion of the spinal cord.
- Pulmonary Arteriovenous Malformations: This type of arteriovenous malformations occurs in the lungs.
Although, the most common places of occurrence of arteriovenous malformation are brain, spinal cord and lung, the other places where it can occur are heart, abdomen, kidney, uterus, shoulders, extremities of limbs and also in the iris.
Symptoms of Arteriovenous Malformations
In most of the cases, the patient is asymptomatic. Arteriovenous malformations are usually revealed during an autopsy or during the treatment of some other underlying medical condition. The severity of the symptoms vary from person to person and in some people these can be even life threatening. The symptoms differ depending on the location of arteriovenous malformations in the body.
Since there is slow buildup of neurological abnormalities, it is observed that symptoms appear at any age and are noticed when people are in their twenties or above. The symptoms are less severe if they appear at a late stage. When they occur in the brain, the associated symptoms include:
- Confusion or dementia
- Bleeding in the skull
- Paralysis of one part of the body
- Loss of balance
- Problem with vision
- Stiff neck
- Sleep disturbances in view of uncomfortable noise at the site of tangling.
Some neurological symptoms appear just after birth and the defect is usually present within a major blood vessel located deep inside the brain. It is often seen as enlargement of the head due to accumulation of fluid inside certain spaces in the brain and seizures. There is heart failure and the baby does not survive. Those who survive, exhibit developmental abnormalities. The child who is born with arteriovenous malformations has a bluish tint on the skin due to absence of oxygenated blood. The skin turns deep red or purple as the child grows.
In case the spinal cord is affected, it causes inability to move the limbs and there is loss of balance. Symptoms such as numbness or pain in the affected area may develop. When it affects other organs, chest or abdomen, it results in pain in the abdomen, back and chest region. Pulmonary arteriovenous malformations usually do not elicit any symptom.
Epidemiology of Arteriovenous Malformation
Globally, the occurrence of arteriovenous malformations is uncommon. It has been studied that incidence rates of arteriovenous malformation in USA is 1.4/100,000. There is lack of information at present in view of its prevalence in terms of gender and ethnic groups.
Prognosis of Arteriovenous Malformation
Arteriovenous malformations cannot be prevented. They can be managed with proper medical care such as medications for controlling bleeding and blood pressure problems arising due to it.
Causes of Arteriovenous Malformation
It is a congenital problem and the exact cause of arteriovenous malformations is unknown.
Pathophysiology of Arteriovenous Malformation
The formation of arteriovenous malformations causes intense pain and leads to serious medical problems. Arteriovenous malformations get progressively larger in size. Increased blood flow through the tangled vessels forces the heart to work harder. This leads to worsening of the tangle and due to extreme fragility, it can lead to bleeding.
The exact cause of arteriovenous malformations is not understood. It is seen that during fetal development a number of new blood vessels are formed and they disappear as the human body develops. These changes are governed by the hormones and other factors that allow new blood vessels formation. Researchers have identified that change in these factors lead to the vascular abnormality.
When studied in familial inherited arteriovenous malformation, there is a genetic mutation in chromosome 7 which affects the vascular development and leads to a type of brain arteriovenous malformation called cavernous malformation.
Risk Factors for Arteriovenous Malformation
If there is a family history then it increases the chances of developing arteriovenous malformations.
Complications of Arteriovenous Malformation
Bleeding in arteriovenous malformations can result in fatal strokes.
Diagnosis of Arteriovenous Malformation
Arteriovenous malformations are detected by the following ways:
- Magnetic resonance imaging (MRI) scan and magnetic resonance angiography (MRA) scans to view the blood vessels and structure of the brain.
- Computed Tomography (CT) scan
- Angiography to check the blood circulation in the body.
Treatment of Arteriovenous Malformation
Headaches, seizures, nausea can be treated with medications. Surgical treatment is the recommended treatment modality for management of arteriovenous malformations. Its main goal is to prevent bleeding, control seizures and prevent other neurological symptoms.
It may involve:
- Endovascular Embolization: To reduce size of arteriovenous malformations.
- Neurosurgery: To attain access to arteriovenous malformations.
- Radiosurgery: To destroy arteriovenous malformations by means of radiation.
The type of treatment depends on the size and location of arteriovenous malformations. In case of neurological arteriovenous malformations, the treatment decision is often challenging. Hence, Spetzler-Martin grading system is used to make a decision between surgical and nonsurgical treatment for management.
Arteriovenous malformations are of rare occurrence and most of which are present in the brain, spinal cord and lungs. It can be treated by surgical intervention which is very challenging. Hence research efforts should aim towards understanding the pathology of this disease and to have better treatment strategies.