INTRODUCTION
Arteriovenous vascular malformation is a collection of poorly formed and abnormal arteries and veins. Though the cause is unknown, they are thought to occur due to developmental anomalies of blood vessels in utero. Compared to other blood vessels, they have an increased tendency to bleed which could be deleterious to the brain and the individual involved. Since they are poorly formed, they are not as strong as the normal vessels, hence their propensity to bleed easily.
EPIDEMIOLOGY
The prevalence and incidence is not really known, the only available data are from autopsy reports which suggests around 4% incidence in the population. Autopsy data is thought to be affected by the aggression of the pathologists in searching for the malformation especially if the patient had neurological symptoms before his death. The age of the patient ,and whatever caused the death are other factors that could increase the pathologists search for it.
Data about cerebral malformations are few. Study carried out in Holland from 1980 to 1990 revealed an incidence per year of symptomatic cerebral malformations as 1.1 per a 100,000 people. Also, another study in Minnesota revealed the detection rates of AVMs as 2.1 per 100 000 in all cases excluding autopsy cases. The commonest malformations detected were intracranial arteriovenous malformations, then venous malformation with cavernous malformations as the third.
CLINICAL PRESENTATION
Though, cranial AVMs are sometimes found in the old, they are usually diagnosed before a patient clocks 40. More than half of the arteriovenous malformation cases present with intracranial haemorrhage, with intracerebral haemorrhage occurring more. Intraventricular and sub-arachnoid haemorrhage can also occur.
Seizures occur in about a quarter of cases seen and they could either be generalized or focal indicating the lesion’s location. Also, about 15% of cases present with headaches, about 5% with focal neurological deficits with pulsatile tinnitus. Children younger than age 2, could present with large head secondary to hydrocephalous, seizures and congestive heart failure.
DIAGNOSIS
Intracranial AVMs can be diagnosed with different diagnostic radiological studies such as computed tomography, magnetic resonance imaging, magnetic resonance angiography, arteriography, and digital subtraction angiography.
Computed tomography without a contrast media has a very low sensitivity, though hypointensity and calcification are revealed. On the other hand, Magnetic resonance imaging is highly sensitive as it provides crucial information about the topography and localisation of the lesion. Also, magnetic resonance angiography done non invasively can reveal presence of feeding aneurysms, detailed information on venous drainage patterns and AVM nidus characterization. However, the gold standard for defining the venous and arterial anatomy is ''Arteriography'' as physiological and functional information pertaining to the clinical decision to be made.
A thorough evaluation of the patient is required. It includes an in depth physical and clinical examination supported with radiological delineation of the anatomical boundaries using the magnetic resonance imaging scanning and arteriography. The investigation results will then be used to take decisions about the best management approach. The lesions natural history is then compared with the morbidity and mortality of the proposed intervention.
TREATMENT OPTIONS
Presently, there are 4 major available treatment options for patients with cerebral arterial malformations. Expectant monitoring could be employed, having it in mind that the patient is at risk of haemorrhage or other symptoms such as focal neurologic deficits and seizures. Also, surgical interventions such as microsurgery, radio surgery and endovascular techniques with the aim of complete obliteration of the arteriovascular malformation can be employed. All the treatment options have associated benefits and risks.
ENDOVASCULAR TREATMENT
Flow-assisted microcatheters and flow -directed microcatheters are employed in this interventional neuroradiology/ neurosurgical procedure. Under general anaesthesia,the microcatheter is inserted via the groin into the brain vessels ,then inside the AVM. Embolic materials such as polyvinyl alcohol particles, microcoils, microballoons, I-butyl cyanoacrylate, ethylene vinyl alcohol and N- butyl cyanoacrylate is used, which then blocks the AVM. The method is often used prior to surgery or radiosurgery since it is not very effective for large AVMs and a cure rate of just 10% recorded by a study by Vinuela et al.
RADIOSURGERY
Stereotactic radiosurgery is employed to irradiate the vessels in the AVMs and cause luminal obliteration, preventing haemorrhage in the process. A study by pollock et al revealed that radiosurgery achieves complete obliteration of the AVM in about 80% of patients.
MICROSURGERY
Microsurgery has become the mainstay in the management of AVMs. A neuronavigation system that is marker based is used for the procedure. It serves as the image guidance intraoperatively, the pathological vessels were segmented preoperatively using the computed tomographic angiographic study to obtain reconstruction of the AVM in the three dimensional pattern. Clips were placed initially on all supplying arteries to reduce intranidal pressure prior to the AVM dissection. The AVMs were then dissected along the draining veins as seen on the neuronavigation. The procedure is effective in more than 80% of cases and complete obliteration is achieved in less than 2 years.
TIMING OF SURGERY AND GRADING SYSTEM
It is recommended that surgeries for AVMs should be elective. The lesions are excised using standard microsurgical techniques using the operating microscope. The arterial feeders are taken on first, then the nidus of the lesion is excised, then the draining vein is excised. Since the goal of a cerebral arteriovenous malformation resection is complete obliteration, intraoperative or postoperative angiography is usually done. A residual lesion requires immediate resection to prevent further haemorrhage. Stereotactic radio surgery is another option for the residual lesion.
Various grading systems have been developed so as to predict risks during obliteration. The important variables are; size of the lesion, number of feeding arteries, speed of flow in the lesion, location, presence of associated aneurysms and the pattern of venous drainage. The grading scales have been applied both post-operatively and pre-operatively. Malik et al proposed a preoperative grading system which was anatomically based. It focussed on the arterial supply and the number of arteries supplying the supratentorial malformations. This system proved to be highly complex for general use though it confirmed that an increasing grade indicated an increased surgical morbidity.
Also, Shi and Chen proposed grading scales based on location of the lesion, the size, depth, how complex the supplying arteries were and how complex the draining veins were. This scale also proved to be too complex for bedside use. However, the Spetzler-Martin grading system is the standard grading system adopted for AVMs. It includes an analysis of size of the lesion, location of the lesion either superficial or deep, the drainage system, whether deep or superficial.
Lesions graded i to iii have low morbidity associated with their treatment but grade iv lesions have about one third morbidity associated with treatment while grade v lesions have about 50% treatment associated morbidity. Hence, surgery was recommended for grades i and ii lesions. Grade iii lesions on a case by case manner while surgery is also recommended for symptomatic and asymptomatic patients. Grades iv and v need a multidisciplinary approach. Though, the grading system is meant to predict outcomes of surgery, it is also used in the combined management of AVMs.
In conclusion, cerebral vascular malformations occur rarely and they are a major cause of intracerebra l haemorrhage and strokes when they occur. They may present with seizures, headaches and stroke like symptoms. The mainstay of treatment is surgery which is sometimes guided by the Spetzler-Martin grading system.
References
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