Arteriovenous Malformations

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Learning objectives

  • Understand the aetiology and risk factors
  • Understand the risks of bleeding
  • Be able to recognise the clinical presentation
  • Discuss diagnostics and treatment strategies
  • Understand the careful balance between bleeding and intervention risks

Introduction

Vascular anomalies are an important cause of primary intracerebral and Subarachnoid Haemorrhage. They tend to be seen in younger patients. Approximately 2% of ICH are due to AVMs. Finally, the act of bleeding and resultant local damage may render a small AVM as being undetectable. The annual risk of ICH from those with known AVM is approximately 4% per year. The prognosis following an AVM related bleed is better than that due to a non-AVM related bleed possibly due to younger age but other factors too. Note that for every 100 MRIs you will find an unruptured aneurysm, for every 500 MRIs you will find an asymptomatic cavernoma, for every 2000 MRIs you will find an asymptomatic AVM.

Aetiology

An arteriovenous malformation is made up of a tangle of arteries and veins which forms the nidus but without the usual intervening capillary bed which protects veins from exposure to systemic blood pressure. The arteries appear to lack the muscularis layer. They can be found anywhere in the brain as well as the spinal cord and dura and surface of the brain. The main concern is that they can bleed intraparencymally, into the Subarachnoid space or into the ventricles depending on anatomy. They can also cause focal seizures. Theories also suggest that they may "steal" and therefore shunt local perfusion away from healthy brian causing ischaemia elsewhere. They can exert the same effects as a space-occupying lesion.

Clinical

An AVM may be associated with developmental defects and learning disability. Some may cause focal seizures which may be secondarily generalised and even TIA-like episodes. Very rarely large superficial AVMs may produce an audible bruit which may be audible to patient and doctor. They come to attention when they cause an acute parenchymal haemorrhagic stroke with a headache. Others may cause bleeding into the subarachnoid space with a thunderclap headache and neck stiffness and coma. Signs and symptoms depend on location and severity and size.

Spetzler-Martin Grading of AVM

The Spetzler-Martin AVM grading system allocates points for various features of intracranial arteriovenous malformations to give a grade between 1 and 5. Grade 6 is used to describe inoperable lesions. The score correlates with operative outcome.

CriteriaScoring
Size of nidus small (<3cm) = 1, medium (3-6cm) = 2, large (> 6cm) = 3
Eloquence of adjacent brain non-eloquent = 0, eloquent = 1. Eloquent brain: sensorimotor, language, visual cortex, hypothalamus, thalamus, brain stem, cerebellar nuclei, or regions immediately adjacent to these structures. non-eloquent brain: frontal and temporal lobe, cerebellar hemispheres
Venous drainage superficial only = 0, deep = 1

Investigations

  • CT(A/V) scan: Initial management is as for a haemorrhagic stroke. They can be seen pre-bleed on CT. These can be about 3-5 cm in size and can exert a mass effect. Many however are asymptomatic. Paradoxically small AVMS have a higher risk of bleeding. Imaging is useful. Non-contrast CT may just show calcification or blood if there has been recent bleeding.
  • MRI(A/V) imaging: is more useful and will show up the vessels and a draining vein.
  • Cerebral Angiography: is the gold standard. Imaging may be delayed until there has been some clot resolution to allow better visualisation. Angiography may allow haemodynamic pressures to be measured directly or indirectly by assessing the passage of contrast through the lesion. A high pressure within the nidus of the lesion is associated with and increased risk of bleeding.

Management

Management depends upon an assessment of the risk of bleeding and that of intervention. It is important to gather as much evidence as possible. The fascinating ARUBA study showed that with a five-year follow-up that in a trial comparing the surgical or endovascular intervention with best medical intervention confirms that despite the higher risk for stroke with an unruptured arteriovenous malformation (AVM), intervention only raises that risk. There was an annual risk of spontaneous haemorrhage rate was 2.1% per year which increased by a factor of 4.5 with intervention. There was increased functional deficit x 2.5 and no reduction in epilepsy or headaches. This was true of all treatment modalities and all AVM subgroups. Superficial and small AVMS with a single feeding arterial branch draining to a cortical vein do best. The following suggests some of the possible management strategies which may at times be combined e.g. embolisation and surgery. Surgical extirpation is suggested as the main treatment for Spetzler-Martin grade I or II if they are surgically accessible with low risk. Focal beam radiotherapy can be used to treat deep lesions with an intention to cause thrombosis of the lesion in AVMs of Spetzler-Martin grade I or II if they < 3 cm in size and surgery has an increased surgical risk based on location and vascular anatomy. Endovascular Embolisation of large deep lesions may also be attempted.


==References and further reading==*<A href="http://www.thelancet.com/pdfs/journals/lancet/PIIS0140-6736(13)62302-8.pdf">Medical management with or without interventional therapy for unruptured brain arteriovenous malformations (ARUBA): a multicentre, non-blinded, randomised trial</a>