Introduction
A 'fistula' describes where an abnormal channel develops within the body connecting two structures. In the case of arteriovenous fistula, there is an abnormal connection between an artery and a vein.
Dural Arteriovenous Fistula
In the case of a dural arteriovenous fistula (often abbreviated to 'dAVF'), this is an abnormal connection between an artery on the dura (the outer membrane that surrounds the brain) with a vein on the inside of the dura, or sometimes within the brain.
Dural arteriovenous fistulas are supplied by feeding arteries from vessels outside the skull, e.g. External Carotid Arteries. dAVF are distinguished from brain arteriovenous malformations, which have feeding arteries from within the skull and contain tangles of vessels known as a nidus.
Carotid-Cavernous Fistula
A carotid-cavernous fistula, or 'CCF' is very similar, but affects the cavernous sinus, which is a large vein situated directly behind the eyes.
These fistulas are relatively rare. dAVFs and CCFs account for roughly 10%-15% of all intracranial malformations (the other malformations are AVMs and cavernous malformations). They tend to develop more often in people over 50 years of age, unlike brain AVMs, which can develop at younger ages including in childhood. In both these conditions, if there is reversal of blood flow into the veins in the brain, there is increased risk of bleeding into the brain.
Causes
These fistulas are often caused when a blood clot (thrombosis) has formed in a dural sinus (one of the large veins surrounding the brain). As the clot heals, new vein pathways are created, and these are the fistulas. These blood clots can form for example…
- After a head injury.
- After an infection, particularly ear infections.
- After brain surgery.
- In conjunction with a brain tumour / benign meningeal tumours.
- Increased clotting tendencies.
- Hereditary factors increasing the risk of developing vein thrombosis.
Symptoms
Many people with either of these forms of fistula may have no symptoms at all. Where symptoms do occur, they include…
Dural Arteriovenous Fistula
- A 'whooshing' sound every time the heart beats (called 'pulsatile tinnitus').
- Headache.
- Problems with vision.
- Stroke-like symptoms (e.g. weakness, numbness, speech disturbance).
Carotid-Cavernous Fistula
- Bulging of one or both eyes ('exophthalmos'), together with increased tearing.
- Reddening of the eye/s.
- Double vision.
- Blurred vision.
- Eye pain.
Other symptoms may appear where there has been resultant bleeding (haemorrhage) in the brain, or where blood supply in a specific part of the brain is compromised. These may include...
- Seizure.
- Nausea / vomiting.
- Numbness / weakness affecting one side of the body.
- Speech / comprehension issues.
- Vision issues.
- Balance issues.
If any of these symptoms are present, immediate medical attention is required.
In some cases, a fistula will cause venous hypertension (high blood pressure) which may result in the headaches and progressive confusion.
Tests / Diagnosis
Initial tests may include a CT (computerised tomography) scan or MRI (magnetic resonance imaging) scan, with CT angiogram or MR angiogram. The best way to diagnose these conditions, however, is with a cerebral angiography (also referred to as 'digital subtraction angiography'), which can determine the exact nature of the condition.