Spinal cord arteriovenous malformations
The aetiology of dural arteriovenous malformations is uncertain. A minority of cases may develop as a secondary consequence of venous thrombosis in the paravertebral venous network; it is rare, however, to identify an underlying coagulation abnormality.
The most common age of onset is late middle age or older, although presentation in young adults does also occur. Males are affected far more often than females.
The great majority of dural arteriovenous malformations are found in the mid to lower thoracic cord, sometimes extending to the conus and, less often, more rostrally, into the cervical region. A typical arteriovenous malformation extends over a number of segments. Macroscopically, large tortuous veins are found on the dorsal aspect of the cord. The feeding artery is usually either the artery of Adamkiewicz, or one or several of the dorsal segmental branches from the aorta which feed into the posterior spinal artery system.
Clinically, a late middle-aged male develops a slowly progressive thoracic cord syndrome. There is often a history of exercise intolerance, with weakness and sensory disturbance in the lower limbs and difficulties with sphincter control.
Examination reveals upper motor neuron signs in the lower limbs but, in addition, it is common to find some lower motor neuron signs involving upper lumbar segments, e.g. wasting, fasciculation, and weakness of quadriceps with reduced or absent knee jerks. This reflects the commonly found caudal extension of the malformation into the conus. It is extremely rare to find a spinal bruit.
There is increased signal with swelling of the cord on T2-weighted images, usually involving several segments in the lower thoracic region. Patchy gadolinium enhancement is often present. The pathognomonic finding is that of multiple, serpiginous, small signal voids which are closely applied to the dorsal surface of the cord at the same level as the intrinsic cord signal changes; these represent the dilated and tortuous dorsal veins. MR angiography or rapid MR imaging after a bolus of gadolinium reveals the lesion more directly. Spinal angiography is still required to determine the level of the fistula, and to determine the suitability of the lesion for embolization.
The natural history for dura arteriovenous malformations is one of gradual progression in clinical deficits, typically over a matter of years. Acute decompensation is less common, and when it does occur is probably due to venous infarction; haemorrhage is a very rare complication.
Given the poor prognosis, therapeutic intervention is recommended once a significant neurological deficit has developed. The main aim of active treatment is to prevent further deterioration, although occasionally there may be a reversal of existing symptoms. Many cases are considered suitable for embolization, an important criterion being that there is adequate alternative arterial supply to the cord apart from the artery that is catheterized for the embolization procedure.
The results of embolization in experienced hands are generally very good, with partial or complete obliteration of the malformation, although there is a measurable risk of inducing cord infarction acutely, and later recurrences of the malformation are sometimes seen.
Surgical extirpation of the malformation can also be achieved in many cases, but is usually chosen only when embolization is unsuccessful or unsuitable, given the greater morbidity of the surgical procedure. In some cases, where an underlying coagulation disorder is found, or where it is thought that there has been venous infarction in the cord secondary to thrombosis within the vascular malformation, anticoagulation may be used; care is needed, however, as there may also be an increased risk of haemorrhage from the malformation.