Patho-mechanical classification of brain AVMS
While there is, therefore, little discussion about the necessity of treating pial AVMs that have bled because of their larger rebleeding risk, pial AVMs that have not bled have to be further subdivided to select those patients in whom therapy is indicated, i.e., in whom the therapeutic risk is lower than the natural history risk. At present a randomized trial is underway that tries to elucidate the risks of treatment compared to a conservative management. In this trial, which was coined ARUBA (A Randomized Trial of Unruptured Brain AVMs), subjects with unruptured AVMs are randomly assigned to either best possible invasive therapy (endovascular, neurosurgical and/or radiation therapy) or medical management without intervention. Patients will be followed for 5 to a maximum of 10 years to get a better understanding of the natural history and the treatment-related risks [15].
In our practice, we try to classify “unruptured” AVMs according to their pathomechanism (printed subsequently in italics) in relation to the angioarchitecture and imaging findings and thereby decide on the necessity of treatment. Due to their high-flow shunt, fistulous pial arteriovenous malformation (Fig. 1), especially when present in childhood, can lead to psychomotor developmental retardation, cardiac insufficiency and, when present later in life, to dementia, and therefore merits treatment [16]. Endovascular treatment should be aimed in these cases at reducing the arteriovenous shunt. Venous congestion (Fig. 2), which can be due to a high input (fistulous lesions) or a reduced outflow (secondary stenosis of the outflow pattern), may be accompanied by a cognitive decline or epilepsy, and we would propose treatment in these cases, with the same aim as stated above [17]. Even if signs of venous congestion are not present, a long pial course of the draining vein may indicate that venous drainage restriction is present over a large area, increasing the risk of venous congestion and subsequent epilepsy. Conversely, a short vein that drains almost directly into a dural sinus is unlikely to interfere with the normal pial drainage. If a patient was to have epilepsy in this kind of angioarchitecture, MRI should be scrutinised for signs of perinidal gliosis. While in the former case (epileptic patient harbouring an AVM with a long pial draining vein), endovascular treatment is warranted to reduce the interference with the normal pial drainage and is likely to reduce the seizure frequency or severity, in the latter cases (epilepsy following perinidal gliosis), endovascular therapies are unlikely to change the seizure frequency or severity, and we would suggest abstention from an endovascular treatment. Mass effect is a rare pathomechanism that may result from large venous ectasias or the nidus proper compressing critical structures, and may lead to epilepsy, neurological deficits and even hydrocephalus (Fig. 3) [18]. Arterial steal has been associated with clinical findings such as migraine and focal neurological symptoms that most often are transitory in nature [19]. With the advent of new imaging techniques such as functional MRI and perfusion weighted MRI, it has now become possible to visualise, whether or not the symptoms of a patient can be attributed to a true steal that can be treated by endovascular means with the aim of reducing the shunt if the symptoms are disabling.
Fig. 1 Fistulous pial arteriovenous malformation with a high-flow shunt. Although they typically present in early childhood when they classically lead to psychomotor developmental retardation or cardiac insufficiency, they can also present later in life; this typically happens with signs of venous congestion, i.e., epilepsy, or even a cognitive decline. Multiple shunts of this type are characteristic of HHT (hereditary haemorrhagic telangiectasia)
Fig. 2 Three-dimensional venous angiogram and venous phases of an ICA injection demonstrate the classical pseudophlebitic aspect of enlarged and tortuous pial veins as a sign of longstanding venous congestion that may be accompanied by epilepsy, headaches, cognitive decline and focal neurological deficits
Fig. 3 In rare cases, mass effect of the nidus proper or, as was present in this case, of the dilated draining vein can lead to neurological deficits or, as in this case, seizures. Following partial treatment with embolisation of the large shunt, the size of the venous pouch regressed, the seizures stopped and the remaining small nidus was treated with radiosurgery