Nidus, AVM

AVFs are formed by direct communication between an enlarged artery and vein without interposed nidus. Lack of a capillary bed in the AVM nidus results in direct arteriovenous communication, which may be unique or multiple (Staff and Mohr 2000). AVFs are much more rare than AVMs (2%, Lasjaunias and Berenstein 1993a) and are always located on the brain surface. They may be present within an AVM nidus as a direct AV shunt surrounded by the network of arteriovenous channels. 

AVM nidus-staged, stepwise embolization. If this option is considered, the first embolization procedure should be targeted at compartments of the nidus fed by arteries harboring the aneurysm. Cases in which the aneurysm has not shrunk at follow-up, despite complete occlusion of the AVM, could be treated with coils. At this point, the treatment decision is as difficult to make as for unruptured regular aneurysm and depends basically on the aneurysm size. 

Direct Arteriovenous Fistulas Direct communication between arteries and veins without interposed nidus may be observed. Two types of direct AVF must be distinguished, pial AVF and AVF within a brain AVM nidus. 

Fig. 18.8a-d. Selective occipital angiogram in lateral view a demonstrates high-flow scalp AVM which was also supplied by branches of the ipsilateral and contralateral superficial temporal arteries and contralateral occipital artery (not shown). Following transarterial partial embolization with glue and particles of PVA into these vessels a percutaneous approach was performed b,c with injection of glue (50% NBCA/ 50% Lipiodol) resulting in complete obliteration of the AVM nidus as shown on the post embolization left external carotid angiogram d... 

In spinal vascular malformations abnormally thick, often tortuous vessels on the surface of the spinal cord can be identified as well as the angiomalike nidus of the AVM. MRI facilitates the differentiation between SDAVF and intramedullary AVM. 

AVMs of the glomerular type are more frequent and characterized by a nidus similar to those of most cerebral AVMs. They may be located superficially on the surface of the spinal cord or deep within the cord parenchyma or extend to both compartments. Due to the numerous anastomoses between the spinal cord arteries, the nidus is always supplied by several arteries or branches derived from the anterior... 

AVMs are composed of a network of channels interposed between feeding arteries and draining veins, without any direct shunt. Two different anatomic types of nidus may be more or less differentiated compact nidus, constituting a tumor-like well-circumscribed network, and diffuse nidus, with sparse, abnormal AV channels spread within normal brain parenchyma (Chin et al. 1992). 

The role of CT angiography in the diagnostic workup of brain AVMs is not precisely defined. Aoki et al. (1998) showed that 3D CT angiography provided precise anatomic information on nidus and draining veins but did not demonstrate small feeders. 

The role of DWI has to be determined (Ducreux et al. 2001). The nidus usually has a low signal with a large and homogeneous increase of the apparent diffusion coefficient (ADC). However, to date DWI does not play a major role in AVMs. 

In the great majority of cases, no activation is detected inside the nidus during the performance of a task. This could be related to the absence of functional tissue within the nidus, but the detection of subtle and minor activation within an AVM could also be obscured by the complex relationships between the BOLD effect and AVM circulatory patterns (Vikingstad et al. 2000). Activation can be observed in the cortical regions adjacent to AVMs. In the majority of cases where brain AVMs are located in eloquent areas, a shift of the activated areas with a frequent interhemispheric transfer is observed. 

Technically, selective angiography has to be performed according to a rigorous protocol. To assess as precisely as possible the anatomic components of the AVM, it is important to inject selectively the internal and external carotid arteries and vertebral arteries. Analysis of the arterial feeders, nidus, and venous drainage is obtained by performing multiple projections (anteroposterior, lateral, and oblique). Three-dimensional angiography may be helpful. 

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