Teaching Case

Incidental detection of a CNS developmental anomaly on the MRI and DSA of two different patients.

The MR imaging and digital subtraction angiographic appearances of simple developmental venous anomalies (venous angiomas) are demonstrated in two different patients. In both of these patients the developmental venous anomaly (DVA) was an incidental finding, unrelated to the patient's symptomatology. The MRI study was performed in a 73-year-old woman presenting with late-onset simple partial seizures and demonstrated multiple, bilateral, enhancing hemispheric lesions (? metastases) in addition to the DVA. The DSA was performed as part of the imaging workup of a 14-year-old girl presenting with a single short episode of intense headache; an aneurysm of the right supracavernous ICA (not shown) was detected as the causative lesion, a right frontal DVA was noted incidentally.

Cerebrovascular malformations (CVMs) can be classified according to specific histological criteria into arteriovenous, cavernous, venous, capillary, and mixed (transitional) malformations (although classical treatises on their pathological definitions do not include a separate category for mixed lesions). Most CVMs have been generally thought of as developmental anomalies or hamartomas, yet, more recent evidence points towards lesion evolution in certain malformations (most conspicuous in cavernous malformations). The venous malformation (occurring in up to 4% of the population, and considered to be the most common form of CVMs) appears to represent the clearest example of a pure developmental anomaly. Indeed, Lasjaunias et al. proposed the term developmental venous anomalies (DVAs), in preference to previously used terms such as venous angiomas (VAs) and medullary venous malformations (proposed by Huang et al.). DVAs comprise both simple (such as the VA), and complex variants. (The term DVA in the following text will be used in relation to simple VAs unless specifically noted otherwise.)

DVAs are composed of histologically abnormal veins (the walls of the veins are thickened and hyalinised and are usually devoid of smooth muscle and elastic tissue), separated by normal brain tissue. These anomalous veins form an essential drainage pathway for the surrounding parenchyma. These lesions are characterised by a caput medusae or an umbrella-like convergence of multiple venules on a single or occasionally multiple, enlarged parenchymal or medullary vein(s) (variably termed collector, drainage or terminal vein), like the trunk of a tree or the shank of an umbrella. The terminal vein can drain into superficial cortical veins or sinuses and/or the deep ventricular veins, no matter where the caput is located (both the superficial cortical and the deep ventricular veins may appear enlarged). The DVA caput may also be classified by location (according to Valavanis et al.) as juxtacortical/superficial (within the grey matter or at the grey-white junction), subcortical, and deep/periventricular (adjacent to the ventricles or within the centre of the structure, such as the pons). The DVA caputs and their draining veins occur in typical locations that could be predicted from the normal medullary venous anatomy. Although frontal, parietal and cerebellar sites are frequent locations for the occurrence of these anomalies, involvement of other areas such as the temporal, occipital, basal ganglia, thalamus, brainstem, and the ventricles has also been noted.

The overwhelming majority of DVAs are considered to be clinically silent. Patients may rarely present with seizures, headaches, and intracerebral haemorrhages (more common with cerebellar lesions). Spontaneous thrombosis of these anomalies (with one case leading to infarction and death of the patient) has been reported in the literature.

These lesions are elegantly demonstrated by MR imaging. Contrast-enhanced MR images can clearly show the caput medusae and direction of the venous drainage (although diagnosis could be made from T2-weighted images). It is the orientation of the DVA and the imaging plane that determine whether the typical caput medusae appearance will be seen. The most common CT manifestation of venous angiomas is visualisation of a linear enhancing transcerebral vein without associated parenchymal abnormality; a caput medusae appearance can occasionally be seen. There is usually no evidence of oedema or mass effect unless recent haemorrhage has occured. Cerebral angiography is no longer judged necessary with uncomplicated DVAs. Although angiography can visalise most DVAs, some of these lesions may be angiographically occult. A conservative approach in treating this type of lesion has been recommended since the report of a case of brainstem infarction following elective removal of a DVA in the posterior fossa.

Complex DVAs represent extreme variations of normal cerebral venous drainage and consist of dilatation of the superficial and/or deep venous system. These rare anomalies can occur unilaterally or bilaterally, supratentorially or infratentorially; they can be either focal or involve the entire hemisphere. A varix is a single dilated vein with histological findings consistent with thickened, fibrotic, and hyalinised walls. Anomalies of this type are generally clinically silent, and cerebral dysfunction is usually absent. Symptoms, when they do occur, most commonly consist of headaches or mild seizure disorders. The angiographic findings are striking, with well-formed but enlarged transcerebral medullary and deep and/or superficial cortical veins. The patients can be reassured that no interventional or surgical therapy is necessary or warranted.

Histopathologically mixed malformations are defined as a combination of two or more CVMs, histologically distinguishable in separate regions of the same lesion. Although reports of well-characterised mixed CVMs have emphasized the rarity of these lesions, Awad et al. believe that the prevalence of these lesions has been vastly underestimated. Different combinations have been reported. Of interest to this discussion is the occurrence of DVAs with either cavernous angiomas (the most common type of mixed CVMs) or arteriovenous malformations (either with or without a typical AVM nidus; such lesions has been variously described as mixed angiomas, arterialised VAs, atypical AVMs or as mixed AVM-venous malformations); or even a case report of the juxtaposition of a cavernous angioma, DVA, and capillary telangiectasia all in the brainstem of a single patient. These mixed malformations have distinct clinical, radiological, and pathological profiles.