An arteriovenous malformation (AVM) is a site of abnormal connectivity between arteries and veins. It is basically like a tangle of worms, where the greatest concentration of worms in the central portion of the AVM (this part is the "nidus") is made up of abnormal blood vessels that are hybrids between true arteries and veins. AVMs are fed by one or several arteries, and are drained by one or more major draining veins; these feeding and draining vessels may be unusually tortuous (winding like rivers), and unusually large. They can occur in the brain (brain AVMs) or along the spinal cord (spinal AVMs).
The vessels of an AVM are abnormal and so may leak or rupture (hemorrhage; that's the main problem). The blood flow and pressure in especially the larger vessels of an AVM are unusually high and may lead to significant shunting of blood to and from the lesion. Higher flow-pressures in addition to abnormal AVM vessel wall structure can lead to the formation of aneurysms on arteries feeding the AVM (i.e., "parent artery" or "pedicle" aneurysms) or within the AVM itself (i.e., "intranidal" aneurysms). These can also rupture. Somewhere around 6-7% of brain AVMs have aneurysms associated with them, and when AVMs rupture, some think it may be these aneurysms which have ruptured (although the abnormal nonaneurysmal components of the AVM can rupture too). About 75% of the aneurysms associated with AVMs are found on arteries feeding the AVM (pedicle aneurysms), while 25% of aneurysms associated with AVMs are found within the core (nidus) of the AVM (intranidal aneurysms). Interestingly, with good treatment of the AVM, pedicle aneurysms can fade away or disappear entirely.
A cavernous malformation, also known as a cavernous hemangioma or cavernoma, is an abnormal vascular entity (or "lesion") made up of many small compartments (lobules) like a bunch of small berries, or a mulberry. These microcomparments contain blood (hemorrhagic) products in different stages of evolution. The "sinusoidal" compartments are enclosed by abnormally thin and quite fragile endothelialized walls. Unlike an arteriovenous malformation, there is no large feeding artery and no large draining vein in a cavernoma. However, there frequently is a venous angioma associated with the cavernoma!
Most cavernomas are found in the larger (supratentorial) parts of the brain hemispheres, but up to 1 in 4 or 5 are found in the hindbrain (posterior fossa; infratentorial), especially in the pons region of the brainstem. Much less commonly a cavernoma may be found in the spinal cord (but this may be more likely to occur in patients with a family history of cavernous malformations).
The risk of hemorrhage from a cavernoma is somewhere between 0.5-1% per year. It may be greater (or at least easier felt symptomatically) in cavernomas located in more "eloquent" brain such as the basal ganglia, thalamus, brainstem, and spinal cord. The rehemorrhage rate varies in the literature, but is probably somewhere between 4-10% per year (some reports indicate an even higher rate in the first year or two following the first diagnosed cavernoma hemorrhage).
The major complications are seizures and progressive neurological impairment, and the type of "deficit" varies according to the size and location of the cavernoma, and the extent and multiplicity of the hemorrhage(s).