And here it was agreed upon by a consortium of experts in phelobology that CCSVI represented a newly discovered form of congenital malformations: www.fondazionehilarescere.org/pdf/consensus-ANGY.pdf
This is a research paper, not a website. They came to the conclusion through their expertise in phlebology, which would include both formal study and years of experience working in blood vessels.
CCSVI is not a symptom. A symptom of what? CCSVI is a disease of its own.
Here is a quote on how they classify venous malformations, which I think may be along the lines of what you're asking (how is it that CCSVI is classified as a venous malformation, thus saying that malformation is the cause of CCSVI):
Classification of venous malformations
Previous classification systems were established
based purely on clinical findings. This was before
the modern technology was available for accurate
diagnosis. These classification systems failed to
provide proper information concerning the etiology/
embryology, anatomy, and pathophysiology
involved in this vascular abnormality.
Numerous classifications of VMs have been proposed,
many based on the appearance of the anomaly,
its anatomy, pathology, or based on the velocity
of blood flow in the lesion. Many VMs are still
named after the clinician who first described the
lesion. Lack of an accepted, universal classification
system resulted in redundant terminology.
For example, terminology such as “cavernous
hemangioma”, “cavernous angioma”, “phlebangioma”,
“lymphangioma”, “Port wine stain”, etc.
only added to the confusion.
Therefore, a new classification system has been
accepted that fulfils the above criteria and provides
information regarding lesion etiology, embryology,
anatomy and pathophysiology of VMs.