In this classification for the first time, they clearly made a difference between defects of the main vessels (like congenital aneurysms, abnormal course, and aplasia) and direct communication between the main artery and vein (arteriovenous fistulas), which he called “troncular” (derived from the embryological term used by several embryologists at the beginning of the century) [11, 12] and “angioma” indicating areas of fistulous tissue or areas of dysplastic veins infiltrating tissues.

He considered that the difference between both groups was due to a different embryological phase in which the vessel development is affected by a pathological process: in the early phase, remnants of the primitive vascular network remain in tissues, while in the late phase anomalies of the main vessels may develop.

Stefan Belov, a Bulgarian pioneer in the treatment of CVM, proposed a classification which was influenced by the publications of Malan and was also based on morphology which he considered crucial to understand CVM. He took over the distinction of Malan between defects of the main vessels and peripheral malformations. However, in a much more clearer and organic classification, he distinguished the defects of the main vessels that referred separately to arteries, veins, and lymphatics, including direct arteriovenous fistulas, in the “troncular” group. Peripheral defects were divided into limited and infiltrating forms of the single types (venous, arteriovenous, or lymphatic) and called “extratruncular,” to differentiate them from the first group [13–15] (Table 22.2). As Malan, he accepted the concept that the truncular and extratruncular forms are the result of a defect in the embryological development of vessels. The new data about genetics may confirm that concepts (see chap. 2). The distinction between the truncular and extratruncular forms, not clearly pointed out by other classifications, proved effective also in the practical approach. This classification was called “Hamburg Classification” because she was discussed and approved during a workshop held in Hamburg (Germany) in 1988 by the working group of vascular anomalies which later became ISSVA (International Society for the Study of Vascular Anomalies (see Chap. 3), as a guidance to diagnosis.

The original Hamburg classification was later modified by adding the capillary malformations, which was missing in the original form and was one of the critics moved to it [16] (Table 22.3).

The difference between hemangiomas and vascular malformations remains unclear until the studies of Judah Volkmann in Boston who demonstrated that hemangiomas are lesions that have a hyperplasia of the endothelium, while malformations have a normal endothelial turnover.

The publication of Mulliken and Glowacki in 1982, which reported the studies of Volkmann, definitively distinguished hemangiomas from vascular malformations as two different entities, based on cellular kinetics, physical examination, and natural history. They proposed a classification which they called “biological classification,” underlining the specific difference between hemangiomas and CVM [17–19].

Their classification of CVM was based on hemodynamics, dividing CVM in high-flow and low-flow lesions, adding syndromes of complex cases in a group of combined complex defects. This classification proved to be an excellent guide to understand the hemodynamics of CVM and is today widely accepted (Table 22.4).

In 1994, during the international workshop in Budapest, ISSVA nominated a commission of two experts, Dirk Loose from Germany and Odile Enjolras from France, in order to propose a common classification. The result of their work was presented during the workshop in Rome, in 1996, and was accepted officially by ISSVA (Table 22.5). Several updates and changes of this classification were published independently by several authors in the latter years. The classification is accepted and extensively used internationally [20–22].