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A Patient and Parent Oriented Compendium

Draft prepared for archival and personal review. All citations subject to verification against primary sources. Always seek medical counsel.

Based on the ISSVA Classification of Vascular Anomalies © 2025 International Society for the Study of Vascular Anomalies. Available at issva.org/classification. Patient-oriented reordering and plain-language narrative by William Anton, patient advocate, kare4rare.wordpress.com. Submitting to ISSVA for review and approval.

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A Note Before You Read

The International Society for the Study of Vascular Anomalies — known as ISSVA — is a world leading medical organization dedicated to the study, classification, and treatment of vascular anomalies. Founded in 1992 and comprised of physicians, researchers, and scientists from across the globe, ISSVA maintains the classification system that doctors worldwide use to diagnose and name conditions involving abnormal blood vessels, lymphatic vessels, and related structures.

Their classification is authoritative, rigorous, and continuously updated as medical science evolves. The 2025 version reflects decades of accumulated clinical and genetic knowledge. It is the most important single reference document in this field.

It is also written by specialists, for specialists. Its organization reflects the diagnostic logic of a physician working through a differential, not the lived experience of a parent holding a newborn with a birthmark, or a patient newly diagnosed after years of misdiagnosis.

This companion guide, adapted by a patient and advocate for the Vascular Malformation community specifically, presents the same classification but in a specific and intentional order — one that likely begins where most patients and families begin: with the conditions they are most likely to be living with. The content is unchanged. The attribution is unchanged. Only the sequence has been adjusted to serve a patient-first readership. This reordering has been submitted to ISSVA for review and approval, and is presented here in that spirit.

Everything in this guide is grounded in the ISSVA classification. Nothing has been invented, simplified to the point of inaccuracy, or presented without attribution. Please bring this to your doctor for professional confirmation and counsel.

Why the Order of This Guide Differs from the Official Classification

The ISSVA classification opens with Vascular Tumors — conditions involving abnormal growth of blood vessel cells, the most well-known of which is the infantile hemangioma. It then addresses Vascular Malformations, followed by Provisionally Unclassified Vascular Anomalies.

That order makes sense for a clinician. A physician encountering an unknown lesion needs to rule out tumors — some of which can be malignant — before classifying a malformation. The diagnostic stakes of missing a tumor are high, so tumors come first in the clinical framework.

For a patient or family, the order of lived experience is almost always reversed. The overwhelming majority of people who find this site are living with a vascular malformation — a birthmark, an enlarged limb, an abnormal vein — that was present from birth and has been present ever since. They are not living with a tumor. They are living with a malformation, most often a slow-flow one.

This guide therefore begins where most readers live: with Vascular Malformations, starting with the slow-flow conditions that represent the vast majority of the patient community. It proceeds to Provisionally Unclassified Anomalies — conditions that have been identified but not yet fully classified — and closes with Vascular Tumors, which, while medically important, describe a different population than most readers of this site.

The order is a patient courtesy, not a clinical revision. The ISSVA classification itself is unchanged and fully respected throughout.

▶  Branch: 1 — Vascular Malformations

The most common presentation for patients and families who find this site.

Vascular malformations are structural errors in how blood vessels or lymphatic vessels form during development. They are present at birth — even when they are not immediately visible — they do not regress on their own, and they tend to grow proportionally as the child grows. They are not tumors. They do not involve abnormal cell proliferation. They are architectural: the vessels are built incorrectly.

The ISSVA classification divides vascular malformations into three major categories based on what the vessels are actually doing — their flow physiology.

1A. Slow-Flow Malformations

Slow-flow malformations involve capillaries, veins, and lymphatic vessels — the lower-pressure systems of the vascular network. Blood or lymphatic fluid moves slowly through these abnormal vessels. They are the most common vascular malformations and the ones most likely to be present in KTS, SWS, and isolated port wine stain presentations.

Capillary Malformations (CM)

The most visible and most common slow-flow malformation. Dilated capillaries in the dermis produce the reddish-purple skin discoloration known as a port wine stain or port wine birthmark. Present at birth. Does not fade. Tends to darken and may thicken over time without treatment.

• Isolated CM: Capillary malformation without deeper involvement. PDL laser has its strongest evidence here.
• CM in Sturge-Weber Syndrome: Facial CM associated with leptomeningeal angiomatosis (brain) and ocular involvement. GNAQ somatic mutation. Requires neurological and ophthalmologic monitoring.
• CM in Klippel-Trenaunay Syndrome: Often bilateral, limb and trunk. The most consistent and visible feature of KTS. PIK3CA-associated in many cases.
• CM-AVM (Capillary Malformation-Arteriovenous Malformation): A specific combined type. Associated with RASA1 and EPHB4 mutations. Classified in both slow-flow and fast-flow categories.
• Cutis Marmorata Telangiectatica Congenita (CMTC) — a reticulate CM variant present at birth.
• Telangiectasias — small dilated superficial vessels. Includes common presentations such as spider angiomas.

Venous Malformations (VM)

Abnormally formed veins — dilated, thin-walled, and often prone to clotting. Venous malformations can occur anywhere in the body and range from small, discrete lesions to extensive networks involving deep tissue. They are compressible, typically enlarge with dependency, and may cause pain, swelling, and functional limitation.

• Common VM — sporadic, localized venous malformation. The most frequent type.
• Familial cutaneomucosal VM — inherited form, associated with TIE2/TEK mutations.
• VM in KTS: Venous malformations are a core component of KTS, varying in severity from discrete varicosities to extensive anomalous veins, including the persistent lateral embryonic vein (vein of Servelle).
• Glomuvenous malformations (GVM) — associated with glomulin gene mutations. Painful, nodular appearance.
• Blue Rubber Bleb Nevus Syndrome — multiple cutaneous and gastrointestinal venous malformations.

Lymphatic Malformations (LM)

Abnormally formed lymphatic channels or cysts. Lymphatic malformations can cause significant swelling (lymphedema), recurrent infection, and in some cases disfigurement. They are present at birth, though not always immediately apparent. May enlarge following infection or trauma.

• Macrocystic LM — large fluid-filled cysts. Often detected prenatally by ultrasound.
• Microcystic LM — small cysts, often in skin or mucosa. More difficult to treat than macrocystic.
• Mixed macrocystic/microcystic LM — combination of both.
• LM in KTS: Lymphatic involvement in KTS varies widely — from absent to severe. Lymphedema, vesicles, and lymphatic channels/lakes may be present and can contribute significantly to limb overgrowth.
• Complex Lymphatic Anomaly (CLA) — a category covering several severe multisystem lymphatic conditions, including Gorham-Stout disease, generalized lymphatic anomaly, and central conducting lymphatic anomaly.

1B. Fast-Flow Malformations

Fast-flow malformations involve arteries or arteriovenous connections — the high-pressure systems of the vascular network. Arterial blood bypasses the normal capillary bed and enters veins directly, exposing those veins to pressures they were not designed to handle. Fast-flow malformations are less common than slow-flow but carry greater risks of bleeding, cardiac strain, and ischemia.

The critical clinical point: PDL laser therapy is contraindicated for fast-flow malformations. Signs of fast-flow — warmth, thrill (vibration over the skin), bruit (sound heard with a stethoscope) — require imaging before any treatment is offered.

Arteriovenous Malformations (AVM)

A tangled network of abnormal connections between arteries and veins — called a nidus — through which blood bypasses the normal capillary bed. AVMs can occur anywhere in the body. The nidus is the defining anatomical feature distinguishing AVM from AVF.

• Parkes Weber Syndrome — AVM type: High-flow AVM with limb overgrowth and capillary malformation. RASA1 mutation. Distinct from KTS.
• Sporadic AVM — non-syndromic arteriovenous malformation.
• Hereditary Hemorrhagic Telangiectasia (HHT) — multiple AVMs, often in lungs, liver, brain. Autosomal dominant.
• CM-AVM — combined capillary malformation with associated AVM. RASA1/EPHB4 mutations.

Arteriovenous Fistulas (AVF)

A direct connection between an artery and a vein without an intervening nidus. One or more discrete channels bypass the capillary bed. The anatomical distinction from AVM matters for imaging characterization and treatment planning.

• Parkes Weber Syndrome — AVF type: High-flow direct fistula with limb overgrowth and capillary malformation. Arterialized veins distal to the fistula. Risks include heart strain, ulceration, and bleeding.
• Sporadic AVF — non-syndromic arteriovenous fistula.

1C. Developmental Anomalies of Named Vessels

A category covering structural abnormalities of specific, named arteries and veins — vessels large enough to have anatomical names — that occur during fetal development. These include absent, duplicated, or anomalously positioned major vessels. They are distinct from malformations of the microvasculature (capillaries, small veins, lymphatics).

• Anomalies of named arteries — including aplasia, stenosis, or anomalous course of major arteries.
• Anomalies of named veins — including absence or anomalous position of major veins. The persistent lateral embryonic vein (vein of Servelle), sometimes seen in KTS is an example of this category.
• Anomalies of named lymphatic vessels — including anomalous central lymphatic channels.

▶  Branch: 2 — Provisionally Unclassified Vascular Anomalies (PUVA)

Conditions identified but not yet fully understood or classified.

PUVA is the ISSVA’s honest acknowledgment that medical knowledge is incomplete. These are conditions that have been described and documented in the literature — patients are living with them — but whose precise classification within the broader framework has not yet been established with sufficient evidence.

This category matters for patients because it names the reality that some diagnoses exist in a space of genuine medical uncertainty. Being in a PUVA category does not mean a condition is not real. It means the science has not yet fully caught up.

• Kaposiform lymphangiomatosis (KLA)
• FAVA — Fibro-Adipose Vascular Anomaly
• Multifocal lymphangioendotheliomatosis with thrombocytopenia
• Cutaneoviseral angiomatosis with thrombocytopenia
• Angiokeratoma
• Papillary intralymphatic angioendothelioma (PILA) / Dabska tumor

The PUVA list evolves with each classification update as conditions move from provisional to fully classified based on accumulating evidence. Checking the current ISSVA classification directly is recommended for the most current status of any specific condition.

▶  Branch: 3 — Vascular Tumors

Conditions involving abnormal proliferation of blood vessel cells.

Noteworthy — Diagnosed medical conditions under this classification are NOT considered Vascular Malformations, per the ISSVA. Proper Care and Treatment depends on optimal understandings of classification and diagnosis. Prescriptive care and remedies follow, accordingly.

Vascular tumors are fundamentally different from vascular malformations. Where malformations are architectural errors — vessels built wrong — tumors involve abnormal cell proliferation: cells growing in ways they should not. This distinction drives everything about how they are diagnosed, monitored, and treated.

The most important practical point for the patient community: infantile hemangiomas — the most common vascular tumor — involute spontaneously over time. This is the defining biological behavior that distinguishes them from malformations, which do not involute. A hemangioma that fades and shrinks in early childhood is behaving exactly as expected. A port wine stain that persists and deepens is also behaving exactly as expected — for a different biological reason.

Benign Vascular Tumors

• Infantile hemangioma (IH): The most common vascular tumor of infancy. Appears in the first weeks of life, grows rapidly, then involutes. Most resolve substantially by age 5–7. Medical treatment (propranolol, timolol) is available when intervention is indicated.
• Congenital hemangiomas — present and fully formed at birth. Unlike infantile hemangiomas, these do not grow postnatally. RICH (rapidly involuting) and NICH (non-involuting) subtypes.
• Tufted angioma
• Spindle cell hemangioma
• Epithelioid hemangioma
• Pyogenic granuloma (lobular capillary hemangioma)

Locally Aggressive or Borderline Vascular Tumors

Tumors that may recur locally or have uncertain behavior but do not typically metastasize.

• Kaposiform hemangioendothelioma (KHE) — associated with Kasabach-Merritt phenomenon (severe platelet trapping and coagulopathy). Requires specialist management.
• Retiform hemangioendothelioma
• Papillary intralymphatic angioendothelioma (PILA)
• Composite hemangioendothelioma

Malignant Vascular Tumors

Rare malignant tumors of vascular origin. These are distinct from the conditions most readers of this site are living with and are included here for completeness of the classification.

• Angiosarcoma
• Epithelioid hemangioendothelioma
• Kaposi sarcoma

How to Use This Guide

This guide is a companion to the ISSVA classification, not a substitute for it. The official classification — available in full at issva.org/classification — is the authoritative reference for clinicians and for patients seeking the most complete and current information.

The patient guides in this series — Port Wine Stain: Three Distinct Biological Causes, and Congenital Vascular Malformation Spectrum: Three Confounded Syndromes — address the specific conditions most common to this site’s readership in greater depth. They are the recommended starting point for parents and patients new to this landscape.

The ISSVA Glossary, linked from the home page, defines every technical term used in the classification in plain language. It is an invaluable companion document and is freely available from ISSVA.

A diagnosis is not a destination. It is the beginning of a conversation with a specialist who understands what the classification means for your specific presentation — your biology, your imaging, your genetics, your life. This guide exists in the hopes it helps you arrive at that conversation better prepared.

Attribution & Sources

Sources identified for corroboration during drafting. Direct verification against primary sources is recommended before final publication.

[1] CLS-001  ISSVA Classification of Vascular Anomalies © 2025 International Society for the Study of Vascular Anomalies.  https://www.issva.org/classification

[2] CLS-002  ISSVA Glossary of Vascular Anomalies. Updated March 2025.  https://www.issva.org/UserFiles/file/ISSVAGLOSSARY_UpdatedFullVersion_03152025_Submission_copyright.pdf

[3] CLS-003  Prendiville J, et al. Updated Classification of Vascular Anomalies: A living document from the ISSVA Classification Group. Journal of Vascular Anomalies. 2025;6(2):e113.  https://journals.lww.com/jova/fulltext/2025/06000/updated_classification_of_vascular_anomalies__a.3.aspx

[4] CLS-004  ISSVA Classification of Vascular Anomalies — Historical 2018 Version.  https://www.issva.org/UserFiles/file/ISSVA-Classification-2018.pdf

[5] KTS-001  NIH MedlinePlus Genetics. Klippel-Trenaunay Syndrome.  https://medlineplus.gov/genetics/condition/klippel-trenaunay-syndrome/

[6] PWS-001  NIH MedlinePlus Genetics. Parkes Weber Syndrome.  https://medlineplus.gov/genetics/condition/parkes-weber-syndrome/

[7] SWS-001  NIH/NCBI StatPearls. Sturge-Weber Syndrome.  https://www.ncbi.nlm.nih.gov/books/NBK459163/

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ISSVA Classification of Vascular Anomalies © 2025 International Society for the Study of Vascular Anomalies. Available at issva.org/classification.

Patient-oriented reordering and plain-language narrative © William Anton, kare4rare.wordpress.com. Submitted to ISSVA for review.

Draft prepared for archival and personal review. All citations subject to verification against primary sources.