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Health condition · Clinically reviewed

Klippel-Trenaunay syndrome, the triad, the PIK3CA cause and modern care.

A rare congenital vascular malformation - port-wine stain, abnormal veins and limb overgrowth - now treatable with compression, targeted intervention and, for selected patients, a PIK3CA inhibitor.

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A radiographer guides a patient onto the bed of an advanced 3 Tesla MRI scanner in a London imaging suite

Why trust this guide

  • 01

    Clinically reviewed

    Written by our editorial team and reviewed by a UK vascular anomaly specialist before publication.

  • 02

    Sourced from guidance

    Checked against NHS England specialist commissioning, ISSVA classification and peer-reviewed sources you can see at the end.

  • 03

    Current for 2026

    Reflects modern UK practice including PIK3CA-targeted therapy with alpelisib (Vijoice) approved for PROS in 2022.

Key facts

KTS at a glance.

The essentials in plain English - what it is, what causes it, and where it is looked after in the UK.

  • What it is

    A rare congenital vascular malformation with a classic triad - capillary malformation (port-wine stain), venous and lymphatic malformation, and limb overgrowth.

  • The cause

    A somatic (mosaic) PIK3CA mutation - part of the PIK3CA-related overgrowth spectrum (PROS). Not inherited from a parent.

  • The lateral vein

    A persistent embryonic vein (the lateral marginal vein of Servelle) runs down the outer thigh and calf in most affected limbs.

  • Not Parkes Weber

    Parkes Weber syndrome is a related but distinct high-flow disorder with arteriovenous malformations - imaging tells them apart.

  • Where you are seen

    Specialist commissioned vascular anomaly centres in the UK - Great Ormond Street, Birmingham Children's and adult tertiary services.

  • A new medical option

    Alpelisib (Vijoice/Piqray), a PIK3CA inhibitor, is approved for PROS and has become practice-changing for selected patients.

Why this guide matters

A lifelong condition with real, modern options.

KTS is rare, but the framework for looking after it is now well-established - and for the first time we have a drug that targets the underlying gene.

  • Compression is not optional

    Daily, well-fitted compression is the single most useful thing anyone with KTS can do. It reduces pain, swelling and complications.

  • Imaging changes the plan

    MRI, MR angiography and selective venography tell surgeons and interventional radiologists what is safe and what is not.

  • Alpelisib has changed the horizon

    A PIK3CA inhibitor, approved in 2022 for PROS, is the first therapy aimed at the cause - not just the consequences.

How the diagnosis is made

From the first port-wine stain to a full specialist plan.

The pathway a UK vascular anomaly team will normally follow, in order - so you know what to expect and why each step matters.

  1. 01

    Assessing

    History and clinical triad

    Port-wine stain from birth, swelling and varicosities in the affected limb, and progressive limb overgrowth as the child grows.

  2. 02

    Assessing

    Vascular and limb examination

    Mapping the capillary stain, palpating for venous ectasia and the marginal vein, and measuring limb length and circumference.

  3. 03

    Assessing

    Doppler ultrasound

    A first-line, low-burden scan to characterise venous flow, valve competence and any deep vein anomalies.

  4. 04

    Confirming

    MRI and MR angiography

    Specialist commissioned imaging maps the venous, lymphatic and soft-tissue components and excludes high-flow disease.

  5. 05

    Confirming

    MRI pelvis for internal disease

    Pelvic venous and lymphatic malformations are common in lower-limb KTS and change surgical planning.

  6. 06

    Confirming

    Selective venography

    A specialist-led catheter study when surgery, sclerotherapy or marginal vein removal is being planned.

  7. 07

    Preparing

    Genetic testing for PIK3CA

    A tissue biopsy is usually needed - the mutation is mosaic, so a standard blood test is often negative. Specialist commissioned genetics service.

Typical pathway: from first referral to a full MDT plan over weeks to a few months.

Symptoms

What KTS actually looks like.

The classic triad plus the day-to-day features that matter - the marginal vein, the swelling and the pain. And the features that mean it is time to escalate.

  • Port-wine stain

    A flat, pink to deep red capillary malformation, present from birth - see our guide to birthmarks.

  • Varicose veins and venous ectasia

    Prominent, tortuous surface veins - often atypical in distribution, appearing on the outer thigh and calf.

  • The lateral marginal vein

    The vein of Servelle - a persistent embryonic vein running down the outside of the leg, a hallmark of KTS.

  • Lymphatic malformation and lymphoedema

    Soft-tissue swelling, blebs and lymph leakage - progressive and a major driver of day-to-day symptoms.

  • Limb overgrowth (hemihypertrophy)

    One limb (usually a leg) is longer and thicker - progressive with growth, worst in childhood and adolescence.

  • Pain and heaviness

    Aching, throbbing, cellulitis flares and skin bleeding from friable vascular blebs.

  • Skeletal knock-on effects

    Leg length discrepancy, pelvic tilt and secondary scoliosis - see our guide to kyphosis and scoliosis.

  • Red flag - swollen, painful limb

    A sudden, tense, painful leg raises deep vein thrombosis - a real risk in KTS and a same-day concern.

Treatment

How KTS is treated in specialist UK care.

Daily compression, targeted intervention on abnormal veins and lymphatics, orthopaedic care for the growing limb, and - for the right patients - a PIK3CA inhibitor.

  • Compression garments

    Custom, medical-grade compression is the backbone of care - specialist lymphoedema-led fitting and review. See our lymphoedema compression clinic.

  • Sclerotherapy and endovenous laser

    Specialist commissioned interventional radiology closes symptomatic venous malformations and superficial varicosities.

  • Surgical vein removal

    Selective removal of the lateral marginal vein and stripping of symptomatic varicosities - only after imaging confirms a competent deep system.

  • Laser for port-wine stain

    Pulsed dye laser (see our vascular laser page) lightens the capillary stain over multiple sessions with specialist dermatology.

  • VTE prophylaxis

    Anticoagulation around surgery, prolonged travel and pregnancy - the risk of deep vein thrombosis and pulmonary embolism is genuinely raised.

  • Orthopaedic care

    Shoe raises for small differences; epiphysiodesis in growing children when the discrepancy will exceed safe limits. Specialist paediatric orthopaedic input.

  • Alpelisib (Vijoice/Piqray)

    The first PIK3CA-targeted therapy - approved in 2022 for PROS. A practice-changing option in a specialist commissioned clinic (see our alpelisib PROS clinic).

  • MDT vascular anomaly care

    Great Ormond Street, Birmingham Children's and adult tertiary services coordinate imaging, intervention, orthopaedics, genetics and lymphoedema in one team.

What this guide is based on

The sources behind every claim on this page.

UK specialist commissioning, international classification and regulatory documents, current at the time of last review.

Key references

Guidelines and standards we relied on.

A quiet reminder

This guide is for information, not medical advice.

Your vascular anomaly team knows your imaging and history and can tell you which parts apply to you. If in doubt, get seen.

  • International Society for the Study of Vascular Anomalies (ISSVA). Classification of vascular anomalies.

  • NHS England. Specialist commissioning: complex vascular anomalies service specification.

  • MHRA and EMA. Alpelisib (Vijoice) approval documents for PIK3CA-related overgrowth spectrum.

  • K-T Support Group UK. Patient information and family resources.

Red flags

When KTS needs urgent attention.

Most day-to-day care is planned and steady. These are the situations that are not - and where a specialist opinion or emergency assessment is needed.

  • Sudden painful swollen limb

    Deep vein thrombosis is genuinely common in KTS - a hot, tense, painful leg needs same-day assessment. See our DVT guide.

  • Chest pain or breathlessness

    Pulmonary embolism is a real risk, especially after surgery, immobility or pregnancy - treat as a medical emergency.

  • Bleeding from vascular blebs

    Persistent or heavy bleeding from skin blebs needs urgent pressure, dressings and specialist review.

  • Spreading redness and fever

    Cellulitis on a lymphoedematous limb can escalate quickly - low threshold for antibiotics and admission.

  • Rapidly changing limb length

    A widening discrepancy needs paediatric orthopaedic assessment before growth plates close.

  • Pelvic pain, bleeding or urinary symptoms

    Internal pelvic vascular malformations can bleed and obstruct - MRI pelvis and specialist review are needed.

  • Pregnancy planning

    A dedicated pre-pregnancy conversation on VTE prophylaxis, delivery planning and anaesthetic access.

  • New neurological symptoms

    Progressive back pain, leg weakness or bladder change - consider cord-related issues and scoliosis complications.

  • Mental-health impact

    The visible and functional burden is real - psychological support is part of good specialist care.

Living with it

A lifelong condition, with a clear plan.

Four things make the biggest difference day to day - compression, movement, skin care and a good team around you.

A quiet reminder

Rare does not mean unsupported.

The K-T Support Group UK and specialist commissioned vascular anomaly services mean nobody has to work this out alone.

  1. 01 Compression

    Wear it, every day

    Consistent, well-fitted compression is the single most useful daily habit - it protects the limb and reduces pain.

  2. 02 Movement

    Move, but plan long journeys

    Walking helps; long flights and drives raise clot risk. Talk to your team about VTE prophylaxis in advance.

  3. 03 Skin

    Protect the skin

    Moisturise, treat any breaks early and have a low threshold for calling about cellulitis - it moves fast.

  4. 04 Team

    Stay in the network

    Regular contact with a vascular anomaly MDT and the K-T Support Group UK makes a lifelong condition much more manageable.

Frequently asked

Everything we get asked about Klippel-Trenaunay syndrome.

Quick answers on the triad, imaging, the marginal vein, genetics, alpelisib and clot risk.

  • What is Klippel-Trenaunay syndrome?

    Klippel-Trenaunay syndrome (KTS) is a rare congenital vascular malformation defined by a classic triad: a capillary malformation (port-wine stain), a venous and lymphatic malformation - usually with a persistent embryonic lateral marginal vein of Servelle - and progressive overgrowth of the affected limb. It is caused by a somatic PIK3CA mutation and sits within the PIK3CA-related overgrowth spectrum (PROS).

  • How is KTS different from Parkes Weber syndrome?

    Parkes Weber syndrome shares the capillary stain and limb overgrowth but adds arteriovenous malformations - a high-flow component that KTS does not have. Parkes Weber tends to be more severe and can affect the heart. MRI and MR angiography reliably distinguish the two, which matters because treatment differs.

  • Why is a blood genetic test often negative?

    The PIK3CA mutation in KTS is somatic (mosaic) - it is only present in a subset of cells, mostly in the affected tissue. A standard blood test usually cannot detect it. Genetic confirmation therefore needs a biopsy of the affected tissue analysed by a specialist commissioned genetics service.

  • What is the lateral marginal vein of Servelle?

    A persistent embryonic vein that runs down the outer aspect of the thigh and calf in most people with KTS. It is a hallmark of the condition and often symptomatic. Selective removal is one of the standard surgical options, but only after imaging confirms the deep venous system is competent.

  • Is alpelisib really a game-changer?

    Approved in 2022 for the PIK3CA-related overgrowth spectrum, alpelisib (Vijoice/Piqray) is the first therapy that targets the root cause. In selected patients it has produced meaningful reduction in limb volume, pain and skin symptoms. It is prescribed in specialist commissioned PROS clinics with careful monitoring.

  • What is the risk of blood clots in KTS?

    Genuinely raised - venous stasis, abnormal veins and the mutation itself all contribute. Any KTS patient facing surgery, immobility, a long flight or pregnancy should have a specialist commissioned conversation about VTE prophylaxis. Sudden limb swelling, chest pain or breathlessness are emergencies.

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