Kasabach-Merritt phenomenon

It can be hard to find accurate information about Kasabach-Merritt phenomenon (KMP) because it is very rare and the term is misused in articles and on the web. Here at Children’s Hospital Boston, we’ve treated more kids with KMP than any other place in the world.

Read on to learn more about how our doctors help shrink your child’s vascular tumor and increase your child’s platelet count to reduce the risk of bleeding.

Are the vascular tumors painful?

As the tumor gets bigger and traps more platelets, your child’s lesion can swell and may be very painful. This may also occur with infection, trauma to the lesion or after platelet transfusion.

How common is KMP?

It’s very rare because it is only associated with very rare vascular tumors. Children’s Vascular Anomalies Center has the largest database in the world of children who have experienced Kasabach-Merritt phenomenon — currently more than 125 cases of vascular anomalies with KMP.

When does it appear?

Kasabach-Merritt phenomenon usually appears during the first year of your child’s life. It affects:

• about 75 percent of kids (in our database) who have Kaposiform hemangioendothelioma, either initially or within a year of diagnosis.
• some children who have tufted angioma.
• nearly all kids who have cutaneovisceral angiomatosis with thrombocytopenia (CAT).

Where do the vascular tumors appear?

These lesions can appear anywhere on your child’s body. They’re often found on your child’s head and neck, but can also appear on the stomach, back, arms and legs — even inside the chest or abdomen.

What do the skin lesions of associated vascular tumors look like?

Associated vascular tumors often have the following characteristics:

• They’re usually a deep, reddish-purple.
• They’re firm and warm to the touch.
• They may be shiny and tense.
• There are often tiny purple or red spots and a bruise-like discoloration near the lesion.

Causes

What causes KMP?

Kasabach-Merritt phenomenon is caused by your child’s vascular tumor as it grows and traps your child’s platelets. Vascular tumors that are larger, more aggressive or more deeply invasive are more likely to have KMP.

Symptoms

What are the signs and symptoms of Kasabach-Merritt phenomenon?

The only sign of Kasabach-Merritt phenomenon is a low platelet count. Some signs of a low platelet count include the following:

• easy, unexplained or worse-than-expected bruising
• a rash with pinpoint red dots called petechiae
• increased bleeding

What’s a low platelet count?

A normal platelet count is in the range of 150,000 to 450,000 platelets per microliter of blood.

• Your child’s doctor may become concerned if your child has a platelet count below this range.
• Most patients with KMP have severely low platelet counts, averaging less than 20,000 platelets per microliter
• Severe thrombocytopenia (low platelets) increases the chances or bruising and bleeding after minor injury and may rarely cause spontaneous bleeding.

Are there any other signs or symptoms of Kasabach-Merritt phenomenon?

Your child’s doctor may look for signs that the tumor that causes Kasabach-Merritt phenomenon is growing. These include:

• a lesion on the skin that’s getting larger
• bleeding or bruising around the lesion
• a lesion that swells and becomes painful

Your child may also experience:

• pain with movement
• decreased range of motion around the area of the lesion

If your primary care physician (and often a hematologist) finds that your child has severely low platelet counts without any other explanation, he or she may recommend an imaging study to look for an undiagnosed vascular tumor.

Long-term outlook

What is the long-term outlook for my child?

While Kasabach-Merritt phenomenon can be very serious, we do have ways to treat your child’s tumor and reduce the risk of a potentially life-threatening emergency.

For cases with severe thrombocytopenia, it is important to get prompt medical attention and consider therapy as quickly as possible. This enables your doctor to attempt to shrink the tumor, which will increase your child’s platelet count.

If treatment is delayed, the tumor may trap and destroy most of the platelets in your child’s blood — leading to potentially serious bleeding risk.

The prognosis for your child greatly depends on the following factors:

• the type of vascular tumor
• the size, extent and location of the tumor
• the tumor's initial response to therapy
• new developments in treatment

Can Kasabach-Merritt phenomenon come back after treatment?

There is some evidence that these tumors and the related KMP can recur, though it is currently unclear how common this is. However, if your child’s KMP does recur, we have good success treating it again, reducing tumor size and stabilizing your child’s platelet count.

What do we do after treatment is over?

Your child should continue with regular, thorough clinic visits at least every year. The purpose of these visits is to monitor for recurrence and rare late effects from therapy.A typical follow-up visit may include some or all of the following:

• a physical exam
• blood tests
• imaging or scans

FAQ

Q: Can Kasabach-Merritt phenomenon be caused by a hemangioma?

A: No. Hemangiomas have a different appearance, grow predictably and shrink and fade on their own. They also look different when biopsied.

KMP is not associated with infantile hemangiomas, although the vascular tumors that cause it are sometimes misdiagnosed as hemangiomas. Take a look at the Diagnosis section for more details.

Q: What’s the main thing I should be concerned about?

A: Though bruising and a rash with tiny red dots (petechiae) are cosmetic issues, they do indicate that your child’s platelet count may be dangerously low. If your child has these symptoms, make a doctor’s appointment and request blood work to check your child’s platelet count. Bleeding complications are the main concern.

Q: What’s are bleeding complications?

A: If your child’s platelet levels drop below 20,000 per microliter, your child has a higher risk of bruising or bleeding after mild trauma and could rarely even begin to bleed spontaneously, which is a life-threatening emergency that requires immediate treatment.

Q: What’s a normal platelet count?

A: A normal platelet count is in the range of 150,000–450,000 platelets per microliter of blood.

Q: Is Kasabach-Merritt phenomenon treatable?

A: Yes. We treat Kasabach-Merritt phenomenon by treating the tumor that causes it. Our doctors use different medications to stop the growth of the tumor and improve your child’s platelet count.

Q: What kinds of medications are used?

A: Our doctors will often recommend steroids first and then move on to chemotherapy or interferon therapy if steroids don’t work.

For detailed information on all aspects of treatment, see the Treatment & Care section for individual vascular tumors, such as Kaposiform hemangioendothelioma and tufted angioma.

Q: What makes Children’s different?

A: We have more experience treating children with Kasabach-Merritt phenomenon than any other hospital in the world. We assist doctors across the globe with diagnosis and treatment for vascular anomalies, evaluating about 1,000 cases of vascular anomalies annually.

Q: My child was diagnosed with an infantile hemangioma, but it seems to be growing, not regressing. Should I be concerned about Kasabach-Merritt phenomenon?

A: KMP is not associated with infantile hemangioma. Infantile hemangiomas typically rapidly grow for 4–5 months and stop growing around 12–15 months of age. Vascular tumors that don’t follow this pattern may warrant a second opinion, which may include a complete blood count to test for a low platelet count, especially if bleeding or bruising symptoms are present. If your child’s platelet count is low and he has a vascular tumor or birthmark, you should consult a vascular anomalies center.

Q: My child has a low platelet count and a birthmark that was diagnosed as hemangioma. Should I get a second opinion to see if my child has Kasabach-Merritt phenomenon?

A: Yes. A low platelet count is never caused by common infantile hemangioma. Low platelets combined with a skin lesion should lead to evaluation for Kaposiform hemangioendothelioma or other benign vascular tumors like tufted angioma or cutaneovisceral angiomatosis with thrombocytopenia (CAT).

Q: My child has a low platelet count and our doctor recommended a transfusion. Now the lesion is bigger than ever and more painful. What happened?

A: What happens in a platelet transfusion is that the new platelets will be quickly attracted to the tumor and trapped. Your child’s lesion will quickly grow and get extremely painful. Over time, the lesion will shrink back to its normal size over time, but your child’s platelet count will not improve because of the transfusion.

We only recommend a platelet transfusion if your child is bleeding badly — not simply because the platelet count is low.

With Kasabach-Merritt phenomenon, it’s important to treat the tumor, not the platelet count. Your child’s platelet count will go up as the tumor shrinks (because it’s not trapping as many platelets). We focus on shrinking the tumor, understanding that the platelet count will normalize as your child’s tumor becomes less aggressive. When your child’s platelet count goes up again, it’s a sign that the Kasabach-Merritt phenomenon has passed.

Questions to ask your child’s doctor

After your child is diagnosed with KMP, you may feel overwhelmed with information. It can be easy to lose track of the questions that occur to you.

Lots of parents find it helpful to jot down questions as they arise – that way, when you talk to your child’s doctors, you can be sure that all of your concerns are addressed.

Some of the questions you may want to ask include:

• How will my child’s KMP be treated?
• Will my child need to stay in the hospital?
• What side effects or complications might my child experience due to treatment?
• What kind of follow-up care should my child receive? How often will she need to come in?
• What is the likelihood of cure?
• How much experience does this center and my child’s doctor have with treating KMP?