Thrombocytopenia-Absent Radius Syndrome: Causes, Symptoms, and Treatment
Medically reviewed by Achuta Uppu, MD, Internal Medicine. Last reviewed: October 23, 2026.
What kind of doctor treats Thrombocytopenia-Absent Radius Syndrome?
A pediatrician, a doctor for children, often leads care, along with a hematologist, a doctor who treats blood problems. An orthopedist, a bone doctor, and a cardiologist, a heart doctor, may also help. A direct primary care or concierge doctor can be a first stop for an evaluation and can refer you to a specialist when needed.
What is thrombocytopenia-absent radius syndrome?
Thrombocytopenia-absent radius syndrome, often called TAR syndrome, is a rare condition that is present at birth. Babies with it have too few platelets, the blood cells that help blood clot. Having too few platelets is called thrombocytopenia. They are also almost always born without the radius, the bone on the thumb side of the forearm, in both arms.
People with TAR syndrome do have thumbs. Other conditions with a missing radius typically do not. TAR syndrome affects fewer than 1 in 100,000 newborns.
Symptoms
Symptoms differ from person to person. Common signs include:
- A radius bone that is almost always missing in both forearms.
- A forearm bone called the ulna that is underdeveloped or missing in one or both arms.
- Hand differences, such as webbed or fused fingers or curved pinky fingers.
- Easy bruising and frequent nosebleeds from the low platelet count.
- Heart or kidney malformations in some people.
- A small lower jaw, a prominent forehead, and low-set ears in some people.
- Allergic reactions to cow's milk in about half of people, which may worsen the low platelet count.
The low platelet count usually shows up in infancy and becomes less severe over time. Severe bleeding can happen in the brain and other organs, especially in the first year of life. Adult women may have heavy or long periods.
Causes and who gets it
Changes in the RBM8A gene cause TAR syndrome. A gene is a set of instructions in your cells. Most people with the condition have a change in one copy of this gene. They are also missing a piece of chromosome 1 that holds the other copy.
TAR syndrome is inherited in an autosomal recessive pattern. This means both copies of the gene in each cell are altered. Parents of an affected child usually do not show signs of the condition. In about 75 percent of cases, the child inherits the missing piece of chromosome 1 from a parent.
How doctors diagnose it
Doctors often diagnose TAR syndrome at birth. They examine the baby and look for typical physical signs. Blood tests can confirm a low platelet count or anemia, which means too few red blood cells. An X-ray of the forearm and an ultrasound of the kidneys may also be done.
Genetic testing can confirm the diagnosis. The first step looks for a missing piece in the region of chromosome 1 that holds the RBM8A gene. If no missing piece is found, doctors can read the gene itself. A heart evaluation may also be recommended to look for heart problems.
Treatment
Treatment is aimed at the specific symptoms each person has. Care is supportive and often needs a team. The team may include pediatricians, surgeons, orthopedists, hematologists, and cardiologists.
- Doctors may suggest ways to help children avoid infection, stress, and other triggers of low platelets.
- Experts say cow's milk should be avoided, since it can trigger low platelet episodes.
- Ongoing monitoring and platelet transfusions or other blood product transfusions may be used when needed.
- Splints, braces, surgery, artificial devices called prosthetics, and mobility aids may help with bone and arm problems.
- Medicines or surgery may be needed for heart defects.
- Early help, such as special education and physical therapy, may help children reach their potential.
Genetic counseling is recommended for affected people and their families.
When to see a doctor
Call 911 or go to the emergency room if bleeding will not stop or someone vomits blood. In a child, also get emergency help for a seizure, a severe headache or vomiting after a head injury, or trouble waking the child. Bleeding in the brain can be life-threatening.
See a doctor soon for frequent nosebleeds, easy bruising, bloody stools, or tiny red or purple spots on the skin. Infections and stress can set off drops in platelets, so tell your child's doctor when your child is sick. Women with heavy or long periods should also see a doctor.
How a direct primary care or concierge doctor fits in
A direct primary care doctor charges a flat monthly fee instead of billing insurance, while a concierge doctor usually charges more and often works alongside insurance. Either kind of doctor can help coordinate the many specialists involved, keep track of test results, and answer questions between specialist visits.
Common questions
Is TAR syndrome inherited?
Yes, TAR syndrome is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell are altered. Parents of an affected child usually do not show signs of the condition.
Does the low platelet count get better with age?
Yes, the low platelet count usually becomes less severe over time. In some people, platelet levels become normal. The risk of serious bleeding is highest during the first year of life.
Do people with TAR syndrome have thumbs?
Yes, people with TAR syndrome have thumbs, even though the radius bone is missing. This helps set it apart from other conditions that involve a missing radius.
Do people with TAR syndrome have normal intelligence?
Yes, most people with TAR syndrome have normal intelligence. Intellectual disability has been reported in some people who had bleeding in the brain.
Sources
- MedlinePlus Genetics - Thrombocytopenia-absent radius syndrome
- National Organization for Rare Disorders (NORD) - Thrombocytopenia Absent Radius Syndrome
Medical Disclaimer
This content is for general educational and informational purposes only and is not medical advice. It is not a substitute for professional medical advice, diagnosis, or treatment, and reading it does not create a doctor-patient relationship. Always seek the advice of your physician or another qualified health provider with any questions about a medical condition, and never disregard or delay seeking professional advice because of something you read here. If you think you may have a medical emergency, call 911 (or your local emergency number) immediately.
