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Thrombocytopenia and Absent Radius is certainly a big mouthful but in fact it is simply a descriptive term.

Thrombocytopenia means low platelets. The thrombocytes are the platelets and "penia" means "not very much of."

There are two bones in the forearm: The radius and the ulna. The radius is the bone that is on the side of the forearm where the thumb is and the ulna is on the side of the arm where the little finger is. In Thrombocytopenia and Absent Radius there are low platelets and absence of the radius on both sides (bilaterally). There may be other bones involved as well but the unique feature of Thrombocytopenia and Absent Radius is that although the radius is absent, the thumb is present. Most other limb anomalies that involve the radius also affect the thumb.

There are a number of other disorders that involve elements of the blood and abnormal structure of bones of the arm and it is important to realize that TAR is a very specific condition. The features of each particular syndrome have been fairly well defined although there are always new things that are learned (perhaps even your family can contribute to new knowledge).

TAR is considered a congenital abnormality since the bone structure is abnormal at birth (congenital means present at birth). But TAR also has an ongoing problem since during childhood most affected individuals continue to have low platelets. In fact, with viral illnesses and other kinds of stress, the platelets may become very low and even require platelet transfusions.

Hematologic Abnormalities

Hematologic refers to the blood and as you probably know, the blood has many different types of cells. The white cells are involved in fighting infections, the red cells are involved in carrying oxygen from the lungs to the rest of the body, and the platelets are involved in the clotting of blood when there is an injury.

The platelets come from a "grandmother" cell that is called the megakaryocyte. In TAR, the megakaryocytes are less active than normal and therefore produce fewer platelets than normal. The rates of growth of various blood cells are controlled by a whole variety of growth factors. It is anticipated that a specific growth factor is missing in TAR that affects both the growth and stimulation of the megakaryocytes but is also missing in very early development of the body so the radius didn't grow normally. It is not known exactly why the maturation of the megakaryocyte is blocked but a great deal of research is presently trying to sort that out. One hundred per cent of individuals with TAR have low platelets but oftentimes as they grow older, it becomes less of a problem and affected individuals can outgrow the risk of bleeding related to low platelets. About ninety per cent of individuals with TAR are symptomatic during the first year of life with easy bruising, bleeding from the GI tract or even bleeding into the brain. The low platelets come in episodes - going up and down over time. A normal platelet count is greater than two hundred thousand platelets per millilitres squared of blood. Individuals with TAR may have less than ten thousand platelets per millilitres squared when they are having severe episodes.

In addition to problems with platelets, some individuals with TAR may make too many white cells at times. This is not leukemia in the sense of being a malignancy but rather is called a leukamoid reaction, that is, a reaction of the leukocytes or white cells so that large numbers of them are made. This most often occurs in early infancy along with low platelets in a very sick child.

Occasionally, the bone marrow makes too much of one type of blood cell called the eosinophil. The eosinophil is a white blood cell that has granules that are reddish. It can be readily recognized in the microscope. It is usually associated with allergies and asthma. Why it can be increased in some individuals with TAR is not known. Many individuals with TAR seem to have an allergy to cow's milk and certainly in that situation, they will have an increase in the eosinophils.

Individuals with TAR often have anemia or low red cells. This can be because of bleeding but also it appears that during the first year of life, they may simply not make as many red cells as the average individual. Again, affected individuals tend to outgrow anemia.

Skeletal Abnormalities

The consistent feature of TAR is absence of the radius but presence of the thumb. One hundred per cent (or as close as you can get to a hundred per cent) of individuals with TAR have bilateral absence of the radius. For practical purposes the diagnosis shouldn't be considered unless there is complete absence of the radius bilaterally. Because of the absence of the radius, the muscles that would normally attach to the radius often go up to the wrist and pull the hand over toward where the radius would be. Sometimes as children get older, it is useful to stabilize the wrist so that it doesn't drift over. This gives the hand more strength.

The other bone in the forearm, the ulna, is often abnormal in TAR. It can be shortened and malformed or it can also be completely absent in about twenty per cent of the cases. In about ten per cent of affected individuals, there is asymmetry with an ulna present on one side but not the other side. There is almost always a five-digit hand, that is, four fingers and a thumb even when the arm is missing. The fingers may be shortened but they are usually quite functional. In some individuals the upper arm is also shortened or absent. Within a family of affected individuals there can be quite different degrees of how severely affected the limbs are.

The legs may be affected as well. There can be dislocation of the hips so that the head of the femur (the big bone in the upper leg) doesn't sit properly into the hip socket. There can be abnormalities of the knees such that they bend abnormally one way or the other, such that the kneecap is loose, or such that the bones of the knee actually slip on each other. Occasionally the bones of the knee are even fused. Frequently there are abnormalities of toe positioning with "scrambling" of the toes and occasionally there is some puffiness to the foot.

It would appear the more severely involved the upper limb is, the more likely the lower limbs are to be involved as well. However, again, if the lower limb is involved, it appears that all five toes and a normal foot is usually seen. Only about twenty per cent of affected individuals have significant lower limb involvement.

Minor abnormalities of the ribs, the spine or the jaw can be seen.


Most individuals with TAR are shorter than other family members. Their average height is usually around the 5th to l0th percentile as compared to the 50th percentile for unaffected individuals. However, most individuals with TAR grow regularly and parallel to the 3rd percentile. No important endocrine abnormalities or growth hormone deficiency have been seen in TAR.

Other Abnormalities

The most frequent other anomaly is to have a hole in the heart. About thirty per cent of affected individuals have some kind of structural abnormality of the heart. This is, of course, particularly complicated when heart surgery is needed and there are low platelets. However there are a number of individuals who have had quite complicated heart surgery and, with the aid of platelet transfusions, have done extremely well.

A variety of other abnormalities have been seen. It is not entirely clear whether this is by chance or whether there really is an increased occurrence of other kinds of structural problems.

Mental retardation can be seen in TAR. It appears to be only related to having had a bleed into the brain.

Natural History

The natural history of TAR is fairly well understood now. Most affected individuals have bruising at birth. Many develop cow's milk allergy and diarrhea in infancy. Diarrheal illness seems to precipitate low platelets and is of real concern. During the first year of life, there seems to be episodes of low platelets. It is important to avoid cow's milk if it is an irritant. It is also probably important to avoid viral illnesses if possible, particularly gastroenterologic ones for the first two years of life.

During the first two years of life, in addition to episodes of low platelets, there can be eosinophilia, extra white cells (leukamoid reactions) and low red cells (anemia). It is important to be followed carefully by a physician and treated symptomatically. If there are problems, it is obviously possible to give blood and/or platelet transfusions but it is important to identify a single donor such as a parent if that is possible. Strenuous support of low platelets should be provided in the first year since after that for most affected individuals, severe low platelets don't seem to be as big a problem. Gradually over childhood, the number of platelets increases and by adulthood can be almost often near normal. Women with TAR often have quite heavy menstrual periods.

When orthopedic surgery is necessary, it is ideal to put it off as long as possible until the child is larger with less danger of thrombocytopenia. Only surgery which is absolutely necessary or life saving should be done in the first few years of life.

When severe intracranial bleeding or bleeding into the brain occurs, it is almost always before one year of age. Individuals with TAR often have delayed motor development because of their short arms but one should expect normal intelligence if there has not been bleeding into the brain.

Bracing, splinting, and stabilizing the wrist or the knees is often quite helpful.


It would appear that Thrombocytopenia and Absent Radius occurs in families in a pattern consistent with autosomal recessive inheritance. That means that we would anticipate that each parent is a carrier of an abnormal gene and when a child receives two abnormal genes they get the condition Thrombocytopenia and Absent Radius. Most individuals who are carriers are perfectly normal and have no evidence that they are carriers. They don't have bruising or skeletal abnormalities. Once there has been an affected child in the family there is a one in four or twenty-five per cent chance with each additional child that the child will be affected. It is very useful to do prenatal diagnosis in that situation to determine whether or not the child of another pregnancy is affected since great care would be given at the time of delivery and many such children have been delivered by cesarean section to avoid bleeding, bruising, and intracranial bleeding.

There are now a large number of adults with Thrombocytopenia and Absent Radius and so it has become quite clear that there are no special problems of adulthood. Many have married and had unaffected children. Most adults are able to live independently and have productive jobs. Although obviously it is important to think through at an early age which jobs will be most appropriate.

Judith G. Hall, OC, MD Head, Department of Pediatrics University of British Columbia and B.C.'s Children's Hospital Vancouver, B.C., Canada

18th January 2000

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