Shwachman-Diamond Syndrome (SDS) is an inherited disorder that impairs pancreatic and bone marrow function and results in decreased height. SDS may also cause other effects including skeletal changes. Effects on the pancreas reduce intestinal nutrient absorption, but this malabsorption is largely reversible with treatment. Impairment of bone marrow (which produces all types of blood cells) most commonly causes decreased numbers of neutrophils, a type of white blood cell that fights infection. However, leukemia and aplastic anemia (in which the bone marrow is unable to generate sufficient quantities of all types of blood cells) also develop in some patients. The course of SDS varies significantly, and every case is different.
Children usually develop symptoms in their first year of life. Many are affected by four months of age. The incidence is unknown but estimated to be 1 in 50,000 births. The disorder is present in people of all racial and ethnic backgrounds. Males reportedly develop the disorder slightly more often than females (1.7:1), but an actual disparity is doubtful since the suspected mode of inheritance (described next) leads to equal incidence in both sexes. SDS is a genetically inherited disorder. A child receives two copies of every gene--one from each parent. For a child to be affected by SDS, that child must inherit two broken copies of the gene associated with SDS. One broken copy must come from the mother and the other from the father. Even though both parents have broken gene copies, they probably are not affected by SDS because they each have a second normal copy of the gene. Thus, it is unlikely that the parents could have known that they might have a child with SDS. Additionally, parents have no control over whether they give their child a normal or broken copy of a gene. In this scenario in which both parents have one normal gene and one altered gene, each child (male or female) has a 25% chance of receiving two broken copies and thus having SDS. The inheritance pattern just described for SDS is known in genetics as “autosomal recessive.” While there has been debate over whether this pattern is the exclusive mode of genetic inheritance in SDS, it appears to be broadly applicable.
Signs and Symptoms
Signs and symptoms of SDS vary. The following list describes features that are associated with SDS, but it is important to remember that not all of these apply to every child. Low birth weight Diarrhea or foul smelling stool (due to increased fat in the stool) Decreased growth and weight loss (failure to thrive) Possible swelling of the abdomen Frequent bacterial infections such as ear infections, pneumonia or other respiratory illnesses, sinus infections, or skin infections Anemia, or low level of oxygen-transporting material in the blood, which may cause fatigue and pale skin color Easy bruising or nose bleeds Continued short stature A bone condition known as metaphyseal dysostosis which affects the growth plates (metaphyses) at the ends of bones. There may be no symptoms, but sometimes bone alterations occur. These include: decreased angle at the hip joint so that the hip cannot be properly rotated outward (coxa vara); outward or inward angling of the lower leg compared to the thigh (knock-knee or bowleg) Other bone effects including alterations in rib shape and rib cage size, and, rarely, permanently bent fingers or joined fingers or toes Delay in development of appropriate voluntary movement and developmental delay Late onset of puberty Improper tooth development or other dental problems Enlarged liver Kidney problems Dry, scaling, or crusted skin
The gene that causes SDS has been identified (called the SBDS gene). However, it is not yet clear how alterations in this gene lead to the symptoms in SDS. It is known that the pancreas of SDS patients does not develop properly. Consequently, it fails to produce sufficient quantities of digestive enzymes (proteins that chop up food components). Inadequate digestion causes decreased nutrient absorption and leads to intestinal symptoms including weight loss and diarrhea as well as anemia (due to lack of iron or other nutrients). Bone marrow is the part of bone that produces blood cells and cells that function in the immune system. Effects on the bone marrow decrease the production of at least one type of blood cell. The most commonly deficient cells are neutrophils, a type of white blood cell which serves the immune system by fighting infection. Neutrophil levels are either consistently or periodically low in nearly all SDS patients. Decreased neutrophil numbers lead to an increased infection rate. Additionally, the neutrophils that are present do not function properly, thus further impairing immune function. In addition to neutrophil deficiency, nearly 50% of SDS patients also have decreased numbers of all other types of blood cells. Besides increased infection rate, this causes anemia (low number of red blood cells), easy bruising, and, possibly, excessive bleeding. Increased bruising and bleeding result from decreased levels of platelets, blood components necessary for clotting. The reason for skeletal alterations is not clear.
Diagnosis is usually made based on a series of tests. Blood will be drawn and blood cells counted. Low neutrophil levels that may be combined with anemia and low platelet levels are consistent with SDS. Fat in the stool may be measured over a 3 day course. SDS results in increased stool fat due to inadequate fat absorption. Additionally, the ability of the pancreas to produce enzymes will be checked by measuring enzymes in the stool or blood. A sweat test, in which a liquid is applied to the skin and sweat collected with a special tool, may also be performed. Sweat tests are normal in SDS patients and thus help distinguish SDS from cystic fibrosis--another childhood cause of pancreatic insufficiency but one with abnormal sweat test results. Decreased blood cell numbers combined with inadequate pancreatic function and a normal sweat test suggest SDS. X-rays of bones may also be done to look for altered growth plates and bone structure. Growth hormone levels may be measured (with a blood test) as these are usually low in children with SDS. Additional studies may also be carried out. Eventually, genetic testing should be available to confirm diagnosis.
Pancreatic insufficiency is treated with pancreatic enzyme pills which replace the missing enzymes. This therapy greatly improves digestion and helps to alleviate diarrhea and weight loss. However, despite better nutrient absorption, growth rate does not increase much. A low-fat diet as well as supplemental fat-soluble vitamins (A, D, E, and K) may also be necessary to treat intestinal symptoms. As a child gets older, the need for pancreatic enzyme replacement may be lessened or eliminated because natural pancreatic enzyme production tends to increase with age. To increase growth, growth hormone injections have been used, but this therapy is ineffective after a short time. In a small number of children, surgery may be necessary to correct improperly angled bones. Antibiotics are used to treat bacterial infections. It is important that a patient seek treatment at the start of an infection. Some people may need antibiotics on a more constant basis to prevent future illness or infection. The growth factor GCSF (granulocyte colony-stimulating factor) may also be used to increase neutrophil levels and decrease infection frequency. Blood transfusions are sometimes needed to alleviate anemia (low red blood cells) and low platelet levels. Additionally, injections of a medication that stimulates red blood cell production also may be recommend. The development of acute myelogenous leukemia or aplastic anemia (inability of the bone marrow to produce sufficient quantities of all blood cells) usually requires bone marrow transplant. In this procedure, bone marrow from a matched donor is transplanted into an SDS patient.
The outcome of children with SDS is not well known. However, it is known that the prognosis is highly variable depending upon the severity of complications from the disease. Children with SDS attend school and can engage in normal activities. (The only exception is when platelet levels are low. At this time, situations that could cause trauma and bleeding, such as contact sports, should be avoided.) Most children are among the smallest 3% of the population for height and weight, but some adults are taller with a height that puts them at the 25th percentile. Because life-threatening infections develop in some people, attainment of prompt treatment is imperative to minimize the risk of serious consequences. Aplastic anemia or other pancytopenia (low numbers of all blood cells) develops in approximately 10-25% of patients and leukemia occurs in about 10%. Unfortunately, these conditions are difficult to treat. They usually require bone marrow transplant, but that procedure is more risky in SDS patients and also requires a matched donor. It has been postulated that the median life expectancy for all SDS patients is beyond 35 years of age. Those who develop aplastic anemia have a life expectancy of 24 years, and the prognosis is poorer when leukemia occurs. The recent identification of the gene causing SDS will allow mechanisms of the disease to be better studied. With improved understanding comes the potential for better treatments.
Connect with other parents
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Shwachman-Diamond Syndrome Foundation
Site provides a concise and clear overview of SDS and includes a registration form for the SDS International Family Conference.