Jacobsen syndrome is associated with a recognisable pattern of features. The children are usually of a pleasant disposition with a characteristic face, somewhat more pear shaped than normal. There may also be some developmental delay (learning difficulties, see entry Learning Disability), varying from mild to more severe. Speech and language may be slower. However, heart problems such as enlarged left heart syndrome (see entry Heart Defects) can be more debilitating, sometimes requiring surgery. Blood disorders, mainly in the form of easy bruising and prolonged bleeding due to low numbers of platelets (thrombocytopenia - reduction in the number of platelets present in the blood and referred to as Paris-Trousseau syndrome) are common. Gastrointestinal problems including pyloric stenosis (a narrowing of the outlet from the stomach to the small intestine) and frequent respiratory problems also are seen. Individuals with Jacobsen syndrome may show some or all of these features although there is great variability in the number and severity of symptoms.
The life expectancy for individuals with Jacobsen syndrome currently remains unknown but is increasing as we understand more about the symptoms and how to manage them. The two most common causes of illness and death are congenital heart defects and bleeding. The improved outcomes in children with most forms of congenital heart disease, however, suggest that the outcomes for children with Jacobsen syndrome with congenital heart defects is also likely to continue to improve.