Prader-Willi syndrome is a congenital (present from birth) disease that involves obesity, decreased muscle tone, decreased mental capacity, and sex glands that produce little or no hormones.

Prader-Willi syndrome is caused by a gene missing on part of chromosome 15. Normally, your parents each pass down a copy of this chromosome. Most patients with Prader-Willi syndrome are missing the genetic material on part of the father's chromosome. The remaining patients frequently have two copies of the mother's chromosome 15.

The genetic changes occur randomly. Patients usually do not have a family history of the condition.

Symptoms may include:

• Almond-shaped eyes
• Delayed motor development
• Floppy newborn infant
• Insatiable appetite, food craving
• Irregular areas of skin that look like bands, stripes, or lines
• Narrow bifrontal skull
• Rapid weight gain
• Skeletal (limb) abnormalities
• Slow mental development
• Small for gestational age
• Undescended testicles in the male infant
• Very small hands and feet in comparison to body

Affected children have an intense craving for food and will do almost anything to get it. This results in uncontrollable weight gain and morbid obesity. Morbid obesity may lead to lung failure with low blood oxygen levels, right-sided heart failure, and death.

Signs of Prader-Willi syndrome may be seen at birth. Newborns with the condition are often small and very floppy. Male infants may have undescended testicles.

As the child grows older, laboratory tests may show signs related to morbid obesity, such as:

• Abnormal glucose tolerance
• Above normal level of insulin in the blood
• Excessive carbon dioxide levels
• Failure to respond to luteinizing hormone releasing factor
• Lack of oxygen supply

There may also be signs of right side heart failure and knee and hip problems.