The characteristics, features and potential complications of Down syndrome vary greatly among patients. All children with the condition are different. However, some degree of mental retardation is nearly universal. In most cases, children with Down syndrome have mild to moderate retardation. Severe retardation is uncommon.

Mild retardation is defined as an intelligence quotient (IQ) of 50 to 70, moderate retardation as an IQ of 35 to 50, severe retardation as an IQ of 20 to 35. The overall development is slow in these children but still occurs. In most cases, children with Down syndrome reach all of the developmental goals of infancy and childhood, just not as quickly as usual.

Down syndrome is commonly associated with a number of physical features. Some children with the condition may have many of these. However, some may have few, if any. These physical features include:

• A small, short and broad head
  
  
• A flat facial profile with a flat nasal bridge
  
  
• Upward slanting eyes with epicanthic folds (eyelid folds at the inner corner of the eyes)
  
  
• Brushfield spots (speckled discoloration on the iris, the colored part of the eye)
  
  
• Small ears that may be misshapen or look lower than usual
  
  
• A small mouth with a large, protruding tongue
  
  
• Excess skin at the back of the neck
  
  
• Short, broad hands with short fingers
  
  
• A curved fifth finger
  
  
• Simian crease (single crease along the center of the palms)
  
  
• Wide gap between the big toe and the second toe

Many medical problems are commonly associated with Down syndrome. Some children with the condition have no other medical problems at all. However, others require a great deal of medical attention. Medical problems commonly associated with Down syndrome include:

• Infections. Most children with Down syndrome are at least slightly more susceptible to infections (e.g., colds, flu) that their peers. When these infections occur, they are likely to last longer than normal.
  
  
• Low muscle tone and loose joints. Many children with Down syndrome have low muscle tone (hypotonia) and loose joints. This can result in excessive flexibility and a floppy, rag-doll like appearance. These problems are often worse in infancy and improve with age. Children with Down syndrome have a 6 percent chance of dislocating their hip, according to the American Academy of Pediatrics (AAP). Some children have a characteristic instability with the bones in the neck that may need to be monitored.
  
  
• Ear problems. Hearing loss occurs in 75 percent of children with Down syndrome, according to the AAP. These children also have between a 50 and 75 percent risk of ear infections. These problems may be due to structural problems or a fluid buildup in the inner ear.
  
  
• Obstructive sleep apnea. Temporary interruptions in breathing during sleep. Children with Down syndrome have a 50 to 75 percent risk of sleep apnea, according to the AAP.
  
  
• Eye problems. The AAP reports an overall risk of eye disease of 60 percent in children with Down syndrome. This includes a 50 percent risk of vision loss due to severe myopia (nearsightedness), hyperopia (farsightedness) or astigmatism (abnormal shape of the retina). These children also have a 15 percent chance of cataracts, according to the AAP. Lazy eye (amblyopia) is also more common in children with Down syndrome.
  
  
• Congenital heart defects. According to the AAP, about 50 percent of infants with Down syndrome are born with heart defects, such as abnormal openings called septal defects. These conditions can also increase a child’s risk of high blood pressure in the lungs (pulmonary hypertension).
  
  
• Thyroid disease. Children with Down syndrome have a 15 percent risk of thyroid disease (e.g., hypothyroidism), according to the AAP.
  
  
• Digestive problems. The AAP reports that 12 percent of infants with Down syndrome are born with incomplete or absent openings (atresia) in their digestive tract. These can include the openings into and out of the stomach, between different parts of the intestines, or a lack of an anus. Poor muscle tone may also increase the risk of digestive problems such as constipation. Intestinal obstruction (blockage) may occur due to problems with function (Hirschsprung disease) in 1 percent of Down syndrome infants, according to the AAP. There may be an increased risk of celiac disease (gluten intolerance).
  
  
• Childhood leukemia. Children with Down syndrome have a greater risk of leukemia than other children. However, this risk is still low, less than 1 percent, according to the AAP. Furthermore, depending on the type of leukemia present, children with Down syndrome may have a higher long-term survival rate than other children with the same type of leukemia.
  
  
• Other problems, such as:
  
  
• • Obesity
    
  • Seizure disorders
    
  • Respiratory problems
    
  • Alopecia areata (patchy baldness)

Some of the developmental delays of Down syndrome may worsen the child’s experience with physical symptoms. For example, many children with Down syndrome have delayed or difficult to understand speech. They also understand language more readily than they can use it and may not be able to say when a body part hurts. They may refuse to walk on a hurt foot if they are unable to tell someone about the pain.