Function of the Autonomic Nervous System in People with Down Syndrome

Key Points

• The autonomic nervous system is the part of the nervous system that functions “automatically” without us needing to think about it.

• The autonomic nervous system works differently in people with Down syndrome.

• This difference in function of the autonomic nervous system impacts blood pressure, heart rate (pulse), blood flow, temperature regulation, opening or constricting of the airways in the lungs, and the function of the gastrointestinal tract and urinary bladder.

 

What is the autonomic nervous system?

The autonomic nervous system is the part of the nervous system that functions automatically. It directs the functioning of body systems that we don’t have to think about controlling, including our heart rate, blood pressure, the opening or constricting of the airways in our lungs, and the function of our gastrointestinal tract and urinary bladder.

The autonomic nervous system tells smooth muscles to contract or relax. Smooth muscles are different than skeletal muscles. Skeletal muscles are the ones we direct to contract or relax (for example, to lift an object, stand up, or walk). The function of smooth muscles is involuntary (we don’t have the same control over them that we do with skeletal muscles). Smooth muscles are found in the walls of hollow organs, such as the digestive tract, blood vessels, and urinary bladder.

The autonomic nervous system also impacts the heart by affecting the “natural pacemaker” (sinoatrial node). The natural pacemaker fires rhythmically, typically once or more per second, sending an electric signal through the heart to cause the heart muscle to contract and the heart to beat. The autonomic nervous system affects how fast the sinoatrial node fires.

The autonomic nervous system consists of two parts:

• Sympathetic nervous system. This is also called the "fight or flight" system. When it is activated, it stimulates the heart to beat faster, blood pressure to rise, and sweating to increase. It also causes decreased digestive function.

• Parasympathetic nervous system. This is also called the "rest and digest" system. When it is activated, some of the body changes include increased digestion and decreased heart rate, blood pressure, and sweating.

 

Is there a difference in the function of the autonomic nervous system in people with Down syndrome?

The autonomic nervous system functions differently in people with Down syndrome. People with Down syndrome tend to have reduced sympathetic (“fight or flight”) activity and increased parasympathetic (“rest and digest”) activity.

 

What findings or conditions result from the differences in autonomic nervous system function? 

Blood pressure

When assessing an adult with Down syndrome, a difference in the blood pressure, is often noted. The blood pressure is often on the lower end of what is considered “normal.”  The upper number of the blood pressure (the systolic blood pressure) often reads low (90-100) or even below normal (70s to 80s).  In daily functioning, most people with Down syndrome do fine with blood pressures at these levels. However, the low or low end of normal blood pressures seem to make individuals with Down syndrome more susceptible to symptoms such as fainting, particularly if additional factors occur such as an illness, dehydration, or situations associated with higher anxiety, fear, pain, and/or temperature.

When noting a blood pressure on the lower end, further evaluation and treatment is usually only necessary if the person has symptoms or if there is a significant change from previous recordings. In addition, the lower blood pressure can have implications when undergoing anesthesia so the anesthesiologist will need to pay particular attention to these differences.

Heart rate

A normal heart rate (pulse) at rest is considered 60-100 beats per minute (although it may be lower for people who are very physically fit). When assessing an adult with Down syndrome, a difference in the pulse is often noted. People with Down syndrome often have pulses on the low end of “normal” (commonly in the 60s) or even a little lower than normal (in the 50s).

A low heart rate is called bradycardia. Like the situation with blood pressure noted above, most of the time people with Down syndrome do fine having a pulse on the low end of normal or even below normal. However, similarly, it needs further investigation if the person is having symptoms (such as dizziness), or it is significantly lower than usual.

More information about heart rate, including how the difference in heart rate for people with Down syndrome impacts heart rate goals for exercise, is available in our Heart Rates and Exercise article.

Acrocyanosis

Acrocyanosis causes a person’s hands and/or feet to turn blue. The small blood vessels in the hands and feet constrict and reduce blood flow. This is a normal response to cold temperatures (to preserve body temperature), but it can also occur when the temperatures are not cold.

Acrocyanosis is typically painless and is not associated with a serious health condition. It is common in people with Down syndrome and typically does not cause a problem. It can be reduced by keeping the extremities (wearing gloves, mittens, and warm socks) and/or the rest of the body warm. It can contribute to dry skin so attention to dry skin is important.

Fainting

As noted above, a typical lower blood pressure and/or pulse seems to predispose some people with Down syndrome to fainting. In addition, some people with Down syndrome seem to have an increased response to stimuli such as fear and anxiety that can further reduce blood pressure and pulse. 

Temperature regulation

When assessing an adult with Down syndrome, a difference in the body temperature is often noted. The body temperature of individuals with Down syndrome may be lower. One study  found that the body temperature of women with Down syndrome was about 2-3^o Fahrenheit lower and the body temperature of men with Down syndrome was about 2-2.5^o Fahrenheit lower (depending on the body region). In our experience, body temperatures of individuals with Down syndrome seem to be lower by about 1^o Fahrenheit.

In daily function, this does not typically cause concern. However, it can make it less likely that some individuals will develop a fever (by the usual definition) since their temperature starts at a lower value. This should be considered when assessing a person with Down syndrome for a possible infection.

More information is available in our Temperature Regulation article.

Gastrointestinal tract

The motility of gastrointestinal (GI) tract is affected by the autonomic nervous system.  How food, drink, and waste materials move through the GI tract is different (typically slower) in people with Down syndrome. This contributes to constipation, gastroesophageal reflux disease (GERD, heartburn), and swallowing dysfunction (dysphagia).

Increased parasympathetic function ("rest and digest") might seem to make constipation less likely. However, there are other factors. In a mouse model  for Down syndrome, a decrease in nerve cells in the colon was found. This could cause less contraction of the colon (which moves stool through the intestines and colon) and more constipation. In addition, differences in collagen (connective tissue), which can affect the structure and function of smooth muscle, may allow increased stretching of the colon. Increased stretching can cause a greater amount of stool to build up in the colon without the body responding to expel it normally. 

Urinary retention

Urinary retention is the inability to empty one’s urinary bladder. This can cause difficulty starting the urinary stream, contribute to bladder infections, contribute to kidney damage, and cause urinary incontinence (urine accidents). As noted for the gastrointestinal tract above, there are likely additional factors such as the bladder getting "stretched out" more readily due to differences in connective tissue. Once the tissue is overstretched, it can be difficult for it to return to normal size. 

Asthma

Asthma, narrowing of the airways causing wheezing, is less common in people with Down syndrome. This is more difficult to understand. When the parasympathetic nervous system function is higher and the sympathetic nervous function lower (as they are in Down syndrome), the airways tend to narrow more. So, the difference in the function of autonomic nervous system would seem to cause more asthma in people with Down syndrome. However, other factors, such as allergies which are less common in Down syndrome, play roles in asthma, so that may explain at least some of the difference in asthma frequency.

 

Are there treatments for the autonomic nervous system?

There are not specific treatments for the differences in function of the autonomic nervous system in people with Down syndrome. However, there are ways to treat or prevent symptoms that can occur due to the differences in the function.

A key factor is the importance for the person with Down syndrome, their family, and their health care professionals to be aware of the differences in the function of the autonomic nervous system. With that awareness, measures can be taken to address ways to avoid or treat symptoms.

More information about treating and preventing symptoms in people with Down syndrome is available in the articles in the Resources section below.

 

Resources

Acrocyanosis

Anesthesia

Connective Tissue

Constipation

Dry Skin

Fainting

Gastroesophageal Reflux Disease (GERD)

Heart Rates and Exercise

Hypertension and Hypotension (high and low blood pressure)

Swallowing Problems (Dysphagia)

Temperature Regulation

Urinary Incontinence

Urinary Retention

 

References

Cilhoroz BT, Receno CN, Heffernan KS, Deruisseau LR. Cardiovascular physiology and pathophysiology in Down syndrome . Physiol Res. 2022;71(1):1-16. doi:10.33549/physiolres.934791

Dębiec-Bąk, A., Wójtowicz, D., Pawik, Ł. et al. Analysis of body surface temperatures in people with Down syndrome after general rehabilitation exercise . J Therm Anal Calorim. 2019;135, 2399–2410. doi:10.1007/s10973-018-7345-1

Graham MF, Drucker DE, Diegelmann RF, Elson CO. Collagen synthesis by human intestinal smooth muscle cells in culture . Gastroenterology. 1987;92(2):400-405. doi:10.1016/0016-5085(87)90134-x

Schill EM, Wright CM, Jamil A, LaCombe JM, Roper RJ, Heuckeroth RO. Down syndrome mouse models have an abnormal enteric nervous system . JCI Insight. 2019;5(11):e124510. doi:10.1172/jci.insight.124510

Sinton JW, Cooper DS, Wiley S. Down syndrome and the autonomic nervous system, an educational review for the anesthesiologist . Paediatr Anaesth. 2022;32(5):609-616. doi:10.1111/pan.14416