What Are Sacral Dimples?

My wife and I had a boy at 26 weeks gestation due to PIH and HELLP syndrome. The baby has had minor complications, but the newest one is what was described as a sacral dimple. It is very small. The doctor so far has only suggested that he will do a kidney work-up closer to discharge, ruling out neural tube defect's or spina-bifida. The baby did have sepsis on and off for the first few weeks of life, and I have heard that UTI's can cause this sacral dimple. Is this true? Should we get a second opinion?

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Robert Steele

Robert W. Steele, MD, is a board certified pediatrician at St. John's Regional Health Center in Springfield, MO. He graduated from medical... Read more

Sacral dimples refer to the divots found on the lower backs of some infants. It is estimated that at least two percent of infants have these dimples or pits most of which are located in the middle of the back just above the buttocks. To understand what the significance of these dimples are (if any) it is helpful to realize the basic way the spinal column develops while the baby is in mom.

During the first eight weeks or so of development, the fetus develops the rudimentary structure of the spinal column called the neural tube. This tube forms sort of in the same manner as when the edges of a flat piece of paper are brought together to form a tube. When the edges or a portion of these edges do not meet, a "neural tube defect" occurs. But it gets a little more complicated. Now picture this tube forming from several sheets of paper that are placed on top of each other. To form a complete neural tube, all the edges of all the layers must meet to be complete. If the outer most paper has edges that meet, the tube may look complete from the outside, but when you look inside, it is possible that the other sheets of paper inside the tube got crumpled and do not have the edges meeting.

This is generally what can occur as the spinal column is forming. If the inner tissues of the tube become malformed, the outer layer including the skin may not be smooth. All the other permutations may be true as well. The outside layer may form with some dimples in it which have no effect on the inner layers. And there may be malformation of the inner layers which have no effect on the outer ones. To put it simply, a dimple in the lower back along the spine may be a simple divot in the skin which causes no problems whatsoever, or it may be the sign of malformation in lower layers including the spine and spinal cord.

The problem with this is that, as I stated above, at least two percent of babies may have these pits. That's one in every 50 babies. And almost all of these pits are insignificant when it comes to serious problems or involvement of deeper tissues. Therefore, what do doctors look for to decide who ought to have additional tests to examine the deeper tissues? There are a number of things, but some of them include:

-- Can the floor of the pit or dimple be easily visualized? Divots in which the bottom portion cannot be seen may be evidence of a neural tube that never closed completely.
-- Does the pit have a tuft of hair growing from it? This again make a pit suspicious for deeper involvement.
-- Is the neurologic exam normal? If there is significant involvement of the spinal column, there may be weakness in the legs.
-- How high up on the back is the dimple? Most dimples very low on the back (just above the buttocks) are of little concern. The higher the dimple is, the more likely it is to be associated with defects in deeper tissues.
-- Are there signs of any other defects? The brain, spinal column, kidneys, as well as other structures form at the same time the neural tube closes. Therefore, if there are any malformations in any other parts of the body, it becomes more concerning that the neural tube may be involved as well.

I'm happy to hear your son is having only minor complications. Premature infants can be quite a challenge as you well know, so it is always pleasing to hear of a premature child who is progressing well. To answer your question, it is the lower portion of the spinal cord that is involved with sending signals to allow the bladder to empty. When a sacral dimple is associated with a spinal cord malformation, there is a risk that the ability to empty the bladder correctly may be impaired. This in turn can lead to urinary tract infection. In other words, the sacral dimple may signal a spinal cord defect which could impair elimination of urine which in turn could cause a urinary tract infection, not the other way around. There is nothing about urinary tract infections that can cause sacral dimples.

Most spinal cord defects signaled by a sacral dimple may be adequately investigated by doing a thorough physical exam and, if needed, an ultrasound. In those in which there is a very high suspicion of spinal cord defect, an MRI is usually done to allow the neurosurgeon to see a very accurate and detailed picture to plan the repair. Ultrasounds are felt by most to be an excellent way to evaluate if there are any spinal cord defects. It is an easier procedure, less expensive, and does not require sedation as opposed to MRI. However, there are certainly variations and differences of opinion as to whom should get an MRI. I suggest you discuss your concerns with your doctor first. And if not satisfied, then certainly a second opinion would not be unreasonable.

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