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Down Syndrome Abstract
of the Month: Jan 2000

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Down syndrome and craniovertebral instability: Topic review and treatment recommendations

Brockmeyer D
Pediatr Neurosurg 1999 Aug;31(2):71-7

Division of Pediatric Neurosurgery, Primary Children's Medical Center, Salt Lake City, Utah, USA.

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The diagnosis and management of occipital-atlantal and atlantoaxial instability in Down syndrome patients is a challenging problem in pediatric spine surgery. To date, no systematic review of this topic has been presented on this confusing and sometimes contentious issue. This topic review will focus on the biomechanical and radiographic foundations for which treatment recommendations in Down syndrome patients are made. In addition, otolaryngologic and anesthetic considerations in Down syndrome are also discussed, as well as advances in surgery that have made the operative fusion of these patients easier and safer.

My comments:

A wonderful and much-needed review of studies to date and some common-sense recommendations. There's a lot here, so I want to hit on the more important points by the author.

First, the background. The craniovertebral joint is made up of the skull and the first two cervical (neck) vertebrae. The first cervical vertebra, C1, is called the atlas and has the shape of a ring. It sits on top of the second vertebra, C2, which is also called the axis. The axis has a protrusion called the odontoid process. In Down syndrome, a combination of bony abnormalities and lax ligaments contribute to the instability called atlantoaxial instability (AAI). For more background on the controversy on the diagnosis and treatment of this condition, see my essay on AAI.

On the topic of radiologic screening, Dr. Brockmeyer reviews the studies to date and contends that on lateral (side view) neck X-rays, the measurement called the "neural canal width" is much better than the commonly used atlanto-dens interval, as it correlates better with findings on the MRIs. Both measurements should be done. If the neural canal width is less than 14 mm or the atlanto-dens interval is greater than 4.5 mm, then the author recommends an MRI be done. The MRI can assess whether or not the vertebrae presses upon the spinal cord. The CT scan has no advantage over the MRI for looking at this condition.

The author then examines the literature to date on the natural history of craniovertebral instability in Down syndrome. He comes to the conclusion that patients with Down syndrome without AAI do not progress to AAI, so children who have been screened and are negative for AAI do not need further screening after the age of 10 years. However, he acknowledges that this age cutoff is controversial.

After reviewing surgical options and experience, the author then makes his recommendations for management of people with DS:

If there are no neurologic symptoms of AAI (see my page on AAI for a list of symptoms):
  • If the neck Xrays show a neural canal width (NCW) of 14 mm or greater, and the atlanto-dens interval (ADI) is 4.5 mm or less, then there is no further need for action at this time.
  • If the NCW is less than 14 mm or the ADI is greater than 4.5 mm, then the patient should have an MRI done of the area.
  • If the MRI shows spinal cord injury, then operative fusion is recommended. If the MRI shows no injury, then the choice for operative fusion becomes an individual decision, based on the patient's specific instability and anatomy. Some children can just be restricted from high-risk activities and restudied in one year's time.
  • The author states that the presence of the bony abnormality "os odontoideum" (a defect in the second cervical vertebra) automatically makes a diagnosis of AAI and requires fusion.
If there are neurologic symptoms of AAI, then lateral neck X-rays and an MRI should be done.
  • If the ADI is 5 mm or more, or the NCW is 14 mm or less, or the MRI shows spinal cord injury, then fusion is recommended.
  • If the ADI is less than 5 mm and the NCW is greater than 14 mm, and the MRI shows spinal cord injury, fusion is still recommended. But if the MRI shows no spinal cord injury, observation is warranted along with a search for other possible causes for the patient's symptoms.

Finally, the author mentions occipitoatlantal instability, which is between the skull and C1, is also a problem for people with Down syndrome. This condition is poorly understood as to its possible danger to people with DS. The author states that his center's policy is to recommend fusion for any person with greater than 8 to 10 mm of subluxation at the occipital-C1 level.

If you're interested in either surgical correction of AAI or Dr Brockmeyer, his website (which discusses all sorts of neurosurgical matters) is http://www.neurosurgery4kids.com/.

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