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Atlanto-axial instability
in Down syndrome

Richard W. Newton M.D., F.R.C.P.

Consultant Paediatric Neurologist,
Royal Manchester Children’s Hospital, Pendlebury, Manchester

Abstract - The spinal column (back bone) is a stack of specially shaped bones, each one moving on the one beneath. The movement occurs at a number of joints and the stability of these joints relies on the strength of the ligaments joining the bones and the tone of the muscles which straddle the bones. The spinal cord is a trunk of thousands of nerve cells which carry information from the brain to muscles in the arms, legs and trunk. The spinal cord lies in a hole running through the centre of each vertebra in the spinal column. In Down syndrome ligaments tend to be lax and muscle tone is often low. This gives one vertebra the potential to move to a greater extent on its neighbour than is seen in the average person. As a consequence the nerve cells of the spinal cord can become distorted and injured.

Keywords - Down Syndrome, Atlanto-axial instability
Introduction

The spinal column (back bone) is a stack of specially shaped bones, each one moving on the one beneath. The movement occurs at a number of joints and the stability of these joints relies on the strength of the ligaments joining the bones and the tone of the muscles which straddle the bones. The spinal cord is a trunk of thousands of nerve cells which carry information from the brain to muscles in the arms, legs and trunk. The spinal cord lies in a hole running through the centre of each vertebra in the spinal column.

In Down syndrome ligaments tend to be lax and muscle tone is often low. This gives one vertebra the potential to move to a greater extent on its neighbour than is seen in the average person. As a consequence the nerve cells of the spinal cord can become distorted and injured.

The greatest potential for this movement is right at the top of the spinal column at the joint that lies between the first vertebra (atlas) which supports the base of the skull and the second vertebra (axis) which allows side to side pivoting of the atlas and skull on its specially designed joint surface. This special design involves a bony spur known as the odontoid peg passing upwards from the body of the axis into the body of the atlas. If the atlas moves too much on the axis the odontoid peg is particularly well placed to cause injury to the spinal cord.

The tendency of the atlas to move on the axis is known as atlanto-axial instability. If the movement is so great that one joint surface falls off the next this is known as atlanto-axial dislocation or subluxation and in this circumstance the risk of spinal cord damage is very high indeed.

In June 1994 a number of doctors gathered at the Royal Society of Medicine to hold a seminar on the subject of atlanto-axial instability. The main speakers were Richard Newton, Paediatric Neurologist from the Manchester Children’s Hospitals, Richard Morton, Paediatrician from Derby Children’s Hospital, Marike Cremers, a Specialist in Public Health from Holland and Mr. McKin Thomas, an Orthopaedic Surgeon from Nottingham. This session was chaired by Professor Sir David Hull, immediate Past President of the British Paediatric Association.

The notes presented here represent the key facts presented on that day:

1. Atlanto-axial instability, defined radiologically as an atlanto-axial distance greater than 4.4mm in flexion, has been reported in between 10% and 40% of people with Down syndrome. The incidence decreases with age.
2. People with Down syndrome are at increased risk of neurological problems secondary to atlanto-axial instability.
3. Acute subluxation of the atlanto-axial joint rarely occurs without prior warning, clinical signs and symptoms. In most published cases of spinal cord injury, symptoms of neck pain, limitation of neck movement, increasing difficulty with walking or bladder and bowel difficulties were often present for some weeks before the dislocation was seen.
4. The number of reported instances world-wide of acute subluxation associated with sporting activities is very small. The incidence of sports injuries of this sort in the general population also needs to be defined.
5. By contrast there are a number of reports of subluxation associated with endotrachael anaesthesia and non-sporting trauma.
6. People with Down syndrome should be fitted with a soft neck collar before anaesthesia or at the time of an RTA to remind everybody that they may be at risk of acute AA subluxation.
7. Neck x-rays taken in routine x-ray departments may give false negative results and are likely to identify only 60% to 80% of those with AAI.
8. Dr. Richard Morton stated that neck x-rays taken erect with the head and neck in maximum flexion are reliable in identifying an increased AA distance. However, this has not been the experience at the Manchester Children’s Hospitals where experienced radiographers found the method to be unreliable.
9. There is no evidence that increased AA distance on x-ray identifies a population at increased risk of acute subluxation.
10. There are several reports of acute subluxation in people whose neck x-rays, even taken in rigorous conditions, have been reported as normal.
11. There is no indication that restricting sporting activities involving neck flexion in people with Down syndrome, with or without radiological abnormality, will reduce the risk of acute subluxation.
12. It is theoretically possible, but unproven, that in the long term, continued participation in activities associated with repeated neck flexion may predispose to the neurological sequelae of chronic instability. Whether those with radiological abnormality are more at risk is not known. In the short term there is evidence that they are not.
13. X-raying the neck in people with Down syndrome in order to identify AAI does not currently fulfil necessary criteria for a screening programme because:
14.
* Neck x-rays taken in routine x-ray departments may identify only 60-80% of those with AAI.
* There is no intervention with proven effect on health care outcome.
* Counselling is rarely available to alleviate anxiety caused by screening procedure.
15. Gait abnormality predicts only 50% of those with radiological instability and excessive generalised joint laxity predicts only 10%.
16. Possible prodromal indicators of risk for subluxation or more serious neurological sequelae are: neck pain, restricted movement, alterations in gait, increasing reflexes, upgoing plantars, deterioration in bladder or bowel control. Onset of any of these in a person with Down syndrome is an indication for urgent referral to an appropriate specialist.

Cervical spine fusion in children with Down syndrome has been reported as carrying a serious level of morbidity and mortality. It should be undertaken only as a clinical necessity.

The Down's Syndrome Association is in the process of collating the different views from the presenters on that day, and that publication will be available from the Down's Syndrome Association in the next two or three months. My personal view is that screening children with Down syndrome with neck x-rays cannot be recommended in the light of the knowledge currently available. But special care needs to be taken of children with Down syndrome who have a general anaesthetic (see 6 above).

You will probably know that currently the Department of Health recommendation is that children with Down syndrome are recommended to have a neck x-ray before participating in "high risk" sports. I can report that representatives of the Department of Health were present at the symposium and medical advisers from the DSA have written to Dr. Kenneth Calman, the Chief Medical Officer, to ask him if he would reconsider department advice in the light of current knowledge.