Craniocervical Instability in Down Syndrome
Original Editor - Beth Potter | Catherine Stanislas | Emily Westwood | Laura Beaman
Introduction[edit | edit source]
Down Syndrome (DS), also known as Trisomy 21, is a condition caused by the presence of an extra chromosome (chromosome 21) which results in atypical physical and cognitive development. It occurs in approximately every 1 in 700 births. Development of secondary health issues such as Craniocervical Instability (CCI) are common within the DS population compared to the general population. Instability of the C0-C1 and C1-C2 joints occur due to malformation of the Axis (C2), specifically the Dens and or laxity of the transverse ligament caused by collagen defects . Both of these structural anomalies allow increased translation of the posterior aspect of the Atlas. The prevalence of CCI in people with DS has been reported to be between 8-63%. CCI can be subdivided in to Occipito-axial Instability (OAI) also known as Atlanto-occipital Instability (AOI) and Atlanto-axial Instability (AAI), affecting 17.5% and between 6.8- 30% of people with DS respectively.   Less than 1-2% of people living with DS who have AAI later develop symptomatic AAI. Symptomatic AAI occurs as a result of excessive cervical movement causing impingement on the spinal cord, with a risk of severe neurological damage if untreated .
Clinically Relevant Anatomy[edit | edit source]
- Occiput (inferior aspect of the skull)
- Atypical Vertebrae:Atlas (C1) &Axis (C2)
- Typical Vertebrae: C3-C7
- Occipitoatlantoaxial ligaments:Apical, Alar, Transverse & Accessory Atlanto-axial
- Vertebral ligaments:Nuchal, Supraspinous, Interspinous, Anterior Longitudinal, Posterior Longitudinal, Intertransverse & Ligamentum Flavum
- Longus Capitis
- Longus Colli
- Rectus Capitis Anterior
- Rectus Capitis Lateralis
- Splenius Capitis
- Splenius Cervicis
- Upper Trapezius
- Levator Scapulae
See Cervical Anatomy page for more information.
Overview of Pathology[edit | edit source]
Occipito-axial Instability in Down Syndrome[edit | edit source]
Occipito-axial instability (OAI) is less researched in the DS population compared to AAI. Bony abnormalities such as undeveloped occipital condyles and superior articular facets of C1 can cause such instability due to decreasing joint congruency.
Ligament laxity has been cited as another explanation for OAI - specifically retropharyngeal ligament laxity.
Atlanto-axial Instability in Down Syndrome[edit | edit source]
Atlanto-axial instability (AAI) is defined as an increased distance between the posterior aspect of the frontal arch of C1 and the anterior aspect of the dens (otherwise known as the odontoid peg) of C2 when measured on radiographs taken in the neutral position. A distance of more than 5mm between the dens and the anterior arch of C2 is considered an abnormality. 
It can occur as a result of increased movement at the Atlantoaxial joint (the atlas and axis joint articulation). The instability arises from bony abnormalities and ligament laxity of the Atlantoaxial joint.
The causes for such ligament laxity have previously been cited as being due to intrinsic collagen defects and/or a chronic inflammatory state which weakens the ligaments. Lax ligaments allow for more movement between the articulating vertebral bodies. Specifically, the laxity of the transverse ligament, which keeps the dens close to the anterior arch of the atlas, is the primary cause.
In terms of bony abnormalities, those concerning the dens are the most commonly cited. For example, odontoid hypoplasia and the presence of accessory ossicles. Odontoid hypoplasia meaning underdevelopment of the odontoid peg and accessory ossicles implies there are secondary ossification centres that remain separate from the adjacent bone (usually round or ovoid in shape). Odontoid hypoplasia may cause slippage of the transverse ligament over the superior aspect of the shortened odontoid peg.
AAI becomes symptomatic when the displaced dens impinges on the spinal cord. 
Signs and Symptoms[edit | edit source]
Symptoms associated with AAI and OAI include:
- Neck pain/discomfort
- Abnormal head posture
- Abnormal gait
- Change in sphincteric control
- Decreased neck range of movement (RoM)
- Frequent falls 
- Increased fatigue on walking
Differential Diagnoses[edit | edit source]
A thorough assessment of the cervical spine should be completed in order to distinguish the cause of neck pain and presenting symptoms. Despite the higher prevalence of CCI among the DS population, the signs and symptoms are not unique to this condition, therefore the following should be considered as a potential cause:
- Cervical Trauma
- Cervical Canal Stenosis
- Cervical Ligament Injury
- Cervical Radiculopathy
- Cervical Spondylosis
- Degenerative Disc Disease
- Disc Herniation
- Juvenile Disc Disorder
Examination and Screening[edit | edit source]
Atlanto-axial Instability Screening[edit | edit source]
Plain Lateral Cervical Radiographs
There is limited evidence to support the use of plain radiography as a screening tool for asymptomatic AAI. Lateral cervical radiographs can be conducted with the individual in neck flexion, extension and neutral position. The distance between the posterior surface of the frontal arcus of C1 and the anterior surface of C2 Dens is measured, otherwise known as the anterior atlanto-odontoid distance (AAOD). However, there are large discrepancies in diagnostic criterial for AAI, with AAI being defined by an AAOD ranging from 3mm-4.5mm across studies.    Several authors have concluded that routine screening using lateral radiographs are not necessary due to the variation in diagnostic criteria, technical difficulties conducting the measurements, the likelihood of AAI decline throughout an individuals' life and the occurrence of symptomatic AAI being extremely rare. Furthermore, Selby et al. found radiographs of the cervical spine was an unreliable and insensitive screening tool in identifying atlantoaxial subluxation in children (aged 6-14 years) with DS.
From a sport participation screening perspective, Cremers et al.  studied 91 children and young adults (4-20 years old) with DS presenting with asymptomatic AAI (>4mm). Participants were randomly assigned to one of two groups. Group one continued with usual sport and exercise and the other group avoided sports deemed 'risky' for a year. Results showed no differences between groups in functional motor scale, neurological signs or atlantoaxial distance, concluding that plain radiographic pre-participation screening is unnecessary for asymptomatic AAI. 
Non-Radiographic Assessment and Screening
British Gymnastics established their own screening criteria for individuals with DS who wish to participate in gymnastics activity (including trampolining). Despite being developed as a screening tool for gymnastics, it has wider application to other sports and physical activity.
A qualified medical practitioner or chartered physiotherapist must complete the following tests:
- Does the person show evidence of progressive Myelopathy? (Yes/No)
- Does the person have poor head/neck muscular control?* (Yes/No)
- Does the person's neck flexion allow the chin to rest on their chest? (Yes/No)
If an individual has a positive test for the first two questions or a negative test for question three, the person should be excluded from participation in gymnastic activity.
- *The neck can be assessed by laying the individual on their back with legs straight, the examiner stands in front of the person and pulls the individual into a sitting position using their hands.
The assessing clinical practitioner should be aware of the signs of progressive Myelopathy and address them during a subjective and objective exam, including: 
- Increase in muscle weakness
- Decrease in co-ordination
- Change in gait
- Loss of sensation
- Altered muscle tone
- Recent onset of incontinence
Additionally, an in-depth subjective history of a person with DS should be undertaken by a qualified health professional before sport participation; ideally a practitioner familiar with the individual's baseline function. Further, Morton et al. have previously recommend a gait assessment and neurological examination including tendon reflexes and plantar responses for AAI screening. Neurological examination is promoted as an alternative to radiographs due to the lack of correlation between X-Ray and neurological findings. 
There are manual tests to measure cervical instability, however, there is a paucity of evidence regarding their diagnostic accuracy:
Collectively, after a detailed subjective history, neurological exam, assessment of neck control and use of the British Gymnastics screening tool, an individual will either have unrestricted sport participation or an adapted, more restricted sport participation (if results indicate symptomatic cervical instability). 
Atlanto-Occipital Instability Screening[edit | edit source]
Plain Lateral Cervical Radiographs
As for AAI, AOI is diagnosed following a lateral cervical X-Ray that must encompass the base of the skull and upper cervical spine (C0-C2). The Rule of 12, otherwise known as the Harris measurement, is used to diagnose AOI.
The Rule of 12 uses the basion-dens interval (BDI) values, with the BDI being the distance from the inferior basion and the superior aspect of the dens. Measurements >12mm indicates AOI or a degree of Atlanto-occipital dissociation.
The consensus statement of the Spine Trauma Study Group concluded that the Rule of 12 is the most sensitive and replicable method in diagnosing AOI in Down Syndrome.
Outcome Measures[edit | edit source]
- Neck Disability Index
- Neck Pain and Disability Scale
- Patient Specific Functional Scale
- Neck Bournemouth Questionnaire
- Measurement of the horizontal displacement and angular displacement of the cervical vertebral body on lateral X‑ray films
Management[edit | edit source]
As many long-term follow-up studies show spontaneous stabilisation of the C1-C2 segment, there is not a demand for the provision of symptom-free stabilisation surgery.
Indications for surgical interventions:
- Neurological manifestations caused by spinal cord compression
- Presence of radiographic signs of obvious AAI. (However, when radiographic signs are present without clinical manifestations, then exclusion of the child's participation in contact sports and annual radiologic examinations are recommended)
- Reduced space around the spinal cord
- Basilar impression
Common surgical fixation strategies are: non-instrumented wiring; wiring with rods; screw fixation; hook and rod fixation; and screw and wire fixation.  In addition to fixation, posterior decompression ie. resection of the posterior arch, may be used as a form of surgical management. The goal of these surgeries being to ameliorate symptoms; stabilise the damaged upper cervical segments; and to eliminate spinal stenosis. 
A retrospective study with a population of 131 patients, specifically identifying outcomes of atlantoaxial fusion in paediatric patients found a rate of failed fusion in 11%, instrumentation failure in 2%, and graft failure in 8%. When identifying factors predictive of fusion failure, the singular significant factor was DS. 
However, the outcomes of surgical interventions in individuals with marked instability has large complication rates, and rarely leads to amelioration of neurological symptoms. 
When management is conservative, routine radiological screening for asymptomatic patients remains imperative, allowing regular reassessment of the management plan.
It is important to ensure education is provided to those with DS, family members and professionals involved with this patient group. Increased awareness of the potential signs and symptoms of symptomatic AAI needs to be raised in these populations.
Strengthening: Neck conditioning exercises should be generally promoted among the DS population. 
Education: Increased awareness of the potential signs and symptoms of symptomatic AAI needs to be raised among patients with DS, family members, and professionals involved with managing this group of patients.
Guidelines for Sport[edit | edit source]
Sport and physical activity are highly beneficial for people with DS in regards to biological, psychological and social spheres. Although the risk of damage to the spinal cord in individuals with AAI during sport is extremely rare, precaution must be taken when advising or prescribing exercise to people with DS with AAI in order to mitigate risk of neurological injury.
At present, special olympics athletes must undergo obligatory X-Ray screening, from which a decision is made whether an athlete can participate.  However, Myśliwiec et al. argue the need for a revision in rules to exclude X-Ray screening and introduce neurological examinations instead, as a safer and more cost-effective method of pre-participation sporting assessment.
Sports that are considered high risk of causing symptomatic AAI, and therefore should be avoided or undertaken with extreme caution in the asymptomatic AAI Down Syndrome population, are as follows:
- Diving (including diving starts during swimming)
- Butterfly stroke during swimming
- Any contact sport such as rugby, football and martial arts
- High jump
Any sport should be undertaken with appropriate supervision in order to facilitate safe sporting participation.
All participants and those involved in the individual's care (such as family, health care and sporting or coaching professionals) should be acutely aware of the aforementioned signs and symptoms of symptomatic AAI. If symptomatic AAI is suspected, the individual's spine should be immobilised and they must be taken to an emergency department for immediate screening. From there, a thorough neurological examination should be commenced by a healthcare professional alongside a radiographic or spinal MRI.
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