Natural History

Pueschel et al.10,11 reported the occurrence of atlantoaxial instability in 14.6% of 404 patients with Down syndrome who underwent dynamic cervical spine radiograph studies. Of these, only six (close to 1%) were symptomatic. Although the authors demonstrated an increase in the atlantodental interval in serial radiographs in a subgroup of patients at a follow-up of 3 to 10 years, none of them developed clinical manifestations. Burke et al.12 reported a progressive increase in the predental space in 6 out of 32 patients followed up over 13 years. Morton et al.,13 however, reported on 67 patients with Down syndrome followed up for ∼5 years and found that none developed de novo atlantoaxial instability. In other reviews, 7 to 40% of patients with Down syndrome have been reported to have atlantoaxial instability, with <1% being symptomatic. Ferguson et al.14 reported no statistical difference between the incidence of symptomatic myelopathy in the subluxator and nonsubluxator groups. Menezes8 commented that in his experience, the presence of atlantoaxial disease in children with Down syndrome does not have a benign relationship to neurological function.

The Committee on Sports Medicine of the American Academy of Pediatrics15 recommended routine screening of patients with Down syndrome with cervical spine radiographs prior to their participation in Special Olympics. The committee further recommended no routine follow-up radiographs in patients who do not have atlantoaxial instability in the initial radiograph. Most authors agree that asymptomatic patients with Down syndrome and atlantoaxial dislocation may be followed up clinically and radiologically.

Radiological Features

The common radiological abnormalities of the CVJ in Down syndrome are atlantoaxial instability, occipitoatlantoaxial instability, rotatory atlantoaxial or occipitoatlantal dislocation, basilar invagination, os odontoideum, and bifid or hypoplastic atlantal arches. The generalized ligamentous laxity associated with Down syndrome results in development of hypermobility that is in part responsible for development of some of the bony anomalies, such as os odontoideum.8–10

Management

The clinical presentation in these patients is akin to those with any CVJ anomaly, with neck pain, torticollis, and features of cervical cord compression. Sudden onset of neurological worsening associated with trauma, intubation procedures, or associated upper respiratory infection has also been documented.8

The goal of treatment in symptomatic patients is reduction of the instability, neural decompression (by transoral decompression in irreducible atlantoaxial instability), and posterior stabilization of the CVJ. Nader-Sepahi et al.16 highlighted the importance of recognizing occipitoatlantal instability coexisting with atlantoaxial instability and recommended occipitocervical fusion in such cases. Incorporation of the occiput into the fusion has also been recommended in cases where there is an abnormality of the atlantal arch or where the atlantal arch has been removed to achieve neural decompression. Menezes8 commented that C1–C2 transarticular fixation may be used in isolated atlantoaxial arthrodesis, supplemented with bilateral interlaminar fusion.

Taggard et al.17 and Menezes8 demonstrated good results with bony fusion in 95% of 64 patients treated over 17 years. Nader-Sepahi et al.16 reported successful fusion in 7 of 12 patients after the first operation but ultimately achieved 100% fusion with repeat surgeries. However, others have reported less encouraging results. In a review of complications of upper cervical spine fusion in children, Smith et al.18 identified Down syndrome as one of the risk factors pre-disposing to incomplete fusion. Segal et al.19 reported that almost all of their patients with Down syndrome who underwent posterior arthrodesis had some complication, ranging from wound infection, incomplete reduction of the atlantoaxial dislocation, instability of the adjacent motion segment, to neurological deterioration. In their series of 10 patients, only 4 developed bony fusion. They commented that the following reasons could be responsible for graft resorption in patients with Down syndrome:

1. 
   

   Patients with this syndrome have immune deficiencies resulting in decreased lymphocyte and monocyte function and may not be able to mount an initial inflammatory response that is essential for bone graft absorption.

   
   
2. 
   

   Intrinsic defects in collagen could contribute to poor fusion.

   
   
3. 
   

   Fibroblasts cultured from patients with Down syndrome have been documented to have increased expression of the β-interferon receptors (the genes that encode this receptor are located in chromosome 21), and this could enhance fibroblast activity, promoting release of collagenase and protease.20 This increased activity could contribute to graft resorption.

Taggard et al.17 found that inadequate postoperative immobilization, failure to recognize and appropriately treat ventral pathology, lack of consideration of bony anomalies, and inadequate bone grafting are the main causes of incomplete fusion. Halo vest immobilization would be ideal but difficult to maintain in these patients due to pin site morbidities in young children and the presence of mental retardation in several of these patients. Figure 44.1 illustrates the radiographs in a patient with Down syndrome and symptomatic atlantoaxial dislocation.