Atlantoaxial instability in Down's syndrome.
Authors: R A Collacott
Journal: BMJ (1987)

SIR,-Dr Richard A Collacott (18 April, p 988) reviewed atlantoaxial instability in Down's syndrome fully but used the term atlantoaxial instability without specifiying its type. In view of the prevalence of this severe complication, the growing awareness of its morphological classification, and the recent development of guidelines for its operative treatment' I we believe that the natural course and treatment of this disorder should be considered. Anterior atlantoaxial dislocation is secondary to the changes in the collagenous tissue that cause laxity in the cervical spine ligaments.' 2 When the transverse ligaments rupture minimal subluxation is possible.3 When the apical and alar ligaments also become stretched considerable instability occurs between C1 and C2, with amplitudes varying by up to 22 mm.4 Anterior subluxation is the cervical instability usually observed in patients with Down's syndrome. In contrast to anterior atlantoaxial subluxation, where soft tissues play an important part in the pathogenesis, vertical subluxation of the odontoid process seems to be a late development of the disease and is related to bilateral destruction of the articular masses in the atlantoaxial facet joints.3 In addition to these two types of atlantoaxial subluxation, four others should be mentioned: posterior atlantoaxial, lateral atlantoaxial, and rotatory atlantoaxial subluxation and nonreducible head tilt deformities, all ofwhich are late developments of the primary anterior type of atlantoaxial subluxation.3-5 The common failure to report their existence in patients with Down's syndrome may reflect the widespread belief that atlantoaxial instability comprises only one of the above changes. Anterior atlantoaxial subluxation is usually a benign condition, causing C2 nerve related occipitocervical headache and signs indicative of myelopathy, which are usually limited to glove like sensory loss in the hands and increased tendon reflexes.3' 6 It is usually the vertical subluxation of the odontoid process and subaxial subluxations that cause the more severe neurological changes.3'5 In most instances focused lateral view extensionflexion radiographs together with open mouth anteroposterior tomograms are required to detect atlantoaxial instabilities. If multiple level instabilities are suspected cervical myelography, computed tomography, and nuclear magnetic resonance imaging may be used to locate and evaluate the cord compression.35 All patients with Down's syndrome should probably be screened radiographically for cervical spine instabilities, although clinical evaluation is still the priority. Gallie type C1-C2 fusion is effective in the treatment of anterior antlantoaxial subluxation.3-5 Postoperatively, however, there is a risk of later subsequent subluxations below the fused segments as a result of laxity of the longitudinal ligaments and mechanical strain.5 The increasing awareness of the clinical relevance of atlantoaxial subluxations should be accompanied by correct use of the term, including its specifications in relation to the morphological classification in Down's syndrome and other diseases. Sports enthusiasts and athletes with Down's syndrome are not the only group at risk. The deleterious effect of cervical instability should also be considered when tooth extraction, physical manipulation, and the induction of general anaesthesia are performed. YRJO T KONTTINEN