Atlantoaxial instability andabnormalities ofthe odontoid inDown'ssyndrome

SUMMARY Radiography ofthelateral neckwas performed on allchildren withDown's syndrome aged4-15intheSouthern Derbyshire health district (n=67) anda randomselection ofadults (n=94). Atlantoaxial instability, defined asa gapofover4mm, was present inseven (10%)children andtwo(2%)adults. Odontoid hypoplasia, defined as an odontoid peg two standard deviations belowthemean inan agematchedpopulation, was present in15(22%) children and14(15%)adults, withaccessoryodontoid ossicles present intwo(2%)andtwo (3%)respectively. No one was foundtohavesymptomsor clinical signs ofspinal cord compression. Atlantoaxial instability was therefore foundtobecommoner inchildren than adults. Different programmes ofmanagementare suggested, intermsofregular clinical examination forsigns andsymptomsandbyradiographic screening. Particular careshouldbe takenwiththose whohavebothatlantoaxial instability andodontoid hypoplasia or accessory ossicles as theyare atparticular riskofspinal corddamage. Interest inuppercervical instability inDown's syndrome hasrecently beenrenewedafter the recommendation bytheCommittee onSports ofthe AmericanAcademyof Pediatrics relating to participation insporting activities.' TheCommittee recommends preparticipation radiographs ofthe cervical spine inflexion, extension, andneutral, withattention beingdirected totheatlantoaxial region inordertoprevent serious damagetothe spinal cord.Similar recommendations havebeen madeinBritain bytheDepartment ofHealth and Social Security (DHSS)2 andMencap.3 Abnormalities oftheuppercervical spinein Down'ssyndrome havebeenrecognised since the original workofTishler andMartel in1965.4 There isa reported incidence ofatlantoaxial instability ofbetween9-5and27%.5-7 People withDown's syndrome alsohavean increased incidence of abnormalities oftheodontoid peg6andthismay predispose todamageiftheyalsohaveatlantoaxial instability.8 We found13casesofsymptomatic atlantoaxial instability inDown'ssyndrome reported intheliterature inwhichfeatures ofthe odontoid pegwerespecifically mentioned. All13 hadaccessory ossicles andeight hadcoexistent odontoid hypoplasia, although thesizeofthe odontoid pegwasnotmentioned intheother five.817