Archives of Disease in Childhood, 1989, 64, 283-288

Special report

Instability of the upper cervical spineSKELETAL DYSPLASIA GROUP*

There are probably about 100 different types ofskeletal dysplasia (osteochondrodystrophy), most ofthem genetic in origin and associated with growthretardation and short stature. Many, in addition,have premature degeneration ofjoints and childhoodosteoarthritis.1 Localised structural bony defects areless common, but odontoid absence has been knownto occur occasionally. Interest and alarm has alsobeen expressed at the high incidence of atlantoaxialinstability in Down's syndrome, variously reported

*R Wynne-Davies, Orthopaedic Department, St Thomas'sHospital, London; CM Hall, Paediatric Radiology Department,Hospital for Sick Children, Great Ormond Street, London;CJ Howell, GCW Baker, Harlow Wood Orthopaedic Hospital,Mansfield, Nottingham; J Crossan, Orthopaedic Department,Victoria Infirmary, Glasgow; GA Evans, Robert Jones OrthopaedicHospital, Oswestry; GE Fulford, Princess Margaret Rose Ortho-paedic Hospital, Edinburgh; MA Smith, Orthopaedic Department,St Thomas's Hospital, London; and PJ Witherow, OrthopaedicDepartment, Royal Hospital for Sick Children, Bristol.

Foramen magnum

arch of atlas

Axis vertebra (C2)

at between 10 and 20%, and its association withserious neurological complications after minimaltrauma.2-4The Skeletal Dysplasia Group at a 1985 meeting

(independently of these latter reports) discussed theproblems connected with odontoid hypoplasia orabsence in generalised disorders of the skeleton andplanned a largely retrospective review of cervicalspine anomalies in all patients on their register,where feasible obtaining up to date radiographswith flexion/extension views. This paper contains ashort description of the anatomical problems and areport of findings from seven skeletal dysplasiaclinics in Britain.

Anatomy and measurement of cervical instability

The normal anatomy at the base of the skull, withthe atlas and axis, is shown in figs 1-4. The main

Atar ligament

Transverse ligamentof atlas holdingodontoid in place

Fig 1 Posterior aspect ofthe upper cervical spine with the posterior arch ofthe atlas cut away to show the transverse andalar ligaments. In Morquio's disease not only does the odontoid fail to develop but the ligaments are deficient, softened, andinefficient.

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284 Skeletal Dysplasia Group

_ Atlanto - odontoidinterval in fullflexion

.-T-| -- Minimum sagittaldiameter of spinalconctl in full flexion

Fig 2 Viewfrom above showing the position oftheodontoid process in relation to the anterior arch andtransverse ligament ofthe atlas.

Ib

Sagittal diameter ofspinal canal

Fig 3 Diagram ofthe lateral aspect ofthe atlas and axis inthe neutral position showing the two most importantmeasurements relating to stability: the atlanto-odontoidinterval and the anteroposterior canal diameter (sagittaldiameter)-see text.

features ensuring stability of the region, whileallowing movements of the head and neck to takeplace, are (1) the bony structure of the axis with itsodontoid process; (2) the transverse ligament of theatlas: a thick, strong band which holds the odontoidprocess in contact with the anterior arch of the atlas;and (3) the alar ligaments of the odontoid process:two strong, rounded cords arising one on each sideof the upper part of the odontoid process, passingupwards obliquely and laterally to be inserted intothe occipital bone.

Fig 4 In fullflexion the atlanto-odontoid interval increasesand the canal diameter decreases. The narrowing ofthespinal canal is obvious compared with fig 3. Increasedmovement on flexion and extension occurs when there isinstability in this region, whether associated with afractured, hypoplastic, or absent odontoid, or deficientligaments, or more than one of these factors (see figs 6-7).

Although vertebral anomalies may be noted onplain radiographs, this does not necessarily meanthere will be spinal instability endangering cordfunction. Lateral radiographs in flexion and exten-sion are necessary in order to carry out measure-ments.5 6 The atlanto-odontoid interval is the dis-tance between the posterior edge of the anteriorarch of the atlas and the front of the odontoidprocess (or, in its absence, a line projected upwardsfrom the front of the body of C2). In normalchildren up to the age of 7 years the measurementshould not exceed 4-5 mm, but in adults, with a fullyossified skeleton and in whom ligaments are lesselastic, it is only 2*5 mm.5The second important measurement is the mini-

mum anteroposterior canal diameter (sagittal dia-meter). This is the distance between the posteriorborder of the body of the axis and the posterior archof the atlas, measured in full (voluntary) flexion. Itis difficult to gain cooperation from small children,and precise figures are not available for them, butGreenberg states that if this distance is 14 mm or lessthen cord compression always occurs; if between 15and 17 mm it may occur, but compression virtuallynever occurs if the diameter exceeds 18 mm.5

Subjects and results

There were 182 patients with osteochondrody-strophies on the register for whom adequate cervical

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Instability of the upper cervical spine 285

Table Upper cervical anomalies in the skeletal dysplasias

No of Age range No with No with No with No with Total No (%)patients (years) odontoid absence other defects: known with

hypoplasia base of neurological bony defectskull to C3 complications

Morquio's disease 15 1-15 7 8 0 5 All 15 (100)Other mucopolysaccharide

disease:Hurler's 6 1-13 4 0 4 0Hunter's 5 3-14 3 0 0 0Scheie's 3 6-14 1 0 1 0 -15 of 20 (75)Sanfilipo's 3 1-11 3 0 0 0Maroteaux Lamy's 3 3-6 3 0 0 0

Spondyloepiphyseal dysplasia:congenita 24 Neonate-54 15 1 4 1 20 of 24 (83)tarda 18 3-62 3 0 1 1 4 of 18

Pseudoachondroplasia 23 Neonate-53 8 0 5 0 13 of 23 (57)Achondroplasia and

hypochondroplasia 23 2 months-65 1 0 1 0 2 of 23Multiple epiphyseal dysplasia 18 4-adult 6 0 0 0 6 of 18Chondrodysplasia punctata 14 Neonate-11 2 0 9 0 11 of 14Metatropic dysplasia and

Kniest disease 7 Neonate-15 1 1 2 0 4 of 7Diastrophic dysplasia 6 Neonate-48 1 0 3 1 4 of 6Spondylometaphyseal dysplasia 3 7-16 1 0 0 0 1 of 3

Total 171 59 10 30 8 80 (47)

spine radiographs were available; findings for 171 ofthem are shown in the table. Eleven patients withsclerosing bone dysplasias or tumour like disorderssuch as osteopetrosis and diaphyseal aclasis showedno abnormality and are not discussed further.Osteogenesis imperfecta was excluded as problemsin this area of the spine are compounded byosteoporosis and possible basilar invagination. Nopatient with Down's syndrome had presented at theskeletal dysplasia clinics. Patients with malforma-tion syndromes were also excluded.

It is clear that not only odontoid dysplasia butother anomalies from the base of the skull to thethird cervical vertebra are not unusual among thisgroup of diseases (48% in total). Twenty six flexionextension radiographs were available for study, butthe only positive signs of instability were found in 15patients with Morquio's disease, all of whom wereeither frankly unstable or borderline, as judged bythe measurements described above (figs 5-7). Fiveof these 15 had known neurological complicationsand some had already undergone surgery for stabi-lisation.Cord compression in the cervical region had

occurred in only three other patients: one withdiastrophic dysplasia and two with spondy-loepiphyseal dysplasia, one with the cogenita formand the other tarda, the latter patient being 62 yearsof age before problems occurred. We are suspicious

that the sudden and unexpected death of one otheradult patient with spondyloepiphyseal congenita,after a road traffic accident, was also due to thiscause, but no postmortem examination was per-formed.The incidence of upper cervical anomalies without

known neurological problems is surprisingly high-particularly in spondyloepiphyseal dysplasia con-genita (83%) (fig 8a and b), the mucopolysaccharidedisorders other than Morquio's disease (75%), andpseudoachondroplasia (57%). The proportion isprobably high also in diastrophic and metatropicdysplasias and Kniest disease, although few caseswith adequate radiology were available in theseextremely rare disorders.

MORQUIO'S DISEASE AND EARLY PRESENTING SIGNSOF NEUROLOGICAL DAMAGEDeterioration begins usually during the middle yearsof childhood (4-9 years of age), but early neuro-logical signs are not immediately and easily differen-tiated from existing mechanical problems in thelower limbs that are associated with severe jointlaxity and often gross genu valgum. At this age,these patients should be reviewed every threemonths and recent physical deterioration noted:parents will be aware of the child's loss of endurance,tiredness, 'going off his feet', and perhaps faintingattacks. It is not unusual for symptoms to arise from

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286 Skeletal Dysplasia Group

Posterior archof atlas

Fig 5a and b Tomogram and tracing ofthe lateral aspect ofthe cervical spine (neutral position) in a child with Morquio'sdisease, indicating the space which should be occupied by the odontoid process.

Posterior archof atlas

Fig 6a and b Radiograph and tracing offlexion view in Morquio's disease. The atlanto-odontoid interval (A) is increased(6mm on the original radiograph)-that is, the body ofthe axis is too far back, impinging on and causing appreciablenarrowing of the anteroposterior canal diameter (B,).

lower down the cervical cord, with 'shocks' of 'pinsand needles' going down the upper limbs. Theposition of the head may be significant, being heldtilted backwards, in extension, where 'it feels moresecure'-as indeed it is (figs 7a and b). The patientmay perhaps refer to a fear of the head 'falling off.Damage (which is irreversible) to the respiratorycentre of the medulla will manifest itself as sleepapnoea or sensitivity to low oxygen concentrations-for example, at high altitudes or during anaesthesia.

Long tract signs come later and urinary signs laterstill.The authors are not all agreed on this point, but as

serious and irreversible neurological damage occurswith great frequency in Morquio's disease, there is acase to be made for taking early flexion/extensionviews of the cervical spine and carrying out prophy-lactic stabilisation, perhaps around 6-8 years of age.Surgery at an earlier age would be even morehazardous than it already is, with the child's excess

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Instability of the upper cervical spine 287

/

Posterior arch %_of atlas

Fig 7 a and b Radiograph and tracing ofextension view in Morquio's disease. The body ofthe axis has now moved too farforward and lies in front of the anterior arch ofthe atlas (A); consequently the anteroposterior canal diameter is muchincreased (B2 and refer to figs 6a and b). It is obvious why a child with cervical instability must hold his headpermanently inextension.

,...;.....~~~~~~~~~~~~~~~~...

Baseof skullI

Hypoplastic odontoid andabsent body of C2 A3

Fig 8a and b Lateral radiograph and tracing ofthe upper cervical spine in a patient with spondyloepiphyseal dysplasiacongenita. The anterior part ofthe body ofthe axis is hypoplastic, the odontoid process absent, and there is appreciablesubluxation.

of cartilage and the consequent difficulty in obtain- Discussioning bony fusion. The area of anatomy described infig 1 occupies only about one square inch in these There is clearly a high incidence (48%) of uppertiny children: it is difficult surgery in a dangerous cervical anomalies in a variety of skeletal dysplasias.area. Skilled anaesthesia is crucial, and the proce- Probably all patients with generalised developmentaldure should be carried out in centres experienced in disorders of the skeleton should have a lateralthe technique. radiograph of the cervical spine to establish whether

-\ot

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288 Skeletal Dysplasia Group

or not there are abnormalities here. In a child thisregion can usually be included on the lateralradiograph of the skull. Observation of a hypoplasticor absent odontoid is useless in itself, however; onlyflexion/extension views (active not passive) canshow instability in this region. In view of the almostinvariable problems in Morquio's disease, theseflexion/extension views should be taken as soon asthe child is old enough to cooperate. In otherdiagnoses, particularly spondyloepiphyseal dysplasiacongenita, these need not be taken routinely, butshould certainly be done before anaesthesia.

All cases of odontoid hypoplasia or absence havea higher than normal risk of spinal cordcompression-the potential is there, but in mostcases strong alar (and other) ligaments maintainstability during normal activity. It is when both bonyand ligamentous tissue is faulty (as in Morquio'sdisease) that dislocation and damage to the cordbecomes almost inevitable. One could imagine asimilar situation arising in other 'joint laxity' condi-tions, such as an Ehlers-Danlos syndrome, if bychance the odontoid process were absent or frac-tured. It is also likely that patients with otherskeletal dysplasias are at risk when reduced mobilityof joints, including those of the neck, are associatedwith upper cervical anomalies, as they will then bemore susceptible to damage in this region fromrelatively minor trauma. It must also be rememberedthat in the skeletal dysplasias anatomical variantsmay not be at (or only at) the level of Cl and 2, butlower down at C3 and 4, thus the initial lateralradiograph must include these lower vertebrae.

In the past upper cervical radiographs have notoften been considered necessary, and this, togetherwith the extreme rarity of some of the skeletaldysplasias means, at present, that information isscanty and incomplete. More detailed investigation

of these patients is required and is continuing in theSkeletal Dysplasia Group by the authors and others,with serial studies and more frequent use of flexion/extension radiographs. Other methods of investiga-tion are available: air myelography,7 computedtomography, or magnetic resonance imaging, butthese are usually for evaluating selected problemsand not practical for routine screening. Meanwhile,because of this potential risk to the cervical cord, wewould urge caution both during anaesthesia andphysical activity for any person suffering from askeletal dysplasia. Walking, cycling, supervisedphysiotherapy, and swimming (but not underwateror diving) are recommended, but not body contactsports.

References

Wynne-Davies R, Hall CM, Apley AG. Atlas of skeletaldysplasias. Edinburgh: Churchill Livingstone, 1985.

2 Department of Health and Social Security. Atlanto-axial insta-bility in people with Down's syndrome. London: DHSS, 1986.(CMO(86)9.)

3 Pueschel SM, Scola FH, Perry CD, Pezzullo JC. Atlanto-axialinstability in children with Down's syndrome. Pediatr Radiol1981 ;10:129-32.

4 Committee of Sports Medicine. American Academy of Pedia-trics. Atlantoaxial instability in Down syndrome. Pediatrics1984;74:152.

5 Greenberg AD. Atlanto-axial dislocations. Brain 1968;91:655-84.

6 McRae DL. Bony abnormalities in region of foramen magnum:correlation of anatomic and neurologic findings. Acta Radio-logica 1953;40:335-55.

7 Perovic MN, Kopits SE, Thompson RC. Radiological evalua-tion of the spinal cord in congenital atlanto-axial dislocation.Radiology 1973;109:713-6.

Correspondence to Dr R Wynne-Davies, 2 Dale Close, OxfordOX1 1TU.

Requests for reprints to Mr MA Smith, Honorary Secretary,Skeletal Dysplasia Group, Orthopaedic Department, St Thomas'sHospital, London SE1 7EH.

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