Pseudosubluxation of the Cervical Spine (Paediatric)

Pseudosubluxation is a physiological anterior displacement of C2 on C3, and sometimes of C3 on C4, of up to about 2-3 mm in young children. It is a normal variant, not an injury, and it is the single most important paediatric cervical normal variant to recognise, so that a normal child is not misdiagnosed as having a fracture-dislocation.

Why it happens. The immature cervical spine has ligamentous laxity, relatively horizontal facet joints and a high fulcrum of flexion-extension motion, around C2-C3, together with a large head-to-body ratio.

Who. It is common and age-related. C2-C3 pseudosubluxation has been reported in around 9% of children younger than 7 years, and it generally resolves as the spine matures.

Drawing the line. The key tool for telling pseudosubluxation from a true injury is the Swischuk posterior cervical (spinolaminar) line. It is drawn between the anterior cortex of the C1 spinous process and the anterior cortex of the C3 spinous process, C1 to C3 only, and C2 is then judged against it.

The commonest error. Drawing the line through C2 destroys the test, because a line through a point always passes through it.

Reading it. In physiological pseudosubluxation the anterior cortex of the C2 spinous process lies within about 2 mm of the C1-C3 line. In a true C2 injury such as a hangman's fracture the posterior arch of C2 stays with C3 while the body displaces forward, so C2 falls more than 2 mm behind the line.

The other reassuring features. Beyond an intact line, pseudosubluxation:

• reduces on an extension (or neutral) view
• has smooth alignment, without a focal kyphotic angulation
• shows no prevertebral soft-tissue swelling beyond the normal range
• shows no fracture

An intact posterior cervical line and reduction on extension confirm a normal variant.

Pseudosubluxation sits among a family of paediatric cervical normal variants, each of which can be mistaken for trauma. Knowing them prevents over-diagnosis.

• Hypermobility and pseudospread of the atlas on the axis
• Absence of the normal cervical lordosis
• Anterior wedging of the still partly cartilaginous vertebral bodies
• Prevertebral soft-tissue pseudowidening, worse on expiration or neck flexion
• Incomplete ossification with open synchondroses

Knowing the values is what avoids over-calling prevertebral swelling and pseudospread, and the paediatric normal ranges differ from the adult. The instability thresholds are developed in Atlantoaxial Instability.

Atlanto-dental interval (ADI). The gap between the anterior arch of C1 and the dens is normally wider in children, up to about 5 mm against around 3 mm in adults, because of greater ligamentous laxity. A value beyond this suggests atlantoaxial instability or transverse-ligament insufficiency.

Prevertebral soft tissue. The shadow is narrow above the larynx and wider below. A working rule in a child is up to about 7 mm at C2, roughly two-thirds of a vertebral-body width, and up to about 14 mm at C6.

The C6 figure is the paediatric one. The adult upper limit is around 21 mm, so applying the adult number to a child under-calls a genuine haematoma.

Pseudowidening. The prevertebral shadow is markedly exaggerated on expiration, neck flexion or crying. Obtain a neutral inspiratory film before calling it a haematoma.

Pseudospread of the atlas (Jackson). In young children the C1 lateral masses can overhang the C2 body by a few millimetres, because the ossification centres of C1 and C2 grow at different rates. The adult rule that combined lateral overhang implies a Jefferson burst fracture does not apply in the young child.

Incomplete ossification, open synchondroses, differential ossification-centre growth and partly cartilaginous bodies all stem from the same developmental anatomy. The open synchondroses are the classic fracture mimics; the odontoid in detail is in Odontoid Fractures.

The centres. The axis (C2) forms from several centres: the body, two neural arches and the dens, which arises from paired centres and carries an apical ossiculum terminale. The atlas (C1) has three: the anterior arch and two posterior (neural) arches.

Unfused C1 and C2 arch synchondroses look like arch fractures.

Synchondrosis or fracture. The distinction rests on four features.

Anterior wedging. The normally partly cartilaginous, anteriorly wedged vertebral bodies explain the anterior-wedging variant. Differential ossification-centre growth explains both anterior wedging and pseudospread.

Correlate. Interpret the paediatric cervical radiograph in light of the child's age, the mechanism of injury and the clinical examination, not in isolation.

Don't over-treat. Recognise physiological pseudosubluxation with the Swischuk line and clinical correlation, rather than immobilising or investigating a normal child unnecessarily.