Anatomical Variation in the Vertebral Column Flashcards
What are the complete anatomical features of ocipitalisation of the atlas?
Fusion of the anterior arch, posterior arch and lateral masses
What are the partial anatomical features of the occipitilisation of the atlas?
Discontinuity between any part of the axis and occiput
What is occipitilisation of the atlas?
Congenital fusion of the atlas to the base of the occiput.
Associated with abnormalities which lead to narrowing of the space available for the spinal cord or brainstem.
Describe the morphologic classification of occipitilisation by Ghlove et al. (2007)
4 patterns according to the anatomic site of occipitilisation (zones 1, 2 and 3).
- Zone 1 - fused anterior arch
- Zone 2 - fused lateral masses
- Zone 3 - fused posterior arch
- 4th pattern - combination of these fused zones
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Describe the effects of occipitilisation found by Ghlove et al. (2007)
- 57% had atlantoaxial instability
- 27% had associated C2-C3 fusion
- 37% had spinal canal encroachment
- Highest prevalence of spinal canal encroachment (63%) was noted in patients with occipitilisation in zone 2.
How does occipitilisation occur in embryological development?
- Resegmentation of sclerotome into caudal and cephalic parts
- Sclerotome fails to differentiate into cranial and caudal parts
- Cranial part of C1 does not sgement from caudal part of O4
What are the implications of occipitilisation of the atlas?
- Anomalous vertebral arteries
- Torticollis
- Neck pain and stifness
- Myelopathy
- Atrophy of rectus capitis posterior major
- Inadequate attachment for obliquus capitis muscles
Describe anterior congenital absence of C1
- 0.1% prevalence
- Unfused anterior synchondrosis
Describe posterior congenital absence of C1
- 4-5% prevalence
- Type A - 2 hemiarches (90%)
- Type B - Unilateral cleft
- Type C - Bilateral cleft
- Type D - Absence of arch with tubercle
- Type E - Absence of arch and tubercle
What are the implications of anterior congenital absence of C1?
- Instability
- Torticollis due to asymmetrical lateral masses
- No articulation site for the dens
- Absence of the transverse atlantal ligament
- Lack of attachment site for:
- Longus colli and anterior longitudinal ligament (anterior tubercle)
- Atlanto-occipital membrane and anterior atlanto-axial ligament (superior and inferior borders).
- Subluxation and cord compression - quadriparesis
What are the implications of posterior congenital absence of C1?
- Usually stable
- Generally asymptomatic
- Spinal canal stenosis
- Myelopathy
- Neck pain
- Pathogenesis unknown
What are the anatomical features of atlantoaxial subluxation?
- Anterior sliding of C1
- 15-20% occurrence with Down’s Syndrome
Describe the variant: arcuate foramen of the atlas
- Prevalence = 16-17%
- No difference between sexes
- Develops due to genetic or environmental factors
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Pathogenesis:
- Calcification of posterior atlanto-occipital membrane
- Ossified primitive ligaments
- Bony feature that forms a bridge over the vertebral artery
- Accessory transverse foramen of atlas
What are possible reasons for anatomical variation of the atlas (arcuate foramen, occipitalisation and defect of the posterior arch)?
- Some suggest that these ponticles may be remnants of the proatlas (occipital vertebra).
- Others have suggested that they represent ossified primitive ligaments or parts of the posterior atlanto-occipital ligament.
- But, ossification of ligaments resulting in the formation of the foramen arcuale is unlikely because ossification centres have not been observed in these structures
Describe the posterior atlanto-occipital membrane
- From the posterior portion of the superior articular process to the posterior arch of the atlas
- Encloses V3 portion of the vertebral artery
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Variants:
- Complete / incomplete
- Bilateral / unilateral
What are the anatomical features of odontoid dysgenesis?
- Hypoplasia
- Short dens height
- Os odontoideum
- Failed fusion of the dens to the centrum
- Os terminale persistens
- Failed fusion of ossiculum terminale to the body of the dens
- Bifid dens
- Failed or partial fusion of 2 parts of the body of the dens
- Bifurcation of os terminale (dens bicornis)
- Agenesis
- Dens fails to develop
What are the implications of odontoid dysgenesis?
- Neck pain
- Torticollis
- Quadraparesis
- Neuropathies
- Brain stem strokes
- Atlantoaxial instability
- Atlantoaxial subluxation
- No anchorage to transverse atlantal ligament
- Hypermobile dens causing spinal cord or brain stem compression
What are the anatomical features of pseudosubluxation of C2?
- Anterior sliding of C2 on C3
- Juvenile skeleton:
- Hypermobility
- Lax ligaments
- Large head in relation to weak neck muscles
- Shallow, angled facet joints
- Incomplete ossification of odontoid
What are the implications of pseudosubluxation of C2?
- Juvenile skeleton:
- None
- Adult skeleton:
- C2 dislocation
- Possible hangman’s fracture
- Associated with:
- Down’s syndrome
- Moquio’s syndrome
What is Klippel-Feil syndrome?
Abnormal congenital fusion of two or more cervical vertebrae.
Carries a risk of injury to the cervical spinal cord.
Describe the epidemiology of Klippel-Feil syndrome
- 0.71% prevalence
- Female predominance
Describe the aetiology of Klippel-Feil syndrome
- Genetic: autosomal recessive or autosomal dominant
- Environmental: alcohol use
Describe the pathogenesis of Klippel-Feil syndrome
Caused by improper segmentation of cervical vertebrae during weeks 3-8 of gestation.
Describe type 1 Klippel-Feil syndrome
Cervical spine fusion in which elements of many vertebrae are incorporated into a single block.
Describe type 2 Klippel-Feil syndrome
Cervical spine fusion in which there is a failure of complete segmentation at only one or two cervical levels and may include an occipito-atlantal fusion.
Describe type 3 Klippel-Feil syndrome
Type 1 or type 2 KFS with co-existing segmentation errors in the lower dorsal or lumbar spine.
What is the most commonly fused spinal level?
- C2 and C3
- Cho et al. (2014) case study:
- Complete fusion on anterior and posterior aspects
- Foramen transversarium present on both vertebrae
- No IV disc
List the implications of Klippel-Feil Syndrome
- Functional:
- Reduced range of motion
- Degenerative changes / pathology:
- Acquired cervical stenosis → Myelopathy
- Osteoarthritis → Radiculopathy
- Dens fracture
- Abnormal curvature
- Scoliosis in 53.3% of young KFS patients
What causes neural arch variations?
What are the usual symptoms?
Changes in the ossification process.
Often asymptomatic.
List the neural arch variations
- Retrosomatic clefts - vertical defects in the pedicle (thickening/thinning).
- Retroisthmic clefts (rare) - dorsal lamina inferior articular process.
- Spinal dysraphism - failure of fusion of the neural arch.
- Articular processes non-existent.
- Failure of posterior midline fusion (atlas) - unilateral or bilateral.
- Primary spondylosis of the axis - clefts through the C2 pedicles.
What is spinal canal stenosis?
In which regions does it occur?
Narrowing of the spinal canal.
Occurs most commonly in the lumbar or cervical region.
What are the symptoms of cervical spinal canal stenosis?
- Gait disturbance resulting in partial paralysis of the lower limbs.
- This can also limit flexion of the neck
What does stenosis of the spinal canal in the lumbar region lead to?
Compression
What is spina bifida?
A congenital condition where the vertebral column in open posteriorly.
What is the epidemiology of spina bifida?
0.5-0.8 live births
Describe the pathogenesis of spina bifida
- Failed closure of the neural tube in the 4th week post-fertilisation.
- Vertebral arch of the spinal column is either incompletely formed or absent.
Describe spina bifida occulta
- Absence of spinous process
- Variable amount of lamina
Describe meningiocele
Meningeal tissue and CSF herniation
Describe myelomeningiocele
Herniation of meningeal tissue, CSF and CNS tissue
What are the clinical signs of spina bifida occulta?
None
What are the clinical signs of meningiocele?
Usually asymptomatic
What are the symptoms of myelomeningiocele?
- Sensory deficit
- Motor deficit
- Urinary incontinence
- Faecal incontinence
- 65-85% associated with hydrocephalus
How common is spina bifida occulta?
Found in 10-15% of the general population - usually an incidental finding.
Describe the pathogenesis of myelomeningiocele
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Primary pathogenesis:
- Failed neural tube closure in the embryonic spinal region.
- Leads to prologned exposure of the open neural tube to the amniotic fluid environment.
- Remarkably, the bifid neuroepithelium initially undergoes relatively normal neuronal differentiation, with the development of spinal motor and sensory function even below the lesion level.
- As gestation progresses however, the exposed spinal cord becomes haemorrhagic and neurons die as a result of toxicity of the amniotic fluid.
Describe butterfly vertebra
- Rare, only a few cases reported
- Considered asymptomatic
- Occurs between 3-6 weeks gestation
- Symmetric fusion defect - ‘split’ vertebra
- Both chondrification centres fail to develop
- May be confused with a compression fracture if there is trapezoidal or cuneiform anterior wedging.
List the associations of butterfly vertebra
- Pfeiffer syndrome
- JarchoLevins syndrome
- Crouzon syndrome
- Alagille syndrome
- Other spinal anomalies such as kyphoscoliosis, hemivertebrae or spina bifida
Describe hemivertebrae
- Occurs during 4-6 weeks development
- One chondrification centre fails to develop, producing a wedge of bone (half a vertebral body, a single pedicle and a hemilamina).
- In anterior ossification fails, posterior hemivertebrae results.
- Occurs in the thoracic and lumbar regions.
- Results in congenital scoliosis.
What are the 4 classifications of hemivertebra?
- Fully segmented - no attachment to any other vertebrae. The most concerning type.
- Semi segmented - half fused with vertebra. No IV disc.
- Incarcerated - joined by the pedicles. Less concerning.
- Non segmented - connected to others. Less concerning.
List the associations of hemivertebrae
- VATER syndrome
- VACTERL syndrome
- Jarcho-Levin syndrome
- Klippel-Feil syndrome
