Atlanto-occipital dislocation

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Atlanto-occipital dislocation, orthopedic decapitation, or internal decapitation describes ligamentous separation of the spinal column from the skull base. It is possible for a human to survive such an injury; however, 70% of cases result in immediate death. It should not be confused with atlanto-axial dislocation, which describes ligamentous separation between the first and second cervical vertebra.

Mechanism

The injury is a result of disruption of the stabilizing ligaments between the

CT due to its faster speed in the acute trauma setting, although MRI can also help with assessment in equivocal cases. The treatment is initial stabilization with a cervical spine collar, and then surgical intervention in cases in which reversal of paralysis is possible. The most common mechanism of injury is high-speed motor vehicle accidents. The injury is more likely in children due to the large size of their heads relative to their bodies, and more horizontal orientation of the occipital condyles. It represents <1% of all cervical spine injuries.[1]

Several subtypes of atlanto-occipital dislocation are known. One suggested categorization scheme includes anterior, vertical and posterior of the head relative to the spine. Other variants with lateral or rotatory displacement have been described, as well as mixed types. Common etiology for such injuries is sudden and severe deceleration leading to a

whiplash-like mechanism.[2]

Diagnosis

The distances between the

dens and surrounding structures are also key features that can suggest the diagnosis, with the normal distance between the dens and basion (i.e., dens–basion interval; BDI) measuring less than 9 mm on CT, and the distance between the atlas and dens (i.e., atlas–dens interval; ADI) measuring less than 3 mm on CT, although this can be increased in cases of rheumatoid arthritis due to pannus formation.[3]

Several indirect measurements on CT can be used to assess ligamentous integrity at the craniocervical junction. The Wackenheim line, a straight line extending along the posterior margin of the

radiograph than CT. The distance between the atlas and the occipital condyles, the atlanto-occipital interval (AOI), should measure less than 4 mm, and is better assessed on coronal images.[4]

The Powers ratio was formerly used, which was the tip of the basion to the spinolaminar line, divided by the distance from the tip of the

opisthion to the midpoint of the posterior aspect of the anterior arch of C1. It is no longer recommended due to low sensitivity and difficulty identifying landmarks. It also will miss vertical or posterior displacement of the cervical spine.[5]

Other measurements include occiput-atlas distance, angle between anterior arch of atlas and axis, vertical distance between posterior arch of atlas and spinous process of axis, vertical atlanto-dens interval, and joint space between C1 and C2.[6]

Treatment

Treatment involves fixation of the cervical spine to the skull base, or occipitocervical fusion, using paramedian rods and transpedicular screws with cross-links for stabilization. The patient is subsequently unable to rotate their head in the horizontal plane.[7] If there is obstructive hydrocephalus, a pseudomeningocele can form, which is decompressed at the time of surgery.[8]

Prognosis

The injury is immediately fatal in 70% of cases, with an additional 15% surviving to the emergency room but dying during the subsequent hospital stay. A

basion-dental interval (BSI) of 16 mm or greater is associated with mortality. In those with neurologic deficits, survival is unlikely.[9]

Most deaths result from mechanical damage to the

cranial nerve deficits, loss of consciousness, and recurrent respiratory and/or cardiac arrests
.

Children are more likely to survive with neurologic compromise than adults. Isolated cases of near-complete recovery after life-threatening symptoms are known.[10][11]

In case of posterior displacement of the head, a concomitant fracture of the atlas with backward migration of the posterior arch is associated with improved chance of survival, as this allows the spinal cord and medulla to migrate backward without getting crushed.[12]

Proximal cervical fractures associated with injury

The

dens of the cervical spine upon which the skull base sits to allow the head to rotate, can also be associated with atlanto-occipital dislocation. Despite its eponym, the fracture is not usually associated with a hanging mechanism of injury.[13]

In Michael Connelly's 2020 thriller novel Fair Warning, protagonist Jack McEvoy investigates occurrences of atlanto-occipital dislocation. It is later learned that these deaths were at the hands of a serial killer, who manually twists the heads of his victims until their necks break and stages their deaths as accidents or suicides. In the TV show The Good Doctor one patient had to undergo surgery for this problem after a car accident. In season 3, episode 18 of the TV show 9-1-1 a teenage boy is treated for this injury after a train derailment. In episode 1 of season 9 of Chicago Med named Row Row Row Your Boat on a Rocky Sea, a 10 year old boy was treated for this injury after a car accident and fully recovered.

References

  1. PMID 23417184
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  2. PMID 28321388
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  3. S2CID 19468690
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  4. S2CID 20243076
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  5. PMID 11959743
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  6. PMID 19098179
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  7. PMID 23417184
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  8. PMID 17973127
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  9. PMID 19765722
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  10. PMID 12704071
    . Retrieved 2018-05-14.
  11. PMID 6787884
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  12. PMID 11806394
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  13. PMID 21206289
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