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post-traumatic symptoms not clearly related to the anomaly, type C or D without symptoms) imaging studies may include 7: isolated posterior ossicle) may be considered a risk factor for neurological morbidity rather than a developmental variant of normal 7. One theory hypothesizes that a reduced distance between occiput and spinous processes in extension causes spinal cord compression by inward buckling of the ligaments and that this is a possible mechanism of acute or chronic injury.Īnother hypothesis for neurological signs is secondary hypertrophy of the ligaments and dura due to a partial or complete absence of the posterior arch.Īnomalies of the posterior arch of C1 are usually considered benign 4, but may give rise to severe neurological compromise. AssociationsĪssociation with congenital anomaly of the posterior arch of the atlas has been reported in several disorders, including: Incomplete posterior fusion may even be normal in children up to 10 years old 7. either with or without the presence of posterior tubercle (see above)įusion of ossicles usually occurs during age 3 to 5 years.varying degrees of posterior arch dysplasia.In ~2% of the population, an additional ossification center develops in the posterior midline, subsequently forming into a posterior tubercle.ĭuring ossification different anomalies can develop, comprising: two lateral centers giving rise to the lateral masses and posterior arch.an anterior center developing into the anterior tubercle.In the embryological period C1 is usually formed from three primary ossification centers: This rare anomaly is a developmental failure of chondrogenesis (lack of chondrification). In contrast, all other morphological types (B to E) are encountered in only 0.69% of the population 7. Type A and subgroup 1 are by far the commonest (approximating 80% of cases) and are encountered in 4% of the general population 7. 5: acute neurological symptoms following minor cervical trauma.4: various chronic neurological problems.
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3: chronic symptoms referable to the neck.2: neck pain or stiffness after trauma to the head or neck.1: incidental imaging finding, asymptomatic.type E: absence of the entire arch, including the tubercle.type D: absence of the posterior arch, with persistent posterior tubercle.type A: failure of posterior midline fusion of the two hemiarches.At the time of writing (August 2016) the Currarino classification remains the most widely used:
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in 1994 consisting of a combination of morphology and clinical presentation 1. However, post-traumatic symptoms have been described in the milder forms 7.Ĭongenital anomalies of the posterior atlas arch can be classified according to a system proposed by Currarino et al. For example, symptoms due to impingement of the cord by a posterior tubercle during extension is well described in the literature 2,7. Not surprisingly, higher morphological groups (especially C and D, owing to a posterior tubercle) tend to be symptomatic, either post-trauma or intermittent to chronic. While most are asymptomatic and present as an incidental finding on imaging studies performed after trauma or for other reasons, severe symptoms such as intermittent tetraparesis after minor cervical trauma have been described in the literature 7,8. Clinical presentationĬlinical presentation is highly variable. Estimates from a recent extensive review range from 0.7-3% 7.
ATLAS ANIMALIA STATBOOK SERIES
The reported incidence in one larger series from 1930 was 4% 3.
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