Age / Sex : 25 / M

Age / Sex : 47 / M

Chief complaint : Headache and posterior neck pain

Congenital defect of posterior arch of the atlas

Discussion

Findings:
Absence of the posterior arch with persistent posterior tubercle

Differential Diagnosis: Post operative change

Diagnosis: Congenital Defect of posterior arch of the atlas

Discussion:

Normally atlas has three primary ossification centers during the embryonic period. The anterior tubercle formed from the anterior ossification center, and two lateral centers form to lateral masses and the posterior arch. Lateral masses unite posteriorly, giving rise to the posterior arch at 3-5 years of age. Anterior arch usually unites with two lateral centers at 5-9 years of ages. Defect of posterior arch of atlas is believed to occur due to failure of local chondrogenesis rather than ossification. Currarino et al. classified this anomaly into five types, depending on the extent of absence of posterior arch and presence of posterior tubercle: type A - failure of posterior midline fusion of the two hemi-arches; type B - unilateral cleft; type C - bilateral cleft; type D - absence of the posterior arch with persistent posterior tubercle; type E - absence of the entire arch including posterior tubercle. Type A anomaly is seen over the 90% of all posterior arch defects and present in 3% to 4% of individuals. In their study, they estimated the Type B-E anomalies as 0.69% of all population. Currarino et al. also divided the posterior arch defects of atlas into five clinical groups: Group I - asymptomatic, their anomaly is found incidentally; Group II - neck pain or/and stiffness after trauma to neck or head; Group III - sudden neurological symptoms after neck or head trauma; Group IV - various neurological symptoms for some time before the diagnosis of the anomaly; Group V - chronic symptoms referable to the neck.

The clinical presentation of congenital aplasia of the posterior arch of atlas can be variable. According to Currarino’s review, almost one third of these patients are asymptomatic. However, the neurological presentation in this anomaly described clearly in a few articles in literature, including sensory symptoms such as paresthesia in all four limbs, in both upper limbs only, ipsilateral upper and lower limbs or motor deficits such as episodic weakness of all four limbs.

The presence of a posterior tubercle is important for determination of clinical aspect. The posterior tubercle can cause transient quadriparesis by impinging on the spinal cord during neck extension or following minor trauma to neck or head.

Sharma et al were first to document the inward mobility of posterior fragment during extension of neck. Their findings supported the hypothesis that an isolated posterior bony fragment is the potential cause of neurological morbidity. The cumulative effects of trauma may cause myelopathy, additional trauma or inappropriate posture related to neck may cause compression on cervical part of spinal cord too.

It is important to determine the type of the defect on the posterior arch of atlas to understand the clinical significance and to prevent further neurological complication. Type A and B do not cause neurological symptoms but Type C-D are likely to cause transient quadriparesis after minor trauma even inappropriate positioning of neck and head. The other considerable matter is that a patient with this anomaly should be warned about avoiding contact sports and other strenuous athletic sports, especially in the presence of Type C-D anomaly and experience of neurological symptoms. Treatment of choice is removal of isolated posterior tubercle, especially type C or D, to prevent cumulative damage to the cord. No treatment is recommended for type A and B.

References:

1.      Klimo P Jr, Blumenthal DT, Couldwell WT. Congenital partial aplasia of the posterior arch of the atlas causing myelopathy: case report and review of the literature. Spine. 2003; 28: E224–228

2.      Currarino G, Rollins N, Diehl JT. Congenital defects of the posterior arch of the atlas: a report of seven cases including an affected mother and son. Am. J. Neuroradiol. 1994; 15: 249–254

3.      Congenital defects of posterior arch of the atlas: a case report. Senay OZDOLAP, PhD.  Neuroanatomy (2007) 6: 72–74

4.      Partial Aplasia of the Posterior Arch of the Atlas with an Isolated Posterior Arch Remnant: Findings in Three Cases. Aseem Sharma. AJNR 21:1167–1171, June/July 2000

5.   Sharma A, Gaikwad SB, Deol PS, Mishra NK, Kale SS. Partial aplasia of the  posterior  arch of the atlas with an isolated posterior arch remnant: findings in three cases. Am. J. Neuroradiol. 2000; 21: 1167–1171