Congenital abnormalities of odontoid can be divided into three groups
- Aplasia – Complete absence of odontoid.
- Hypoplasia – Partial development of odontoid
- Os odontoideum – In os odontoideum, the odontoid is an oval or round separated from the axis vertebra by a transverse gap. The lesion is mostly discovered as an incidental finding. [Recent studies have revealed that Os odontoideum is actually result of trauma in early age and not a congenital abnormality]
Congenital abnormalties of odontoid are more common in patients of
- Down syndrome
- Klippel-Feil syndrome
- Morquio syndrome
- Spondyloepiphyseal dysplasia.
Diagnosis of Congenital Abnormalities of Odontoid
A number of patients may be asymptomatic. Patients may present with neck pain, torticollis, or headaches. Neural deficits may be present. Proprioceptive and sphincter disturbances are common findings.
Vertebral artery compression may cause cervical and brainstem ischemia of brainstem andcervical spine resulting in seizures, syncope, vertigo, and visual disturbances.
Routine radiographs can help to diagnose anomalies of odontoid . CT scans with reconstruction views and MRI help to define the lesion compression better.
Odontoid aplasia appears as a slight depression between the superior articulating facets on the open-mouth odontoid view.
Odontoid hypoplasia is seen as a short, bony remnant.
With os odontoideum, a space is present between the body of the axis and a bony ossicle. The free ossicle of os odontoideum usually is half the size of a normal odontoid and is oval or round with smooth, sclerotic borders.
Os odontoideum can be confused with fracture and can be differentiated by its wide and smooth margin as compared thin and irregular in case of fracture.
Measurement of instability is done by noting the distance between a projecting a line superiorly from the body of the axis and a line projected inferiorly from the posterior border of the anterior arch of the atlas.
Measurements greater than 3 mm in adults and 4 to 5 mm in children indicate significant instability.
MRI can be useful in identifying reactive retrodental [behind the dens] .
The prognosis of os odontoideum depends on the clinical presentation. The prognosis is good if only mechanical symptoms (torticollis or neck pain) or transient neurological symptoms exist. It is poor if neurological deficits slowly progress.
Treatment of Congenital Abnormalities of Odontoid
A congenital abnormalities of odontoid may reveal abnormal atlantoaxial joint. This can subluxate or dislocate with minor trauma and cause permanent neurological damage or even death. This is the major concern with atlantoaxial instability
Conservative treatment, such as cervical traction or immobilization helps the patients with local symptoms.
Operative intervention is required in following cases with congenital abnormalities of odontoid
- Neurological involvement including transient episodes.
- Instability of more than 5 mm anteriorly or posteriorly
- Progressive instability
- Persistent neck complaints associated with atlantoaxial instability not relieved by conservative treatment.
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