Odontoid Hypoplasia

Description

The odontoid process, or dens, is a bony projection from the axis (C2) upward into the ring of the atlas (C1) at the top of the spine. During embryogenesis, the body of the odontoid derives from the centrum of the atlas and separates from the atlas, fusing with the superior portion of the axis. If the odontoid is hypoplastic or absent, the attachments for the apical and alar ligaments are missing, allowing for excessive rotation of the atlas, craniocervical instability, and possibly cord compression (summary by Stevens et al., 2009).

Clinical Features

Stevens et al. (2009) reported a 3-generation family in which 4 individuals had odontoid hypoplasia. The proband was a 16-year old girl who had dislocation of the hips as an infant and mild limitation of elbow supination. Cervical radiographs showed odontoid hypoplasia with 8-mm C1-C2 instability, and MRI showed a small, blunted odontoid and articulation of a dystopic os odontoideum with the skull base. There was craniocervical instability and kinking of the medulla. Other radiographic studies showed bilateral radial head abnormalities and short metacarpals, particularly the fifth. The proband was asymptomatic but evaluated due to a positive family history. Her mother had similar radiographic studies with 9-mm cervical spine mobility, and was diagnosed at age 5 years. She first experienced severe cervical pain radiating to the extremities after diving at a whitewater park at age 36 years. She underwent fusion of C1-C2 at age 39, and has remained neurologically asymptomatic. A maternal uncle of the proband was briefly paralyzed after head-first diving at age 12. He underwent C1-C2 fusion and has remained asymptomatic. The maternal grandfather was also affected, and underwent fusion of C1-C2. He had progressive neurologic symptoms and underwent additional surgery for occipital to C4 fusion. He has quadriparesis, more severe in the upper extremities, and is not able to walk or stand. Stevens et al. (2009) noted that most individuals with this finding are asymptomatic and may only experience problems after trauma.

Inheritance

The presence of odontoid hypoplasia in the family reported by Stevens et al. (2009) was consistent with autosomal dominant transmission. Stevens et al. (2009) referred to an earlier report of a family with autosomal dominant transmission of odontoid hypoplasia (Shepard, 1966), suggesting a genetic etiology.