Original classification as described by Feil is based on the extent of fusion but is not clinically useful. Etiology is unknown. What is known is that there is a failure of the normal segmentation of cervical somites during the third and eighth weeks of gestation. Some hidden anomalies can be fatal. Generally flexion and extension are better preserved than side- bending and rotation.
Patients have the ability to compensate at the unfused segments.
These inter-spaces can become extremely hypermobile. Ptosis of the eye, Duane's eye contracture, lateral rectus palsy, facial nervepalsy and cleft palate. Upper extremity abnormalities, ie. These may show flattening and widening of vertebrae, hemivertebrae or block vertebrae, instability. MRI with head flexed and extended should be part of the work-up.