Platybasia is malformation of the base of the skull due to softening of skull bones or a developmental anomaly, with bulging upwards of the floor of the posterior cranial fossa, upward displacement of the upper cervical vertebrae, and bony impingement on the brainstem. It results in abnormal obtuseness of the basal angle that can be demonstrated radiographically. If platybasia is associated with basilar invagination, compression of the brainstem and upper cervical cord can result.
Basal Angle formed by:
line joining the nasion with the centre of the pituitary fossa
line joining the anterior border of the foramen magnum with the centre of the pituitary fossa
normal: 125 degrees-143 degrees
platybasia: > 143 degrees
basilar kyphosis: < 125 degrees