Isolated Sixth Nerve Palsy: A Case of Pseudotumor Cerebri and an Overview of the Evolutionary Dynamic Geometry of Dorello's Canal

The dynamics of increased intracranial pressure (ICP) and sixth cranial nerve palsy has undergone a paradigm shift, with emphasis shifting from a length hypothesis to a theory based on novel anatomic findings pertaining to the geometry of Dorello's canal. In particular, the sixth cranial nerve resides in a transfixed coaxial cylinder within the canal. The cistemal portion of the nerve is intradural and the rest of the nerve is extradural; therefore, with increased ICP, the former is stretched, thereby pulling on the rest of the nerve, which is anchored in Dorello's canal. We present a case of pseudotumor cerebri secondary to minocycline presenting with an isolated sixth nerve palsy. This case is used as a platform to segue into the recent findings outlined above, in particular, the evolutionary transformation of Dorello's canal from a circular outline with a bony roof to an elliptic profile with a fibro-osseus roof during hominid basocranial expansion. The fibro-osseus roof, being elastic, is particularly susceptible to the influence of raised ICP, thereby narrowing the canal and injuring the sixth cranial nerve.