"When present, it is seen in continuity with a long syrinx that demonstrates syringomyelia, syringobulbia, and syringopontia 1. As with syringopontia, the syrinx may or may not have a communication with the fourth ventricle 1. In rare cases when it is present without syringomyelia, the term keyhole aqueduct syndrome has been employed by some authors (see syringopontia for further discussion) 2."
"There are numerous causes and associations, and these are discussed in more depth in the general article on syrinx."