What is the differential diagnosis?
The differential is between two main entities. 1) a spinal arachnoid cyst located posterior to the cord pushing the cord forward; 2) ventral thecal defect with adherence or herniation of the cord anteriorly.
Which do you favour?
An arachnoid cyst is most likely, as the cord is not focally distorted and no herniation is visible (although both can be subtle).
True or false: Arachnoid cysts often appear as hyperintense compared to CSF elsewhere on T2 fast spin echo sequences.
True. This is due to normal CSF losing some signal due to pulsation induced flow, whereas arachnoid cysts confine the CSF, preventing significant movement and thus preserving T2 signal.
How can this be sorted out?
Myelography / CT myelography is helpful in identifying the margins of the arachnoid cyst. CSF flow studies or high resolution targeted axial T2 images can help delineate an arachnoid cyst if present, or visualise the herniated cord.
How is a CT myelogram performed?
A spinal needle is introduced into the theca and water soluble contrast medium introduced (e.g. 10 mL). The needle is removed and the patient is asked to roll a few times to aid in mixing the contrast. They are then scanned without intravenous contrast.
True or false: Arachnoid cysts do not communicate with the subarachnoid space.
False. Most arachnoid cyst eventually opacify with contrast, although the rate at which they do so is variable. In many instances, the cyst opacifies readily and as such it can be challenging to diagnose with certainty even with prompt scanning.
MRI of the spine demonstrates the upper thoracic cord to be displaced anteriorly, focally abutting the posterior aspect of the vertebral column. Axial T2 images suggest that the cord is being pushed anteriorly by a CSF intensity mass, rather than pulled forward.