Syrinx (pl. syringes or syrinxes 7) is the collective name given to hydromyelia, syringomyelia, syringobulbia, syringopontia, syringomesencephaly, and syringocephalus.

The use of the general term "syrinx" has grown out of the difficulty in distinguishing between hydromyelia and syringomyelia using imaging modalities. 

Strictly speaking, in hydromyelia, there is dilatation of the central canal of the spinal cord, and thus the lesion is lined by ependyma. While in syringomyelia there is cystic dissection through the ependymal lining of the central canal and a CSF collection within the cord parenchyma itself, and thus this lesion is not lined by ependyma. 

As clinically, there is no difference between the two, and the severity of symptoms is related to the location and size of the lesion, the word "syrinx" may be used as a general descriptor and has the benefit of brevity. 

It is, however, worth knowing the related terms and their distinguishing features. 

hydromyelia: fluid accumulation/dilatation within the central canal, therefore, lined by ependyma

syringomyelia: cavitary lesion within cord parenchyma, of any cause (there are many); located adjacent to the central canal, therefore not lined by ependyma

syringohydromyelia: a term used for either of the above, since the two may overlap and cannot be discriminated on imaging, also known as hydrosyringomyelia

syringobulbia: extension of syringomyelia into the medulla oblongata, although used by some authors to refer to any syrinx in the brainstem

syringopontia: extension of syringomyelia into the pons

syringomesencephaly: extension of syringomyelia into the midbrain

syringocephalus: extension of syringomyelia into the cerebrum, also known as syringoencephalomyelia

syrinx: common name for the cavity in all of the above

Neurological symptoms vary considerably depending on where the syrinx is located in the neuraxis. Classically there is a mix of motor and sensory features, with both acute and chronic effects. 

See the individual articles on syringomyelia, syringobulbia, syringopontia, syringomesencephaly, and syringocephalus for further details of the clinical presentation.

As mentioned, a syrinx refers to any cavity within the spinal cord which may or may not communicate with the central canal. When a syrinx is present rostrally into the neuraxis, syringomyelia is generally also present and the resultant syrinx is generally appreciated as an "extension" from this.

They may be congenital (90%) or acquired in etiology.

Congenital causes include:

myelomeningocele

Chiari I malformation

Chiari II malformation

Dandy-Walker malformation

Klippel-Feil syndrome

Acquired (secondary) causes include:

post-traumatic: occurs in ~5% of patients with spinal cord injury usually from a whiplash-type injury; symptoms may start many months or years after an injury 

cervical canal stenosis

post-inflammatory

secondary to a spinal cord tumor

secondary to a hemorrhage

due to vascular insufficiency

spondylosis 6,8

Radiographic investigations may reveal many anomalies depending on the cause of the syrinx. This section will solely describe the imaging characteristics of the syrinx itself. 

Plain films of the spine may show a subtle widening of the spinal canal in both the sagittal and coronal planes.

The syrinx may be appreciated as an area of decreased attenuation, similar to that of CSF, within the spinal cord.

The syrinx follows CSF signal characteristics on all sequences:

T1: hypointense

T2: hyperintense, although there may be hypointense regions representing flow or pulsation artifact

In many instances, a syrinx can be safely evaluated without post-contrast sequences 6. This is particularly the case when the cause of the syrinx is obvious (e.g. a prominent Chiari I malformation) and the quality of the T2-weighted imaging is high. It is, however, worth noting that if this is not the case then imaging with contrast on the first study is probably prudent to ensure a small enhancing tumor is not present. 

When symptomatic, neurosurgical intervention may be required.