Tubular cavitation of the spinal cord, along with associated progressive neurologic symptoms, has long attracted medical interest. An accumulation of cerebrospinal fluid (CSF) may lead to simple distention of the central canal of the spinal cord; in such cases, the cavity is lined by ependymal cells (ie, hydromyelia). Alternatively, an accumulation of CSF may dissect into the surrounding white matter to form a paracentral cavity, in which case none of the cavity is lined by ependyma (ie, syringomyelia). In many, if not most, cases, both hydromyelia and syringomyelia are present (ie, syringohydromyelia). As the syringomyelic cavity expands, the initial hydromyelic cavity compresses, and the communication between the 2 cavities may atrophy and be lost. Hydromyelia and syringomyelia may be difficult to distinguish even after a detailed histologic examination. Characteristics of syringohydromyelia are demonstrated in the images below. [1, 2, 3, 4]
Barnett et al published a classic article on the etiology of syringomyelia.  The authors classified syringohydromyelic cavities into 5 types:
- Communicating (with the subarachnoid space, usually at the level of the obex at the inferior aspect of the fourth ventricle)
In a study by Milhorat et al, the intramedullary cavities were classified into communicating, noncommunicating, and atrophic types to conform to the classification of the ventricular enlargements of the brain. 
Overall, the etiologies of communicating hydrocephalus include subarachnoid hemorrhage, meningitis, neoplastic seeding of the leptomeninges, and idiopathic causes. A model of the communicating type may be produced by injecting sclerosing agents into the cisterna magna. Obstruction of the basilar subarachnoid cisterns results in symmetric enlargement of the CSF cavities proximal to the blockage, including all 4 cerebral ventricles and the central canal of the spinal cord. [7, 8, 9]
Noncommunicating syringes are subdivided into 6 types:
- Chiari II malformation with hydrocephalus
- Chiari I malformation without hydrocephalus
- Extramedullary compressive lesions at the craniocervical junction or along the length of the spinal canal
- Spinal cord trauma
- Intramedullary tumors and intraperimedullary infections
- Multiple sclerosis
Adhesive spinal arachnoiditis may develop postoperatively or after trauma; following pyogenic, tuberculous, or luetic (syphilitic) meningitis; or after bleeding into the subarachnoid space. Atrophic syringes, which occur with myelomalacia (following trauma or infarction), are associated with a reduction in the transverse dimensions of the spinal cord. Atrophic syringes are limited to a discrete area of myelomalacia in a manner similar to the porencephalic cysts that occur in some cases of encephalomalacia.
Other associated findings include Klippel-Feil anomaly, basilar impression, scoliosis, craniovertebral fusion, and hydrocephalus.
For patients in whom syringomyelia is suggested by clinical criteria, the diagnosis should be confirmed by radiographic investigation.
Associated congenital anomalies include the following:
Basilar impression and platybasia
Spina bifida (which occurs primarily at the C1 vertebra)
Scoliosis, widening of the central spinal canal, and diastematomyelia are additional associated findings.
Peripheral neuropathic joints or even neuropathic alteration of the spine itself may develop in long-standing cases of syringomyelia.
The most common abnormal radiographic musculoskeletal findings are those of joint distention in dislocation, degeneration, and frank joint destruction.
An atrophic form with resorption of the proximal humerus is described frequently in syringomyelia.
The scoliosis in this category is a progressive change that results from the cord cavitation, associated loss of anterior horn cells, and perispinal muscular imbalance, ultimately leading to scoliosis as a secondary phenomenon.
Scoliosis associated with markedly destructive changes of shoulder joints should prompt a search for syringomyelia.
Magnetic Resonance Imaging
The advent of magnetic resonance imaging (MRI) has revolutionized the diagnosis and follow-up imaging of syringohydromyelia. Not only is the disorder more common than previously recognized, but it can now be shown to occur in association with a wide variety of congenital and acquired lesions. With the use of MRI, classifying intramedullary cavities according to anatomic and pathologic types is possible (see the images below). [10, 11, 12, 13, 14, 15, 16]
A significant number of syringes are found in association with a variety of intraspinal lesions; the belief that an abnormality of the cervicomedullary junction is almost always present is not correct.
Most syringes do not communicate with the fourth ventricle. Even syringes that obstruct the cervicomedullary junction tend to be found at some distance below the foramen magnum and are separated from it by a syrinx-free segment of normal spinal cord. Septations within the syrinx cavity are common with more extensive lesions; however, these septa are always incomplete and do not separate one part of a syrinx cavity from another.
Syringohydromyelic cavities are frequently associated with intramedullary neoplasia. Nontumoral cysts extending into the parenchyma of the surrounding spinal cord are located at the poles of the tumors and do not enhance on intravenous (IV) contrast-enhanced MRI, whereas cysts within the substance of the tumor are considered intratumoral cysts and typically demonstrate some degree of peripheral contrast enhancement.
Many MRI findings in syringomyelia associated with adhesive spinal arachnoiditis have been described. The most striking findings include deformity of the cord at the site of adhesive arachnoiditis, poor definition of part of the syrinx wall, a fluid flow void within the syrinx cavity or cavities, and an absence of associated contrast enhancement. Meningeal thickening is a rare finding.
The presence of fluid flow voids in the syrinx cavity is not a differentiating characteristic of syringomyelia; flow voids may be seen in cases associated with Chiari malformations, trauma, and adhesive arachnoiditis. Most cavities associated with intramedullary tumors do not demonstrate a flow void, although rare exceptions have been reported.
The presence of fluid flow voids in syrinx cavities is common (40%); such flow voids reflect pulsatile movements of the syrinx fluid. The appearance of these flow voids may vary, depending on the imaging parameters used. As proposed by some authors, these dynamic movements may cause syrinx propagation; therefore, the observation of fluid flow voids within a syrinx cavity may have prognostic significance.
Detection of fluid movement in a posttraumatic spinal cord lesion may lead to changes in therapy. The preoperative assessment is made on the basis of the presence or absence of cyst fluid flow voids on T2-weighted MRI scans.
Quantitative measurement of oscillating CSF flow  in the spinal canal is possible using an optimized MRI flow-sensitive protocol, which may reveal stenosis-induced velocity changes. Phase contrast flow imaging has now emerged as the technique of choice.
Obstruction of the foramen magnum in patients with Chiari I malformation causes an abrupt systolic downward displacement of the spinal cord and impairs the recoil of CSF during diastole. These observations may have pathophysiologic implications.
Cyst fluid flow measurement is a method for direct serial monitoring of patients with syringomyelic cysts. Diastolic and systolic cyst velocities may assist in evaluating the efficacy of surgery.
Asymptomatic localized hydromyelia has been reported; it probably represents the persistence of a fetal configuration of the central canal of the spinal cord. The central canal of the spinal cord is present at birth and normally becomes progressively obliterated. Cadaver studies have shown that it may persist partially or completely. For this reason, an "incidental" syringomyelic cavity may occasionally be encountered in practice.
Gadolinium-based contrast agents have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Systemic Fibrosis. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see Medscape.