Discussion Author(s): Ashley Conway,
Geoffrey Gerow, D.C.
Joanna Garvey, D.C.
Krishnan Kartha, M.D.
Joseph Serghany, M.D.
Greg Feld, M.D.
The Central canal is an ependymal lined tube within the spinal cord from the obex to the conus medullaris. It is usually present at birth and undergoes segmental occlusion with age and is obliterated most adults, but persists and is visible on MRI in about 1.5% of the adult population.(1,2) MRI has increased the number of incidental findings of fluid cavities within the spinal cord. The nomenclature used to describe these findings has lead to physician-physician miscommunication and patient confusion. (3)
Radiological use of syringomyelia, syringohydromyelia and syrinx as umbrella terms for these incidental findings which are likely not true syringomyelia, may imply a prognosis of progressive neurological deterioration to clinicians and patients, when in fact the patient may have a benign dilatation of a persistent central canal. (3, 4,5)
Clinically, syringomyelia is used to describe a characteristic central cord syndrome presenting with loss of pain and temperature sensation in a cape distribution (neck, shoulders and upper arms) or in the hands, with sparing of touch and vibration as well as areflexic weakness in the upper extremities (dissociated sensory loss). Patients may present with painless injuries or burns to hands from lack of pain sensation or experience central cord and dysesthetic pain. (6,7)
Syringomyelia is associated with developmental abnormalities of vertebral column, base of the skull or cerebellum and brainstem: 90% of the time, syringomyelia is in association with a Chiari I malformation.(8) There are also acquired cases, such as post-traumatic syringomyelia occurring after serious spinal cord injury, sometimes many years later. It is estimated that 21-28% of patients with spinal cord injuries develop syringes, 30-50% will have some cystic spinal cord changes and 1-9% will have symptoms. (9)
Syringomyelia was coined by Ollivier d'Angers in 1827, before the recognition of the central canal as a normal structure. 10 After the central canal was recognized as a normal structure, it was assumed that the cavitations of the spinal cord previously described as syringomyelia were dilatations of the canal, and the term hydromyelia came into use. By the late 1800s, cavities unconnected to the central canal were described and it was suggested that syringomyelia be reserved for extracanicular cavities and that the term hydromyelia be used to describe dilatation of the central canal. (8)
At present, several classification schemes exist. Several factors likely contribute to lack of consensus concerning the nomenclature, including controversy regarding the pathogenesis, natural history of disease, and best treatment.
A classification scheme in Adams and Victorâ€™s Principles of Neurology textbook is based on patency of the foramen magnum and distinguishes between developmental/acquired, but not between intra/extracanicular cavities: (8)
â€˘ Type I. Syringomyelia with obstruction of the foramen magnum and dilatation of the central canal (developmental type)
â€˘ Type II. Syringomyelia without obstruction of the foramen magnum (idiopathic developmental type)
â€˘ Type III. Syringomyelia with other diseases of the spinal cord (acquired types)
D. Secondary myelomalacia from cord compression
â€˘ Type IV. Pure hydromyelia (developmental dilatation of the central canal)
Milhorat et al. published a classification scheme based on communication with the fourth ventricle:(11)
â€˘ 1. Communicating syringomyelia - dilations of the central canal that are continuous with the fourth ventricle
â€˘ 2. Noncommunicating syringomyelia - dilations of the central canal that do not communicate with the fourth ventricle. Primary and parenchymal cavitations - extracanalicular syringes that originate in the spinal cord parenchyma and do not communicate with the central canal or fourth ventricle.
â€˘ 3. Atrophic syringes occurring with myelomalacia (syringomyelia ex vacuo)
â€˘ 4. Neoplastic cysts.
Why does differentiating between post-traumatic syringomyelia and a persistent central canal/hydromyelia matter?
â€˘ 1. For patients: to prevent unnecessary anxiety, testing and follow-up in those with persistent central canals found incidentally on imaging that are unlikely to progress or be the cause of patient symptoms.
â€˘ 2. For clear communication between physicians, eg radiologists and primary care physicians/neurologists/neurosurgeons.
â€˘ 3. For legal purposes eg, in the case of potential post-traumatic syringomyelia in patients involved in litigation related to their injury.
â€˘ 4. For identifying true syringomyelia subgroups likely to share a common pathogenesis for research purposes.
MECHANISM OF INJURY
Despite a body of research, syringomyelia pathogenesis remains controversial. Acquired syringomyelia is associated with conditions that alter CSF flow or irritate the cord such as trauma, hemorrhage, meningitis, neoplasms or arachnoiditis. Post-traumatic syringes are commonly found in vascular watershed areas, suggesting ischemia may play a role in formation of a syrinx. Another possibility may be inflammation leading to edema and cyst formation or absorption of a hematoma leading to a cyst. Within the cyst, passive fluid accumulation is unlikely because syringes have high opening pressures.(9) The theory that obstructing CSF flow is involved in the pathogenesis fits with the association of syringomyelia with subarachnoid adhesions and the improvement in patients that undergo arachnoidolysis. For a detailed review of the postulated hydrodynamic mechanism of syrinx formation, see Greitz et al.(12)
80% of post-traumatic syringomyelia patients are males, which can be attributed to the male preponderance in spinal cord injury. 9
PROGNOSIS & TREATMENT
Prognosis: Usually slow progressive decline in neurological function, however some estimates for static course over 10 years are as high as 50%.(9)
Treatment: When to intervene surgically remains debatable. Some advocate for surgical intervention as soon as a cavity is identified to prevent further permanent loss of function, while others recommend waiting until the condition is symptomatic and progressing. Patients managed nonsurgically should have magnetic resonance imaging every 6â€“12 months and close clinical observation.(9) Post-traumatic syringomyelia arachnoidolysis or untethering was shown to be superior to shunting for patients with progressing symptoms. 94% were at least stabilized with arachnoidolysis. 13
Persistent central canal:
Holly and Batzdorf retrospectively reviewed 45 patients with slit-like syrinx cavities with an average size of 2mm who were imaged for mechanical spinal pain, radicular pain, paresthesia, numbness, and muscle spasm. Patients were followed for an average of about 3 years. The authors concluded that these cavities were remnants of the central canal â€“ not true syringomyelia â€“ were not the cause of patient symptoms and were unlikely to change in size. 4 Roser et al. also followed hydromyelia patients for 36.9 months, none had radiological progression of hydromyelia.3
Description of syringomyelia from Spinal Cord Medicine, Principles and Practice: 14
â€śAn intramedullary cyst with well-defined margins, which is eccentric, extends beyond the limits of the original cord injury, does not communicate with the central canal or the fourth ventricle, and is isodense with CSF (i.e., low-density T1-weighted image, high-density T2-weighted image) is characteristic. Syrinx MRI features that are associated with neurologic decline include long and wide, CSF flow from cardiac-gated images, poorly demarcated T2-weighted signal hyperintensity at the rostral extent, flow void sign on T2-weighted images suggesting high pressure, and association with spinal stenosis.â€ť
A study of axial MRIs from 115 patients found the following correlations: 15
â€ś1) Symmetrically enlarged central cavities were asymptomatic or produced nonspecific neurological signs.
2) Central cavities that expanded paracentrally were associated with segmental signs referable to the paracentral component.
3) Eccentric cavities produced various combinations of long tract and segmental signs that could usually be related to the level, side, and specific quadrant of spinal cord cavitation.â€ť
Persistent central canal is defined by Holly and Batzdorf as having a consistently thin diameter of the cavity (average 2mm) and its lack of pathogenic factors that alter CSF dynamics. 4
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