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Home , Cerebrospinal fluid, Ventricular system

Syringomyelia and CSF flow

P.M. Parizel, L. van den Hauwe, F. Vanhevel, J. Van Goethem, F. De Belder, C. Venstermans, T. Van der Zijden, M. Voormolen, P. Pullens

Dept of Radiology & Neuroradiology Antwerp University Hospital & University of Antwerp Belgium

Introduction and definitions

Syringomyelia refers to the presence of a fluid-filled cystic cavity within the . The intramedullary cyst, which is called a syrinx, can increase in size over time, and progressively destroys the spinal cord. The word “syrinx” is derived from the ancient Greek word for “tube” (the same root is found in the word “syringe”). The term syringomyelia was first coined by Ollivies d’ Angers (1827). The reported prevalence of syringomyelia is 8.4 per 100,000. The condition occurs more frequently in men than in women, and is most common in the third and fourth decades of life. Rarely, it may develop in childhood or late adulthood. Neuropathologically, a syrinx refers to a spinal cord cavity, which is lined with astrocytic gliosis, and is independent of the ependymal canal. The term hydromyelia indicates excess in the of the spinal cord. Hydromyelia is characterized by a tubular, elongated dilatation of the ependymal canal. This cavity remains lined with ependymal cells. When the fluid enters the surrounding parenchyma of the spinal cord, forming a fluid-filled cystic cavity or syrinx, the term syringomyelia is applied. Thus, hydromyelia can be seen as the spinal cord equivalent of in the : dilatation of a pre-existing space, filled with CSF and lined with ependymal cells. Conversely, syringomyelia could be interpreted as the equivalent of a porencephalic cyst or encephalomalacia in the brain: a cavity within the substance of the parenchyma. On most neuro-imaging studies of the spine, which are performed with a slice thickness of 2-5 mm, it is impossible to accurately distinguish between syringomyelia and hydromyelia. Thus, some authors have used the hybrid terminology of “syringohydromyelia”, which covers both types of cavity or cyst in the spinal cord.

Clinical presentation

The symptoms of syringomyelia depend on the size, extent, and location of the syrinx within the spinal cord. Damage to spinal cord by the presence of a cystic cavity may lead to a wide array of symptoms including (neck) pain, paralysis, weakness, stiffness in the back and shoulders. A typical finding on clinical neurologic examination is the so-called “thermo-analgesic dissociation”, meaning that the patient is unable to feel extremes of hot or cold. Loss of pain and sensation indicates involvement of the lateral . The disorder can lead to a cape-like loss of pain and temperature

1 sensation along the back and arms, though the presentation may be different from patient to patient. Involvement of the corticospinal tracts is heralded by the appearance of upper motor signs in the lower limbs. Advanced disease is usually characterised by presence of posterior column signs. Symptoms usually start in young adulthood, and the condition tends to develop slowly. Sudden onset of symptoms has been associated with coughing or straining.

Etiology of syringomyelia

Syringomyelia is associated with several diseases, including: • spinal cord tumors • • post-traumatic • post-arachnoiditis • idiopathic

Syringomyelia associated with tumors

Spinal cord tumors are associated with formation with cyst(s) or syrinx in 25 to 58% of cases. Conversely, in autopsy series of patients with syringomyelia, a spinal cord tumor is found in 8 to 16% of cases. Intramedullary fluid accumulation is usually caused by secretion from neoplastic cells or hemorrhage. The tumors most often associated with syringomyelia are ependymoma and hemangioblastoma (cysts in 50 to 70%). Syringomyelia may also occur in patients with extramedullary intradural or extradural tumors. However, in these cases, the mechanism of syrinx formation is most likely due to blockage of the CSF pathway. A full discussion of spinal cord tumors is beyond the scope of this presentation.

Non-neoplastic causes of syringomyelia

Non-neoplastic causes of syringomyelia are most commonly associated with disorders of cerebrospinal fluid (CSF) flow, either at the craniocervical junction, or in the subarachnoid spaces surrounding the spinal cord.

Chiari type I malformation is the most common non-neoplastic cause of an intramedullary cyst or cavity. This developmental malformation characterized by 3 to 5 mm downward displacement of cerebellar tonsils (and ) into , below the level of the foramen magnum, which is defined on sagittal images by a line that connects the bottom of the clivus (basion) to the posterior rim of the foramen magnum (opisthion). A Chiari malformation can be complicated by osseous abnormalities of the base and craniocervical junction (platybasia, basilar invagination, occipitalization of C1, Paget’s disease, …). Syringomyelia is most commonly seen, and is presumably caused by blockage of the CSF flow at the level of the (the opening of the central canal of the spinal cord,

2 at the level of the medulla oblongata). In fact, the downward herniation of the cerebellar tonsils causes them to obstruct the free pulsatile flow of CSF at the level of the foramen magnum and the obex. Whenever a cystic cavity of the spinal cord is discovered on MR - most frequently at the cervical level - the first reflex of the radiologist should be to look at the craniocervical region: if a Chiari malformation is present, then a diagnosis of a communicating syrinx (syringohydromyelia) is the most likely explanation. However, if there is no Chiari malformation, and if the craniocervical junction and skull base appear normal, then a spinal cord tumor should be suspected, and it is necessary to inject a gadolinium-based contrast agent.

Other non-neoplastic causes of syringo(hydro)myelia include conditions which cause obstruction of CS flow in the subarachnoid spaces, such as spinal trauma, radiation necrosis, (e.g. rupture or arteriovenous malformation, cavitation following ischemic injury or severe degenerative disease, (bacterial or tuberculous , arachnoiditis). In those cases, the syrinx develops and expands in the involved segment(s) of the spinal cord. This sometimes referred to as non-communicating syringomyelia. Symptoms may take months and even years to develop.

Post-traumatic syringomyelia (PTS) is recognized as a specific subtype, which characterized by neuropathic symptoms, including pain, numbness, weakness, and disruption in temperature sensation. Syringomyelia can also adversely affect sweating, sexual function, and, later, bladder and bowel control. A typical cause of PTS would be a car accident or similar trauma involving a whiplash injury. PTS is difficult to diagnose due to the fact that the symptoms may first appear a long time (months or even years) after the causative accident.

Syringomyelia may also be associated with congenital malformations of the spine, such as spinal dysraphism. Identification and treatment of associated dysraphism has the greatest impact on arresting progression of syringomyelia.

CSF Flow

CSF circulates within the of the brain, as well as the subarachnoid spaces around the brain and the spinal cord. CSF is produced at a rate of approximately 0,3-0,4 mL/min (about 500 mL per 24 hours). Total CSF volume is 90-150 mL in adults. CSF is produced in the of ventricles, parenchyma of the brain and spinal cord, and the ependymal lining of the ventricles. In CSF motion, two components can be distinguished: 1) bulk flow (circulation) and 2) pulsatile flow (back and forth motion). Bulk CSF flow is so slow, that it cannot be visualised on imaging studies. Conversely, it is possible to demonstrate the pulsatile motion of CSF, which is driven by the systolic-diastolic variations in the volume of intracranial .

In bulk flow theory, CSF flows from the through the foramina of Monro into the , then through the aqueduct of Sylvius into the .

3 From the fourth ventricle, the fluid passes through the foramina of Magendie (midline) and Luschka (laterally) into the and subarachnoid spaces, which cover the brain and spinal cord. In the spinal cord, the CSF flows through the central canal, and exits at the level of the obex (located at the back of the lower medulla oblongata). Lesions at this location can result in obstructive hydrocephalus or syringomyelia. In a Chiari I malformation, the egress of CSF from the central ependymal canal in the cord is blocked at the level of the obex, by the downward herniation of the cerebellar tonsils, which act like a “cork in a bottle”.

In order to understand the pulsatile flow of CSF, it is useful to review the Monro-Kellie doctrine, which states that the and its accompanying fluids are enclosed in a rigid container (i.e. the skull and the spinal canal), whose total volume tends to remain constant. An increase in volume of one component (e.g., brain, blood, or cerebrospinal fluid) will elevate pressure and thus decrease the volume of one of the other elements. During systole, as blood is pumped into the brain (which receives approximately 15% of cardiac output), the volume of the intracranial blood, thereby causing an outflow of CSF through the foramen magnum into the spinal canal. During SAS : subarachnoid spaces diastole, the opposite movement occurs, LV: : lateral ventricle causing upward flow of CSF from the spinal TV : third ventricle canal into the skull, through the foramen FV : fourth ventricle magnum. The downward-upward pulsatile CSF CM : cisterna magna Arrows : indicate direction of CSF flow flow is synchronous with the heart beat. This pulsatile flow pattern can be shown by ECG- or pulse-triggered phase contrast MRI. This technique generates signal contrast between flowing and stationary nuclei by sensitising the phase of the transverse magnetisation to the velocity of motion. For each anatomic location, typically three images are obtained: a re-phased gradient echo image, a magnitude image, and a phase image. Phase contrast MRI can discriminate between syringomyelia and cystic myelomalacia, and allows evaluation of CSF flow patterns in posterior fossa cystic malformations as well as in the pre-operative work-up of Chiari type I malformation and many other conditions. During the presentation and the workshop, we shall review the phase-contrast MRI technique, CSF flow physiology and cerebrospinal space anatomy, congenital and acquired disorders that can alter the cerebrospinal fluid dynamics.

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