radiological pathology of congenital syringomyelia

INDEX

INTRODUCTION,PATHOGENESIS,PATHOLOGY

THE CRANIOCERVICALANOMALIES

o ARNOLD-CHIARIMALFORMATION

o SYRINGOBULBIA

o BASILARINVAGINATION

o ASSIMILATION OFATLAS

THE SYRINGOMYELICCAVITIES

CONGENITAL SYRINGOMYELIA

Syringomyelia is a chronic disorder involving the spinal cord or the medulla or both.Pathologically it is characterized by the development of cavitations and gliosis within thesestructures. When cavitations and gliosis extend to the medulla (bulb), the termsyringobulbia is applicable.

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Professor Yasser Metwallywww.yassermetwally.com

The present term "syringomyelia" was devised by ollivier in 1837 from the two Greekwords "to become hollow" and "marrow". This term was intended to describe anycavitation in the spinal cord including even the central canal which had not beenrecognized as a normally occurring structure until stifling described it in 1859.

In order to understand the pathogenesis of congenital syringomyelia it is necessary tounderstand the dynamics of the CSF flow in the central canal of the spinal cord and thesurrounding subarachnoid spaces.

The bulk flow of the CSF follows a downward route behind the spinal cord and posterior tothe dentate ligament from the cervical region and down to the lumber region and thenupward in front of the spinal cord to the basilar cisterns. Pressure waves are generated bythe distension and the collapse of the cerebrovascular and the spinovascular beds and arefelt to be responsible for the CSF pulsation. As in case of the blood, the propagation of thepulse waves is independent of and much faster than the blood velocity. Also thepropagation of the CSF pulse wave is much more rapid than the actual CSF movement.

The CSF down flow, which occur behind the spinal cord, begins during systole and ceasesduring diastole and is of 10 times greater volumetric magnitude than the ventricular pulse.

The ventricular CSF pulse wave is generated by the pulsation of the choroid plexus in thelateral ventricles which then escapes through the foramen of magendi into thesubarachnoid spaces and is progressively damped as it passes down behind the spinal cordthrough the foramen magnum; in this way the central canal of the spinal cord is bypassedand is not subjected to the ventricular fluid pulse wave and is left behind as a potentiallydistensible vestigial structure.

Around 30% of the CSF is formed in the central canal of the spinal cord and flow upwardby the milking action of the CSF pressure waves that are transmitted to the walls of thespinal cord. These pressure waves are thought to be caused by engorgement of the spinalvenous plexus and are most marked during coughing, straining and other valsalvas effectproducing maneuver.

THE CRANIOCERVICAL ANOMALIES1

The following craniocervical anomalies are found in congenital syringomyelia:

Arnold-Chiarimalformation

(100%) Cerebellar tonsillar ectopia

Basilar invagination (25%) Complete intracranial invagination of the atlasand axis

Syringobulbia (15%) Medullary cavitation

Hydrocephalus (10%) mmm

Klippel feil anomaly (5%) Complete fusion of bodies and arches of



adjacent vertebrae

Assimilation of the atlas (5%) Atlanto-occipital fusion

Arnold-Chiari malformation

Stenosis of the foramen magnum, due to cerebellar tonsillar ectopia, with the resultant ofreduction of the volume of the foramen of magendi, as it opens anatomically between thetwo cerebellar tonsils, will disrupt the normal CSF circulations and pulse waves. This willinterfere with the escape of the ventricular pulse waves and pressure waves generated bythe choroid plexus situated at the obex of the 4th ventricle (this escape normally occursthrough the foramen of magendi and the foramen magnum) and direct them into thecentral canal of the spinal cord, resulting in syrinx formation.

Figure 1. Chiari Imalformation

Figure 2. The anatomicsubstrate of congenitalsyringomyelia and/orhydromyelia is based uponcerebellar tonsillar ectopiain fetal life. Blockade ofthe foramen of magendiand stenosis of theforamen magnum willfunnel the CSF arterialpulse waves into the spinalcanal, distending it andeventually creatinghydrosyringomyelia .

Stenosis of the foramen magnum and the foramen of magendi can also inhibit the CSFupflow from the central canal of the spinal cord towards its intracranial sites of resorption



and causes increased spinal CSF pressure. The CSF, driven by the high intraspinalpressure, will thin filter into the spinal cord resulting in longstanding cord oedema thateventually causes cavitations within the spinal cord parenchyma.

Figure 3. A, CT myelography, B, MRI T1 image showing Arnold Chiari malformation(A,B), basilar invagination (A) and syringobulbic slit (B)



Figure 4. MRI T1 images (A,B) showing Arnorld-Chiari malformation.



Interestingly the foramen of magendi (which is situated at thecaudal end of the 4th ventricle and can easily be appreciated onthe T1 MRI sagittal images) is appreciated as being markedlydiminished in volume and occasionally totally obliterated andunidentifiable in all patients with tonsillar ectopia. In somepatient the foramen of magendi is transformed into a long slitthat opened below the level of the foramen magnum.

Figure 5. MRI T1 image showing a case with Arnold-Chiarimalformation, notice the tonsillar ectopia, adhesions betweenthe herniated tonsils and the medulla, resulting in markedstenosis of the foramen magnum and the foramen of magendi

Adhesion between the herniated cerebellar tonsils and the posterior aspect of the cervico-medullary junction could be appreciated in all cases, so besides stenosis of the foramenmagnum and the foramen of magendi that is induced mechanically by the crowding of theforamen magnum by the herniated cerebellar tonsils, these adhesions will furthercompromise the volume of the foramen of magendi.



Figure 6. Normal MRI T1 image (left) and two cases with Arnold Chiari malformation(middle and right images), notice the tonsillar ectopia,adhesions between the herniatedtonsils and the medulla, resulting in marked stenosis of the foramen magnum and theforamen of magendi,also notice the syringobulbic slit (right image)

Figure 7. Arnold Chiari malformationassociated with hydrocephalus andsyringomyelia, notice evidence of surgicalintervention

Syringobulbia

Syringobulbic slits are demonstrated in 15% of cases with syringomyelia and they arecomposed of two types, the first type extends asymmetrically into the lateral tegmentum ofthe medulla, in the presumed area of the descending root of the trigeminal nerve. The othertype extends along the median raphe. Direct communication between the bulbic slits andthe 4th ventricle is occasionally demonstrated in some cases.



Figure 8. MRI T1 images showing a lateral syringobulbic slit with definite communicationwith the 4TH ventricle and the cervical syringomyelic cavity.

Figure 9. CT myelography showing amedian syringobulbic slit, notice thebasilar invagination.



Direct communication between the bulbic slits and the cervical syringomyelic cavities iscommonly demonstrated in almost all cases, this is because syringobulbia is formed byfluid in the cervical syringomyelic cavities breaking upward through the pyramidaldecussation region to form a cavity in the bulb. Pulsatile fluid shifts in the cervical syringesare responsible for the upward extension of these syringes. all the bulbic slits aredemonstrated on both the T1 and T2 weighted MRI images, both in the axial as well as thesagittal planes.

Hydrocephalus

Hydrocephalus is demonstrated in about 10% ofcases with congenital syringomyelia and isoccasionally associated with an abnormally elongatedcerebellum and a markedly distorted 4th ventriclewith significant reduction of its volume. Thisprobably indicates the existence of a graver degree ofstenosis at the 4th ventricular level. This markeddegree of stenosis apparently results inhydrocephalus in addition to hydrosyringomyelia.Occasionally hydrocephalus is associated withabnormally large cisterna magna .

Figure 10. A postmortem specimen showing Arnold-Chiari malformation and hydrocephalus

Figure 11. A, This figure illustrates the position of the downwardly displaced portions ofcerebellum. The lower medulla has a congenital "kink." The position of the foramenmagnum, as it appeared in life, is marked on the image. B, This figure illustrates the brainstem and cerebellum cut sagitally in a case of Arnold-Chiari malformation. The arrowpoints to the cerebellar tonsils or "pegs" of cerebellum which have been displaced caudallyover the roof of the medulla.



Basilar invagination

Complete intracranial invagination of the atlas and axis (basilar impression orinvagination) is demonstrated in about 25% of cases with congenital syringomyelia. Basilarimpression acts by reducing the volume capacity of the posterior fossa and crowds thecerebellum thus producing cerebellar tonsillar herniation, thereby sitting up the substratefor the funneling of the CSF pressure waves into the central canal of the spinal cord thuscreating hydrosyringomyelia. In all cases with basilar impression the odontoid process wasfixed with no evidence of subluxation.

Figure 12. Basilar invagination plain x ray (left) plain CT scan (middle) and MRI T1image(right), notice that the hyperintense odontoid (due to increased fat content) can beseen in touch with the pons

Assimilation of atlas (atlanto-occipital fusion)

Assimilation of the atlas (complete fusion betweenthe atlas and the occipital condyles) occur in about5% of cases of congenital syringomyelia. In atlanto-occipital fusion, the odontoid process is abnormallyhigh, subluxated and compressing the cervico-medullary junction posteriorly. The most significantfinding in assimilation of the atlas with neurologicalsymptoms is an odontoid process with abnormalsize, in abnormal position and with abnormalmobility .

Figure 13. High subluxated odontoid compressingthe cervico-medullary zone in a case of assimilationof atlas



Figure 14. High subluxated odontoid compressing the cervico-medullary zone in a case ofassimilation of atlas (left plain x ray and right two images CT myelography)

When the atlas is fused with the occiput, flexion of the head results in partial forwardsubluxation of the fused atlas on the axis. Posterior displacement of the odontoid processthen occurs, resulting in compression of the cervico-medullary junction. As the posteriorluxation of the odontoid process is intermittent (only during head flexion), so intermittentcompression of the cervico-medullary junction might result, initially, in intermittentneurological manifestations.

Regarding the cervico-dorsal syringomyelic cavities, two types are demonstrated. The firstone was composed of a single, continuous slit-like or tubular cavity that extended throughthe whole cervico-dorsal region. The walls of these cavitations are thin and smooth. Signalloss on the T2 weighted images (CSF flow void sign) is occasionally demonstrated insidethese cavitations. Patients harboring this type of cavitations are younger.

Figure 15. High subluxated odontoidcompressing the cervico-medullaryzone in a case of assimilation of atlas(left, plain x ray,and right MRI T1image)



THE SYRINGOMYELIC CAVITIES

Figure 16. MRI T1 image showing TYPE I syringomyelic cavity

Regarding the cervico-dorsal syringomyelic cavities, two types are demonstrated. The firstone was composed of a single, continuous slit-like or tubular cavity that extended throughthe whole cervico-dorsal region. The walls of these cavitations are thin and smooth. Signalloss on the T2 weighted images (CSF flow void sign) is occasionally demonstrated insidethese cavitations. Patients harboring this type of cavitations are younger.



Figure 17. MRI T1 images showing TYPE I syringomyelic cavity composed of a single,continuous slit-like or tubular cavity that extended through the whole cervico-dorsalregion. The walls of these cavitations are thin and smooth.



Figure 18. MRI T2 image (A) and CT myelography (B) showing TYPE I syringomyeliccavity composed of a single, continuous slit-like or tubular cavity that extended through thewhole cervico-dorsal region. The walls of these cavitations are thin and smooth. Signal losson the T2 weighted image (CSF flow void sign) is evident on the T2 image

On the other hand, the second type is characterized by thick walls and extensive intra-cavitary septations, transverse bands and CSF multiloculations. The CSF flow void sign isnot demonstrated inside these cavitations on the T 2 weighted images. Patients harboringthis type of cavitations are older.



Figure 19. MRI T1 images showing type II syringomyelic cavity characterized by thickwalls and extensive intra-cavitary septations, transverse bands and CSF multiloculations.



Figure 20. MRI T1 images (A) and MRI T2 image (B) showing type II syringomyelic cavitycharacterized by thick walls and extensive intra-cavitary septations, transverse bands andCSF multiloculations. The CSF flow void sign can not be demonstrated inside thesecavitations on the T 2 weighted image.

From the pathological point of view syringomyelia is a chronic spinal cord disordercharacterized by progressive cavitations and gliosis. Cavitations occur early and progressup and down by the pulsatile fluid shifts inside the syringomyelic cavities. Pulsatile fluidshifts inside the cavitations are caused by CSF pulsation in the subarachnoid spaces that istransmitted to the fluids inside the syringomyelic cavities through the walls of the spinalcord. These pulsatile fluid shifts are increased by coughing and straining and result inrecurrent sucking and sloshing of the CSF inside the syringomyelic cavities.



Figure 21. Type II syringomyelic cavities, notice the thick walls, and traverses septations

Such sucking and sloshing movements of the CSF inside the cavitations result, on shortterm basis, in progressive extension of the cavitations up and down within the substance ofthe spinal cord. The pulsatile fluid shifts in the syrinx cavities are thought to be responsiblefor the loss of signal on the T2 weighted images that has been termed CSF flow void sign(CFVS). However on long term basis pulsatile fluid shifts in the syringomyelic cavitiesinduce reactive gliosis that ultimately results in the formation glial bands, septations andCSF multiloculations inside the syringomyelic cavities. It also results in progressivethickening of the walls of the cavities and probably also reduction of the volume of thecavitations.



Figure 22. Type I syringomyelic cavities.

These glial bands and septations will interfere with the CSF flow inside the cavitations bydamping the pulsatile CSF movement. This ultimately results in stasis of the CSF inside thecavitations and loss of the CSF flow void sign that was initially present at a younger age.

Symptoms of syringomyelia are initially caused by the progressive cephalo-caudalextension of cavitations caused by the pulsatile fluids shifts inside these cavitations. Thepulsatile fluid shifts vary in intensity from time to time and from one position to another.Engorgement of the epidural venous plexus by prolonged recumbency and during coughingand straining also increases the intensity of the pulsatile fluid shifts. The variability of theintensity of the pulsatile fluid shifts is reflected clinically as marked fluctuation of theintensity of the clinical symptomatology of some of these patients to the point that some ofthem were initially misdiagnosed as MS.

At an older age group symptomatology of syringomyelia is caused mainly by the reactivegliosis that can interfere with the blood supply of the affected segments and with thephysiological function of the myelin sheath and neurons at the affected zones, thusresulting in progressive deterioration of the clinical neurological deficits.

In congenital syringomyelia, shunting of the syringomyelic cavitations is probablyindicated only when the CSF flow void sign could be appreciated inside these cavitations onthe T2 weighted images, especially when MRI also demonstrates absence of any significantglial septations and/or CSF multiloculations, i.e. shunting is indicated only for type Isyringomyelic cavitations.

Early surgical treatment can relieve the distending force (i.e. sucking and sloshing CSFmovements) and perhaps - on long term basis - can prevent the gliotic zone which latersurround the syringomyelic cavities. Post-operatic absence of the CSF flow void sign thatwas initially present pre-operatively is an indication of a successful shunting.



REFERENCE

1- Metwally,MYM : Imaging of syringomyelia, a comparative study. Read at the scientificmeeting of the Egyptian society of neurology, psychiatry and Neurosurgery, Port Said,October 1993



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radiological pathology of congenital syringomyelia

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