Diagnostic MRI in Dogs and Cats PDF

CHAPTER 7.9 SYRINGOMYELIA VetBooks.ir Wil...

CHAPTER 7.9 SYRINGOMYELIA VetBooks.ir Wilfried Mai 595 CONTENTS Normal cerebrospinal fluid production and flow....................................................................................................................................................595 Pathophysiology of syringomyelia........................................................................................................................................................................595 MRI appearance of syringomyelia.........................................................................................................................................................................598 References.............................................................................................................................................................................................................601 From a strict standpoint, ‘hydromyelia’ refers to pathologic CSF accumulating in the subarachnoid space is progres- dilation of the central canal of the spinal cord, whereas in sively removed through absorption by the arachnoid villi, ‘syringomyelia’ the fluid dissects through the ependymal which are finger-like endothelium-lined protrusions of lining of the central canal, which creates a focal fluid collec- the arachnoid outer layer through the dura mater and tion within the spinal cord outside of the central canal.1,2 The into the lumen of calvarial venous sinuses (cranial arach- clinical manifestations are identical between both entities, noid villi) and epidural venous sinuses (spinal arachnoid and there is often some overlap between them: a dilated cen- villi). Alternative pathways of CSF absorption include tral canal can rupture, allowing cerebrospinal fluid (CSF) to the cranial and spinal nerve sheaths, the cribriform plate, extend into the parenchyma, and, conversely, a parenchymal and the adventitia of cerebral arteries.4 cavitation adjacent to the central canal can rupture into it. The CSF flow from the sites of production to the sites In addition, research has shown that the abnormal collec- of absorption is pulsatile, corresponding to the systolic tion of fluid is not in fact pure CSF, but extracellular fluid. pulse wave in the choroidal arteries.4 As a result, the term ‘syringomyelia’ is now accepted to refer to all spinal cord cavitations containing fluid that is identical PATHOPHYSIOLOGY OF SYRINGOMYELIA to or closely resembling CSF.2,3 Synonymous terms com- monly used include ‘syringohydromyelia’ and ‘syrinx’. There are a number of theories to explain the develop- ment of syringomyelia, but they all rely on the premise NORMAL CEREBROSPINAL FLUID that there is some form of obstruction or impairment to PRODUCTION AND FLOW the normal pulsatile flow of CSF in the subarachnoid space that is normally associated with the cardiac cycle.2,3,5–33 The majority of the CSF is produced by the choroid plex- Regardless of etiology, the driving force of syringomy- uses located in the lateral, 3rd, and 4th ventricles. A small elia is the systolic CSF pulse pressure: the pressure wave amount also comes from the brain interstitial fluid, the of CSF displaced from the head during each arterial parenchymal capillaries, and the ependymal lining of the pulsation.2 ventricles,4 although their exact contribution to the total With any compression of the spinal subarachnoid space, amount of CSF produced is controversial. there is, during the systolic pressure wave, a high-pressure In normal mammals, the flow of CSF is directed from compartment cranial to the obstructive site and a lower- the lateral ventricles through the interventricular fora- pressure compartment caudal to the obstructive site:2,20 men into the 3rd ventricle, and then caudally through the If there is complete obstruction to flow of CSF mesencephalic aqueduct into the 4th ventricle. The CSF (e.g., circumferential constriction due to an extra- then enters the subarachnoid space through the lateral dural lesion or arachnoid adhesions), the systolic apertures of the 4th ventricle, while only a small amount pressure wave is transmitted through the spinal enters the central canal of the spinal cord. Once in the cord parenchyma caudal to the obstruction, and cranial subarachnoid space, the CSF circulates rostrally some is reflected backwards (i.e., cranially) into the to the cranial villous sites of absorption, or caudally into spinal cord parenchyma cranial to the obstruction. the spinal subarachnoid space.4 This causes repeated, cyclic mechanical distension  596 CHAPTER 7.9 VetBooks.ir of the cord, resulting in accumulation of extracel- encroachment of the craniocervical or spinal subarach- lular fluid originating from the microcirculation of noid space. the spinal cord. After a potentially reversible phase A very common cause for syringomyelia in dogs is of spinal cord edema, this fluid eventually coalesces the Chiari-like malformation/syringomyelia complex into cavities (i.e., the syrinx) (Fig. 7.9.1). A syrinx can (see Chapter 5.1), which is a multifactorial disease pro- form both caudal and cranial to the obstructive site cess governed by the effects of increased hindbrain due to the bidirectional pressure wave transmission volume and impaired occipital bone development.33 into the spinal cord parenchyma.2,20 This causes a volume mismatch between the caudal If there is an incomplete obstruction, an open, albeit fossa and brain parenchyma it contains, resulting in narrow, subarachnoid space channel is left for the CSF crowding of the hindbrain parenchyma. The main mor- to flow through during each systolic pressure wave; phologic features of the condition are indentation of the the narrowed window causes the velocity of CSF cerebellum by the supraoccipital bone and/or hernia- flow to increase through the opening (CSF flow jet). tion of a part of the cerebellum through the foramen The Venturi effect then takes place, a fluid dynam- magnum (Fig. 7.9.2).33 However, cerebellar indentation ics law stipulating that the total mechanical energy can be seen in 37%–51% of non-Cavalier King Charles of flowing fluids remains constant, so that if there is Spaniels without signs of Chiari-like malformation, and an area of increased velocity, the hydrostatic pressure therefore may not in isolation constitute an accurate decreases accordingly. This causes a ‘suction effect’ MRI indicator of this anomaly.34 The herniation of the on the spinal cord that distends it during each systole, cerebellum through the foramen magnum causes vari- thereby contributing to the increase in the mechani- able degrees of encroachment of the dorsal subarach- cal distension of the spinal cord and leading to syrinx noid space at that level. In turn, this causes disturbance formation.2,20 to CSF flow dynamics in the craniocervical area, lead- Such alterations of the CSF hydrodynamics can result ing to progressive syringomyelia in the spinal cord.33 from a variety of conditions causing some degree of Abnormal CSF flow dynamics have been demonstrated (a) Fig. 7.9.1 Transverse T2W images at the level of caudal C1 (a) and mid-T1 (b) in a 7-year-old Jack Russell Terrier with Chiari-like malformation and secondary syringomyelia in the cervical spine. At the level of C1, a small syrinx is present (arrow) and there is pre-syrinx interstitial edema in the dorsal parenchyma (arrowhead). At the level of T1, a large well-formed syrinx (dashed arrow) is visible. The syrinx is markedly T2 hyperintense (b) due to its fluid content. (1.5T MRI system)  Sy r i ng om y e l i a 597 VetBooks.ir (a) (b) (c) Fig. 7.9.2 Sagittal T2W image of the cervical spine (a) and transverse T2W (b), T1W (c), and T2-FLAIR (d) images at the level of C2-C3 in a 5-year-old female Cavalier King Charles Spaniel with Chiari-like malformation and secondary syringomyelia. There is herniation of the cerebellar vermis through the foramen magnum (arrow, a). A large syrinx extends from cranial C2 to C5, which has a circular shape on the transverse images and is T2 hyperintense, T1 hypointense, and suppressed on T2-FLAIR (arrowheads, b–d). (d) (1.5T MRI system) with phase-contrast cine MRI in Cavalier King Charles affected with Chiari-like malformation and syringo- Spaniels affected by Chiari-like malformation, includ- myelia compared with control dogs and Cavalier King ing interrupted flow at the foramen magnum and inho- Charles Spaniels with Chiari-like malformation only, mogeneous flow (turbulence or jets) at the foramen supporting the pulse pressure theory (see above) as an magnum or the C2-C3 space.9 Inhomogeneous flow important contributing factor to syrinx development.17 is associated with the presence and severity of syrin- Chiari-like malformation is common in the Cavalier gomyelia.9 Cardiac-gated cine balanced fast field echo King Charles Spaniel, but is reported in other breeds studies demonstrated significantly greater pulsation such as the Griffon Bruxellois and many other small- of the cerebellum in Cavalier King Charles Spaniels breed dogs as well.3,19,25,34–36  598 CHAPTER 7.9 VetBooks.ir Other anomalies of the caudal fossa or craniocervical flank (resulting in ‘phantom scratching’), scoliosis, signs junction such as quadrigeminal cysts (see Chapter 5.5), of cervical myelopathy, or cerebellovestibular signs.19,36 atlanto-occipital overlapping, atlantoaxial instability, occipitoatlantoaxial malformations, atlantoaxial dural MRI APPEARANCE OF SYRINGOMYELIA bands, or dens abnormalities (see Chapter 7.4) can lead to the development of syringomyelia through similar Although a recent study showed that CT performed as mechanisms (Figs. 7.9.3, 7.9.4).37 well as MRI in identifying and measuring syringomyelia Other conditions such as tethered spinal cord (e.g., sec- lesions in dogs,47 MRI is still considered a better imag- ondary to dermoid sinus or myelomeningocele),38,39 ing modality than CT to assess the presence and extent trauma, arachnoiditis, caudal fossa space-occupying of the syrinx and, in particular, allow assessment of pre- lesions (such as brain stem tumors40,41 or epidermoid syrinx interstitial edema, which is not visible on CT. cyst42), and spinal arachnoid diverticula43–46 can also be A syrinx appears as longitudinally elongated T2 hyper- associated with syringomyelia (Fig. 7.9.5). intense/T1 hypointense cavities within the spinal Treatment of the underlying cause can lead to resolution cord, with a similar signal intensity to that of the CSF of the syrinx on follow-up imaging, underscoring the (Figs. 7.9.2–7.9.4).36 On T2-FLAIR images the signal dynamic process at play in the formation of syringomy- within these cavities is suppressed. These characteristics elia (Fig. 7.9.3).40,46 are due to the fluid-filled nature of the lesions. In some cases, syringomyelia can be present without obvi- The shape of the spinal cord cavities may be complex ous identifiable craniocervical junction abnormality.19,25 with septations. Portions of the central canal may be As some of these dogs are found to have concomitant dilated and can communicate with syrinx cavities. dilation of the ventricular system, it is hypothesized that The longitudinal extent of the syrinx can be well appre- they may have some form of obstruction of the fora- ciated on sagittal images, especially T2W images, due men magnum (e.g., arachnoiditis) other than Chiari- to the hyperintense signal of the lesions. However, the like malformation or classic craniocervical junction dimensions of the syrinx may be overestimated on T2W anomalies.19,25 images due to summation with areas of pre-syrinx inter- Clinical signs of syringomyelia associated with cranio- stitial spinal cord edema that precede the cavitation, and cervical abnormalities may occur at any age and include some authors advocate the use of T1W series to obtain hyperesthesia, allodynia and paresthesia of neck and more accurate measurements of the lesions.48 * (a) (b) (c) (d) Fig. 7.9.3 Sagittal T1W image of the head (a), initial sagittal T2W images over the cervicothoracic (b) and thoracolumbar (c) spine, and sagittal T2W image (d) of the cervicothoracic spine a few weeks after placement of a ventriculoperitoneal shunt in a 10-year-old Toy Poodle with a quadrigeminal cyst, hydrocephalus, and secondary syringomyelia. The quadrigeminal cyst is indicated in (a) by the arrow and is causing compression of the cerebellum (asterisk). There is irregular discontinuous syringomyelia involving the cervical, thoracic, and lumbar spine (dashed arrows, b and c). After treatment (d), there is mild improvement of the cervical syringomyelia and marked improvement of the cranial thoracic syringomyelia (arrowhead). (1.5T MRI system)  Sy r i ng om y e l i a 599 VetBooks.ir (a) (b) Fig. 7.9.4 Sagittal T2W (a) and transverse T1W (b) and T2W (c) images at the level of cranial C3 in a 10-year-old Pug with an atlantoaxial fibrous band (arrow, a) causing dorsal compression of the subarachnoid space and mild dorsal cord compression; there is a long irregular syrinx in the cervical spine, which appears T1 hypointense and T2 hyperintense (c) (dotted arrows, b and c). (1.5T MRI system) Although in dogs with craniocervical junction anomalies features of these conditions (e.g., arachnoid diverticula, (e.g., Chiari-like malformation) and concurrent syrin- quadrigeminal cyst, space-occupying neoplastic lesion, gomyelia the cervical spinal cord is often affected, other meningitis with adhesions) will be present. These are parts of the spinal cord are commonly involved as well, described in detail in other chapters (Figs. 7.9.4, 7.9.5). and therefore the entire spine should be imaged to evalu- The potential association between morphologic fea- ate the extent of the condition.48 In Cavalier King Charles tures of the syrinx on MRI and clinical signs has been Spaniels with Chiari-like malformation, syringomyelia studied:23,30 when present affects the C1-C4 segment in 100% of cases, Although the presence of a syrinx is not necessarily the C5-L2 segment in 76%, and the L3-L7 segment in associated with neurologic signs,11,12 larger syringo- 49% of cases (Fig. 7.9.3).48 The maximal dorsoventral myelia lesions increase the likelihood of these signs.11 syrinx size can occur in any region of the spinal cord.48 According to one study, maximum syrinx width is a In Cavalier King Charles Spaniels with Chiari-like mal- strong predictor of pain, scratching behavior, and sco- formation/syringomyelia, there is a correlation between liosis; 95% of Cavalier King Charles Spaniels with a syrinx size and extent and the patient’s age, indicating maximum syrinx width of 0.64 cm or more will have that the condition is progressive and not static.14,48 associated clinical signs.23 However, other studies In cases of Chiari-like malformation, the cerebellum failed to demonstrate a relationship between syrinx and medulla are seen on sagittal MR images to extend width and persistent pain.30 into or through the foramen magnum, which is occluded Association of syrinx size with neurologic deficits was with little or no CSF around the neural structures also found in American Brussels Griffon dogs.19 (see Chapter 5.1) (Fig. 7.9.2). The size of the cerebellar Larger lesions with involvement of the dorsal horns herniation does not correlate with the severity of clinical of the gray matter of the spinal cord and asymmetric signs.11,12 Ventricular dilation is common, and the degree appearance of the syrinx on transverse images may be of dilation correlates with the syrinx dimensions.15,36 associated with persistent neuropathic pain, although When syringomyelia is developing secondary to condi- the relative contribution of the asymmetry and syrinx tions other than Chiari-like malformations, specific MRI size is controversial (Fig. 7.9.6).23,30  Sy r i ng om y e l i a 601 VetBooks.ir REFERENCES malformation and syringomyelia using cardiac-gated cine 1. Kirberger RM, Jacobson LS, Davies JV et al. (1997). magnetic resonance imaging. Vet J 198(1):88–91. Hydromyelia in the dog. Vet Radiol Ultrasound 38(1):30–8. 18. Fenn J, Schmidt MJ, Simpson H et al. (2013). 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