Management of Chiari Malformations and Syringomyelia

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Chapter 182 Management of Chiari Malformations and Syringomyelia

Definition and History

In 1891, Hans Chiari described three types of cerebellar malformations that were associated with hydrocephalus.1,2 He added a fourth type 5 years later.1,2 The type I Chiari malformation was described as herniation restricted to the tonsils and adjacent cerebellum, which was associated with elongation of these structures into a conical shape. The cerebellar tonsils were atrophic and attached to the dorsal medulla by fibrous adhesions.3 Chiari malformation type II included protrusion of the medulla, fourth ventricle, cerebellar vermis, and cerebellar tonsils into the spinal canal. The insertion of the tentorium was low, the tentorial hiatus widened, and posterior fossa small in these patients.4 This malformation was present at birth and arose in association with myelomeningocele and hydrocephalus. Chiari III malformation consisted of a rare deformity in which the cerebellum herniated through a defect in the suboccipital bone or upper cervical lamina and created an occipitocervical encephalocele. Beaking of the tectum, elongation and kinking of the brain stem, hydrocephalus, and lumbar spina bifida were also present.3 This condition resulted in severe neurologic deficits and early mortality. Chiari type IV malformation was extremely rare and was characterized by severe cerebellar hypoplasia, but with the cerebellum and brain stem remaining within the posterior fossa.5 Mortality in infancy is the rule with this malformation.

In recent years other authors have expanded the Chiari classification system to include other types of lesions involving the hindbrain and cerebrospinal fluid (CSF) pathways at the foramen magnum. The “Chiari 0 malformation” is a condition in which the obex, but not the cerebellar tonsils, are inferiorly located and a syrinx is found within the cervical segments of the spinal cord in an identical anatomic location as syringomyelia associated with Chiari I malformation.6 In this condition a membrane covers the foramen of Magendie or bridges the subarachnoid space, and the volume of the posterior fossa volume is smaller than normal.7,8 The “Chiari 1.5 malformation” is a condition in which cerebellar ectopia is restricted to the cerebellar tonsils, as in Chiari I malformation, but contrary to Chiari’s original description of the Chiari I malformation, the brain stem is also caudally displaced.9

In syringomyelia a cyst forms within the spinal cord and produces myelopathy. The word syrinx is derived from the Greek word for “reed or pipe,” which in classical mythology is the form that the nymph Syrinx assumed to escape pursuit from the Greek god Pan.10 Surgical treatment for syringomyelia was initiated by Abbe and Coley, who performed a syringostomy in 1892.11 In 1938 the Chiari I malformation was first described in adults12 and in a patient without hydrocephalus13 based on intraoperative and postmortem observations.14

In 1950 James Gardner and associates at the Cleveland Clinic recognized the association of the Chiari I malformation with syringomyelia.15 They postulated that the outlets of the fourth ventricle were occluded by the Chiari I malformation and that a water-hammer pulsation was directed from the fourth ventricle, through the obex, and into the central canal of the spinal cord, leading to pulsatile expansion of the central canal to form a syrinx. To reverse this process, Gardner performed a surgical procedure that removed the bone from the posterior aspect of the foramen magnum, opened the fourth ventricle to the subarachnoid space, and plugged the obex.15

In the 1970s Logue introduced a less-invasive alternative to Gardner’s procedure. His procedure consisted of simple bony decompression and expansion of the dura with a tissue graft, and avoidance of opening of the arachnoid membrane and entrance into the subarachnoid space or fourth ventricle. His group performed a clinical study comparing Gardner’s procedure with their procedure of simple decompression and duraplasty and demonstrated that there was no difference in syrinx resolution between the procedures, although Gardner’s operation resulted in a higher complication rate.16,17 Since then, some investigators have advocated a decompressive procedure that opens the arachnoid membrane, removes or shrinks the inferior portion of the cerebellar tonsils, and attempts to enlarge the CSF pathways beyond what is achievable with bony decompression and duraplasty alone.18 Syringomyelia resolves following this latter procedure in about 80% of cases, which is similar to the results reported for simple decompression and duraplasty with preservation of the arachnoid membrane and tonsils (Table 182-1).

Development

In patients with the Chiari I malformation, the bones of the skull base often are underdeveloped, which results in reduced volume of the posterior fossa.1923 The Chiari I malformation therefore appears to result not from a primary cerebellar anomaly but rather from a smaller-than-normal posterior fossa, the volume of which is inadequate to contain the entire cerebellum; as a result, the cerebellar tonsils are displaced into the cervical spinal canal. Genetic disorders and diseases that affect skull development and reduce intracranial volume can result in the development of secondary Chiari I malformation.2428 However, most patients with Chiari I malformation do not have an associated disease to explain the development of this condition. In them, environmental and genetic factors presumably contribute to underdevelopment of the posterior fossa and development of the Chiari I malformation.

Chiari type II malformation is associated with myelomeningocele and primary and secondary brain anomalies. Cerebellar gliosis and atrophy occur, along with distortion and hypoplasia of cranial nerve, olivary, and pontine nuclei. Cerebral findings include focal cortical dysplasia, gray heterotopias in the hemispheric white matter and subependymal zone of the lateral ventricles, and thickening of the massa intermedia.3 The posterior fossa in Chiari II malformation is even smaller than in Chiari I malformation. The CSF cisterns are poorly developed in these patients, theoretically because the CSF cisterns did not expand during development because the outlets of the fourth ventricle are obstructed29 and the CSF circulation is down the central canal and into the fistula in the myelomeningocele, rather than into the basilar cisterns.30 Drainage of CSF through the myelomeningocele in utero encourages herniation of the posterior fossa structures through the foramen magnum. Following repair of the myelomeningocele, obstruction of the CSF pathways in the basilar cisterns, and often at the cerebral aqueduct, prevent normal CSF flow, which results in hydrocephalus.30 Ventricular shunting effectively treats hydrocephalus in these patients. Symptomatic syringomyelia can subsequently arise years later in the context of shunt malfunction, spinal cord tethering, or compression of CSF pathways by the malformation at the foramen magnum. Treatment for syringomyelia is directed toward its etiology and includes restoring CSF drainage by shunt revision or craniocervical decompression, relieving tension on the spinal cord by spinal cord untethering, or draining the syrinx directly using a syringopleural or syringoperitoneal shunt. Medullary compression can develop and requires prompt craniocervical decompression.

Syringomyelia occurs as a consequence of another abnormal process. There is general agreement that in syringomyelia the CSF pathways are encroached upon by an underlying condition. Because syrinx fluid is identical in chemical composition to CSF, syrinx formation likely results from a process that increases the movement of CSF into the spinal cord.31 Autopsy and radiographic studies rarely demonstrate a patent central canal in adult patients with syringomyelia,32 so it appears that development of syringomyelia does not occur by expansion of the central canal of the spinal cord by CSF that is transmitted from the fourth ventricle (Gardner’s water hammer theory).

Another mechanism of syrinx formation and progression has been proposed that does not require a patent central canal.33 The mechanism is initiated by the Chiari malformation partially obstructing CSF pathways at the foramen magnum, which prevents the normally rapid efflux and influx of CSF between the head and the spine that compensates for brain expansion and contraction during the cardiac cycle. In lieu of CSF, the cerebellar tonsils are displaced during the cardiac cycle, creating a piston effect on the partially enclosed spinal subarachnoid space that produces enlarged cervical subarachnoid pressure waves, which compress the spinal cord from without, direct CSF into the spinal cord, and cause pulsatile syrinx flow, which leads to syrinx progression (Figs. 182-1 and 182-2).33 A prospective study of patients with Chiari I malformation and syringomyelia that were evaluated before, during, and after surgery provided radiographic and physiologic findings that were consistent with this mechanism.20

Improved understanding of the pathophysiology of syringomyelia encourages the design and implementation of procedures directed toward eliminating the obstruction of CSF pathways, that is, strategies designed to reverse the pathophysiologic process underlying syringomyelia. Procedures that effectively open the CSF pathways at the foramen magnum, in the case of Chiari I malformation and basilar invagination, or in the spinal canal, in the setting of post-traumatic and postinflammatory syringomyelia, provide effective and lasting treatment of syringomyelia with low morbidity (Figs. 182-3 and 182-4). Since the 1990s the use of syrinx shunts for the treatment of syringomyelia has declined to the extent that shunts are now used only as a last resort.

Clinical Characteristics

In many patients, Chiari I malformation is diagnosed on the basis of non-neurologic symptoms such as suboccipital headache and cough headache; headache and neck pain are typical symptoms in young children.34 Neurologic signs and symptoms arising from the cerebellum, medulla, or central spinal cord are often found in patients with Chiari I malformation, either from direct compression of the cerebellum or medulla at the foramen magnum or from syringomyelia or syringobulbia. Lateral or downbeat nystagmus, ataxia, and reduced gag reflex are neurologic findings consistent with Chiari I malformation. In cases in which neither neurologic findings nor typical symptoms occur, the clinical significance of a mild degree of tonsillar ectopia cannot be determined. Generalized headache, chronic fatigue, and similar symptoms usually have etiologies other than Chiari I malformation. Thus, it is important not to overinterpret the magnetic resonance imaging (MRI) finding of a mild degree of tonsillar ectopia in the absence of syringomyelia.

Prevalence of syringomyelia is not clearly established, but one estimate is that symptomatic syringomyelia occurs in about 8.4 cases per 100,000 population.35 The Chiari I malformation is present in 70% of patients with syringomyelia.18 Basilar invagination causes 10% of cases of syringomyelia.36 Syringomyelia associated with conditions of the spine below the craniocervical junction is known as primary spinal syringomyelia and accounts for about 16% of all cases of syringomyelia. This type of syringomyelia may be posttraumatic, postinflammatory, or compressive.36,37 Patients with posttraumatic syringomyelia present several months or years after the initial trauma.36,3845 The incidence of syringomyelia following trauma that produces paraplegia was estimated to be 1% to 4% in the pre-MRI era, when it was difficult to detect and could not be detected with noninvasive diagnostic techniques, although with the availability of MRI the incidence is much higher.46

Postinflammatory syringomyelia results from a delayed reaction to chronic meningitis, either infectious or chemical.4751 Syringomyelia also occurs in association with compression of the CSF pathways by extramedullary tumors or cysts, osteophytes, or herniated intervertebral disks.5262 Intramedullary spinal cord tumors cause about 4% of the cases of syringomyelia and can do so without narrowing the CSF pathways.36 Syrinx fluid in primary spinal syringomyelia is identical to CSF, whereas syrinx fluid in syringomyelia associated with an intramedullary tumor is highly proteinaceous.31,63,64

Patients with syringomyelia present with symptoms of paralysis, sensory loss, and chronic pain, which most commonly develop during the second through the fifth decades of life. The natural history of syringomyelia is typically one of gradual, stepwise neurologic deterioration over many years.6567 Although syringomyelia is uncommon, typical signs and symptoms often suggest its diagnosis. The cervical segments of the spinal cord are affected first in syringomyelia associated with the Chiari I malformation, resulting in upper extremity symptoms of pain, weakness, atrophy, and loss of pain and temperature sensation. Early in its course, the signs and symptoms of syringomyelia may be mild and confined to a restricted area of the body. As the syrinx displaces and destroys the central gray matter of the spinal cord, the upper extremities become weak, atrophic, hyporeflexic, and devoid of normal pain and temperature sensation (see Fig. 182-2). Sensory loss is considered to be disassociated, because sensation of pain and temperature is lost, while light touch is preserved, and suspended, because the sensory loss hangs between regions of normal sensation.46 Both sides of the body are usually affected, but asymmetric extension of the syrinx to one side of the spinal cord results in more-severe involvement of the upper extremity on that side.

If left untreated, the upper extremity dysfunction progresses over months to years and eventually is accompanied by spasticity in the lower extremities as the syrinx expands outside the gray matter and into the corticospinal tracts.68 In less-advanced cases, the diagnosis of Chiari I malformation and syringomyelia is often made by MRI while evaluating a patient in whom another, more common condition, such as a herniated cervical disc, cervical spondylosis, or scoliosis, is suspected.34

In untreated patients with Chiari I malformation and syringomyelia, the pace of neurologic deterioration is more rapid initially and slows after the signs of syringomyelia are well established.66 This time course is consistent with an incidence of neurologic deterioration of 10% to 24% per year in studies in the MRI era, in which deterioration is measured early in the disease,6971 compared with 2% to 3% per year in the pre-MRI era,65,66 when deterioration was measured later in the disease and after definite central myelopathy had developed.

Primary spinal syringomyelia is associated with lesions within the spinal subarachnoid space or with deformity of the spinal canal and is often recognized months or years after spinal trauma or meningitis. It manifests as ascending loss of motor and sensory function, which usually signals involvement of the spinal cord above the original level of injury.32

Diagnostic Evaluation

Radiographic evaluation of syringomyelia should include T1- and T2-weighted MRI studies of the brain and cervical and thoracic spine, performed with and without contrast (see Figs. 182-3 and 182-4). Brain MRI detects hydrocephalus or a posterior fossa mass. Lumbar spine MRI may be included if there is consideration of tethering by the filum terminale, although the conus medullaris is usually in its normal position in patients with Chiari I malformation.72,73 MRI clearly defines the presence of syringomyelia and detects associated conditions such as the Chiari I malformation, basilar invagination, an intracranial mass that is causing cerebellar herniation, or a spinal tumor. In general, tonsillar ectopia of 5 mm or less is considered normal,74 although up to 6 mm is normal in those younger than 10 years of age.75 Symptomatic Chiari I malformation patients have a mean tonsillar ectopia of 13 mm, although symptoms have been reported with as little as 3 mm of ectopia.74

Many persons have MRI findings of “incidental Chiari I malformation” and are asymptomatic but meet the radiographic criteria for diagnosis of the condition. One study found that 0.9% of normal adults undergoing MRI studies of the brain have tonsillar herniation extending more than 5 mm below the foramen magnum.76 In a retrospective review of more than 22,000 hospitalized patients, 14% of patients with radiographic findings of Chiari I malformation were clinically asymptomatic.77 In another retrospective series of 68 patients with MRI findings of Chiari I malformation, 30% of patients were asymptomatic. In this study, ectopia over 12 mm was always associated with symptoms.78 Radiographic diagnosis should also take into account Chiari’s original description that the tonsils should be conical rather than round1,2 and the extent of narrowing of the CSF pathways by tonsillar impaction into the foramen magnum. Although cine MRI has helped clarify the pathophysiology of syringomyelia, it is unclear whether phase-contrast cine-MRI (CSF flow study) adds diagnostic information beyond that provided by structural MRI studies.

On T1- and T2-weighted MRI, syringomyelia appears as a well-circumscribed intramedullary fluid-filled mass (see Figs. 182-3 and 182-4). Enhancement of the spinal cord indicates the presence of an associated intramedullary tumor. In patients with primary spinal syringomyelia of unknown etiology, axial imaging of the spine using T2-weighted and fast imaging employing steady-state acquisition (FIESTA) MRI sequences might reveal previously undetected arachnoid cysts and arachnoid adhesions (Fig. 182-5, and see Fig. 182-4). Water-soluble CT-myelography has little clinical use in patients with syringomyelia associated with Chiari I malformation, but it is often useful in primary spinal syringomyelia because of its ability to localize and outline the extent of abnormalities within the spinal subarachnoid space. Certain findings on the spinal MRI predict more-rapid neurologic progression, particularly distention of the spinal cord by a syrinx of large diameter (more than 5 mm) and the presence of associated spinal cord edema.79

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