23 Surgical Management of Syringomyelia Occurring with Chiari Malformations

Chiari malformations are named after the Austrian pathologist Hans Chiari, who published two seminal reports on hindbrain herniation in 18911 and 1896.2 Chiari provided detailed pathological descriptions from postmortem cases, with hindbrain descent through the foramen magnum associated with hydrocephalus. The current classification of Chiari malformations involves four distinct subtypes ( Table 23.1 ).

The focus of this chapter will be on the most common Chiari malformation, type I. The definition of this anomaly has evolved since Chiari’s original description of tonsillar herniation occurring with hydrocephalus. Most cases now are diagnosed in adults without hydrocephalus. Unfortunately, the term Chiari I malformation currently applies to virtually any type of tonsillar herniation except for cases related to myelodysplasia (Chiari II) or cervical encephalocele (Chiari III). This practice does not emphasize the etiologic factors that produce the tonsillar herniation and can lead to inappropriate treatment. The current definition of the Chiari I malformation depends on a single criterion of tonsillar descent > 5 mm, based on a magnetic resonance imaging (MRI) diagnosis.3

Although tonsillar herniation is the common anatomical marker, it is important to recognize that Chiari I malformations are heterogeneous conditions with different pathophysiological mechanisms, which can often cause overlapping symptoms. Tonsillar herniation may arise from congenital causes with small posterior cranial fossae (classic Chiari I malformations and craniosynostotic syndromes) and acquired causes with normal posterior cranial fossae (hydrocephalus, Paget disease, posterior fossa tumors, tethered cord, and spinal hypo-tension). Because of the varied manifestations for Chiari malformations, there have been differences in treatments and surgical outcomes. To appropriately treat these patients, each patient’s tonsillar descent must be evaluated in the setting of morphometric analysis of the posterior fossa and stability of the craniovertebral junction. Ultimately, successful management depends on appropriate patient selection, tailoring the surgical intervention to treat the underlying anatomical disorder, and complication avoidance.

Chiari malformations are the leading cause of syringomyelia ( Fig. 23.1 ). The association between Chiari malformations and syringomyelia was first recognized by Russell and Donald4 in 1935 and Lichtenstein5 in 1943. The early surgical strategies for syringomyelia were directed at the syrinx cavity, and there was little attention given to any associated hindbrain anomalies. It was Gardner and Goodall6 in the 1950s who were credited with demonstrating that correction of the hindbrain hernia by sub-occipital decompression could lead to improved surgical outcomes. Gardner’s investigations appropriately directed attention to the craniovertebral junction.

According to Gardner’s hydrodynamic theory, a syrinx develops as a result of obstruction of the outlets of the fourth ventricle, which causes a caudal cerebrospinal fluid (CSF) pulse wave, producing a “water hammer” Effect that dilates the lumen of the central canal. As it became known, Gardner’s operation included a foramen magnum decompression with plugging of the obex to close the hypothetical communication between the syrinx and the fourth ventricle. His technique revealed significant clinical improvement in some patients, and his operation laid the basis for contemporary surgical management of syringomyelia occurring with Chiari malformations.

Williams7 and others8 noted that decompression alone was sufficient for good clinical results. His critical evaluation of the Gardner technique revealed that plugging the obex was an additional source of morbidity and occasional mortality. Williams developed an alternative explanation that obstructions of CSF, occurring at the level of the foramen magnum, produce a dissociation of pressure between the cranial and spinal CSF compartments. Williams theorized that the pressure gradient between cranial and spinal compartments “sucks” fluid from the fourth ventricle into the central canal, as a consequence of a relatively lower CSF pressure caudal to the block. Valsalva maneuvers, such as sneezing, coughing, and straining, accentuate this pressure gradient phenomenon by producing changes in venous volume and pressure.

With the advent of MRI technology, the classical theories of Gardner and Williams have been challenged. Although MRI firmly established the diagnosis and the link between Chiari malformations as the leading cause of syringomyelia, there remains some question regarding the pathogenesis of these disorders. Unlike Gardner’s initial contention, it has become clear from analyses of pathological autopsy specimens that only a minority of syrinxes communicate directly with the fourth ventricle, and that most syrinxes are separated by the fourth ventricle by a long segment of syrinx-free spinal cord.9 Further advances in MR technology improved the temporal imaging resolution, particularly during the cardiac cycle. Oldfield et al.10 demonstrated that the tonsils behave like a piston moving during systole and diastole. During systole the tonsils are impacted into the foramen magnum, leading to a CSF pressure wave in the spinal subarachnoid space which drives CSF into the cord through Virchow-Robin spaces or the dorsal roots. Although some questions remain, Oldfield’s theory has gained wide acceptance.

Toward a better understanding of the pathogenesis, there were observations that patients with Chiari I malformations have a relatively small posterior cranial fossa.11 With a small cranial vault, the normal-sized brain matter is constricted, causing tonsillar descent through the foramen magnum. This tonsillar ectopia leads to obstruction of the CSF flow at the foramen magnum and a noncommunicating syringomyelia in 20% to 65% of cases. Further morphometric studies involving quantitative comparisons of the size of the posterior cranial fossa confirmed this theory.12–15 Goel et al. in 1998 described additional radiological morphometric parameters and discussed the clinical symptoms and therapeutic implication of the reduction in posterior cranial volume and its relationship with pathogenesis of initially Chiari I malformation and subsequently syringomyelia.13

By the early 1990s (100 years after Chiari’s initial description), there emerged multiple treatments for syringomyelia associated with Chiari malformations. There were proponents for a variety of different approaches, which included (1) foramen magnum decompression only, (2) foramen magnum decompression with plugging of the obex, (3) syrinx shunting only, and (4) simultaneous decompression and syrinx shunting. With the proliferation of different surgeries, there has been great difficulty in assessing outcomes. Operative strategies for treating Chiari I malformation and syringomyelia have yet to be fully standardized. The traditional approach is to perform a posterior fossa decompression comprising a suboccipital craniectomy, upper cervical laminectomy, and duraplasty. Unfortunately, specific details, such as the size of the craniectomy, the extent of the laminectomy, and the preferred integument of duraplasty (e.g., cadaveric dura, pericranium, bovine pericardium, or synthetic material), are rarely discussed in the literature. Also controversial are additional steps, such as lysis of adhesions, plugging of the obex, terminal ventriculostomy, drainage of the fourth ventricle, leaving the dura open, and resection or shrinkage of the cerebellar tonsils. No one procedure has been uniformly successful, and it is estimated that a significant improvement in preoperative symptoms and a reduction in syrinx size occurs in only 40% to 60% of patients.

In surveys sent to international pediatric neurosurgeons16 and American pediatric neurosurgeons,17 sub-occipital decompression was considered the standard surgical procedure. The majority of respondents favored routine dural opening at surgery and closure with a pericranial or synthetic patch graft. This technique has been adopted by many surgeons, because it improves flow of CSF at the level of the foramen magnum.

Contemporary neurosurgical debate rests on whether or not to perform a duraplasty. The controversy arises from a higher associated morbidity with duraplasty and intradural manipulations. As a result, some surgeons advocate bony decompression only, as there appears to be a subset of patients who do respond to this intervention.13,15,18–20 In Effect, the surgeon’s decision is often determined by balancing the risk of a complication against the risk of undertreatment, necessitating a return to the operating room.

Duraplasty and intradural manipulations have been associated with higher morbidity in certain series. When the arachnoid is opened, there is an increased risk of bleeding and adhesion formations, which can lead to arachnoiditis, pseudomeningoceles, hydrocephalus, and persistent syringomyelia. In turn, these conditions may cause persistence in symptoms or new posterior fossa syndrome complaints. The advantage of opening the dura is that it provides the necessary exposure to allow for internal decompression. The Effect of chronic severe foramen magnum impaction by the cerebellar tonsils is the formation of arachnoidal adhesions, which may be the primary pathological focus. The adhesions may be quite pervasive, involving the brainstem, posteroinferior cerebellar artery, and spinal cord. Microlysis of the adhesions is an important part of the internal decompression. Additionally, we advocate tonsillar reduction if the obex area is closed and if there is no evidence of pulsatile flow of the CSF from the fourth ventricle. At the end of the operation, the tonsils are ideally positioned slightly above the level of the putative foramen magnum. Those surgeons who elect not to open the dura13,15 may not address the potential significant impact of arachnoidal scarring.

Most surgeons have moved away from various intradural techniques, including posterior fossa stenting, catheterization, and plugging of the obex, which have not been associated with improved outcomes.21,22 The one popular exception is tonsillar shrinkage or complete tonsillar amputation. Some have argued that neural tissue should be preserved whenever possible.23 Reduction of the cerebellar tonsils appears to be well tolerated. At the time of surgery, cystic changes are often apparent as a consequence of chronic compression and ischemia.24 There is evidence that the cerebellar tonsils have no neurological function, and bilateral tonsillectomy is not associated with neurological deficits.

Although the majority of surgeons who perform duraplasties close the dura, there have been some reports with good outcomes supporting leaving the dura opened25 or stitched laterally to the muscles.26 According to the authors, it is important to preserve the arachnoid plane. Postoperative complications were related to arachnoidal violations. Limonadi et al. reported that a dura-splitting decompression compared with duraplasty can result in reduced operative time, hospital stay, and cost with equivalent early outcome.27 The counterargument is that one return to the operating room for incomplete decompression would significantly tip the cost-Effective analysis in favor of duraplasty.

Duraplasty material is another variable. Duraplasties include autologous and nonautologous graft materials. Autologous grafts may be harvested from the fascia lata, ligamentum nuchae, and pericranium. Nonautologous materials include, in decreasing order of use, bovine pericardium, cadaveric dura, and synthetics. These products may be favorable because they decrease operative time. We prefer autologous pericranium because of the decreased incidence of inflammatory reactions and CSF leaks.28,29

With all of these variables, perhaps the central question is what is necessary and sufficient in an operation to achieve surgical goals. To help answer this question, we have adopted the use of color Doppler ultrasonography (CDU) to guide us intraoperatively.30 Many of the technical decisions are patient specific. The particular variables in our practice include the size of the craniectomy, the levels of the laminectomy, the degree of tonsillar reduction, and the size of the duraplasty. CDU is an important tool because it gives real-time feedback and intraoperative confirmation of the restoration of CSF flow before surgery is completed.30 Some surgeons have also reported that an intraoperative ultrasound may help in deciding whether or not to remove bone only or perform a craniectomy with duraplasty.31

Indications

In this era of MRI and other commonly available imaging technologies, Chiari malformations and syringomyelia are being diagnosed with increasing frequency. In a study of 2000 MRI scans, Chiari malformations comprised 0.9% of unexpected, asymptomatic brain abnormalities on imaging.32 Although the correct diagnosis can usually be established by an MRI scan of the cervical spine, the following workup is strongly recommended: (1) an MRI scan of the brain to rule out hydrocephalus and other causes of acquired tonsillar herniation ( Fig. 23.2 ), (2) an MRI scan of the thoracolumbar spine to rule out spinal cord tethering ( Fig. 23.3 ), (3) cine-MRI to assess CSF velocity/flow at the cervicomedullary junction ( Fig. 23.4 ), and (4) a three-dimensional CT scan to reveal any bony variations (e.g., an incomplete bifid C1 lamina or occipital–C1 assimilation) ( Fig. 23.5 ). The anatomical factors that need to be assessed are the level of cerebellar tonsillar descent, degree of cervicomedullary compression and foramen magnum impaction, presence of skeletal anomalies (basilar impression, platybasia, odontoid alignment, pannus formation, joint hypermobility, and craniocervical instability), and disturbance of CSF circulation (hydro-cephalus or syringomyelia).

Patients develop symptoms related to Chiari malformations from two primary mechanisms: direct neural compression and disturbance of CSF flow.14 The most common symptom is the suboccipital headache that may radiate to the vertex, behind the eyes, or to the shoulders and neck. Cranial nerve signs may include impaired gag reflexes, facial sensory loss, and vocal cord paralysis. Ocular, otoneurological, and cerebellar disturbances are also varied and common. Pediatric patients may demonstrate different clinical manifestations. The youngest patients may present with poor Karnofsky scores related to failure to thrive, because of poor oral intake. Other pediatric patients may have vague behavioral problems before they are diagnosed. Some adolescent patients may present after having a workup for scoliosis, which is associated with syringomyelia. Finally, an incidental Chiari malformation may be found after a traumatic event. These patients are first given conservative treatment, which often involves pain management and clinical monitoring of the syrinx if present.

After conservative remedies have been exhausted, the three main indicators for surgical interventions are poor Karnofsky score (≤ 70), new-onset or progression of syringomyelia (particularly syrinxes occupying > 75% transverse diameter), and severe neurological deficit.

Related posts:

24 Chiari Malformations and Syringomyelia 25 Is Syringomyelia Pathologic, or a Natural Protective Phenomenon? 28 Vertical Mobile and Reducible Atlantoaxial Dislocation 29 Musculoskeletal Changes in Basilar Invagination 30 “Fixed” (Irreducible) Atlantoaxial Dislocation 31 Rotatory Atlantoaxial Dislocation

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